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 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
