Microwave Technology

First Edition, 2011

ISBN 978-93-81157-23-7

© All rights reserved.

Published by: The English Press 4735/22 Prakashdeep Bldg, Ansari Road, Darya Ganj, Delhi - 110002 Email: [email protected] 

Table of Contents Chapter 1- Introduction to Microwaves Chapter 2 - Cosmic Microwave Background Radiation Chapter 3 - Microwave Transmission Chapter 4 - Microwave Frequency Bands Chapter 5 - Other Microwave Frequency Bands Chapter 6 - Waveguide Chapter 7 - Klystron Chapter 8 - Microstrip Chapter 9 - Waveguide Flange Chapter 10 - Cavity Magnetron Chapter 11 - Diverse Microwave Technologies

Chapter- 1

Introduction to Microwaves

A microwave telecommunications tower on Wrights Hill in Wellington, New Zealand Microwaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz. This broad definition includes both UHF and EHF (millimeter waves), and various sources use different boundaries. In all cases, microwave includes the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum, with RF engineering often putting the lower boundary at 1 GHz (30 cm), and the upper around 100 GHz (3mm). Apparatus and techniques may be described qualitatively as "microwave" when the wavelengths of signals are roughly the same as the dimensions of the equipment, so that lumped-element circuit theory is inaccurate. As a consequence, practical microwave technique tends to move away from the discrete resistors, capacitors, and inductors used with lower frequency radio waves. Instead, distributed circuit elements and transmissionline theory are more useful methods for design and analysis. Open-wire and coaxial transmission lines give way to waveguides and stripline, and lumped-element tuned circuits are replaced by cavity resonators or resonant lines. Effects of reflection, polarization, scattering, diffraction and atmospheric absorption usually associated with visible light are of practical significance in the study of microwave propagation. The same equations of electromagnetic theory apply at all frequencies. While the name may suggest a micrometer wavelength, it is better understood as indicating wavelengths much shorter than those used in radio broadcasting. The boundaries between far infrared light, terahertz radiation, microwaves, and ultra-highfrequency radio waves are fairly arbitrary and are used variously between different fields of study.

Stripline techniques become increasingly necessary at higher frequencies Electromagnetic waves longer (lower frequency) than microwaves are called "radio waves". Electromagnetic radiation with shorter wavelengths may be called "millimeter waves", terahertz radiation or even T-rays. Definitions differ for millimeter wave band, which the IEEE defines as 110 GHz to 300 GHz. Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is effectively opaque, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.

Microwave sources

Vacuum tube devices operate on the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the magnetron, klystron, traveling-wave tube (TWT), and gyrotron. These devices work in the density modulated mode, rather than the current modulated mode. This means that they work on the basis of clumps of electrons flying ballistically through them, rather than using a continuous stream.

Cutaway view inside a cavity magnetron as used in a microwave oven Low power microwave sources use solid-state devices such as the field-effect transistor (at least at lower frequencies), tunnel diodes, Gunn diodes, and IMPATT diodes. A maser is a device similar to a laser, which amplifies light energy by stimulating the emitted radiation. The maser, rather than amplifying light energy, amplifies the lower frequency, longer wavelength microwaves. The sun also emits microwave radiation, and most of it is blocked by Earth's atmosphere. The Cosmic Microwave Background Radiation (CMBR) is a source of microwaves that supports the science of cosmology's Big Bang theory of the origin of the Universe.

Uses Communication

Before the advent of fiber-optic transmission, most long distance telephone calls were carried via networks of microwave radio relay links run by carriers such as AT&T Long Lines. Starting in the early 1950s, frequency division multiplex was used to send up to 5,400 telephone channels on each microwave radio channel, with as many as ten radio channels combined into one antenna for the hop to the next site, up to 70 km away. Wireless LAN protocols, such as Bluetooth and the IEEE 802.11 specifications, also use microwaves in the 2.4 GHz ISM band, although 802.11a uses ISM band and U-NII frequencies in the 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services have been used for almost a decade in many countries in the 3.5– 4.0 GHz range. The FCC recently carved out spectrum for carriers that wish to offer services in this range in the U.S. — with emphasis on 3.65 GHz. Dozens of service providers across the country are securing or have already received licenses from the FCC to operate in this band. The WIMAX service offerings that can be carried on the 3.65 GHz band will give business customers another option for connectivity. Metropolitan area networks: MAN protocols, such as WiMAX (Worldwide Interoperability for Microwave Access) based in the IEEE 802.16 specification. The IEEE 802.16 specification was designed to operate between 2 to 11 GHz. The commercial implementations are in the 2.3 GHz, 2.5 GHz, 3.5 GHz and 5.8 GHz ranges. Wide Area Mobile Broadband Wireless Access: MBWA protocols based on standards specifications such as IEEE 802.20 or ATIS/ANSI HC-SDMA (e.g. iBurst) are designed to operate between 1.6 and 2.3 GHz to give mobility and in-building penetration characteristics similar to mobile phones but with vastly greater spectral efficiency. Some mobile phone networks, like GSM, use the low-microwave/high-UHF frequencies around 1.8 and 1.9 GHz in the Americas and elsewhere, respectively. DVB-SH and SDMB use 1.452 to 1.492 GHz, while proprietary/incompatible satellite radio in the U.S. uses around 2.3 GHz for DARS. Microwave radio is used in broadcasting and telecommunication transmissions because, due to their short wavelength, highly directional antennas are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies). There is also more bandwidth in the microwave spectrum than in the rest of the radio spectrum; the usable bandwidth below 300 MHz is less than 300 MHz while many GHz can be used above 300 MHz. Typically, microwaves are used in television news to transmit a signal from a remote location to a television station from a specially equipped van. Most satellite communications systems operate in the C, X, Ka, or Ku bands of the microwave spectrum. These frequencies allow large bandwidth while avoiding the crowded UHF frequencies and staying below the atmospheric absorption of EHF frequencies. Satellite TV either operates in the C band for the traditional large dish fixed satellite service or Ku band for direct-broadcast satellite. Military communications run primarily over X or Ku-band links, with Ka band being used for Milstar.

Radar Radar uses microwave radiation to detect the range, speed, and other characteristics of remote objects. Development of radar was accelerated during World War II due to its great military utility. Now radar is widely used for applications such as air traffic control, weather forecasting, navigation of ships, and speed limit enforcement. A Gunn diode oscillator and waveguide are used as a motion detector for automatic door openers.

Radio astronomy Most radio astronomy uses microwaves. Usually the naturally-occurring microwave radiation is observed, but active radar experiments have also been done with objects in the solar system, such as determining the distance to the Moon or mapping the invisible surface of Venus through cloud cover.

Galactic background radiation of the Big Bang mapped with increasing resolution

Navigation

Global Navigation Satellite Systems (GNSS) including the Chinese Beidou, the American Global Positioning System (GPS) and the Russian GLONASS broadcast navigational signals in various bands between about 1.2 GHz and 1.6 GHz.

Power A microwave oven passes (non-ionizing) microwave radiation (at a frequency near 2.45 GHz) through food, causing dielectric heating by absorption of energy in the water, fats and sugar contained in the food. Microwave ovens became common kitchen appliances in Western countries in the late 1970s, following development of inexpensive cavity magnetrons. Water in the liquid state possesses many molecular interactions which broaden the absorption peak. In the vapor phase, isolated water molecules absorb at around 22 GHz, almost ten times the frequency of the microwave oven. Microwave heating is used in industrial processes for drying and curing products. Many semiconductor processing techniques use microwaves to generate plasma for such purposes as reactive ion etching and plasma-enhanced chemical vapor deposition (PECVD). Microwave frequencies typically ranging from 110 – 140 GHz are used in stellarators and more notably in tokamak experimental fusion reactors to help heat the fuel into a plasma state. The upcoming ITER Thermonuclear Reactor is expected to range from 110– 170 GHz and will employ Electron Cyclotron Resonance Heating (ECRH). Microwaves can be used to transmit power over long distances, and post-World War II research was done to examine possibilities. NASA worked in the 1970s and early 1980s to research the possibilities of using Solar power satellite (SPS) systems with large solar arrays that would beam power down to the Earth's surface via microwaves. Less-than-lethal weaponry exists that uses millimeter waves to heat a thin layer of human skin to an intolerable temperature so as to make the targeted person move away. A twosecond burst of the 95 GHz focused beam heats the skin to a temperature of 130 °F (54 °C) at a depth of 1/64th of an inch (0.4 mm). The United States Air Force and Marines are currently using this type of Active Denial System.

Spectroscopy Microwave radiation is used in electron paramagnetic resonance (EPR or ESR) spectroscopy, typically in the X-band region (~9 GHz) in conjunction typically with magnetic fields of 0.3 T. This technique provides information on unpaired electrons in chemical systems, such as free radicals or transition metal ions such as Cu(II). The microwave radiation can also be combined with electrochemistry, microwave enhanced electrochemistry.

Microwave frequency bands

The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain (RSGB), are shown in the table below:

ITU Radio Band Numbers 1 2 3 4 5 6 7 8 9 10 11

ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF

NATO Radio bands ABCDEFGHIJKLM

IEEE Radar bands HF VHF UHF L S C X Ku K Ka Q V W

Microwave frequency bands Letter Designation Frequency range L band 1 to 2 GHz S band 2 to 4 GHz C band 4 to 8 GHz X band 8 to 12 GHz Ku band 12 to 18 GHz K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Q band 33 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz E band 60 to 90 GHz W band 75 to 110 GHz F band 90 to 140 GHz D band 110 to 170 GHz Footnote: P band is sometimes incorrectly used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521.

Microwave frequency measurement Microwave frequency can be measured by either electronic or mechanical techniques.

Frequency counters or high frequency heterodyne systems can be used. Here the unknown frequency is compared with harmonics of a known lower frequency by use of a low frequency generator, a harmonic generator and a mixer. Accuracy of the measurement is limited by the accuracy and stability of the reference source. Mechanical methods require a tunable resonator such as an absorption wavemeter, which has a known relation between a physical dimension and frequency.

Wavemeter for measuring in the Ku band In a laboratory setting, Lecher lines can be used to directly measure the wavelength on a transmission line made of parallel wires, the frequency can then be calculated. A similar technique is to use a slotted waveguide or slotted coaxial line to directly measure the

wavelength. These devices consist of a probe introduced into the line through a longitudinal slot, so that the probe is free to travel up and down the line. Slotted lines are primarily intended for measurement of the voltage standing wave ratio on the line. However, provided a standing wave is present, they may also be used to measure the distance between the nodes, which is equal to half the wavelength. Precision of this method is limited by the determination of the nodal locations.

Health effects Microwaves do not contain sufficient energy to chemically change substances by ionization, and so are an example of nonionizing radiation. The word "radiation" refers to the fact that energy can radiate. The term in this context is not to be confused with radioactivity. It has not been shown conclusively that microwaves (or other nonionizing electromagnetic radiation) have significant adverse biological effects at low levels. Some but not all studies suggest that long-term exposure may have a carcinogenic effect. This is separate from the risks associated with very high intensity exposure, which can cause heating and burns like any heat source, and not a unique property of microwaves specifically. During World War II, it was observed that individuals in the radiation path of radar installations experienced clicks and buzzing sounds in response to microwave radiation. This microwave auditory effect was thought to be caused by the microwaves inducing an electric current in the hearing centers of the brain. Research by NASA in the 1970s has shown this to be caused by thermal expansion in parts of the inner ear. When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in the body. Exposure to microwave radiation can produce cataracts by this mechanism, because the microwave heating denatures proteins in the crystalline lens of the eye (in the same way that heat turns egg whites white and opaque) faster than the lens can be cooled by surrounding structures. The lens and cornea of the eye are especially vulnerable because they contain no blood vessels that can carry away heat. Exposure to heavy doses of microwave radiation (as from an oven that has been tampered with to allow operation even with the door open) can produce heat damage in other tissues as well, up to and including serious burns which may not be immediately evident because of the tendency for microwaves to heat deeper tissues with higher moisture content.

History and research The existence of electromagnetic waves was predicted by James Clerk Maxwell in 1864 from his equations. In 1888, Heinrich Hertz was the first to demonstrate the existence of electromagnetic waves by building an apparatus that produced and detected microwaves in the UHF region. The design necessarily used horse-and-buggy materials, including a horse trough, a wrought iron point spark, Leyden jars, and a length of zinc gutter whose parabolic cross-section worked as a reflection antenna. In 1894 J. C. Bose publicly

demonstrated radio control of a bell using millimeter wavelengths, and conducted research into the propagation of microwaves. Perhaps the first, documented, formal use of the term microwave occurred in 1931: "When trials with wavelengths as low as 18 cm were made known, there was undisguised surprise that the problem of the micro-wave had been solved so soon." Telegraph & Telephone Journal XVII. 179/1 In 1943: the Hungarian engineer Zoltán Bay sent ultra-short radio waves to the moon, which, reflected from there worked as a radar, and could be used to measure distance, as well as to study the moon. Perhaps the first use of the word microwave in an astronomical context occurred in 1946 in an article "Microwave Radiation from the Sun and Moon" by Robert Dicke and Robert Beringer. This same article also made a showing in the New York Times issued in 1951. In the history of electromagnetic theory, significant work specifically in the area of microwaves and their applications was carried out by researchers including: Specific work on microwaves Work carried out by Area of work Barkhausen and Kurz Positive grid oscillators Hull Smooth bore magnetron Varian Brothers Velocity modulated electron beam → klystron tube Randall and Boot Cavity magnetron

Chapter- 2

Cosmic Microwave Background Radiation

In cosmology, cosmic microwave background (CMB) radiation (also CMBR, CBR, MBR, and relic radiation) is a form of electromagnetic radiation filling the universe. With a traditional optical telescope, the space between stars and galaxies (the background) is pitch black. But with a radio telescope, there is a faint background glow, almost exactly the same in all directions, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum, hence the name cosmic microwave background radiation. The CMB's serendipitous discovery in 1964 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s, and earned them the 1978 Nobel Prize. The CMBR is well explained as radiation left over from an early stage in the development of the universe, and its discovery is considered a landmark test of the Big Bang model of the universe. When the universe was young, before the formation of stars and planets, it was smaller, much hotter, and filled with a uniform glow from its whitehot fog of hydrogen plasma. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled enough, stable atoms could form. These atoms could no longer absorb the thermal radiation, and the universe became transparent instead of being an opaque fog. The photons that existed at that time have been propagating ever since, though growing fainter and less energetic, since exactly the same photons fill a larger and larger universe. This is the source for the term relic radiation, another name for the CMBR. Precise measurements of cosmic background radiation are critical to cosmology, since any proposed model of the universe must explain this radiation. The CMBR has a thermal black body spectrum at a temperature of 2.725 K, thus the spectrum peaks in the microwave range frequency of 160.2 GHz, corresponding to a 1.9 mm wavelength. This holds if you measure the intensity per unit frequency, as in Planck's law. If instead you measure it per unit wavelength, using Wien's law, the peak will be at 1.06 mm corresponding to a frequency of 283 GHz. The glow is almost but not quite uniform in all directions, and shows a very specific pattern equal to that expected if a fairly uniformly distributed hot gas is expanded to the current size of the universe. In particular, the spatial power spectrum (how much

difference is observed versus how far apart the regions are on the sky) contains small anisotropies, or irregularities, which vary with the size of the region examined. They have been measured in detail, and match what would be expected if small thermal variations, generated by quantum fluctuations of matter in a very tiny space, had expanded to the size of the observable universe we see today. This is still a very active field of study, with scientists seeking both better data (for example, the Planck spacecraft ) and better interpretations of the initial conditions of expansion. Although many different processes might produce the general form of a black body spectrum, no model other than the Big Bang has yet explained the fluctuations. As a result, most cosmologists consider the Big Bang model of the universe to be the best explanation for the CMBR.

Features

The cosmic microwave background spectrum measured by the FIRAS instrument on the COBE satellite is the most-precisely measured black body spectrum in nature. The data points and error bars on this graph are obscured by the theoretical curve.

The cosmic microwave background is isotropic to roughly one part in 100,000: the root mean square variations are only 18 µK, after the dipole anisotropy, which is due to the Doppler shift of the microwave background radiation due to our peculiar velocity relative to the comoving cosmic rest frame, has been subtracted out. This feature is consistent with the Earth moving at some 627 km/s towards the constellation Virgo. The FarInfrared Absolute Spectrophotometer (FIRAS) instrument on the NASA Cosmic Background Explorer (COBE) satellite has carefully measured the spectrum of the cosmic microwave background. The FIRAS project members compared the CMB with an internal reference black body and the spectra agreed to within the experimental error. They concluded that any deviations from the black body form that might still remain undetected in the CMB spectrum over the wavelength range from 0.5 to 5 mm must have a weighted rms value of at most 50 parts per million (0.005%) of the CMB peak brightness. This made the CMB spectrum the most precisely measured black body spectrum in nature. The cosmic microwave background is perhaps the main prediction of the Big Bang model. In addition, Inflationary Cosmology predicts that after about 10−37 seconds the nascent universe underwent exponential growth that smoothed out nearly all inhomogeneities. The exception is inhomogeneities caused by quantum fluctuations in the inflaton field. This was followed by symmetry breaking; a type of phase transition that set the fundamental forces and elementary particles in their present form. After 10−6 seconds, the early universe was made up of a hot plasma of photons, electrons, and baryons. The photons were constantly interacting with the plasma through Thomson scattering. As the universe expanded, adiabatic cooling caused the plasma to cool until it became favorable for electrons to combine with protons and form hydrogen atoms. This recombination event happened at around 3000 K or when the universe was approximately 379,000 years old. This is equivalent to a redshift of z = 1,088. At this point, the photons no longer interacted with the now electrically neutral atoms and began to travel freely through space, resulting in the decoupling of matter and radiation. The color temperature of the photons has continued to diminish ever since; now down to 2.725 K, their temperature will continue to drop as the universe expands. According to the Big Bang model, the radiation from the sky we measure today comes from a spherical surface called the surface of last scattering. This represents the collection of spots in space at which the decoupling event is believed to have occurred, less than 400,000 years after the Big Bang, and at a point in time such that the photons from that distance have just reached observers. The estimated age of the Universe is 13.75 billion years. However, because the Universe has continued expanding since that time, the comoving distance from the Earth to the edge of the observable universe is now at least 46.5 billion light years. The Big Bang theory suggests that the cosmic microwave background fills all of observable space, and that most of the radiation energy in the universe is in the cosmic microwave background, which makes up a fraction of roughly 6×10−5 of the total density of the universe (the photon density is 4.7×10−31 kg/m3, while the critical density is 7.9×10−27 kg/m3 ).

Two of the greatest successes of the big bang theory are its prediction of its almost perfect black body spectrum and its detailed prediction of the anisotropies in the cosmic microwave background. The recent Wilkinson Microwave Anisotropy Probe has precisely measured these anisotropies over the whole sky down to angular scales of 0.2 degrees. These can be used to estimate the parameters of the standard Lambda-CDM model of the big bang. Some information, such as the shape of the Universe, can be obtained straightforwardly from the cosmic microwave background, while others, such as the Hubble constant, are not constrained and must be inferred from other measurements. The latter value gives the redshift of galaxies (interpreted as the recessional velocity) as a proportion of their distance.

Timeline of the CMB Important people and dates

1941

Andrew McKellar was attempting to measure the average temperature of the intestellar medium, and reported the observation of an average bolometric temperature of 2.3 K based on the study of interstellar absorption lines.

1946

Robert Dicke predicts ".. radiation from cosmic matter" at <20 K but did not refer to background radiation

1948

George Gamow calculates a temperature of 50 K (assuming a 3-billion-year old Universe), commenting it ".. is in reasonable agreement with the actual temperature of interstellar space", but does not mention background radiation.

1948

Ralph Alpher and Robert Herman estimate "the

temperature in the Universe" at 5 K. Although they do not specifically mention microwave background radiation, it may be inferred. 1950

Ralph Alpher and Robert Herman re-estimate the temperature at 28 K.

1953

George Gamow estimates 7 K.

1955

Émile Le Roux of the Nançay Radio Observatory, in a sky survey at λ=33 cm, reported a near-isotropic background radiation of 3 kelvins, plus or minus 2.

1956

George Gamow estimates 6 K.

1957

Tigran Shmaonov reports that "the absolute effective temperature of the radioemission background ... is 4±3K". It is noted that the "measurements showed that radiation intensity was independent of either time or direction of observation... it is now clear that Shmaonov did observe the cosmic microwave background at a wavelength of 3.2 cm"

1960s

Robert Dicke re-estimates a MBR (microwave background radiation) temperature of 40 K

1964

A. G. Doroshkevich and Igor Novikov publish a brief paper, where they name the CMB radiation phenomenon

as detectable. 1964– 65

Arno Penzias and Robert Woodrow Wilson measure the temperature to be approximately 3 K. Robert Dicke, P. J. E. Peebles, P. G. Roll, and D. T. Wilkinson interpret this radiation as a signature of the big bang.

1983

RELIKT-1 Soviet CMB anisotropy experiment was launched.

1990

FIRAS on COBE measures the black body form of the CMB spectrum with exquisite precision.

Apr 1992

Scientists who analyzed data from COBE DMR announce the discovery of the primary temperature anisotropy.

1999

First measurements of acoustic oscillations in the CMB anisotropy angular power spectrum from the TOCO, BOOMERANG, and Maxima Experiments.

2002

Polarization discovered by DASI.

2004

E-mode polarization spectrum obtained by the CBI.

2005

Ralph A. Alpher is awarded the National Medal of Science for his groundbreaking work in nucleosynthesis and prediction that the universe expansion leaves behind

background radiation, thus providing a model for the Big Bang theory. 2006

Two of COBE's principal investigators, George Smoot and John Mather, received the Nobel Prize in Physics in 2006 for their work on precision measurement of the CMBR.

History The cosmic microwave background was predicted in 1948 by George Gamow, Ralph Alpher, and Robert Herman. Alpher and Herman were able to estimate the temperature of the cosmic microwave background to be 5 K, though two years later they re-estimated it at 28 K. This high estimate was due to an mis-estimate of the Hubble constant by Alfred Behr, which could not be replicated and was later abandoned for the earlier estimate. Although there were several previous estimates of the temperature of space, these suffered from two flaws. First, they were measurements of the effective temperature of space and did not suggest that space was filled with a thermal Planck spectrum. Next, they depend on our being at a special spot at the edge of the Milky Way galaxy and they did not suggest the radiation is isotropic. The estimates would yield very different predictions if Earth happened to be located elsewhere in the Universe. The 1948 results of Alpher and Herman were discussed in many physics settings through about 1955, when each left the Applied Physics Laboratory at Johns Hopkins University. The mainstream astronomical community, however, was not intrigued at the time by cosmology. Alpher and Herman's prediction was rediscovered by Yakov Zel'dovich in the early 1960s, and independently predicted by Robert Dicke at the same time. The first published recognition of the CMB radiation as a detectable phenomenon appeared in a brief paper by Soviet astrophysicists A. G. Doroshkevich and Igor Novikov, in the spring of 1964. In 1964, David Todd Wilkinson and Peter Roll, Dicke's colleagues at Princeton University, began constructing a Dicke radiometer to measure the cosmic microwave background. In 1965, Arno Penzias and Robert Woodrow Wilson at the Crawford Hill location of Bell Telephone Laboratories in nearby Holmdel Township, New Jersey had built a Dicke radiometer that they intended to use for radio astronomy and satellite communication experiments. Their instrument had an excess 3.5 K antenna temperature which they could not account for. After receiving a telephone call from Crawford Hill, Dicke famously quipped: "Boys, we've been scooped." A meeting between the Princeton and Crawford Hill groups determined that the antenna temperature was indeed due to the microwave background. Penzias and Wilson received the 1978 Nobel Prize in Physics for their discovery.

The interpretation of the cosmic microwave background was a controversial issue in the 1960s with some proponents of the steady state theory arguing that the microwave background was the result of scattered starlight from distant galaxies. Using this model, and based on the study of narrow absorption line features in the spectra of stars, the astronomer Andrew McKellar wrote in 1941: "It can be calculated that the 'rotational temperature' of interstellar space is 2 K." However, during the 1970s the consensus was established that the cosmic microwave background is a remnant of the big bang. This was largely because new measurements at a range of frequencies showed that the spectrum was a thermal, black body spectrum, a result that the steady state model was unable to reproduce.

The Holmdel Horn Antenna on which Penzias and Wilson discovered the cosmic microwave background. Harrison, Peebles, Yu and Zel'dovich realized that the early universe would have to have inhomogeneities at the level of 10−4 or 10−5. Rashid Sunyaev later calculated the observable imprint that these inhomogeneities would have on the cosmic microwave background. Increasingly stringent limits on the anisotropy of the cosmic microwave background were set by ground based experiments during the 1980s. The NASA COBE mission clearly detected the primary anisotropy with the Differential Microwave

Radiometer instrument, publishing their findings in 1992. The team received the Nobel Prize in physics for 2006 for this discovery. Inspired by the COBE results, a series of ground and balloon-based experiments measured cosmic microwave background anisotropies on smaller angular scales over the next decade. The primary goal of these experiments was to measure the scale of the first acoustic peak, which COBE did not have sufficient resolution to resolve. This peak corresponds to large scale density variations in the early universe that are created by gravitational instabilities, resulting in acoustical oscillations in the plasma. The first peak in the anisotropy was tentatively detected by the Toco experiment and the result was confirmed by the BOOMERanG and MAXIMA experiments. These measurements demonstrated that the geometry of the Universe is approximately flat, rather than curved. They ruled out cosmic strings as a major component of cosmic structure formation and suggested cosmic inflation was the right theory of structure formation. The second peak was tentatively detected by several experiments before being definitively detected by WMAP, which has also tentatively detected the third peak. As of 2010, several experiments to improve measurements of the polarization and the microwave background on small angular scales are ongoing. These include DASI, WMAP, BOOMERanG, QUaD, Planck spacecraft, Atacama Cosmology Telescope, South Pole Telescope and the QUIET telescope.

WMAP image of the CMB temperature anisotropy.

Relationship to the Big Bang The cosmic microwave background radiation and the cosmological red shift are together regarded as the best available evidence for the Big Bang theory. Measurements of the CMB have made the inflationary Big Bang theory the standard model of the earliest eras

of the universe. The discovery of the CMB in the mid-1960s curtailed interest in alternatives such as the steady state theory. The Big Bang theory predicts that the initial conditions for the universe are originally random in nature, and inhomogeneities follow a roughly Gaussian probability distribution, which, when graphed in cross-section, form bell-shaped curves. By analyzing this distribution at different frequencies, a spectral density or power spectrum is generated. The power spectrum of these fluctuations has been calculated, and agrees with the observations. The resulting standard model of the Big Bang uses a Gaussian random field with a nearly scale invariant or Harrison-Zel'dovich spectrum to represent the primeval inhomogeneities. Certain observables, for example the overall amplitude of the fluctuations, are more or less free parameters of the cosmic inflation model. Therefore, meaningful statements about the inhomogeneities in the universe need to be statistical in nature. This leads to cosmic variance in which the uncertainties in the variance of fluctuations at the largest scale observed are difficult to accurately compare to theory.

Temperature The CMB gives a snapshot of the Universe when, according to standard cosmology, the temperature dropped enough to allow electrons and protons to form hydrogen atoms, thus making the universe transparent to radiation. When it originated some 380,000 years after the Big Bang—this time is generally known as the "time of last scattering" or the period of recombination or decoupling—the temperature of the Universe was about 3000 K. This corresponds to an energy of about 0.25 eV, which is much less than the 13.6 eV ionization energy of hydrogen. Since decoupling, the temperature of the background radiation has dropped by a factor of roughly 1,100 due to the expansion of the Universe. As the Universe expands, the CMB photons are redshifted, making the radiation's temperature inversely proportional to a parameter called the Universe's scale length. The temperature Tr of the CMB as a function of redshift, z, can be shown to be proportional to the temperature of the CMB as observed in the present day (2.725 K or 0.235 meV): Tr = 2.725(1 + z)

Primary anisotropy

The power spectrum of the cosmic microwave background radiation temperature anisotropy in terms of the angular scale (or multipole moment). The data shown come from the WMAP (2006), Acbar (2004) Boomerang (2005), CBI (2004), and VSA (2004) instruments. Also shown is a theoretical model (solid line). The anisotropy of the cosmic microwave background is divided into two sorts: primary anisotropy, due to effects which occur at the last scattering surface and before; and secondary anisotropy, due to effects such as interactions of the background radiation with hot gas or gravitational potentials, which occur between the last scattering surface and the observer. The structure of the cosmic microwave background anisotropies is principally determined by two effects: acoustic oscillations and diffusion damping (also called collisionless damping or Silk damping). The acoustic oscillations arise because of a competition in the photon-baryon plasma in the early universe. The pressure of the photons tends to erase anisotropies, whereas the gravitational attraction of the baryons—moving at speeds much slower than light—makes them tend to collapse to form dense haloes. These two effects compete to create acoustic oscillations which give the microwave background its characteristic peak structure. The peaks correspond, roughly, to resonances in which the photons decouple when a particular mode is at its peak amplitude.

The peaks contain interesting physical signatures. The angular scale of the first peak determines the curvature of the Universe (but not the topology of the Universe). The next peak—ratio of the odd peaks to the even peaks—determines the reduced baryon density. The third peak can be used to pull information about the dark matter density. The locations of the peaks also give important information about the nature of the primordial density perturbations. There are two fundamental brands of density perturbations—called adiabatic and isocurvature. A general density perturbation is a mixture of both, and different theories that purport to explain the primordial density perturbation spectrum predict different mixtures. •

Adiabatic density perturbations the fractional overdensity in each matter component (baryons, photons ...) is the same. That is, if there is 1% more energy in baryons than average in one spot, then with a pure adiabatic density perturbations there is also 1% more energy in photons, and 1% more energy in neutrinos, than average. Cosmic inflation predicts that the primordial perturbations are adiabatic.

•

Isocurvature density perturbations the sum of the fractional overdensities is zero. That is, a perturbation where at some spot there is 1% more energy in baryons than average, 1% more energy in photons than average, and 2% less energy in neutrinos than average, would be a pure isocurvature perturbation. Cosmic strings would produce mostly isocurvature primordial perturbations.

The CMB spectrum is able to distinguish these two because these two brands of perturbations produce different peak locations. Isocurvature density perturbations produce a series of peaks whose angular scales (l-values of the peaks) are roughly in the ratio 1:3:5:..., while adiabatic density perturbations produce peaks whose locations are in the ratio 1:2:3:... Observations are consistent with the primordial density perturbations being entirely adiabatic, providing key support for inflation, and ruling out many models of structure formation involving, for example, cosmic strings. Collisionless damping is caused by two effects, when the treatment of the primordial plasma as fluid begins to break down: • •

the increasing mean free path of the photons as the primordial plasma becomes increasingly rarefied in an expanding universe the finite depth of the last scattering surface (LSS), which causes the mean free path to increase rapidly during decoupling, even while some Compton scattering is still occurring.

These effects contribute about equally to the suppression of anisotropies on small scales, and give rise to the characteristic exponential damping tail seen in the very small angular scale anisotropies. The depth of the LSS refers to the fact that the decoupling of the photons and baryons does not happen instantaneously, but instead requires an appreciable fraction of the age of the Universe up to that era. One method to quantify exactly how long this process took uses the photon visibility function (PVF). This function is defined so that, denoting the PVF by P(t), the probability that a CMB photon last scattered between time t and t+dt is given by P(t)dt. The maximum of the PVF (the time where it is most likely that a given CMB photon last scattered) is known quite precisely. The first-year WMAP results put the time at which P(t) is maximum as 372±14 ka. This is often taken as the "time" at which the CMB formed. However, to figure out how long it took the photons and baryons to decouple, we need a measure of the width of the PVF. The WMAP team finds that the PVF is greater than half of its maximum value (the "full width at half maximum", or FWHM) over an interval of 115±5 ka. By this measure, decoupling took place over roughly 115,000 years, and when it was complete, the universe was roughly 487,000 years old.

Late time anisotropy Since the CMB came into existence, it has apparently been modified by several subsequent physical processes, which are collectively referred to as late-time anisotropy, or secondary anisotropy. When the CMB photons became free to travel unimpeded, ordinary matter in the universe was mostly in the form of neutral hydrogen and helium atoms. However, observations of galaxies today seem to indicate that most of the volume of the intergalactic medium (IGM) consists of ionized material (since there are few absorption lines due to hydrogen atoms). This implies a period of reionization during which some of the material of the universe was broken into hydrogen ions. The CMB photons scatter off free charges such as electrons that are not bound in atoms. In an ionized universe, such charged particles have been liberated from neutral atoms by ionizing (ultraviolet) radiation. Today these free charges are at sufficiently low density in most of the volume of the Universe that they do not measurably affect the CMB. However, if the IGM was ionized at very early times when the universe was still denser, then there are two main effects on the CMB: 1. Small scale anisotropies are erased. (Just as when looking at an object through fog, details of the object appear fuzzy.) 2. The physics of how photons scatter off from free electrons (Thomson scattering) induces polarization anisotropies on large angular scales. This broad angle polarization is correlated with the broad angle temperature perturbation. Both of these effects have been observed by the WMAP spacecraft, providing evidence that the universe was ionized at very early times, at a redshift more than 17. The detailed

provenance of this early ionizing radiation is still a matter of scientific debate. It may have included starlight from the very first population of stars (population III stars), supernovae when these first stars reached the end of their lives, or the ionizing radiation produced by the accretion disks of massive black holes. The time following the emission of the Cosmic Microwave Background—and before the observation of the first stars—is semi-humorously referred to by cosmologists as the dark age, and is a period which is under intense study by astronomers. Two other effects which occurred between reionization and our observations of the Cosmic Microwave Background, and which appear to cause anisotropies, include the Sunyaev-Zel'dovich effect, where a cloud of high energy electrons scatters the radiation, transferring some of its energy to the CMB photons, and the Sachs-Wolfe effect, which causes photons from the Cosmic Microwave Background to be gravitationally redshifted or blueshifted due to changing gravitational fields.

E polarization measurements as of March 2008 in terms of angular scale (or multipole moment). The polarization is much more poorly measured than the temperature anisotropy.

Polarization The cosmic microwave background is polarized at the level of a few microkelvins. There are two types of polarization, called E-modes and B-modes. This is in analogy to

electrostatics, in which the electric field (E-field) has a vanishing curl and the magnetic field (B-field) has a vanishing divergence. The E-modes arise naturally from Thomson scattering in a heterogeneous plasma. The B-modes, which have not been measured and are thought to have an amplitude of at most a 0.1 µK, are not produced from the plasma physics alone. They are a signal from cosmic inflation and are determined by the density of primordial gravitational waves. Detecting the B-modes will be extremely difficult, particularly given that the degree of foreground contamination is unknown, and the weak gravitational lensing signal mixes the relatively strong E-mode signal with the B-mode signal.

Microwave background observations Subsequent to the discovery of the CMB, hundreds of cosmic microwave background experiments have been conducted to measure and characterize the signatures of the radiation. The most famous experiment is probably the NASA Cosmic Background Explorer (COBE) satellite that orbited in 1989–1996 and which detected and quantified the large scale anisotropies at the limit of its detection capabilities. Inspired by the initial COBE results of an extremely isotropic and homogeneous background, a series of ground- and balloon-based experiments quantified CMB anisotropies on smaller angular scales over the next decade. The primary goal of these experiments was to measure the angular scale of the first acoustic peak, for which COBE did not have sufficient resolution. These measurements were able to rule out cosmic strings as the leading theory of cosmic structure formation, and suggested cosmic inflation was the right theory. During the 1990s, the first peak was measured with increasing sensitivity and by 2000 the BOOMERanG experiment reported that the highest power fluctuations occur at scales of approximately one degree. Together with other cosmological data, these results implied that the geometry of the Universe is flat. A number of ground-based interferometers provided measurements of the fluctuations with higher accuracy over the next three years, including the Very Small Array, Degree Angular Scale Interferometer (DASI), and the Cosmic Background Imager (CBI). DASI made the first detection of the polarization of the CMB and the CBI provided the first E-mode polarization spectrum with compelling evidence that it is out of phase with the T-mode spectrum. In June 2001, NASA launched a second CMB space mission, WMAP, to make much more precise measurements of the great scale anisotropies over the full sky. WMAP used symmetric, rapid-multi-modulated scanning, rapid switching radiometers to minimize non-sky signal noise. The first results from this mission, disclosed in 2003, were detailed measurements of the angular power spectrum to below degree scales, tightly constraining various cosmological parameters. The results are broadly consistent with those expected from cosmic inflation as well as various other competing theories, and are available in detail at NASA's data bank for Cosmic Microwave Background (CMB). Although WMAP provided very accurate measurements of the great angular-scale fluctuations in the CMB (structures about as broad in the sky as the moon), it did not have the angular resolution to measure the smaller scale fluctuations which had been observed by former ground-based interferometers.

A third space mission, the Planck Surveyor, launched in May, 2009. Planck employs both HEMT radiometers as well as bolometer technology and will measure the CMB on smaller scales than WMAP. Unlike the previous two space missions, Planck is run by the ESA (the European Space Agency). Its detectors got a trial run at the Antarctic Viper telescope as ACBAR (Arcminute Cosmology Bolometer Array Receiver) experiment— which has produced the most precise measurements at small angular scales to date—and at the Archeops balloon telescope. Additional ground-based instruments such as the South Pole Telescope in Antarctica and the proposed Clover Project, Atacama Cosmology Telescope and the QUIET telescope in Chile will provide additional data not available from satellite observations, possibly including the B-mode polarization.

Data reduction and analysis Raw CMBR data coming down from the space vehicle (i.e., WMAP) contain foreground effects that completely obscure the fine-scale structure of the Cosmic Microwave background. The fine-scale structure is superimposed on the raw CMBR data but is too small to be seen at the scale of the raw data. The most prominent of the foreground effects is the dipole anisotropy caused by the Sun's motion relative to the CMBR background. The dipole anisotropy and others due to Earth's annual motion relative to the Sun and numerous microwave sources in the galactic plane and elsewhere must be subtracted out to reveal the extremely tiny variations characterizing the fine-scale structure of the CMBR background. The detail analysis of CMBR data to produce maps, an angular power spectrum, and ultimately cosmological parameters is a complicated, computationally difficult problem. Although computing a power spectrum from a map is in principle a simple Fourier transform, decomposing the map of the sky into spherical harmonics, in practice it is hard to take the effects of noise and foreground sources into account. In particular, these foregrounds are dominated by galactic emissions such free-free, synchrotron, and dust that emit in the microwave band; in practice, the galaxy has to be removed resulting in a CMB map that is not a full-sky map. In addition, point sources like galaxies and clusters represent another source of foreground which must be removed lest they distort the short scale structure of the CMB power spectrum. Constraints on many cosmological parameters can be obtained from their effects on the power spectrum, and results are often calculated using Markov Chain Monte Carlo sampling techniques.

CMBR dipole anisotropy From the CMB data it is seen that our local group of galaxies (the galactic cluster that includes the Solar System's Milky Way Galaxy) appears to be moving at 627±22 km/s relative to the reference frame of the CMB (also called the CMB rest frame, or the frame of reference in which there is no motion through the CMB) in the direction of

galactic longitude l = 276±3°, b = 30±3°. This motion results in an anisotropy of the data (CMB appearing slightly warmer in the direction of movement than in the opposite direction). The standard interpretation of this temperature variation is a simple velocity redshift and blueshift due to motion relative to the CMB, but alternative cosmological models can explain some fraction of the observed dipole temperature distribution in the CMB.

Low multipoles and other anomalies With the increasingly precise data provided by WMAP, there have been a number of claims that the CMB suffers from anomalies, such as very great-scale anisotropies, anomalous alignments, and non-Gaussian distributions. The most longstanding of these is the low-l multipole controversy. Even in the COBE map, it was observed that the quadrupole (l=2 spherical harmonic) has a low amplitude compared to the predictions of the big bang. Some observers have pointed out that the anisotropies in the WMAP data did not appear to be consistent with the big bang picture. In particular, the quadrupole and octupole (l=3) modes appear to have an unexplained alignment with each other and with the ecliptic plane, an alignment sometimes referred to as the axis of evil. A number of groups have suggested that this could be the signature of new physics at the greatest observable scales. Ultimately, due to the foregrounds and the cosmic variance problem, the greatest modes will never be as well measured as the small angular scale modes. The analyses were performed on two maps that have had the foregrounds removed as best as is possible: the "internal linear combination" map of the WMAP collaboration and a similar map prepared by Max Tegmark and others. Later analyses have pointed out that these are the modes most susceptible to foreground contamination from synchrotron, dust, and free-free emission, and from experimental uncertainty in the monopole and dipole. A full Bayesian analysis of the WMAP power spectrum demonstrates that the quadrupole prediction of Lambda-CDM cosmology is consistent with the data at the 10% level and that the observed octupole is not remarkable. Carefully accounting for the procedure used to remove the foregrounds from the full sky map further reduces the significance of the alignment by ~5%.

Chapter- 3

Microwave Transmission

The atmospheric attenuation of microwaves in dry air with a precipitable water vapor level of 0.001 mm. The downward spikes in the graph correspond to frequencies at which microwaves are absorbed more strongly, such as by oxygen molecules Microwave transmission refers to the technology of transmitting information by the use of the radio waves whose wavelengths are conveniently measured in small numbers of centimeters, by using various electronic technologies. These are called microwaves. This part of the radio spectrum ranges across frequencies of roughly 1.0 gigahertz (GHz) to 30 GHz. Also by using the formula λ = c/f , these correspond to wavelengths from 30 centimeters down to 1.0 cm. [In the above equation, the Greek letter λ ( lambda ) is

the wavelength in meters; c is the speed of light in meters per second; and f is the frequency in hertz (Hz).] In the microwave frequency band, antennas are usually of convenient sizes and shapes, and also the use of metal waveguides for carrying the radio power works well. Furthermore, with the use of the modern solid-state electronics and traveling wave tube technologies that have been developed since the early 1960s, the electronics used by microwave radio transmission have been readily used by expert electronics engineers. Microwave radio transmission is commonly used by communication systems on the surface of the Earth, in satellite communications, and in deep space radio communications. Other parts of the microwave radio band are used for radars, radio navigation systems, sensor systems, and radio astronomy. The next higher part of the radio electromagnetic spectrum, where the frequencies are above 30 GHz and below 100 GHz, are called "millimeter waves" because their wavelengths are conveniently measured in millimeters, and their wavelengths range from 10 mm down to 3.0 mm. Radio waves in this band are usually strongly attenuated by the Earthly atmosphere and particles contained in it, especially during wet weather. Also, in wide band of frequencies around 60 GHz, the radio waves are strongly attenuated by molecular oxygen in the atmosphere. The electronic technologies needed in the millimeter wave band are also much more difficult to utilize than those of the microwave band.

Properties • • •

Suitable over line-of-sight transmission links without obstacles Provides good bandwidth Affected by rain, vapor, dust, snow, cloud, mist and fog, heavy moisture, depending on chosen frequency

Uses • • •

Backbone or backhaul carriers in cellular networks. Used to link BTS-BSC and BSC-MSC. Communication with satellites Microwave radio relay links for television and telephone service providers

A parabolic antenna for Erdfunkstelle Raisting, based in Raisting, Bavaria, Germany.

Military microwave set in Switzerland

Parabolic (microwave) antenna A parabolic antenna is a high-gain reflector antenna used for radio, television and data communications, and also for radiolocation (radar), on the UHF and SHF parts of the electromagnetic spectrum. The relatively short wavelength of electromagnetic radiation at these frequencies allows reasonably sized reflectors to exhibit the desired highly directional response for both receiving and transmitting.

Microwave power transmission

Microwave power transmission (MPT) is the use of microwaves to transmit power through outer space or the atmosphere without the need for wires. It is a sub-type of the more general wireless energy transfer methods.

History Following World War II, which saw the development of high-power microwave emitters known as cavity magnetrons, the idea of using microwaves to transmit power was researched. In 1964, William C. Brown demonstrated a miniature helicopter equipped with a combination antenna and rectifier device called a rectenna. The rectenna converted microwave power into electricity, allowing the helicopter to fly. In principle, the rectenna is capable of very high conversion efficiencies - over 90% in optimal circumstances. Most proposed MPT systems now usually include a phased array microwave transmitter. While these have lower efficiency levels they have the advantage of being electrically steered using no moving parts, and are easier to scale to the necessary levels that a practical MPT system requires. Using microwave power transmission to deliver electricity to communities without having to build cable-based infrastructure is being studied at Grand Bassin on Reunion Island in the Indian Ocean.

Common safety concerns The common reaction to microwave transmission is one of concern, as microwaves are generally perceived by the public as dangerous forms of radiation - stemming from the fact that they are used in microwave ovens. While high power microwaves can be painful and dangerous as in the United States Military's Active Denial System, MPT systems are generally proposed to have only low intensity at the rectenna. Though this would be extremely safe as the power levels would be about equal to the leakage from a microwave oven, and only slightly more than a cell phone, the relatively diffuse microwave beam necessitates a large rectenna area for a significant amount of energy to be transmitted. Research has involved exposing multiple generations of animals to microwave radiation of this or higher intensity, and no health issues have been found.

Proposed uses MPT is the most commonly proposed method for transferring energy to the surface of the Earth from solar power satellites or other in-orbit power sources. MPT is occasionally proposed for the power supply in [beam-powered propulsion] for orbital lift space ships. Even though lasers are more commonly proposed, their low efficiency in light generation and reception has led some designers to opt for microwave based systems.

Current status Wireless Power Transmission (using microwaves) is well proven. Experiments in the tens of kilowatts have been performed at Goldstone in California in 1975 and more recently (1997) at Grand Bassin on Reunion Island. In 2008 a long range transmission experiment successfully transmitted 20 watts 92 miles from a mountain on Maui to the main island of Hawaii.

Microwave radio relay

Heinrich-Hertz-Turm in Germany

Microwave radio relay is a technology for transmitting digital and analog signals, such as long-distance telephone calls and the relay of television programs to transmitters, between two locations on a line of sight radio path. In microwave radio relay, radio waves are transmitted between the two locations with directional antennas, forming a fixed radio connection between the two points. Long daisy-chained series of such links form transcontinental telephone and/or television communication systems.

How microwave radio relay links are formed

Relay towers on Frazier Mountain, Southern California Because a line of sight radio link is made, the radio frequencies used occupy only a narrow path between stations (with the exception of a certain radius of each station).

Antennas used must have a high directive effect; these antennas are installed in elevated locations such as large radio towers in order to be able to transmit across long distances. Typical types of antenna used in radio relay link installations are parabolic reflectors, shell antennas and horn radiators, which have a diameter of up to 4 meters. Highly directive antennas permit an economical use of the available frequency spectrum, despite long transmission distances.

Danish military radio relay node

Planning considerations Because of the high frequencies used, a quasi-optical line of sight between the stations is generally required. Additionally, in order to form the line of sight connection between the two stations, the first Fresnel zone must be free from obstacles so the radio waves can propagate across a nearly uninterrupted path. Obstacles in the signal field cause unwanted attenuation, and are as a result only acceptable in exceptional cases. High mountain peak or ridge positions are often ideal: Europe's highest radio relay station, the Richtfunkstation Jungfraujoch, is situated atop the Jungfraujoch ridge at an altitude of 3,705 meters (12,156 ft) above sea level.

Multiple antennas provide space diversity Obstacles, the curvature of the Earth, the geography of the area and reception issues arising from the use of nearby land (such as in manufacturing and forestry) are important issues to consider when planning radio links. In the planning process, it is essential that "path profiles" are produced, which provide information about the terrain and Fresnel zones affecting the transmission path. The presence of a water surface, such as a lake or river, in the mid-path region also must be taken into consideration as it can result in a near-perfect reflection (even modulated by wave or tide motions), creating multipath distortion as the two received signals ("wanted" and "unwanted") swing in and out of phase. Multipath fades are usually deep only in a small spot and a narrow frequency band, so space and frequency diversity schemes were usually applied in the third quarter of the 20th century.

The effects of atmospheric stratification cause the radio path to bend downward in a typical situation so a major distance is possible as the earth equivalent curvature increases from 6370 km to about 8500 km (a 4/3 equivalent radius effect). Rare events of temperature, humidity and pressure profile versus height, may produce large deviations and distortion of the propagation and affect transmission quality. High intensity rain and snow must also be considered as an impairment factor, especially at frequencies above 10 GHz. All previous factors, collectively known as path loss, make it necessary to compute suitable power margins, in order to maintain the link operative for a high percentage of time, like the standard 99.99% or 99.999% used in 'carrier class' services of most telecommunication operators.

Portable microwave rig for television news

Over-horizon microwave radio relay In over-horizon, or tropospheric scatter, microwave radio relay, unlike a standard microwave radio relay link, the sending and receiving antennas do not use a line of sight transmission path. Instead, the stray signal transmission, known as "tropo - scatter" or simply "scatter," from the sent signal is picked up by the receiving station. Signal clarity obtained by this method depends on the weather and other factors, and as a result a high level of technical difficulty is involved in the creation of a reliable over horizon radio relay link. Over horizon radio relay links are therefore only used where standard radio relay links are unsuitable (for example, in providing a microwave link to an island).

Usage of microwave radio relay systems During the 1950s the AT&T Communications system of microwave radio grew to carry the majority of US Long Distance telephone traffic, as well as intercontinental television network signals. The prototype was called TDX and was tested with a connection between New York City and Murray Hill, the location of Bell Laboratories in 1946. The TDX system was set up between New York and Boston in 1947. The TDX was improved to the TD2, which still used klystrons, and then later to the TD3 that used solid state electronics. The main motivation in 1946 to use microwave radio instead of cable was that a large capacity could be installed quickly and at less cost. It was expected at that time that the annual operating costs for microwave radio would be greater than for cable. There were two main reasons that a large capacity had to be introduced suddenly: Pent up demand for long distance telephone service, because of the hiatus during the war years, and the new medium of television, which needed more bandwidth than radio. Similar systems were soon built in many countries, until the 1980s when the technology lost its share of fixed operation to newer technologies such as fiber-optic cable and optical radio relay links, both of which offer larger data capacities at lower cost per bit. Communication satellites, which are also microwave radio relays, better retained their market share, especially for television. At the turn of the century, microwave radio relay systems are being used increasingly in portable radio applications. The technology is particularly suited to this application because of lower operating costs, a more efficient infrastructure, and provision of direct hardware access to the portable radio operator.

Microwave link A microwave link is a communications system that uses a beam of radio waves in the microwave frequency range to transmit video, audio, or data between two locations, which can be from just a few feet or meters to several miles or kilometers apart. Microwave links are commonly used by television broadcasters to transmit programmes across a country, for instance, or from an outside broadcast back to a studio.

Mobile units can be camera mounted, allowing cameras the freedom to move around without trailing cables. These are often seen on the touchlines of sports fields on Steadicam systems. Properties of microwave links • • • • •

Involve line of sight (LOS) communication technology Affected greatly by environmental constraints, including rain fade Have limited penetration capabilities Sensitive to high pollen count Signals can be degraded during Solar proton events

Uses of microwave links • • •

In communications between satellites and base stations As backbone carriers for cellular systems In short range indoor communications

Tunable microwave device A tunable microwave device is a device that works at radio frequency range with the dynamic tunable capabilities, especially an electric field. The material systems for such a device usually have multilayer structure. Usually, magnetic or ferroelectric film on ferrite or superconducting film is adopted. The former two are used as the property tunable component to control the working frequency of the whole system. Devices of this type include tunable varators, tunable microwave filters, tunable phase shifters, and tunable resonators. The main application of them is re-configurable microwave networks, for example, reconfigurable wireless communication, wireless network, and reconfigurable phase array antenna.

Chapter- 4

Microwave Frequency Bands

L band

L band Frequency range

IEEE: ~1 – 2 GHz NATO: 40 – 60 GHz

ITU Radio Band Numbers 1 2 3 4 5 6 7 8 9 10 11

ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF

NATO Radio bands ABCDEFGHIJKLM

IEEE Radar bands HF VHF UHF L S C X Ku K Ka Q V W

L band refers to four different bands of the electromagnetic spectrum: 40 to 60 GHz (NATO), 1 to 2 GHz (IEEE), 1565 nm to 1625 nm (optical), and around 3.5 micrometres (infrared astronomy).

NATO L band The NATO L band is defined as the frequency band between 40 and 60 GHz (5– 7.5 mm).

IEEE L band

The IEEE L band (20-cm radar long-band) is a portion of the microwave band of the electromagnetic spectrum ranging roughly from 1 to 2 GHz. It is used by some communications satellites, and for some terrestrial Eureka 147 digital audio broadcasting (DAB). The amateur radio service also has an allocation between 1240 and 1300 MHz (23-centimeter band). The L band refers to the frequency range of 950 MHz to 1450 MHz. Satellite modems and television receivers work in this range, and the signal is translated to and from the band the satellite uses by either dedicated upconverters/downconverters or a solid-state Low-noise block converter and Block upconverter.

Military use In the United States and overseas territories, the L band is held by the military for telemetry, thereby forcing digital radio to in-band on-channel (IBOC) solutions. DAB is typically done in the 1452–1492-MHz range as in most of the world, but other countries also use VHF and UHF bands.

GNSS The Global Positioning System carriers are in the L band, centered at 1176.45 MHz (L5), 1227.60 MHz (L2), 1381.05 MHz (L3), and 1575.42 MHz (L1) frequencies. • •

The Galileo Navigation System uses the L-band similarly to GPS. The GLONASS System uses the L-band similarly to GPS.

Telecommunications use GSM mobile phones operate at 800–900 and 1800–1900 MHz. Iridium Satellite LLC phones use frequencies between 1616 and 1626.5 MHz to communicate with the satellites

Digital Audio Broadcasting (Earth Orbital) WorldSpace satellite radio broadcasts in the 1467–1492 MHz L sub-band.

Amateur radio •

The Radio Regulations of the International Telecommunication Union allow amateur radio operations in the frequency range from 1240 to 1300 MHz.

DAB L band usage The following blocks are used for T-DAB (terrestrial) broadcasts: Block Center Frequency

LA LB LC LD LE LF LG LH LI LJ LK LL LM LN LO LP

1452.960 MHz 1454.672 MHz 1456.384 MHz 1458.096 MHz 1459.808 MHz 1461.520 MHz 1463.232 MHz 1464.944 MHz 1466.656 MHz 1468.368 MHz 1470.080 MHz 1471.792 MHz 1473.504 MHz 1475.216 MHz 1476.928 MHz 1478.640 MHz

The following blocks are used for S-DAB (satellite) broadcasts: Block Center Frequency LQ 1480.352 MHz LR 1482.064 MHz LS 1483.776 MHz LT 1485.488 MHz LU 1487.200 MHz LV 1488.912 MHz LW 1490.624 MHz Note: Canada uses slightly different central frequencies for L-band DAB while in many European countries DAB is limited part of Band III due to television and mobile two way radio using the rest.

Physics issues relating to band use The band also contains the hyperfine transition of neutral hydrogen (the hydrogen line, 1420 MHz), which is of great astronomical interest as a means of imaging the normally invisible neutral atomic hydrogen in interstellar space. Consequently parts of the L-band are protected radio astronomy allocations worldwide.

Optical communications L band

L band is also used in optical communications to refer to the wavelength range 1565 nm to 1625 nm.

Infrared astronomy

Atmospheric windows in the infrared. The L band is the transmission window centred on 3.5 micrometres In infrared astronomy, the L band refers to an atmospheric transmission window centred on 3.5 micrometres (in the mid-infrared).

Other microwave bands The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain (RSGB), are shown in the table below: L band 1 to 2 GHz S band 2 to 4 GHz C band 4 to 8 GHz X band 8 to 12 GHz Ku band 12 to 18 GHz K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz

Q band U band V band E band W band F band D band

30 to 50 GHz 40 to 60 GHz 50 to 75 GHz 60 to 90 GHz 75 to 110 GHz 90 to 140 GHz 110 to 170 GHz

Footnote: P band is sometimes incorrectly used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521.

S band S band Frequency range

2 – 4 GHz

ITU Radio Band Numbers 1 2 3 4 5 6 7 8 9 10 11

ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF

NATO Radio bands ABCDEFGHIJKLM

IEEE Radar bands HF VHF UHF L S C X Ku K Ka Q V W

The S band (named for Short wave) is defined by an IEEE standard for radio waves with frequencies that range from 2 to 4 GHz, crossing the conventional boundary between UHF and SHF at 3.0 GHz. It is part of the microwave band of the electromagnetic spectrum. The S band is used by weather radar, surface ship radar, and some communications satellites, especially those used by NASA to communicate with the Space Shuttle and the International Space Station. The 10-cm radar short-band ranges roughly from 1.55 to 5.2 GHz. •

Amateur radio and amateur television operators use 2300-2310 MHz and 23902450 MHz also 3300-3500 MHz.

In the U.S., the FCC approved Digital Audio Radio Satellite (DARS) broadcasts in the S band from 2.31 to 2.36 GHz, currently used by Sirius XM Radio. More recently, it has approved for portions of the S band between 2.0 and 2.2 GHz the creation of Mobile

Satellite Service (MSS) networks in connection with Ancillary Terrestrial Components (ATC). There are presently a number of companies attempting to deploy such networks, including ICO Satellite Management and TerreStar. The 2.6 GHz range is used for China Multimedia Mobile Broadcasting, a satellite radio and mobile TV standard which, as with proprietary systems in the U.S., is incompatible with the open standards used in the rest of the world. In May 2009, Inmarsat and Solaris mobile (a joint venture between Eutelsat and Astra) were awarded each a 2×15 MHz portion of the S band by the European Commission. The two companies are allowed two years to start providing pan-European MSS services for 18 years. Allocated frequencies are 1.98 to 2.01 GHz for Earth to space communications, and from 2.17 to 2.2 GHz for space to Earth communications. In some countries, S band is used for Direct-to-Home satellite television (unlike similar services in most countries, which use Ku band). The frequency typically allocated for this service is 2.5 to 2.7 GHz (LOF 1.570 GHz). Wireless network equipment compatible with IEEE 802.11b and 802.11g standards use the 2.4 GHz section of the S band. Digital cordless telephones operate in this band too. Microwave ovens operate at 2495 or 2450 MHz. IEEE 802.16a and 802.16e standards utilize a part of the frequency range of S band, under WiMAX standards most vendors are now manufacturing equipment in the range of 3.5 GHz. The exact frequency range allocated for this type of use varies between countries. In North America, 2.4 - 2.483 GHz is an ISM band used for unlicensed spectrum devices such as cordless phones, wireless headphones, and video senders, among other consumer electronics uses, including Bluetooth which operates between 2.402 GHz and 2.480 GHz.

Optical communications S band S band is also used in optical communications to refer to the wavelength range 1460 nm to 1530 nm.

C band C band Frequency range

NATO: 500 – 1000 MHz IEEE: 4 – 8 GHz

ITU Radio Band Numbers 1 2 3 4 5 6 7 8 9 10 11

ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF

NATO Radio bands ABCDEFGHIJKLM

IEEE Radar bands HF VHF UHF L S C X Ku K Ka Q V W

The C band is a name given to certain portions of the electromagnetic spectrum, as well as a range of wavelengths of microwaves that are used for long-distance radio telecommunications. The IEEE C-band - and its slight variations - contains frequency ranges that are used for many satellite communications transmissions; by some Wi-Fi devices; by some cordless telephones; and by some weather radar systems. For satellite communications, the microwave frequencies of the C-band perform better in comparison with Ku band (11.2 GHz to 14.5 GHz) microwave frequencies, under adverse weather conditions, which are used by another large set of communication satellites. The adverse weather conditions all have to do with moisture in the air, such as during rainfalls, thunderstorms, sleet storms, and snowstorms.

The NATO C-band The NATO C-band is that portion of the radio spectrum between 500 megahertz (MHz) and 1000 MHz, but this terminology is rarely used in the two very large NATO members that are located in North America.

The IEEE C-band The IEEE C-band is a portion of the electromagnetic spectrum in the microwave range of frequencies ranging from 4.0 to 8.0 gigahertz (GHz)., but this definition is the one that is followed by radar manufacturers and users, but not necessarily by microwave radio telecommunications users. The communications C-band was the first frequency band that was allocated for commercial telecommunications via satellites. Nearly all C-band communication satellites use the band of frequencies from 3.7 to 4.2 GHz for their downlinks, and the band of frequencies from 5.925 GHz to 6.425 GHz for their uplinks. Note that by using the band from 3.7 to 4.0 GHz, this C-band overlaps somewhat into the IEEE S-band for radars. The C-band communication satellites typically have 24 radio transponders spaced 20 MHz apart, but with the adjacent transponders on opposite polarizations. Hence, the transponders on the same polarization are always 40 MHz apart. Of this 40 MHz, each transponder utilizes about 36 MHz. (The unused 8.0 MHz between the pairs of transponders acts as "guard bands" for the likely case of imperfections in the microwave electronics.)

The C-band is primarily used for open satellite communications, whether for full-time satellite TV networks or raw satellite feeds, although subscription programming also exists. This use contrasts with direct broadcast satellite, which is a completely closed system used to deliver subscription programming to small satellite dishes that are connected with proprietary receiving equipment. The satellite communications portion of the C-band is highly associated with television receive-only satellite reception systems, commonly called "big dish" systems, since small receiving antennas are not optimal for C-band systems. Typical antenna sizes on C-band capable systems ranges from 7.5 to 12 feet (2.5 to 3.5 meters) on consumer satellite dishes, although larger ones also can be used. The C-band frequencies of 5.4 GHz band [5.15 to 5.35 GHz, or 5.47 to 5.725 GHz, or 5.725 to 5.875 GHz, depending on the region of the world] is used for IEEE 802.11a WiFi and cordless telephone applications, leading to occasional interference with some weather radars that are also allocated to the C-band.

C-band variations Slight variations in the assignments of C-band frequencies have been approved for use in various parts of the world, depending on their locations in the three International Telecommunications Union radio regions. Note that one region includes all of the Americas; a second includes all of Europe and Africa, plus all of Russia, and the third region includes all of Asia outside of Russia, plus Australia and New Zealand. This latter region is the most populous one, since it includes the People's Republic of China, India, Pakistan, Japan, and Southeast Asia. C-Band Variations Around The World Transmit Frequency Receive Frequency Band (GHz) (GHz) Standard C-Band 5.850–6.425 3.625–4.200 Extended C-Band 5.850–6.725 3.400–4.200 INSAT / Super-Extended C-Band 6.725–7.025 4.500–4.800 Russian C-Band 5.975–6.475 3.650–4.150 LMI C-Band 5.7250–6.025 3.700–4.000

Amateur radio •

The Radio Regulations of the International Telecommunication Union allow amateur radio operations in the frequency range from 5.650 to 5.925 GHz.

Other Microwave bands

The microwave spectrum is usually defined as the electromagnetic spectrum that ranges from 1.0 GHz to 30 GHz in frequency, but some antiquated usages includes lower frequencies. Most common applications are within the 1.0 to 30 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain (RSGB), are shown in the table below. Note that frequencies above 30 GHz are typically said to be in the "millimeter wave". because their wavelengths can be conveniently measured in millimeters (mm). The frequency of 30 GHz corresponds quite closely to a wavelength of 10 mm, or 1.0 centimeter. L band 1 to 2 GHz S band 2 to 4 GHz C band 4 to 8 GHz X band 8 to 12 GHz Ku band 12 to 18 GHz K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Q band 30 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz E band 60 to 90 GHz W band 75 to 110 GHz F band 90 to 140 GHz D band 110 to 170 GHz Footnote: "P-band" is sometimes incorrectly used for the Ku-band. "P" for "previous" was a radar band used in the United Kingdom that ranged from 250 to 500 MHz, which is now completely obsolete by the IEEE Standard 521.

Fiberoptic Communications In infrared optical communications, C-band refers to the wavelength range 1530 - 1565 nm, which corresponds to the amplification range of erbium doped fiber amplifiers (EDFAs) .

X band X band Frequency range

8.0 – 12.0 GHz (IEEE radar)

ITU Radio Band Numbers

1 2 3 4 5 6 7 8 9 10 11

ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF

NATO Radio bands ABCDEFGHIJKLM

IEEE Radar bands HF VHF UHF L S C X Ku K Ka Q V W

The X-band is a segment of the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of X-band is rather indefinitely set at approximately 7.0 to 11.2 gigahertz (GHz). In radar engineering, the frequency range is specified by the IEEE at 8.0 to 12.0 GHz. The term "X-band" is also used informally and inaccurately to refer to the extended AM broadcast band, where the "X" stands for "extended".

Satellite communications For military communications satellites, the International Telecommunications Union (ITU) has assigned the X-band uplink frequency band (for sending modulated signals) as from 7.9 to 8.4 GHz. The ITU-assigned downlink frequency band (for receiving signals) is from 7.25 to 7.75 GHz. The typical local oscillator frequency of an X-band low-noise block converter (LNB) is 6300 MHz. Both of these frequency bands are 500 megahertz wide. In engineering, this pair of frequency bands may be referred to as the 8 / 7 GHz X-band satellite communications system.

Radar X-band is used in radar applications including continuous-wave, pulsed, singlepolarization, dual-polarization, synthetic aperture radar, and phased arrays. X-band radar frequency sub-bands are used in civil, military, and government institutions for weather monitoring, air traffic control, maritime vessel traffic control, defense tracking, and vehicle speed detection for law enforcement. X-band is often used in modern radars. The shorter wavelengths of the X-band allow for higher resolution imagery from high-resolution imaging radars for target identification and discrimination.

Terrestrial communications and networking

In Ireland, Libya, Saudi Arabia and Canada, the X-band 10.15 to 10.7 segment is used for terrestrial broadband. Alvarion, Cambridge, and Ogier make systems for this, though these are all incompatible. The Ogier system is a full duplex Transverter used for DOCSIS over microwave. The home / Business CPE has a single coaxial cable with a power adapter connecting to an ordinary cable modem. The local oscillator is usually 9750 MHz, the same as for Ku-band satellite TV LNB. Two way applications such as broadband typically use a 350 MHz TX offset.

Space communications Portions of the X-band are assigned by the International Telecommunications Union (ITU) exclusively for deep space telecommunications. The primary user of this allocation is the American NASA Deep Space Network (DSN). DSN facilities are located in Goldstone, California (in the Mojave Desert), near Canberra, Australia, and near Madrid, Spain. These three stations, located approximately 120 degrees apart in longitude, provide continual communications from the Earth to almost any point in the Solar System independent of Earth rotation. DSN stations are capable of using the older and lower Sband deep-space radio communications allocations, and some higher frequencies on a more-or-less experimental basis, such as in the K-band. Notable deep space probe programs that have employed X-band communications include the Viking Mars landers; the Voyager missions to Jupiter, Saturn, and beyond; the Galileo Jupiter orbiter; and the Cassini-Huygens Saturn orbiter. An important use of the X-band communications came with the two Viking program landers. When the planet Mars was passing near or behind the Sun, as seen from the Earth, a Viking lander would transmit two simultaneous continuous-wave carriers, one in the S-band and one in the X-band in the direction of the Earth, where they were picked up by DSN ground stations. By making simultaneous measurements at the two different frequencies, the resulting data enabled theoretical physicists to verify the mathematical predictions of Albert Einstein's General Theory of Relativity. These results are some of the best confirmations of the General Theory of Relativity.

Amateur radio The Radio Regulations of the International Telecommunication Union allow amateur radio operations in the frequency range from 10.000 to 10.500 GHz.

Motion detection Motion detectors often use 10.525 GHz. 10.4 GHz is proposed for traffic light crossing detectors.

Other microwave bands The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain (RSGB), are shown in the table below: L band 1 to 2 GHz S band 2 to 4 GHz C band 4 to 8 GHz X band 8 to 12 GHz Ku band 12 to 18 GHz K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Q band 30 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz E band 60 to 90 GHz W band 75 to 110 GHz F band 90 to 140 GHz D band 110 to 170 GHz Footnote: P band is sometimes incorrectly used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521.

Chapter- 5

Other Microwave Frequency Bands

Ku band

Ku band Frequency range

12 to 18 GHz

Related bands

K-band

ITU Radio Band Numbers 1 2 3 4 5 6 7 8 9 10 11

ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF

NATO Radio bands ABCDEFGHIJKLM

IEEE Radar bands HF VHF UHF L S C X Ku K Ka Q V W

The Ku band is a portion of the electromagnetic spectrum in the microwave range of frequencies. This symbol refers to "K-under" (originally German: Kurz-unten)—in other words, the band directly below the K-band. In radar applications, it ranges from 12 to 18 GHz according to the formal definition of radar frequency band nomenclature in IEEE Standard 521-2002. Ku band is primarily used for satellite communications, most notably for fixed and broadcast services, and for specific applications such as NASA's Tracking Data Relay Satellite used for both space shuttle and ISS communications. Ku band satellites are also used for backhauls and particularly for satellite from remote locations back to a television network's studio for editing and broadcasting. The band is split into multiple segments that vary by geographical region by the International Telecommunication Union (ITU).

NBC was the first television network to uplink a majority of its affiliate feeds via Ku band in 1983. Some frequencies in this radio band are used for vehicle speed detection by law enforcement, especially in Europe.

Segments and regions The Americas Segments in most of The Americas are represented by ITU Region 2 from 11.7 to 12.2 GHz (Local Oscillator Frequency (LOF) 10.750 GHz), allocated to the FSS (fixed service satellite), uplink from 14.0 to 14.5 GHz. There are more than 22 FSS Ku band satellites orbiting over North America, each carrying 12 to 48 transponders, 20 to 120 watts per transponder, and requiring a 0.8-m to 1.5-m antenna for clear reception. The 12.2 to 12.7 GHz (LOF 11.250 GHz) segment is allocated to the BSS (broadcasting satellite service). BSS (DBS direct broadcast satellites) normally carry 16 to 32 transponders of 27 MHz bandwidth running at 100 to 240 watts of power, allowing the use of receiver antennas as small as 18 inches (450 mm).

Europe and Africa Segments in those regions are represented by ITU Region 1 and they are, the 11.45 to 11.7 and 12.5 to 12.75 GHz bands are allocated to the FSS (fixed satellite service, uplink 14.0 to 14.5 GHz). In Europe Ku band is used from 10.7 to 12.75 GHz (LOF Low 9.750 GHz, LOF High 10.600 GHz) for direct broadcast satellite services such as those carried by the Astra satellites. The 11.7 to 12.5 GHz segment is allocated to the BSS (broadcasting satellite service).

Australia Australia is part of ITU Region 3 and the Australian regulatory environment provides a class license that covers downlinking from 12.25 GHz to 12.75 GHz and uplinking from 14.0 GHz to 14.5 GHz.

Indonesia The ITU has categorized Indonesia as Region P, countries with very high rain precipitation. This statement has made many people unsure about using Ku-band (11 – 18 GHz) in Indonesia. If frequencies higher than 10 GHz are used in a heavy rain area, a decrease in communication availability results. This problem can be solved by using an appropriate link budget when designing the wireless communication link. Higher power can overcome the loss to rain fade.

Measurements of rain attenuation in Indonesia have been done for satellite communication links in Padang, Cibinong, Surabaya and Bandung. The DAH Model for rain attenuation prediction is valid for Indonesia, in addition to the ITU model. The DAH model has become an ITU recommendation since 2001 (Recommendation No. ITU-R P.618-7). This model can create a 99.7% available link so that Ku-band can be applied in Indonesia. The use of the Ku-band for satellite communications in tropical regions like Indonesia is becoming more frequent. Several satellites above Indonesia have Ku-band transponders, and even Ka-band transponders. Newskies (NSS 6), launched in December 2002 and positioned at 95° East, contains only Ku-band transponders with a footprint on Indonesia (Sumatra, Java, Borneo, Celebes, Bali, Nusa Tenggara, Moluccas). The iPSTAR satellite, launched in 2004 also uses Ku band footprints. MEASAT has named the Ku-band footprint directed towards Indonesia Ku-band for Indonesi. MEASAT-3 plans to cover the whole of Indonesia from West to East. This satellite was launched by Malaysia in December 2006.

Others Other ITU allocations have been made within the Ku band to the fixed service (microwave towers), radio astronomy service, space research service, mobile service, mobile satellite service, radiolocation service (radar), amateur radio service, and radionavigation. However, not all of these services are actually operating in this band and others are only minor users.

Advantages Compared with C-band, Ku band is not similarly restricted in power to avoid interference with terrestrial microwave systems, and the power of its uplinks and downlinks can be increased. This higher power also translates into smaller receiving dishes and points out a generalization between a satellite’s transmission and a dish’s size. As the power increases, the dish’s size can decrease. This is because the purpose of the dish element of the antenna is to collect the incident waves over an area and focus them all onto the antenna's actual receiving element, mounted in front of the dish (and pointed back towards its face); if the waves are more intense, less of them need to be collected to achieve the same intensity at the receiving element. The Ku band also offers a user more flexibility. A smaller dish size and a Ku band system’s freedom from terrestrial operations simplifies finding a suitable dish site. For the End users Ku band is generally cheaper and enables smaller antennas (both because of the higher frequency and a more focused beam). Ku band is also less vulnerable to rain fade than the Ka band frequency spectrum. The satellite operator's Earth Station antenna do require more accurate position control when operating at Ku band than compared to C band. Position feedback accuracies are

higher and the antenna may require a closed loop control system to maintain position under wind loading of the dish surface.

Disadvantages There are, however, some disadvantages of Ku band system. Especially at frequencies higher than 10 GHz in heavy rain fall areas, a noticeable degradation occurs, due to the problems caused by and proportional to the amount of rainfall (commonly known as "rain fade"). This problem can be mitigated, however, by deploying an appropriate link budget strategy when designing the satellite network, and allocating a higher power consumption to compensate rain fade loss. The Ku band is not only used for television transmission, which some sources imply, but also very much for digital data transmission via satellites, and for voice/audio transmissions. The higher frequency spectrum of the Ku band is particularly susceptible to signal degradation, considerably more so than C-band satellite frequency spectrum. A similar phenomenon, called "snow fade" (where snow or ice accumulation significantly alters the focal point of a dish) can also occur during winter precipitation. Also, the Kuband satellites typically require considerably more power to transmit than the C-band satellites. Under both "rain fade" and "snow fade" conditions, Ka and Ku band losses can be marginally (but significantly) reduced using super-hydrophobic Lotus effect coatings.

Other Microwave Bands The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain (RSGB), are shown in the table below: L band 1 to 2 GHz S band 2 to 4 GHz C band 4 to 8 GHz X band 8 to 12 GHz Ku band 12 to 18 GHz K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Q band 30 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz D band 110 to 170 GHz

Footnote: P band is sometimes incorrectly used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521.

K band

K band Frequency range

NATO: 20 – 40 GHz IEEE: 18 – 27 GHz

ITU Radio Band Numbers 1 2 3 4 5 6 7 8 9 10 11

ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF

NATO Radio bands ABCDEFGHIJKLM

IEEE Radar bands HF VHF UHF L S C X Ku K Ka Q V W

NATO K band The NATO K band is defined as a frequency band between 20 and 40 GHz (7,500– 15,000 nanometer wavelength).

IEEE K band The IEEE K band is a portion of the electromagnetic spectrum in the microwave range of frequencies ranging between 18 and 27 GHz. K band between 18 and 26.5 GHz is absorbed easily by water vapor (H2O resonance peak at 22.24 GHz, 1.35 cm).

Amateur radio •

The Radio Regulations of the International Telecommunication Union allow amateur radio operations in the frequency range from 24.500 to 24.250 GHz.

Subdivisions The IEEE K band is conventionally divided into three sub-bands:

• • •

Ka band: K-above band, 26.5–40 GHz, mainly used for radar and experimental communications. K-band 18-27 GHz Ku band: K-under band, 12–18 GHz, mainly used for satellite communications, terrestrial microwave communications, and radar, especially police traffic-speed detectors.

Infrared astronomy In infrared astronomy, the K band refers to a different frequency range atmospheric transmission window centered on 2.2 microns (in the near-infrared 136 THz range).

Other Microwave bands The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain (RSGB), are shown in the table below: L band 1 to 2 GHz S band 2 to 4 GHz C band 4 to 8 GHz X band 8 to 12 GHz Ku band 12 to 18 GHz K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Q band 30 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz E band 60 to 90 GHz W band 75 to 110 GHz F band 90 to 140 GHz D band 110 to 170 GHz Footnote: P band is sometimes incorrectly used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521.

Ka band

Ka band Frequency range

26.5 – 40 GHz

Related bands

K band · Ku band

ITU Radio Band Numbers 1 2 3 4 5 6 7 8 9 10 11

ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF

NATO Radio bands ABCDEFGHIJKLM

IEEE Radar bands HF VHF UHF L S C X Ku K Ka Q V W

The Ka band (Pronounced: "Kay-A Band") covers the frequencies of 26.5-40 GHz. The Ka band is part of the K band of the microwave band of the electromagnetic spectrum. This symbol refers to "K-above" — in other words, the band directly above the K-band. The so-called 30/20 GHz band is used in communications satellites, uplink in either the 27.5 GHz and 31 GHz bands, and high-resolution, close-range targeting radars aboard military airplanes. Some frequencies in this radio band are used for vehicle speed detection by law enforcement. Kepler Mission uses this frequency range to downlink the scientific data collected by the space telescope. The designation "Ka-band" is from Kurz-above, which stems from the German word "kurz" meaning short.

Other Microwave bands The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain (RSGB), are shown in the table below: L band S band C band X band

1 to 2 GHz 2 to 4 GHz 4 to 8 GHz 8 to 12 GHz

Ku band 12 to 18 GHz K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Q band 30 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz E band 60 to 90 GHz W band 75 to 110 GHz F band 90 to 140 GHz D band 110 to 170 GHz Footnote: P band is sometimes incorrectly used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521.

Q band

Q band Frequency range

33 to 50 GHz

Related bands

Ka band

ITU Radio Band Numbers 1 2 3 4 5 6 7 8 9 10 11

ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF

NATO Radio bands ABCDEFGHIJKLM

IEEE Radar bands HF VHF UHF L S C X Ku K Ka Q V W

The Q band of the microwave part of the electromagnetic spectrum and ranges from 33 to 50 GHz. It sits above, and partly overlaps with, the U.S. IEEE designated Ka band (26.5 to 40 GHz). It sits below the U.S. IEEE designated V band (50–75 GHz) in frequency. The Q band is mainly used for satellite communications, terrestrial microwave communications and for radio astronomy studies such as the QUIET telescope. It is also used in automotive radar, and radar investigating the properties of the Earth's surface

Other Microwave bands The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain (RSGB), are shown in the table below: L band 1 to 2 GHz S band 2 to 4 GHz C band 4 to 8 GHz X band 8 to 12 GHz Ku band 12 to 18 GHz K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Q band 33 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz E band 60 to 90 GHz W band 75 to 110 GHz F band 90 to 140 GHz D band 110 to 170 GHz Footnote: P band is sometimes incorrectly used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521.

V band V band Frequency range

50 to 75 GHz

ITU Radio Band Numbers 1 2 3 4 5 6 7 8 9 10 11

ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF

NATO Radio bands ABCDEFGHIJKLM

IEEE Radar bands HF VHF UHF L S C X Ku K Ka Q V W

The V band (vee-band) of the electromagnetic spectrum ranges from 50 to 75 GHz. The V band is not heavily used, except for millimeter wave radar research and other kinds of scientific research. It should not be confused with the 600–1000 MHz range of Band-V (band-five) of the UHF frequency range. The V band is also used for high capacity terrestrial millimeter wave communications systems. In the United States, the Federal Communications Commission has allocated the frequency band from 57 to 64 GHz for unlicensed wireless systems. These systems are primarily used for high capacity, short distance (less than 1 mile) communications. In addition, frequencies at 70, 80, and 90 GHz have been allocated as "lightly licensed" bands for multi-gigabit wireless communications. All communications links in the V band require unobstructed line of sight between the transmit and receive point, and rain fade must be taken into account when performing link budget analysis.

Notable Uses On Dec. 15, 1995 the V band at 60 GHz was used by the world's first crosslink communication between satellites in a constellation. This communication was between the U.S. Milstar 1 and Milstar 2 military satellites.

Other Microwave bands The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain (RSGB), are shown in the table below: L band 1 to 2 GHz S band 2 to 4 GHz C band 4 to 8 GHz X band 8 to 12 GHz Ku band 12 to 18 GHz K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Q band 30 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz E band 60 to 90 GHz W band 75 to 110 GHz

F band 90 to 140 GHz D band 110 to 170 GHz Footnote: P band is sometimes incorrectly used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521.

E band ITU Radio Band Numbers 1 2 3 4 5 6 7 8 9 10 11

ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF

NATO Radio bands ABCDEFGHIJKLM

IEEE Radar bands HF VHF UHF L S C X Ku K Ka Q V W

The NATO E band is the range of radio frequencies from 2 GHz to 3 GHz in the electromagnetic spectrum. This is equal to wave lengths between 15 cm and 10 cm. The E band is in the upper UHF range of the radio spectrum. The NATO E band lies in the S band (2—4 GHz) of the older IEEE classification system. The newer designation of "E-Band" lies in the extremely high frequency bands from 71 to 76 gigahertz (GHz), 81 to 86 GHz and 92 to 95 GHz. It is being used for short range, high bandwidth communications.

Atmospheric Effects At these high frequencies the short wavelengths give the radiation a very directional quality, similar to visible light. Many molecules possess rotational and vibrational states excited by very specific wavelengths in this band, thus the atmospheric gasses such as Oxygen, Water Vapor, Carbon Dioxide and Nitrogen can absorb, and be excited causing variable beam attenuation effects dependent on meteorological and atmospheric conditions.

Other Microwave bands

The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain (RSGB), are shown in the table below:

Amateur radio •

The Radio Regulations of the International Telecommunication Union allow amateur radio operations in the frequency range from 76.000 to 81.000 GHz.

L band 1 to 2 GHz S band 2 to 4 GHz C band 4 to 8 GHz X band 8 to 12 GHz Ku band 12 to 18 GHz K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Q band 30 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz E band 60 to 90 GHz W band 75 to 110 GHz F band 90 to 140 GHz D band 110 to 170 GHz Footnote: P band is sometimes incorrectly used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521.

W band W band Frequency range

75 to 110 GHz

Related bands

V band · M band

ITU Radio Band Numbers 1 2 3 4 5 6 7 8 9 10 11

ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF

NATO Radio bands ABCDEFGHIJKLM

IEEE Radar bands HF VHF UHF L S C X Ku K Ka Q V W

The W band of the microwave part of the electromagnetic spectrum ranges from 75 to 110 GHz. It sits above the U.S. IEEE designated V band (50–75 GHz) in frequency, yet overlaps the NATO designated M band (60–100 GHz). The W band is used for satellite communications, millimeter wave radar research, military radar targeting and tracking applications, and some non-military applications. A number of passive millimetre-wave cameras for concealed weapons detection operate at 94 GHz. A frequency around 77 GHz is used for automotive cruise control radar. The atmospheric radio window at 94 GHz is used for imaging millimetre-wave radar applications in astronomy, defense, and security applications. Less-than-lethal weaponry exists that uses millimeter waves to heat a thin layer of human skin to an intolerable temperature so as to make the targeted person move away. A twosecond burst of the 95 GHz focused beam heats the skin to a temperature of 130 °F (54 °C) at a depth of 1/64th of an inch (0.4 mm). The United States Air Force and Marines are currently using this type of Active Denial System. In terms of communications capability, W-band offers high data rate throughput when used at high altitudes and in space. (The 71 - 76 GHz / 81 - 86 GHz segment of the Wband is allocated by the International Telecommunication Union to satellite services.) Because of increasing spectrum and orbit congestion at lower frequencies, W-band satellite allocations are of increasing interest to commercial satellite operators, although no commercial project has yet been implemented in these bands.

Other Microwave bands The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain (RSGB), are shown in the table below: L band 1 to 2 GHz S band 2 to 4 GHz C band 4 to 8 GHz X band 8 to 12 GHz Ku band 12 to 18 GHz

K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Q band 30 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz E band 60 to 90 GHz W band 75 to 110 GHz F band 90 to 140 GHz D band 110 to 170 GHz Footnote: P band is sometimes incorrectly used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521.

F band

F band Frequency range

90 – 140 GHz

ITU Radio Band Numbers 1 2 3 4 5 6 7 8 9 10 11

ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF

NATO Radio bands ABCDEFGHIJKLM

IEEE Radar bands HF VHF UHF L S C X Ku K Ka Q V W

The F band is the range of radio frequencies from 90 GHz to 140 GHz in the electromagnetic spectrum. This is equal to wave lengths between 2.1 mm and 3.3 mm. The F band is in the lower parts of the SHF range of the radio spectrum. The F band lies in the S band of the older classification system. A new F Band lies up between 90 and 140 GHz.

Other Microwave bands The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain (RSGB), are shown in the table below: L band 1 to 2 GHz S band 2 to 4 GHz C band 4 to 8 GHz X band 8 to 12 GHz Ku band 12 to 18 GHz K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Q band 30 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz E band 60 to 90 GHz W band 75 to 110 GHz F band 90 to 140 GHz D band 110 to 170 GHz Footnote: P band is sometimes incorrectly used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521.

D band

D band Frequency range

110 – 170 GHz

ITU Radio Band Numbers 1 2 3 4 5 6 7 8 9 10 11

ITU Radio Band Symbols ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF

NATO Radio bands ABCDEFGHIJKLM

IEEE Radar bands

HF VHF UHF L S C X Ku K Ka Q V W

D band is the range of radio frequencies from 110 GHz to 170 GHz in the electromagnetic spectrum. This is equal to wave lengths between 1.8 mm and 2.7 mm. The D band is in the EHF range of the radio spectrum. The modern D band intersects with the L band (0.5—1.55 GHz) of the older IEEE classification system. A newer D-Band lies at the approach to upper frequency limit of contemporary electronic oscillator technology, between 110 and 170 GHz.

Other Microwave bands The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain (RSGB), are shown in the table below: L band 1 to 2 GHz S band 2 to 4 GHz C band 4 to 8 GHz X band 8 to 12 GHz Ku band 12 to 18 GHz K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Q band 30 to 50 GHz U band 40 to 60 GHz V band 50 to 75 GHz E band 60 to 90 GHz W band 75 to 110 GHz F band 90 to 140 GHz D band 110 to 170 GHz Footnote: P band is sometimes incorrectly used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521.

Chapter- 6

Waveguide

A section of flexible waveguide with a pressurizable flange

Electric field inside an x-band hollow metal waveguide. A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguide for each type of wave. The original and most common meaning is a hollow conductive metal pipe used to carry high frequency radio waves, particularly microwaves. Waveguides differ in their geometry which can confine energy in one dimension such as in slab waveguides or two dimensions as in fiber or channel waveguides. In addition, different waveguides are needed to guide different frequencies: an optical fiber guiding light (high frequency) will not guide microwaves (which have a much lower frequency). As a rule of thumb, the width of a waveguide needs to be of the same order of magnitude as the wavelength of the guided wave. There are structures in nature which act as waveguides: for example, a layer in the ocean can guide whale song to enormous distances.

Principle of operation Waves in open space propagate in all directions, as spherical waves. In this way they lose their power proportionally to the square of the distance; that is, at a distance R from the source, the power is the source power divided by R2. The waveguide confines the wave to propagation in one dimension, so that (under ideal conditions) the wave loses no power while propagating. Waves are confined inside the waveguide due to total reflection from the waveguide wall, so that the propagation inside the waveguide can be described approximately as a "zigzag" between the walls. This description is exact for electromagnetic waves in a rectangular or circular hollow metal tube.

History The first structure for guiding waves was proposed by J. J. Thomson in 1893, and was first experimentally tested by O. J. Lodge in 1894. The first mathematical analysis of electromagnetic waves in a metal cylinder was performed by Lord Rayleigh in 1897. For sound waves, Lord Rayleigh published a full mathematical analysis of propagation modes in his seminal work, “The Theory of Sound”. The study of dielectric waveguides (such as optical fibers, see below) began as early as the 1920s, by several people, most famous of which are Rayleigh, Sommerfeld and Debye. Optical fiber began to receive special attention in the 1960s due to its importance to the communications industry.

Uses The uses of waveguides for transmitting signals were known even before the term was coined. The phenomenon of sound waves guided through a taut wire have been known for a long time, as well as sound through a hollow pipe such as a cave or medical stethoscope. Other uses of waveguides are in transmitting power between the components of a system such as radio, radar or optical devices. Waveguides are the fundamental principle of guided wave testing (GWT), one of the many methods of non-destructive evaluation. Specific examples: • • • •

•

Optical fibers transmit light and signals for long distances and with a high signal rate. In a microwave oven a waveguide leads power from the magnetron where waves are formed to the cooking chamber. In a radar, a waveguide leads waves to the antenna, where their impedance needs to be matched for efficient power transmission (see below). A waveguide called stripline can be created on a printed circuit board, and is used to transmit microwave signals on the board. This type of waveguide is very cheap to manufacture and has small dimensions which fit inside printed circuit boards. Waveguides are used in scientific instruments to measure optical, acoustic and elastic properties of materials and objects. The waveguide can be put in contact with the specimen (as in a Medical ultrasonography), in which case the waveguide ensures that the power of the testing wave is conserved, or the specimen may be put inside the waveguide (as in a dielectric constant measurement), so that smaller objects can be tested and the accuracy is better.

A sketch of the theoretical analysis Electromagnetic wave propagation along the axis of the waveguide is described by the wave equation, which is derived from Maxwell's equations, and where the wavelength

depends upon the structure of the waveguide, and the material within it (air, plastic, vacuum, etc.), as well as on the frequency of the wave. The spatial distribution of the time-varying electric fields and magnetic fields within the waveguide depends on boundary conditions imposed by the shape and materials of the waveguide. Let us assume that the waveguide is made of a metal that is such a good conductor that we can consider it to be a perfect conductor. Nearly all waveguides have copper interiors, but some of them are even plated with silver or gold on the inside excellent conductors, and also resistant to corrosion. Now, the boundary conditions are these: 1). Electromagnetic waves do not pass through conductors, but rather, they are reflected. 2). Any electric field that touches a conductor must be perpendicular to it. 3). Any magnetic field close to a conductor must be parallel to it. These boundary conditions eliminate an infinite number of solutions to the wave equation, and the ones that remain are the possible solutions to the wave equation inside the waveguide. The rest of the analysis of the solutions of the electromagnetic waves inside a waveguide gets very mathematical. All that remains that can be said without getting very mathematical is that commonlyused waveguides are only of a few categories. The most common kind of waveguide is one that has a rectangular cross-section, one that is usually not square. It is common for the long side of this cross-section to be twice a long as its short side. These are useful for carrying electromagnetic waves that have a horizontal or vertical polarization to them. The second most commonly used kind of waveguide has a circular cross-section. These turn out to be quite useful when carrying electromagnetic waves with a rotating, circular polarization to them. Then, its electrical field traces out a helical pattern as a function of time. The third kind of a waveguide - actually a seldom-used one - has an elliptical crosssection.

Propagation modes and cutoff frequencies A propagation mode in a waveguide is one solution of the wave equations, or, in other words, the form of the wave. Due to the constraints of the boundary conditions, there are only limited frequencies and forms for the wave function which can propagate in the waveguide. The lowest frequency in which a certain mode can propagate is the cutoff frequency of that mode. The mode with the lowest cutoff frequency is the basic mode of the waveguide, and its cutoff frequency is the waveguide cutoff frequency.

Impedance matching In circuit theory, the impedance is a generalization of electrical resistivity in the case of alternating current, and is measured in ohms (Ω). A waveguide in circuit theory is

described by a transmission line having a length and self impedance. In other words the impedance is the resistance of the circuit component (in this case a waveguide) to the propagation of the wave. This description of the waveguide was originally intended for alternating current, but is also suitable for electromagnetic and sound waves, once the wave and material properties (such as pressure, density, dielectric constant) are properly converted into electrical terms (current and impedance for example). Impedance matching is important when components of an electric circuit are connected (waveguide to antenna for example): The impedance ratio determines how much of the wave is transmitted forward and how much is reflected. In connecting a waveguide to an antenna a complete transmission is usually required, so that their impedances are matched.

The reflection coefficient can be calculated using: , where Γ is the reflection coefficient (0 denotes full transmission, 1 full reflection, and 0.5 is a reflection of half the incoming voltage), Z1 and Z2 are the impedance of the first component (from which the wave enters) and the second component, respectively. An impedance mismatch creates a reflected wave, which added to the incoming waves creates a standing wave. An impedance mismatch can be also quantified with the standing wave ratio (SWR or VSWR for voltage), which is connected to the impedance ratio and reflection coefficient by:

, where

are the minimum and maximum values of the voltage absolute value, and the VSWR is the voltage standing wave ratio, which value of 1 denotes full transmission, without reflection and thus no standing wave, while very large values mean high reflection and standing wave pattern.

Electromagnetic waveguides Waveguides can be constructed to carry waves over a wide portion of the electromagnetic spectrum, but are especially useful in the microwave and optical frequency ranges. Depending on the frequency, they can be constructed from either conductive or dielectric materials. Waveguides are used for transferring both power and communication signals.

Optical waveguides Waveguides used at optical frequencies are typically dielectric waveguides, structures in which a dielectric material with high permittivity, and thus high index of refraction, is surrounded by a material with lower permittivity. The structure guides optical waves by total internal reflection. The most common optical waveguide is optical fiber.

Other types of optical waveguide are also used, including photonic-crystal fiber, which guides waves by any of several distinct mechanisms. Guides in the form of a hollow tube with a highly reflective inner surface have also been used as light pipes for illumination applications. The inner surfaces may be polished metal, or may be covered with a multilayer film that guides light by Bragg reflection (this is a special case of a photoniccrystal fiber). One can also use small prisms around the pipe which reflect light via total internal reflection —such confinement is necessarily imperfect, however, since total internal reflection can never truly guide light within a lower-index core (in the prism case, some light leaks out at the prism corners).

Acoustic waveguides An acoustic waveguide is a physical structure for guiding sound waves. A duct for sound propagation also behaves like a transmission line. The duct contains some medium, such as air, that supports sound propagation.

Sound synthesis Uses digital delay lines as computational elements to simulate wave propagation in tubes of wind instruments and the vibrating strings of string instruments.

Chapter- 7

Klystron

High-power klystron used at the Canberra Deep Space Communications Complex. (Klystrons used for generating heterodyne reference frequencies in radar receivers are about the size of a whiteboard pen.)

A klystron is a specialized linear-beam vacuum tube (evacuated electron tube). Klystrons are used as amplifiers at microwave and radio frequencies to produce both low-power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for modern particle accelerators. Klystron amplifiers have the advantage (over the magnetron) of coherently amplifying a reference signal so its output may be precisely controlled in amplitude, frequency and phase. Many klystrons have a waveguide for coupling microwave energy into and out of the device, although it is also quite common for lower power and lower frequency klystrons to use coaxial couplings instead. In some cases a coupling probe is used to couple the microwave energy from a klystron into a separate external waveguide. All modern klystrons are amplifiers, since reflex klystrons, which were used as oscillators in the past, have been surpassed by alternative technologies. The name klystron comes from the stem form κλυσ- (klys) of a Greek verb referring to the action of waves breaking against a shore, and the end of the word electron.

History The brothers Russell and Sigurd Varian of Stanford University are the inventors of the klystron. Their prototype was completed in August 1937. Upon publication in 1939, news of the klystron immediately influenced the work of US and UK researchers working on radar equipment. The Varians went on to found Varian Associates to commercialize the technology (for example to make small linear accelerators to generate photons for external beam radiation therapy). In their 1939 paper, they acknowledged the contribution of A. Arsenjewa-Heil and Oskar Heil (wife and husband) for their velocity modulation theory in 1935. The work of physicist W.W. Hansen was instrumental in the development of klystron and was cited by the Varian brothers in their 1939 paper. His resonator analysis, which dealt with the problem of accelerating electrons toward a target, could be used just as well to decelerate electrons (i.e., transfer their kinetic energy to RF energy in a resonator). During the second World War, Hansen lectured at the MIT Radiation labs two days a week, commuting to Boston from Sperry gyroscope company on Long Island. His resonator, called a "hohlraum" by nuclear physicists and coined "rhumbatron" by the Varian brothers, is used in 2009 in the National Ignition Facility investigating nuclear fusion. Hansen died in 1949 as a result of exposure to beryllium oxide (BeO). During the second World War, the Axis powers relied mostly on (then low-powered) klystron technology for their radar system microwave generation, while the Allies used the far more powerful but frequency-drifting technology of the cavity magnetron for microwave generation. Klystron tube technologies for very high-power applications, such as synchrotrons and radar systems, have since been developed.

Explanation Klystrons amplify RF signals by converting the kinetic energy in a DC electron beam into radio frequency power. A beam of electrons is produced by a thermionic cathode (a heated pellet of low work function material), and accelerated by high-voltage electrodes (typically in the tens of kilovolts). This beam is then passed through an input cavity. RF energy is fed into the input cavity at, or near, its natural frequency to produce a voltage which acts on the electron beam. The electric field causes the electrons to bunch: electrons that pass through during an opposing electric field are accelerated and later electrons are slowed, causing the previously continuous electron beam to form bunches at the input frequency. To reinforce the bunching, a klystron may contain additional "buncher" cavities. The RF current carried by the beam will produce an RF magnetic field, and this will in turn excite a voltage across the gap of subsequent resonant cavities. In the output cavity, the developed RF energy is coupled out. The spent electron beam, with reduced energy, is captured in a collector.

Two-cavity klystron amplifier

In the two-chamber klystron, the electron beam is injected into a resonant cavity. The electron beam, accelerated by a positive potential, is constrained to travel through a cylindrical drift tube in a straight path by an axial magnetic field. While passing through the first cavity, the electron beam is velocity modulated by the weak RF signal. In the moving frame of the electron beam, the velocity modulation is equivalent to a plasma oscillation. Plasma oscillations are rapid oscillations of the electron density in conducting media such as plasmas or metals.(The frequency only depends weakly on the wavelength). So in a quarter of one period of the plasma frequency, the velocity modulation is converted to density modulation, i.e. bunches of electrons. As the bunched electrons enter the second chamber they induce standing waves at the same frequency as the input signal. The signal induced in the second chamber is much stronger than that in the first.

Two-cavity klystron oscillator The two-cavity amplifier klystron is readily turned into an oscillator klystron by providing a feedback loop between the input and output cavities. Two-cavity oscillator klystrons have the advantage of being among the lowest-noise microwave sources available, and for that reason have often been used in the illuminator systems of missile targeting radars. The two-cavity oscillator klystron normally generates more power than the reflex klystron—typically watts of output rather than milliwatts. Since there is no reflector, only one high-voltage supply is necessary to cause the tube to oscillate, the voltage must be adjusted to a particular value. This is because the electron beam must produce the bunched electrons in the second cavity in order to generate output power. Voltage must be adjusted to vary the velocity of the electron beam (and thus the frequency) to a suitable level due to the fixed physical separation between the two cavities. Often several "modes" of oscillation can be observed in a given klystron.

Reflex klystron

In the reflex klystron (also known as a 'Sutton' klystron after its inventor), the electron beam passes through a single resonant cavity. The electrons are fired into one end of the tube by an electron gun. After passing through the resonant cavity they are reflected by a negatively charged reflector electrode for another pass through the cavity, where they are then collected. The electron beam is velocity modulated when it first passes through the cavity. The formation of electron bunches takes place in the drift space between the reflector and the cavity. The voltage on the reflector must be adjusted so that the bunching is at a maximum as the electron beam re-enters the resonant cavity, thus ensuring a maximum of energy is transferred from the electron beam to the RF oscillations in the cavity. The voltage should always be switched on before providing the input to the reflex klystron as the whole function of the reflex klystron would be destroyed if the supply is provided after the input. The reflector voltage may be varied slightly from the optimum value, which results in some loss of output power, but also in a

variation in frequency. This effect is used to good advantage for automatic frequency control in receivers, and in frequency modulation for transmitters. The level of modulation applied for transmission is small enough that the power output essentially remains constant. At regions far from the optimum voltage, no oscillations are obtained at all. This tube is called a reflex klystron because it repels the input supply or performs the opposite function of a klystron. There are often several regions of reflector voltage where the reflex klystron will oscillate; these are referred to as modes. The electronic tuning range of the reflex klystron is usually referred to as the variation in frequency between half power points—the points in the oscillating mode where the power output is half the maximum output in the mode. The frequency of oscillation is dependent on the reflector voltage, and varying this provides a crude method of frequency modulating the oscillation frequency, albeit with accompanying amplitude modulation as well. Modern semiconductor technology has effectively replaced the reflex klystron in most applications.

Multicavity klystron

Large klystrons as used in the storage ring of the Australian Synchrotron to maintain the energy of the electron beam In all modern klystrons, the number of cavities exceeds two. A larger number of cavities may be used to increase the gain of the klystron, or to increase the bandwidth.

Tuning a klystron Some klystrons have cavities that are tunable. Tuning a klystron is delicate work which, if not done properly, can cause damage to equipment or injury to the technician. By adjusting the frequency of individual cavities, the technician can change the operating

frequency, gain, output power, or bandwidth of the amplifier. The technician must be careful not to exceed the limits of the graduations, or damage to the klystron can result. Manufacturers generally send a card with the unique calibrations for a klystron's performance characteristics, that lists the graduations to be set to attain any of a set of listed frequencies. No two klystrons are exactly identical (even when comparing like part/model number klystrons), and so every card is specific to the individual unit. Klystrons have serial numbers on each of them to uniquely identify each unit, and for which manufacturers may (hopefully) have the performance characteristics in a database. If not, loss of the calibration card may be an economically insoluble problem, making the klystron unusable or perform marginally un-tuned. Other precautions taken when tuning a klystron include using nonferrous tools. Some klystrons employ permanent magnets. If a technician uses ferrous tools, (which are ferromagnetic), and comes too close to the intense magnetic fields that contain the electron beam, such a tool can be pulled into the unit by the intense magnetic force, smashing fingers, injuring the technician, or damaging the unit. Special lightweight nonmagnetic (aka diamagnetic) tools made of beryllium alloy have been used for tuning U.S. Air Force klystrons. Precautions are routinely taken when transporting klystron devices in aircraft, as the intense magnetic field can interfere with magnetic navigation equipment. Special overpacks are designed to help limit this field "in the field," and thus allow such devices to be transported safely.

Optical klystron In an optical klystron the cavities are replaced with undulators. Very high voltages are needed. The electron gun, the drift tube and the collector are still used.

Floating drift tube klystron The floating drift tube klystron has a single cylindrical chamber containing an electrically isolated central tube. Electrically, this is similar to the two cavity oscillator klystron with a lot of feedback between the two cavities. Electrons exiting the source cavity are velocity modulated by the electric field as they travel through the drift tube and emerge at the destination chamber in bunches, delivering power to the oscillation in the cavity. This type of oscillator klystron has an advantage over the two-cavity klystron on which it is based. It only needs one tuning element to effect changes in frequency. The drift tube is electrically insulated from the cavity walls, and DC bias is applied separately. The DC bias on the drift tube may be adjusted to alter the transit time through it, thus allowing some electronic tuning of the oscillating frequency. The amount of tuning in this manner is not large and is normally used for frequency modulation when transmitting.

Collector

After the RF energy has been extracted from the electron beam, the beam is destroyed in a collector. Some klystrons include depressed collectors, which recover energy from the beam before collecting the electrons, increasing efficiency. Multistage depressed collectors enhance the energy recovery by "sorting" the electrons in energy bins.

Applications Klystrons produce microwave power far in excess of that developed by solid state. In modern systems, they are used from UHF (hundreds of MHz) up through hundreds of gigahertz (as in the Extended Interaction Klystrons in the CloudSat satellite). Klystrons can be found at work in radar, satellite and wideband high-power communication (very common in television broadcasting and EHF satellite terminals), medicine (radiation oncology), and high-energy physics (particle accelerators and experimental reactors). At SLAC, for example, klystrons are routinely employed which have outputs in the range of 50 megawatts (pulse) and 50 kilowatts (time-averaged) at frequencies nearing 3 GHz Popular Science's "Best of What's New 2007" included a company Global Resource Corporation using a klystron to convert the hydrocarbons in everyday materials, automotive waste, coal, oil shale, and oil sands into natural gas and diesel fuel.

Chapter- 8

Microstrip

Cross-section of microstrip geometry. Conductor (A) is separated from ground plane (D) by dielectric substrate (C). Upper dielectric (B) is typically air. Microstrip is a type of electrical transmission line which can be fabricated using printed circuit board [PCB] technology, and is used to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric layer known as the substrate. Microwave components such as antennas, couplers, filters, power dividers etc. can be formed from microstrip, the entire device existing as the pattern of metallization on the substrate. Microstrip is thus much less expensive than traditional waveguide technology, as well as being far lighter and more compact. The disadvantages of microstrip compared with waveguide are the generally lower power handling capacity, and higher losses. Also, unlike waveguide, microstrip is not enclosed, and is therefore susceptible to cross-talk and unintentional radiation. For lowest cost, microstrip devices may be built on an ordinary FR-4 (standard PCB) substrate. However it is often found that the dielectric losses in FR4 are too high at microwave frequencies, and that the dielectric constant is not sufficiently tightly controlled. For these reasons, an alumina substrate is commonly used. On a smaller scale, microstrip transmission lines are also built into monolithic microwave integrated circuits [MMIC]s.

Microstrip lines are also used in high-speed digital PCB designs, where signals need to be routed from one part of the assembly to another with minimal distortion, and avoiding high cross-talk and radiation. Microstrip is very similar to stripline and coplanar waveguide [CPW], and it is possible to integrate all three on the same substrate.

Inhomogeneity The electromagnetic wave carried by a microstrip line exists partly in the dielectric substrate, and partly in the air above it. In general, the dielectric constant of the substrate will be different (and greater) than that of the air, so that the wave is travelling in an inhomogeneous medium. In consequence, the propagation velocity is somewhere between the speed of radio waves in the substrate, and the speed of radio waves in air. This behaviour is commonly described by stating the effective dielectric constant (or effective relative permittivity) of the microstrip; this being the dielectric constant of an equivalent homogeneous medium (i.e. one resulting in the same propagation velocity). Further consequences of an inhomogeneous medium include: •

The line will not support a true TEM wave; at non-zero frequencies, both the E and H fields will have longitudinal components (a hybrid mode). The longitudinal components are small however, and so the dominant mode is referred to as quasiTEM.

•

The line is dispersive. With increasing frequency, the effective dielectric constant gradually climbs towards that of the substrate, so that the phase velocity gradually decreases. This is true even with a non-dispersive substrate material (the substrate dielectric constant will usually fall with increasing frequency).

•

The characteristic impedance of the line changes slightly with frequency (again, even with a non-dispersive substrate material). The characteristic impedance of non-TEM modes is not uniquely defined, and depending on the precise definition used, the impedance of microstrip either rises, falls, or falls then rises with increasing frequency. The low-frequency limit of the characteristic impedance is referred to as the quasi-static characteristic impedance, and is the same for all definitions of characteristic impedance.

•

The wave impedance varies over the cross-section of the line.

Characteristic Impedance A closed-form approximate expression for the quasi-static characteristic impedance of a microstrip line was developed by Wheeler:

where weff is the effective width, which is the actual width of the strip, plus a correction to account for the non-zero thickness of the metallization. The effective width is given by

with Z0 = impedance of free space, dielectric constant of substrate, w = width of strip, h = thickness ('height') of substrate and t = thickness of strip metallization. This formula is asymptotic to an exact solution in three different cases 1. 2. 3.

, any , ,

(parallel plate transmission line), (wire above a ground-plane) and

It is claimed that for most other cases, the error in impedance is less than 1%, and is always less than 2%. By covering all aspect-ratios in one formula, Wheeler 1977 improves on Wheeler 1965 which gives one formula for w / h > 3.3 and another for (thus introducing a discontinuity in the result at w / h = 3.3). Nevertheless, the 1965 paper is perhaps the more often cited. A number of other approximate formulae for the characteristic impedance have been advanced by other authors. However, most of these are applicable to only a limited range of aspect-ratios, or else cover the entire range piecewise. Curiously, Harold Wheeler disliked both the terms 'microstrip' and 'characteristic impedance', and avoided using them in his papers.

Bends In order to build a complete circuit in microstrip, it is often necessary for the path of a strip to turn through a large angle. An abrupt 90° bend in a microstrip will cause a

significant portion of the signal on the strip to be reflected back towards its source, with only part of the signal transmitted on around the bend. One means of effecting a lowreflection bend, is to curve the path of the strip in an arc of radius at least 3 times the strip-width. However, a far more common technique, and one which consumes a smaller area of substrate, is to use a mitred bend.

Microstrip 90° mitred bend. The percentage mitre is 100x/d To a first approximation, an abrupt un-mitred bend behaves as a shunt capacitance placed between the ground plane and the bend in the strip. Mitring the bend reduces the area of metallization, and so removes the excess capacitance. The percentage mitre is the cutaway fraction of the diagonal between the inner and outer corners of the un-mitred bend. The optimum mitre for a wide range of microstrip geometries has been determined experimentally by Douville and James. They find that a good fit for the optimum percentage mitre is given by,

subject to and the with the substrate dielectric constant . This formula is entirely independent of . The actual range of parameters for which Douville and James present evidence is and . They report a VSWR of better than 1.1 (i.e. a return better than -26dB) for any percentage mitre within 4% (of the original d) of that given by the formula. Note that for the minimum w / h of 0.25, the percentage mitre is 96%, so that the strip is very nearly cut through. For both the curved and mitred bends, the electrical length is somewhat shorter than the physical path-length of the strip.

Chapter- 9

Waveguide Flange

Figure 1. A UBR320 flange on R320 (WG22, WR28) guide. This type of flange has no choke or gasket grooves. The through-mounted assembly is made evident by the distinct colours of the copper waveguide-tube and brass flange. A waveguide flange is a connector for joining sections of waveguide, and is essentially the same as a pipe flange—a waveguide, in the context of this article, being a hollow metal conduit for microwave energy. The connecting face of the flange is either square, circular or (particularly for large or reduce-height rectangular waveguides), rectangular. The connection between a pair of flanges is usually made with four or more bolts, though alternative mechanisms, such as a threaded collar, may be used where there is a need for rapid assembly and disassembly. Dowel pins are sometimes used in addition to bolts, to ensure accurate alignment, particularly for very small waveguides.

Key features of a waveguide join are; whether or not it is air-tight, allowing the waveguide to be pressurized, and whether it is a contact or a choke connection. This leads to three sorts of flange for each size of rectangular waveguide. For rectangular waveguides there exist a number of competing standard flanges which are not entirely mutually compatible. Standard flange designs also exist for double-ridge, reduced-height, square and circular waveguides.

Pressurization The atmosphere within waveguide assemblies is often pressurized to raise its breakdown voltage and so increase the power that may be carried by the guide. This requires that all joins in the waveguide be airtight, something which is usually achieved by means of a rubber O-ring recessed into a groove in the face of at least one of flanges forming each join. Gasket, gasket/cover or pressurizable flanges (such as that on the right of figure 2), are identifiable by the single circular groove which accommodates the O-ring. It is only necessary for one of the flanges in each pressurizable connection to be of this type; the other may have a plain flat face (like that in figure 1). This ungrooved type is known as a cover, plain or unpressurizable flange. It is also possible to form air-tight seal between a pair of otherwise unpressurizable flanges using a flat gasket made out of a special electrically conductive elastomer. Two plain cover flanges may be mated without such a gasket, but the connection is then not pressurizable.

Electrical continuity

Figure 2. A UG-1666/U (MIL-standard) choke flange (left), and matching gasket/cover flange (right). These flanges are aluminium and are socket-mounted onto aluminium WG18 (WR62) waveguide. Electric current flows on the inside surface of the waveguides, and must cross the join between them if microwave power is to pass through the connection without reflection or loss.

Contact connection A contact connection is formed by the union of any combination of gasket and cover flanges, and ideally creates a continuous inner surface from one waveguide to the other, with no crack at the join to interrupt the surface currents. The difficulty with this sort of connection is that any manufacturing imperfections or dirt or damage on the faces of the flanges will result in a crack. Arcing of the current across the crack will cause further damage, loss of power, and may give rise to arcing from one side of the guide to the other, thereby short circuiting it.

Choke connection

Figure 3. E-plane cross-section of connected choke and gasket/cover waveguide flanges from figure 2. The gap between the flange faces has been exaggerated by a factor of four to make it clearly visible. Legend: a. waveguide tubing socket-mounted into... b. choke flange and... c. gasket/cover flange d. gap between flange faces (width exaggerated by factor of 4) e. point of contact of flange faces f. short at bottom of choke ditch g. O-ring gaskets to allow pressurization The choke flange can be also be mated with a plain cover flange and still form a pressurizable join A choke connection is formed by mating one choke flange and one cover (or gasket/cover) flange (but never two choke flanges). The central region of the choke flange face is very slightly recessed so that it does not touch the face of the cover flange, but is separated from it by a narrow gap. The recessed region is bounded by a deep choke trench (or ditch or groove) cut into the face of the flange. Choke flanges are only used with rectangular waveguide, and are invariably pressurizable, having a gasket groove encircling the choke ditch. The presence of these two concentric circular grooves makes choke flanges easily recognizable. The left-hand flange in figure 2 is a choke flange. In the absence of unpressurizable choke flanges, all flanges fall into one of three categories: choke, gasket/cover and cover. An E-plane cross section of an assembled choke connection is shown in figure 3. This is the plane cutting each of the broad walls of the waveguide along its centre-line, which is where the longitudinal surface currents—those that must cross the join—are at their strongest. The choke ditch and the gap between the flange faces together form a somewhat convoluted side-branch to the path of the main guide. This side branch is designed to present a low input impedance where it meets the broad walls of the waveguide, so that the surface currents there are not obstructed by the gap, but instead flow onto and off of the separated faces of the flanges. Conversely, on the outer edge of the choke ditch, at the point where the two flanges come into physical contact, the ditch presents a high series impedance. The current through the contact point is thus reduced to

a small value, and the danger of arcing across any crack between the flanges is likewise reduced. Theory At the operational frequency of the choke flange, the depth of the ditch is approximately one quarter of a wavelength. This is somewhat longer than a quarter of the free-space wavelength, since the electric field also varies in going around the ditch, having two changes of polarity, or one complete wave in the circumference. The ditch thus constitutes a quarter-wave resonant short-circuit stub, and has a high (ideally infinite) input impedance at its mouth. This high impedance is in series with the metal-to-metal connection between the flanges, and minimizes the current across it. The distance from the main waveguide through the gap to the ditch is likewise one quarter of a wavelength in the E-plane. The gap thus forms a quarter-wave transformer, transforming the high impedance at the top of the ditch to a low (ideally zero) impedance at the broad wall of the waveguide.

Figure 4. Plastic caps over disconnected flanges prevent dirt and moisture entering the waveguide, in addition to protecting the face of the flange from damage. Frequency dependence

Because the working of a choke connection depends on the wavelength, its impedance can be zero at at most one frequency within the operating band of the waveguide. However, by making the gap extremely narrow, and the choke ditch relatively wide, the input impedance can be kept small over a broad frequency band. For gap and ditch widths in a fixed proportion, the connection input impedance is approximately proportional to either width (doubling both widths is like having two connections in series). Increasing just the ditch width, increases its input impedance proportionately, and to a some extent decreases the transformed impedance, though the effect is limited when the gap-length is not exactly one quarter wavelength. The MIL-spec choke flanges have a gap width of between 2% and 3% of the waveguide height (the smaller inner dimension of the guide), which for WR28 waveguide (WG22) amounts to a gap of just 3 thousandths of an inch. The choke ditch in these flanges is some 8 times wider (around 20% of the waveguide height), although the proportions vary considerably, as the width-to-height ratio of the standard mid-size guides deviates from 2:1. MIL-Spec choke flanges are intended for use over the full recommended operational frequency band of the waveguide (that is roughly from 1.3 to 1.9 time the guide cutoff). History Claimants to the invention of the choke connection include Norman Ramsey with the assistance of Shep Roberts while the two were working at the MIT Radiation Lab during World War II. Winfield Salisbury also claims to have made the invention while leader of the Radio Frequency Group at the MIT Radiation Lab between 1941 and 1942. The invention was not patented.

Performance Choke connections can achieve a VSWR of 1.01 (a return of -46 dB) over a useful bandwidth, and eliminate the danger of arcing at the join. Nevertheless, better performance is possible with a carefully made contact-connection between undamaged plain flanges.

Attachment to waveguide

Figure 5. RCSC 5985-99-083-0003 choke flange through-mounted on WG16 (WR90) waveguide. Machining down the end of the waveguide tube has left a clear pattern across the recessed face and the end of the tube. The O-ring for pressurization is in place. Flanges are either through-mounted or socket-mounted on the end of the waveguide tube.

Through-mounting In through-mounting, the waveguide tube passes all the way through to the front face of the flange. Initially the tube is allowed to protrude slightly beyond the face of the flange, then after the two pieces have been soldered or brazed together, the end of the tube is machined down so that it is perfectly level with the face. This type of construction can be seen in figures 1, 4 and 5.

Socket-mounting In socket-mounting, the aperture in the front face of the flange matches the inside dimensions of the waveguide. At the back, the aperture is rabbeted to form a socket which fits onto the end of the waveguide tubing. The two pieces are soldered or brazed together to ensure an uninterrupted conducting path between the inside surface of the waveguide tube and the mouth of the flange. This type of construction can be seen in figure 2, and is shown diagramatically in figure 3. A variation on this is butt-mounting, in which the waveguide tube abuts the back face of the flange. The back of the flange has a number of protrusions, sufficient to align the tube, but without forming an unbroken socket-wall around it. Socket mounting avoids the need to machine the face of the flange during attachment. For choke flanges this means that the depth to which the face is recessed, and the width of the resulting gap is fixed when the flange is manufactured and will not change when it is attached. MIL-spec choke flanges are socket-mounted.

Standards

Figure 5. Non-standard quick-disconnect (threaded collar) flanges on WR102 guide

MIL-Spec MIL-DTL-3922 is a United States Military Standard giving detailed descriptions of choke, gasket/cover and cover flanges for rectangular waveguide. MIL_DTL-39000/3 describes flanges for double-ridge waveguide, and formerly also for single-ridge guide. MIL-Spec flanges have designations of the form UG-xxxx/U where the x's represent a variable-length catalogue number, not in itself containing any information about the flange. These standards are works of the U.S. government, and are freely available online from the U.S. Defense Logistics Agency.

IEC

International Electrotechnical Commission (IEC) standard IEC 60154 describes flanges for square and circular waveguides, as well as for what it refers to as flat, medium-flat, and ordinary rectangular guides. IEC flanges are identified by an alphanumeric code consisting of; the letter U, P or C for Unpressurizable (plain cover), Pressurizable (with a gasket groove) and Choke (also with a gasket groove); a second letter, indicating the shape and other details of the flange and finally the IEC identifier for the waveguide. For standard rectangular waveguide the second letter is A to E, where A and C are round flanges, B is square and D and E are rectangular. So for example UBR220 is a square plain cover flange for R220 waveguide (that is, for WG20, WR42), PDR84 is a rectangular gasket flange for R84 waveguide (WG15, WR112) and CAR70 is a round choke flange for R70 waveguide (WG14, WR137). The IEC standard is endorsed by a number of European standards organizations, such as the British Standards Institution.

EIA The Electronic Industries Alliance (EIA) is the body that defined the WR designations for standard rectangular waveguides. EIA flanges are designated CMR (for Connector, Miniature, Rectangular waveguide) or CPR (Connector, Pressurizable, Rectangular waveguide) followed by the EIA number (WR number) for the relevant waveguide. So for example, CPR112 is a gasket flange for waveguide WR112 (WG15).

RCSC The Radio Components Standardization Committee (RCSC) is the body that originated the WG designations for standard rectangular waveguides. It also defined standard choke and cover flanges with identifiers of the form 5985-99-xxx-xxxx where the x's represent a catalogue number, not in itself containing any information about the flange.

Chapter- 10

Cavity Magnetron

The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field. The 'resonant' cavity magnetron variant of the earlier magnetron tube was invented by Randall and Boot in 1940. The high power of pulses from the cavity magnetron made centimetre-band radar practical. Shorter wavelength radars allowed detection of smaller objects. The compact cavity magnetron tube drastically reduced the size of radar sets so that they could be installed in anti-submarine aircraft and escort ships. At present, cavity magnetrons are commonly used in microwave ovens and in various radar applications.

Construction and operation

Magnetron with section removed (magnet is not shown)

A similar magnetron with a different section removed (magnet is not shown). All cavity magnetrons consist of a hot cathode with a high (continuous or pulsed) negative potential by a high-voltage, direct-current power supply. The cathode is built into the center of an evacuated, lobed, circular chamber. A magnetic field parallel to the filament is imposed by a permanent magnet. The magnetic field causes the electrons, attracted to the (relatively) positive outer part of the chamber, to spiral outward in a circular path rather than moving directly to this anode. Spaced around the rim of the chamber are cylindrical cavities. The cavities are open along their length and connect the common cavity space. As electrons sweep past these openings, they induce a resonant, high-frequency radio field in the cavity, which in turn causes the electrons to bunch into groups. A portion of this field is extracted with a short antenna that is connected to a waveguide (a metal tube usually of rectangular cross section). The waveguide directs the extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high-gain antenna in the case of radar.

A cross-sectional diagram of a resonant cavity magnetron. Magnetic lines of force are parallel to the geometric axis of this structure. The sizes of the cavities determine the resonant frequency, and thereby the frequency of emitted microwaves. However, the frequency is not precisely controllable. The operating frequency varies with changes in load impedance, with changes in the supply current, and with the temperature of the tube. This is not a problem in uses such as heating, or in some forms of radar where the receiver can be synchronized with an imprecise magnetron frequency. Where precise frequencies are needed, other devices such as the klystron are used. The magnetron is a self-oscillating device requiring no external elements other than a power supply. A well-defined threshold anode voltage must be applied before oscillation will build up; this voltage is a function of the dimensions of the resonant cavity, and the applied magnetic field. In pulsed applications there is a delay of several cycles before the oscillator achieves full peak power, and the build-up of anode voltage must be coordinated with the build-up of oscillator output. The magnetron is a fairly efficient device. In a microwave oven, for instance, a 1.1 kilowatt input will generally create about 700 watt of microwave power, an efficiency of around 65%. (The high-voltage and the properties of the cathode determine the power of a magnetron.) Large S-band magnetrons can produce up to 2.5 megawatts peak power with an average power of 3.75 kW. Large magnetrons can be water cooled. The magnetron remains in widespread use in roles which require high power, but where precise frequency control is unimportant.

Applications

Magnetron from a microwave oven with magnet in its mounting box. The horizontal plates form a heat sink, cooled by airflow from a fan

Radar In radar devices the waveguide is connected to an antenna. The magnetron is operated with very short pulses of applied voltage, resulting in a short pulse of high power microwave energy being radiated. As in all radar systems, the radiation reflected off a target is analyzed to produce a radar map on a screen. Several characteristics of the magnetron's power output conspire to make radar use of the device somewhat problematic. The first of these factors is the magnetron's inherent instability in its transmitter frequency. This instability is noted not only as a frequency

shift from one pulse to the next, but also a frequency shift within an individual transmitter pulse. The second factor is that the energy of the transmitted pulse is spread over a wide frequency spectrum, which makes necessary its receiver to have a corresponding wide selectivity. This wide selectivity permits ambient electrical noise to be accepted into the receiver, thus obscuring somewhat the received radar echoes, thereby reducing overall radar performance. The third factor, depending on application, is the radiation hazard caused by the use of high power electromagnetic radiation. In some applications, for example a marine radar mounted on a recreational vessel, a radar with a magnetron output of 2 to 4 kilowatts is often found mounted very near an area occupied by crew or passengers. In practical use, these factors have been overcome, or merely accepted, and there are today thousands of magnetron aviation and marine radar units in service. Recent advances in aviation weather avoidance radar and in marine radar have successfully implemented semiconductor transmitters that eliminate the magnetron entirely.

Heating In microwave ovens the waveguide leads to a radio frequency-transparent port into the cooking chamber.

Lighting In microwave-excited lighting systems, such as a sulfur lamp, a magnetron provides the microwave field that is passed through a waveguide to the lighting cavity containing the light-emitting substance (e.g., sulfur, metal halides, etc.)

History The first simple, two-pole magnetron was developed in 1920 by Albert Hull at General Electric's Research Laboratories (Schenectady, New York), as an outgrowth of his work on the magnetic control of vacuum tubes in an attempt to work around the patents held by Lee De Forest on electrostatic control. Hull's magnetron was not originally intended to generate VHF (very-high-frequency) electromagnetic waves. However, in 1924, Czech physicist August Žáček (1886-1961) and German physicist Erich Habann (1892-1968) independently discovered that the magnetron could generate waves of 100 megahertz to 1 gigahertz. Žáček, a professor at Prague's Charles University, published first; however, he published in a journal with a small circulation and thus attracted little attention. Habann, a student at the University of Jena, investigated the magnetron for his doctoral dissertation of 1924. Throughout the 1920s, Hull and other researchers around the world worked to develop the magnetron. Most of these early magnetrons were glass vacuum tubes with multiple anodes. However, the two-pole magnetron, also known as a split-anode magnetron, had relatively low efficiency. The cavity version (properly referred to as a resonant-cavity magnetron) proved to be far more useful.

While radar was being developed during World War II, there arose an urgent need for a high-power microwave generator that worked at shorter wavelengths (around 10 cm (3 GHz)) rather than the 150 cm (200 MHz) that was available from tube-based generators of the time. It was known that a multi-cavity resonant magnetron had been developed and patented in 1935 by Hans Hollmann in Berlin. However, the German military considered its frequency drift to be undesirable and based their radar systems on the klystron instead. But klystrons could not achieve the high power output that magnetrons eventually reached. This was one reason that German night fighter radars were not a match for their British counterparts. In 1940, at the University of Birmingham in the United Kingdom, John Randall and Harry Boot produced a working prototype similar to Hollman's cavity magnetron, but added liquid cooling and a stronger cavity. Randall and Boot soon managed to increase its power output 100 fold. Instead of abandoning the magnetron due to its frequency instability, they sampled the output signal and synchronized their receiver to whatever frequency was actually being generated. In 1941, the problem of frequency instability was solved by coupling alternate cavities within the magnetron. Because France had just fallen to the Nazis and Britain had no money to develop the magnetron on a massive scale, Churchill agreed that Sir Henry Tizard should offer the magnetron to the Americans in exchange for their financial and industrial help (the Tizard Mission). An early 6 kW version, built in England by the General Electric Company Research Laboratories, Wembley, London (not to be confused with the similarly named American company General Electric), was given to the US government in September 1940. At the time the most powerful equivalent microwave producer available in the US (a klystron) had a power of only ten watts. The cavity magnetron was widely used during World War II in microwave radar equipment and is often credited with giving Allied radar a considerable performance advantage over German and Japanese radars, thus directly influencing the outcome of the war. It was later described as "the most valuable cargo ever brought to our shores". The Bell Telephone Laboratories made a producible version from the magnetron delivered to America by the Tizard Mission, and before the end of 1940, the Radiation Laboratory had been set up on the campus of the Massachusetts Institute of Technology to develop various types of radar using the magnetron. By early 1941, portable centimetric airborne radars were being tested in American and British planes. In late 1941, the Telecommunications Research Establishment in Great Britain used the magnetron to develop a revolutionary airborne, ground-mapping radar codenamed H2S. The H2S radar was in part developed by Alan Blumlein and Bernard Lovell. Centimetric radar, made possible by the cavity magnetron, allowed for the detection of much smaller objects and the use of much smaller antennas. The combination of smallcavity magnetrons, small antennas, and high resolution allowed small, high quality radars to be installed in aircraft. They could be used by maritime patrol aircraft to detect objects as small as a submarine periscope, which allowed aircraft to attack and destroy submerged submarines which had previously been undetectable from the air. Centimetric

contour mapping radars like H2S improved the accuracy of Allied bombers used in the strategic bombing campaign. Centimetric gun-laying radars were likewise far more accurate than the older technology. They made the big-gunned Allied battleships more deadly and, along with the newly developed proximity fuze, made anti-aircraft guns much more dangerous to attacking aircraft. The two coupled together and used by antiaircraft batteries, placed along the flight path of German V-1 flying bombs on their way to London, are credited with destroying many of the flying bombs before they reached their target. Since then, many millions of cavity magnetrons have been manufactured; while some have been for radar the vast majority have been for microwave ovens. The use in radar itself has dwindled to some extent, as more accurate signals have generally been needed and developers have moved to klystron and traveling-wave tube systems for these needs.

Health hazards

Caution: radiowaves hazard Among more speculative hazards, at least one in particular is well known and documented. As the lens of the eye has no cooling blood flow, it is particularly prone to overheating when exposed to microwave radiation. This heating can in turn lead to a higher incidence of cataracts in later life. A microwave oven with a warped door or poor microwave sealing can be hazardous. There is also a considerable electrical hazard around magnetrons, as they require a high voltage power supply. Some magnetrons have beryllium oxide (beryllia) ceramic insulators, which are dangerous if crushed and inhaled, or otherwise ingested. Single or chronic exposure can lead to berylliosis, an incurable lung condition. In addition, beryllia is listed as a confirmed human carcinogen by the IARC; therefore, broken ceramic insulators or magnetrons should not be directly handled.

Chapter- 11

Diverse Microwave Technologies

Isolator (microwave)

Resonance absorption isolator consisting of WG16 waveguide containing two strips of ferrite (black rectangle near right edge of each broad wall), which are biased by a

horseshoe permanent magnet external to the guide. Transmission direction is indicated by arrow on label on right An isolator is a two-port device that transmits microwave or radio frequency power in one direction only. It is used to shield equipment on its input side, from the effects of conditions on its output side; for example, to prevent a microwave source being detuned by a mismatched load.

Non-reciprocity An isolator in a non-reciprocal device, with a non-symmetric scattering matrix. An ideal isolator transmits all the power entering port 1 to port 2, while absorbing all the power entering port 2, so that to within a phase-factor its S-matrix is

To achieve non-reciprocity, an isolator must necessarily incorporate a non-reciprocal material. At microwave frequencies this material is invariably a ferrite which is biased by a static magnetic field. The ferrite is positioned within the isolator such that the microwave signal presents it with a rotating magnetic field, with the rotation axis aligned with the direction of the static bias field. The behaviour of the ferrite depends on the sense of rotation with respect to the bias field, and hence is different for microwave signals travelling in opposite directions. Depending on the exact operating conditions, the signal travelling in one direction may either be phase-shifted, displaced from the ferrite or absorbed.

Types

An X band isolator consisting of a waveguide circulator with an external matched load on one port

Two isolators each consisting of a coax circulator and a matched load

Resonance absorption In this type the ferrite absorbs energy from the microwave signal travelling in one direction. A suitable rotating magnetic field is found in the TE10 mode of rectangular waveguide. The rotating field exists away from the centre-line of the broad wall, over the full height of the guide. However, to allow heat from the absorbed power to be conducted away, the ferrite does not usually extend from one broad-wall to the other, but is limited to a shallow strip on each face. For a given bias field, resonance absorption occurs over a fairly narrow frequency band, but since in practice the bias field is not perfectly uniform throughout the ferrite, the isolator functions over a somewhat wider band.

Using a circulator A circulator is a non-reciprocal three- or four-port device, in which power entering any port is transmitted to the next port in rotation (only). So to within a phase-factor, the scattering matrix for a three-port circulator is

A two-port isolator is obtained simply by terminating one of the three ports with a matched load, which absorbs all the power entering it. The biassed ferrite is part of the circulator. The bias field is lower than that needed for resonance absorption, and so this type of isolator does not require such a heavy permanent magnet. Because the power is absorbed in an external load, cooling is less of a problem than with a resonance absorption isolator.

Microwave Power Module A Microwave Power Module (MPM) is a microwave device used to amplify radio frequency signals to high power levels. It is a hybrid combination of solid-state and vacuum tube electronics, which encloses a solid-state driver amplifier (SSPA), traveling wave tube amplifier (TWTA) and electronic power conditioning (EPC) modules into a single unit . Their average output power capability falls between that of solid-state power amplifiers (SSPAs) and dedicated Traveling Wave Tube (TWT) amplifiers. They may be applied wherever high power microwave amplification is required, and space is at a premium. They are available in various frequency ranges, from S band up to W band. Typical output power at Ku band ranges from 20W to 1kW.

History The microwave power module concept was designed for use in active phased array antennas, where their compact size permits packing a large number of modules into the radiating face of the antenna. The concept was explored in detail by the 1989 Microwave Power Module Panel, supported by the US Naval Research Laboratory. While the eventual goal was to design a power module with a cross section as small as a half square inch, most MPMs today are larger, and suitable only for line arrays, partially distributed arrays and single-module applications.

Typical Specifications Microwave power modules are available at various frequencies, from S band up to W band . Both CW and pulsed MPMs are available, the pulsed MPMs having a wide duty cycle range. Power levels range from less than 20W to over 1 kW. MPMs are lightweight compared to traditional TWTAs, and the power supply requirements are typically low-voltage DC (28 - 270V DC).

Construction

Block diagram of an MPM A microwave power module consists of a solid state power amplifier, which drives a vacuum power booster, typically a traveling wave tube. The high voltage power supply required by the TWT is provided by an electronic power conditioner. In pulsed-mode MPMs, the power conditioner provides a pulsed high voltage that is triggered by a trigger input. MPMs also include a microcontroller, which is responsible for controlling the operation of the module, such as making sure the various power supply voltages come up in the appropriate sequence to prevent damage to the TWT. It also reports the module status, including the various voltages, currents and temperatures.

Applications Microwave power modules are used in • • •

Active phased array antennas Radar transmitters where relatively low power, but long pulse widths are needed (such as Synthetic Aperture Radars) Commercial and military satellite communications

Microwave cavity A microwave cavity is a closed metal structure that confines electromagnetic fields in the microwave region of the spectrum. Such cavities act as resonant circuits with extremely low loss at their frequency of operation. Their Q factor may reach several hundred thousand compared to a few hundred for resonant circuits made with inductors and capacitors at the same frequency. For frequencies over a few hundred megahertz in the VHF range, conventional inductors and capacitors present difficult problems. The losses of both increase with frequency. This type of inductor is usually wound from wire in the shape of a helix with no core. Skin effect causes the high frequency resistance of inductors to be many times their direct current resistance. In addition, capacitance between turns causes dielectric losses in the insulation which coats the wires. These effects make the high frequency resistance greater and decrease the "Q".

This type of capacitor will use air, mica, ceramic or perhaps teflon for a dielectric. Even with a low loss dielectric, capacitors are also subject to skin effect losses in their leads and plates. Both effects increase their equivalent series resistance and reduce their Q. Even if the Q of VHF inductors and capacitors is high enough to be useful, each suffers from the problem of being composed of some of the other. The shunt capacitance of an inductor may be more significant than its desirable series inductance. The series inductance of a capacitor may be more significant than its desirable shunt capacitance. As a result, in the VHF or microwave regions, a capacititor may appear to be an inductor and an inductor may appear to be a capacitor. The energy of an air core inductor should be almost totally in its magnetic field. Some energy is stored in the electric field due to the capacitance between its turns. The latter energy is an unwanted feature. The energy of a capacitor should be almost totally in the electric field of its dielectric. Some is stored in the magnetic field from the current in its leads. This is unwanted as well. Air is almost loss free for high frequency electric or magnetic fields. Microwave cavities confine electric and magnetic fields almost exclusively to the air spaces between their walls. The currents in the cavity walls are small because they are at a high impedance point. While losses are small from these currents, cavities are frequently plated with silver to increase their electrical conductivity and reduce the losses even further. Copper cavities frequently oxidize, which increases their loss. Silver or gold plating will prevent this. Even though gold is not quite as good a conductor as copper, it prevents oxidation and the resulting deterioration of Q with aging. Because of its much higher cost, it is used only in the most demanding applications. Comment: I disagree with the note about oxides destroying the Q of the resonator. The currents will flow under the oxide layer. The problem is if the oxide layer becomes resistive. Silver will oxidize and this does not destroy the Q. I do not know how copper oxide behaves. Some satellite resonators are silver plated, that are covered with a gold flash layer. The current will then mostly flow in the silver, while the gold protects the silver layer from oxidizing.

Monolithic microwave integrated circuit

Photograph of a GaAs MMIC (a 2-18GHz upconverter)

MMIC MSA-0686. A Monolithic Microwave Integrated Circuit, or MMIC (sometimes pronounced "mimic"), is a type of integrated circuit (IC) device that operates at microwave frequencies (300 MHz to 300 GHz). These devices typically perform functions such as microwave mixing, power amplification, low noise amplification, and high frequency switching. Inputs and outputs on MMIC devices are frequently matched to a characteristic impedance of 50 ohms. This makes them easier to use, as cascading of MMICs does not then require an external matching network. Additionally most microwave test equipment is designed to operate in a 50 ohm environment. MMICs are dimensionally small (from around 1 mm² to 10 mm²) and can be mass produced, which has allowed the proliferation of high frequency devices such as cellular phones. MMICs were originally fabricated using gallium arsenide (GaAs), a III-V compound semiconductor. It has two fundamental advantages over Silicon (Si), the traditional material for IC realisation: device (transistor) speed and a semi-insulating substrate. Both factors help with the design of high frequency circuit functions. However, the speed of Si-based technologies has gradually increased as transistor feature sizes have reduced and MMICs can now also be fabricated in Si technology. The primary advantage of Si technology is its lower fabrication cost compared with GaAs. Silicon wafer

diameters are larger (typically 8" or 12" compared with 4" or 6" for GaAs) and the wafer costs are lower, contributing to a less expensive IC. Other III-V technologies, such as Indium Phosphide (InP), have been shown to offer superior performance to GaAs in terms of gain, higher cutoff frequency, and low noise. However they also tend to be more expensive due to smaller wafer sizes and increased material fragility. Silicon Germanium (SiGe) is a Si-based compound semiconductor technology offering higher speed transistors than conventional Si devices but with similar cost advantages. Gallium Nitride (GaN) is also an option for MMICs. Because GaN transistors can operate at much higher temperatures and work at much higher voltages than GaAs transistors, they make ideal power amplifiers at microwave frequencies.

Rat-race coupler

Rat-race coupler

A rat-race coupler (also known as a hybrid ring coupler) is a type of coupler used in RF and Microwave systems. In its simplest form it is a 3dB coupler and is thus an alternative to a magic tee. Compared to the magic tee, it has the advantage of being easy to realize in planar technologies such as microstrip and stripline, although waveguide rat races are also practical. Unlike magic tees, a rat-race needs no matching structure to achieve correct operation. The rat-race coupler has four ports, each placed one quarter wavelength away from each other around the top half of the ring. The bottom half of the ring is three quarter wavelengths in length. A signal input on port 1, will be split between ports 2 and 4, and port 3 will be isolated. The full scattering matrix for an ideal 3dB rat-race is

Arithmetics with rat-race coupler

Rat-race couplers are used to sum two in-phase combined signals with essentially no loss or to equally split an input signal with no resultant phase difference between out and inputs. It is also possible to configure the coupler as a 180 degree phase-shifted output divider or to sum two 180 degree phase-shifted combined signals with almost no loss.

RF switch matrix RF Switch Matrix or Microwave Switch Matrix or Switch Matrix An RF/Microwave Switch Matrix is used in test systems, in both design verification and manufacturing test, to route high frequency signals between the device under test (DUT) and the test and measurement equipment. Besides signal routing, the RF/Microwave Switch Matrix may also contain signal conditioning including passive signal conditioning devices, such as attenuators, filters, and directional couplers, as well as active signal conditioning, such as amplification and frequency converters. Since the signal routing and signal conditioning needs of a test system differ from design to design, RF/Microwave Switch Matrices typically have to be custom designed by the test system engineer or a hired contractor for each new test system. The Switch Matrix is made up of switches and signal conditioners that are mounted together in a mechanical infrastructure or housing. Cables are employed to interconnect the switches and signal conditioners. The switch matrix then employs some type of driver circuit and power supply to power and drive the switches and signal conditioners. The switch matrix uses connectors or fixtures to route the signal paths of the sourcing and measurement equipment to the DUT. The switch matrix is typically located close to DUT in the test system to shorten the signal paths to the DUT thus reducing insertion loss and signal degradation.

Benefits of an RF/Microwave Switch Matrix The purpose of a switch matrix is to move the signal routing and signal conditioning to one central location in the test system versus having it all distributed at various places in the test system. Moving the signal routing and signal conditioning to a single location in the test system has the following advantages: •

Calibration plane between the DUT and test equipment becomes smaller and centralized making it easier to characterize.

•

• • •

Switches and signal conditioners have similar power, mounting, and driver requirements so moving them to a single location means you will only need a single power supply and driver circuit to power and control them. Short signal paths reduce insertion loss and increase signal integrity. Exact length signal paths are possible to control phase issues. Simplifies service and support.

Making It vs Buying It Switch matrices present a unique problem to test system designers because the signal conditioning needs, the frequency range, the bandwidth, and power aspects change from application to application. So test and measurement companies cannot provide a one size fits all solution. This leaves test system designers with two choices for their switch matrix design: Create an in-house solution or contract it out. Advantages of creating your switch matrix in-house:

•

• •

•

Proprietary concerns can be a big issue especially in the Aerospace Defense industry. Creating a switch matrix in-house makes proprietary concerns a nonissue. Using spare human resources may be less costly. Being the first to develop an emerging technology into a finished product can be very profitable for a company. When building a switch matrix in-house the timely process of shopping around for the right contractor is bypassed. A company is in control of the amount of daily man hours spent developing a switch matrix. Successive switch matrix designs can be highly leveragable from design to design. The switch driver hardware and software, the mechanical designs, the power supply, etc. can all be leveraged from design to design with little or no modification.

Contracting out advantages: • • •

Company lacks spare human resources. System integrators (contractors) tend to have more experience and expertise. They can design within tight specs and can handle complicated designs. System integrators can provide guaranteed work as well as product support.

Signal routing

A PIN Diode RF Microwave Switch. Picture courtesy of Herley There are two types of switches typically used in switch matrices: Coaxial Electromechanical Switches and Solid State Switches, also known as electronic switches. Coaxial electromechanical switches can be further divided into two categories based on their architecture, latching relay and non-latching relay. Solid state switches come in three types: PIN diode, FET, and hybrid. The advantages of solid state switches over EM switches include they have much faster switching speed (at least 10,000 times faster), they have an almost infinite life, and they are very stable and repeatable. On the other hand, since solid state switches have non-linear portions over their frequency range their bandwidth is limited. Also, EM switches provide better insertion loss, VSWR, power handling, and isolation specifications. For these reasons EM switches are used much more often in switch matrix designs.

Example applications Custom Switch Matrices are used extensively throughout test systems in the wireless and aerospace defense sectors for design verification and manufacturing test. They can range from the simple to the complex. An example of a simple design switch matrix application

would be a 1:16 MUX configuration that routes 12 satellite TV feeds to a single spectrum analyzer input that is used to perform signal integrity checks on the satellite feeds. Such a design would require 5 SP4T coaxial EM switches as well as interconnecting coax cable for the signal routing along with a mechanical infrastructure, power supply, and switch driver circuit to mount, power, and operate the switches. An example of a more complex switch matrix is an application that is measuring jitter on multiple high speed serial data buses. The switch matrix inputs the data bus signals then provides the proper switching and signal conditioning for the signals before feeding the signals to test and measurement instruments. This custom switch matrix employed 14 EM switches and a number of different signal conditioners including: power splitters, amplifiers, mixers, filters, and attenuators.

Design challenges There are six main challenges when designing a custom RF/Microwave Switch Matrix from beginning to end: 1. Mechanical Design: design of an electrically shielded enclosure or box, internal component mounting brackets, as well as component and cabling layout. 2. RF/Microwave Design: RF/Microwave signal routing and signal conditioning design and testing. A calibration plan for the switch matrix would need to be developed to properly characterize the signal paths. 3. Power and Control Hardware: The power supply and switch driver circuitry will need to be designed and developed. 4. Software Control: A software driver will need to be developed to provide an interface between the control hardware and test system program. 5. Documentation: The whole switch matrix design will have to be documented to support maintenance and possible future design leveraging. 6. Servicing Plan: A servicing plan will need to be developed to ensure the life of the switch matrix lasts as long as the life of the test system. Test equipment manufacturers offer instruments that provide a power supply, driver circuitry, and software drivers that essentially saves a test system designer time and cost by eliminating two of the six switch matrix design challenges: power and control hardware design as well as software driver development. Many companies have introduced new product concepts that aid in custom switch matrix design. These new products offers test system designers a power supply, driver circuitry, and software drivers all wrapped together in a mainframe. The mainframe provides flexible mounting for switches and other components as well as blank front and rear panel that can be easily modified to fit a design need. These new products eliminates 3 of the 6 design challenges: mechanical design, power and control hardware design, and software driver development

Vircator A vircator (VIRtual CAthode oscillaTOR) is a microwave generator that is capable of generating brief pulses of tunable, narrow band microwaves at very high power levels.

A typical vircator is built inside an evacuated resonant cavity or waveguide. An electrode at one end injects an intense electron beam, such as from a Marx generator or a flux compression generator. The electrons are attracted to a thin anode, such as an aluminized PET film, that is connected to the grounded waveguide body. The unit is surrounded by a magnet. Due to the intensity of the electron beam, many electrons pass through the anode into the region beyond it, forming a virtual cathode. The electron beam must be so intense as to exceed the space charge limiting current in that region, causing oscillations that generate microwaves. The frequency, efficiency and other characteristics of the emitted beam depend on the precise physical configuration and operating parameters. Vircators have been used as electromagnetic pulse generators and for generating X-rays. Power levels on the order of 1010 to 1012 watts are possible.

Backward wave oscillator A backward wave oscillator (BWO), also called carcinotron (a trade name for tubes manufactured by CSF, now Thales) or backward wave tube, is a vacuum tube that is used to generate microwaves up to the terahertz range. It belongs to the traveling-wave tube family. It is an oscillator with a wide electronic tuning range.

An electron gun generates an electron beam that is interacting with a slow-wave structure. It sustains the oscillations by propagating a traveling wave backwards against the beam. The generated electromagnetic wave power has its group velocity directed oppositely to the direction of motion of the electrons. The output power is coupled out near the electron gun. It has two main subtypes, the M-type, the most powerful, (M-BWO) and the O-type (OBWO). The O-type delivers typically power in the range of 1 mW at 1000 GHz to 50 mW at 200 GHz. Carcinotrons are used as powerful and stable microwave sources. Due to the good quality wavefront they produce, they find use as illuminators in terahertz imaging. The backward wave oscillators were demonstrated in 1951, M-type by Bernard Epsztein, (French patent 1,035,379; British patent 699,893; US patent 2,880,355) and O-type by Rudolf Kompfner. The M-type BWO is a voltage-controlled non-resonant extrapolation of magnetron interaction, both types are tunable over a wide range of frequencies by varying the accelerating voltage. They can be swept through the band fast enough to be appearing to radiate over all the band at once, which makes them suitable for effective radar jamming, quickly tuning into the radar frequency. Carcinotrons allowed airborne radar jammers to be highly effective. However, frequency-agile radars can hop frequencies fast enough to force the jammer to use barrage jamming, diluting its output power over a wide band and significantly impairing its efficiency. Carcinotrons are used in research, civilian and military applications. For example, the Kopac passive sensor and Ramona passive sensor employed carcinotrons in their receiver systems.

The Slow-wave structure

(a) Forward fundamental space harmonic (n=0), (b) Backward fundamental The needed slow-wave structures must support a Radio Frequency (RF) electric field with a longitudinal component; the structures are periodic in the direction of the beam and behave like microwave filters with passbands and stopbands. Due to the periodicity of the geometry, the fields are identical from cell to cell except for a constant phase shift Φ. This phase shift, a purely real number in a passband of a lossless structure, varies with frequency. According to Floquet's theorem, the RF electric field E(z,t) can be described at an angular frequency ω, by a sum of an infinity of "spatial or space harmonics" En

E(z,t)= where the wave number or propagation constant kn of each harmonic is expressed as: kn=(Φ+2nπ)/p (-π<Φ<+п) z being the direction of propagation, p the pitch of the circuit and n an integer. Two examples of slow-wave circuit characteristics are shown, in the ω-k or Brillouin diagram: •

on figure (a), the fundamental n=0 is a forward space harmonic (the phase velocity vn=ω/kn has the same sign as the group velocity vg=dω/dkn), synchronism

condition for backward interaction is at point B, intersection of the line of slope ve - the beam velocity - with the first backward (n = -1) space harmonic, •

on figure (b) the fundamental (n=0) is backward

A periodic structure can support both forward and backward space harmonics, which are not modes of the field, and cannot exist independently, even if a beam can be coupled to only one of them. As the magnitude of the space harmonics decreases rapidly when the value of n is large, the interaction can be significant only with the fundamental or the first space harmonic.

M-type BWO

Schematic of an M-BWO The M-type carcinotron, or M-type backward wave oscillator, uses crossed static electric field E and magnetic field B, similar to the magnetron, for focussing an electron sheet beam drifting perpendicularly to E and B, along a slow-wave circuit, with a velocity E/B. Strong interaction occurs when the phase velocity of one space harmonic of the wave is equal to the electron velocity. Both Ez and Ey components of the RF field are involved in the interaction (Ey parallel to the static E field). Electrons which are in a decelerating Ez electric field of the slow-wave, lose the potential energy they have in the static electric field E and reach the circuit. The sole electrode is more negative than the cathode, in order to avoid collecting those electrons having gained energy while interacting with the slow-wave space harmonic.

O-type BWO The O-type carcinotron, or O-type backward wave oscillator, uses an electron beam longitudinally focused by a magnetic field, and a slow-wave circuit interacting with the beam. A collector collects the beam at the end of the tube.

Spurline The spurline is a type of radio-frequency and microwave distributed element filter with band-stop (notch) characteristics, most commonly used with microstrip transmission lines. Spurlines usually exhibit moderate to narrow-band rejection, at about 10% around the central frequency. Spurline filters are very convenient for dense integrated circuits because of their inherently compact design and ease of integration: they occupy surface that corresponds only to a quarter-wavelength transmission line.

Structure Description It consists of a normal microstrip line breaking into a pair of smaller coupled lines that rejoin after a quarter-wavelength distance. Only one of the input ports of the coupled lines is connected to the feed microstrip, as shown in the figure below. The orange area of the illustration is the microstrip transmission line conductor and the gray color the exposed dielectric.

Figure : Microstrip Spurline Notch Filter (Top View) Where λg is the wavelength corresponding to the central rejection frequency of the bandstop filter, measured - of course - in the microstrip line material. This is the most important parameter of the filter that sets the rejection band. The distance between the two coupled lines can be selected appropriately to fine-tune the filter. The smaller the distance, the narrower the stop-band in terms of rejection. Of course that is limited by the circuit-board printing resolution, and it is usually considered at about 10% of the input microstrip width. The gap between the input microstrip line and the one open-circuited line of the coupler has a negligible effect on the frequency response of the filter. Therefore it is considered approximately equal to the distance of the two coupled lines.

Circulator

A waveguide circulator used as an isolator by placing a matched load on port 3. The label on the permanent magnet indicates the direction of circulation A circulator is a passive non-reciprocal three- or four-port device, in which microwave or radio frequency power entering any port is transmitted to the next port in rotation (only). Thus, to within a phase-factor, the scattering matrix for an ideal three-port circulator is

When one port of a three-port circulator is terminated in a matched load, it can be used as an isolator, since a signal can travel in only one direction between the remaining ports. There are circulators for LF, VHF, UHF, microwave frequencies and for light, the latter being used in optical fiber networks. Circulators fall into two main classes: 4-port waveguide circulators based on Faraday rotation of waves propagating in a magnetised material, and 3-port "Y-junction" circulators based on cancellation of waves propagating over two different paths near a magnetised material. Waveguide circulators may be of either type, while more compact devices based on striplines are of the 3-port type.

Sometimes two or more Y-junctions are combined in a single component to give four or more ports, but these differ in behaviour from a true 4-port circulator. In radar, circulators are used to route outgoing and incoming signals between the antenna, the transmitter and the receiver. In a simple system, this function could be performed by a switch that alternates between connecting the antenna to the transmitter and to the receiver. The use of chirped pulses and a high dynamic range may lead to temporal overlap of the sent and received pulses, however, requiring a circulator for this function. Radio frequency circulators are composed of magnetised ferrite materials. A permanent magnet produces the magnetic flux through the waveguide. Ferrimagnetic garnet crystal is used in optical circulators. There have also been investigations into making "active circulators" which are based on electronics rather than passive materials. However, the power handling capability and linearity and signal to noise ratio of transistors is not as high as those made from ferrites. It seems that transistors are the only (space efficient) solution for low frequencies.

British Telecom microwave network The British Telecom microwave network was a network of point-to-point microwave radio links in the United Kingdom, operated at first by the General Post Office, and subsequently by its successor BT plc. From the late 1950s to the 1980s it provided a large part of BT's trunk communications capacity, and carried telephone, television and radar signals and digital data, both civil and military. Its use of line-of-sight microwave transmission was particularly important during the Cold War for its resilience against nuclear attack. It was rendered obsolete, at least for normal civilian purposes, by the installation of a national fibre optic communication network with considerably higher reliability and vastly greater capacity. BT remains one of the largest owners of transmission and microwave towers in the UK. The most famous of these is the BT Tower in London, which was the tallest building in the UK from its construction in the 1960s until the early 1980s, and a major node in the BT microwave network.

Television links The earliest operational GPO microwave links were provided for 405-line BBC television.

Experimental systems London to Birmingham pre-war

In 1939 the Post Office placed a contract with EMI for an experiment in the relaying of television signals to Birmingham. In this case, the signals from Alexandra Palace were to be received at Dunstable and transmitted over a radio link to Sharmans Hill, Charwelton, some 40 miles distant towards Birmingham; thus carrying the signal two-thirds of the way from London to Birmingham. World War II intervened and this early experiment had to be abandoned. London to Castleton 195 MHz The GPO built in an experimental chain of radio relay stations for television, which used the relatively low VHF frequency of 195 MHz and frequency modulation with a deviation of 6 MHz per volt. Each relay station consisted essentially of back-to-back rhombic antennas on opposite sides of a hilltop, connected via an amplifier. The frequency was not changed. The system was first tested on March 24, 1949. The stations were located at: • • • • • •

Rowley Lodge, near Barnet Green Hailey Widley Hook Wotton-under-Edge Post Office Radio Laboratory at Castleton, near Cardiff

London to Castleton 4 GHz The GPO built an experimental 4 GHz system. This was used operationally to feed TV pictures to the Wenvoe transmitter during the latter's first four months on air in late 1952, until a coaxial feed became available. Some of this equipment from this link was recovered, refurbished, modified and used to provide a permanent link from London to Rowridge in 1954.

London to Birmingham 900 MHz A chain of stations was built between telephone exchanges in London and Birmingham to connect the Sutton Coldfield transmitting station to Alexandra Palace. The contract for this was placed with GEC in mid-1947. The stations were located at: • • • • • •

London Museum exchange Harrow Weald Zouches Farm Charwelton Turner's Hill Birmingham Telephone House

Manchester to Kirk o'Shotts

The GPO placed a contract in July 1950 for a chain of microwave links to feed BBC television from Manchester to the Kirk o'Shotts transmitting station. This was the first permanent GPO system to use the 4 GHz band. The chain was routed near the east coast in order to be close to Leeds, Newcastle and Edinburgh. The stations were located at: • • • • • • • • •

Manchester Telephone House Windy Hill Tinshill Arncliffe Wood Pontop Pike Corby's Crags Blackcastle Hill Blackford Hill Kirk o'Shotts

Backbone

Backbone as proposed in 1956 The term 'backbone' is often applied to the core of a communications network, i.e. the part that provides high-capacity links over long distances between major nodes. In the early 1950s, the term was used by the General Post Office (BT's predecessor) to describe a chain of microwave links designed to provide resilient communications in the event of nuclear war. It was originally designed as a chain of stations between south-east England and Scotland. The exact location of the Backbone sites changed as the project was developed, but in July 1956 there were 14 planned sites at (from south to north): •

Tring (Herts.)

• • • • • • • • • • • • •

Charwelton, Northants. Coalville, Leics. Pye Green BT Tower Sutton Common Saddleworth Hunters Stones (Nr. Skipton, Yorks.) Azerley, Yorks. Richmond, Yorks. Muggleswick, Co. Durham Cold Fell, Cumberland Lockerbie, Dumfries Green Lowther, Dumfries Kirk o'Shotts (GPO site near BBC site)

Two additional 'Backbone spur' sites were planned for Shrewsbury and Grantham, which connected to the main Backbone spine at Pye Green and Coalville respectively.

Radio standby to line The 1956 GPO paper referred to under 'Backbone' above also described a series of links called 'radio standby to line'. These were spur links between the GPO Backbone sites and defence 'customer' sites. They were designed to carry between 25 and 150 'private wire' (a.k.a. leased line) circuits each, by radio. The paper contains a list of sites and a network map, showing the following radio standby to line links: • • •

• • • • • • •

Kirk o'Shotts to Gailes GCI radar station near Ayr Muggleswick to Boulmer GCI station, ROC and regional communications, Seaton Snook GCI station Hunters Stones to Forest Moor Admiralty radio receiving station, Shipton RAF 'Sector Operations Centre' (SOC), Preston SOC, Regional Commissioner's HQ and Admiralty radio transmitting station Grantham to RAF bomber bases and US Air Force bases Norwich to RAF SOC (Bawburgh), US Air Force bases, GCI stations, naval headquarters, Continental communications Kelvedon Hatch to RAF SOC, RAF bomber stations, RAF radar stations West Malling to naval headquarters at Chatham and Dover, RAF radar and Fighter Command headquarters, Continental communications Upavon to Army establishments on Salisbury Plain Sopley and Portsmouth to naval headquarters at Portsmouth and naval radio stations at Horsea and Flowerdown Box to Admiralty establishment at Bath, RAF SOC and Signals centre, Army signals centres at Cheltenham and Droitwich and Army radio stations, Foreign Office GCHQ and radio stations, important radio stations and miscellaneous radar stations in south-west England, South Wales and the Border Counties.

Antennas and towers Various types of antenna have been used in the network's history. At first, prime-focus parabolic reflectors were used. In about 1960, dual-band horn antennas started to be used widely, and a few of these survive to the present day. They began to go out of fashion at the end of the 1960s, when types of parabolic antenna with an improved performance became available. Many of the towers were designed with particular types of antenna in mind. Many towers were designed to carry horn antennas but no longer do so, and look rather odd as a result.