A telecommunications tower with a variety of dish antennas formicrowave relay links onFrazier Peak, Ventura County,California. The apertures of the dishes are covered by plastic sheets (radomes) to keep out moisture.
Theprefixmicro- inmicrowave indicates that microwaves are small (having shorter wavelengths), compared to theradio waves used in priorradio technology. Frequencies in the microwave range are often referred to by theirIEEE radar band designations:S,C,X,Ku,K, orKa band, or by similar NATO or EU designations.
Microwaves travel byline-of-sight; unlike lower frequencyradio waves, they do not diffract around hills, follow the Earth's surface asground waves, or reflect from theionosphere, so terrestrial microwave communication links are limited by the visual horizon to about 40 miles (64 km). At the high end of the band, they are absorbed by gases in the atmosphere, limiting practical communication distances to around a kilometer.
In descriptions of theelectromagnetic spectrum, some sources classify microwaves as radio waves, a subset of the radio wave band, while others classify microwaves and radio waves as distinct types of radiation.[1] This is an arbitrary distinction.
Frequency bands
Bands of frequencies in the microwave spectrum are designated by letters. Unfortunately, there are several incompatible band designation systems, and even within a system the frequency ranges corresponding to some of the letters vary somewhat between different application fields.[7][8] The letter system had its origin in World War 2 in a top-secret U.S. classification of bands used in radar sets; this is the origin of the oldest letter system, the IEEE radar bands. One set of microwave frequency bands designations by theRadio Society of Great Britain (RSGB), is tabulated below:
satellite communications, millimeter-wave radar research, military radar targeting and tracking applications, and some non-military applications, automotive radar
SHF transmissions: Radio astronomy, microwave devices/communications, wireless LAN, most modern radars, communications satellites, satellite television broadcasting,DBS, amateur radio
The term P band is sometimes used forUHF frequencies below the L band but is now obsolete per IEEE Std 521.
When radars were first developed at K band during World War 2, it was not known that there was a nearby absorption band (due to water vapor and oxygen in the atmosphere). To avoid this problem, the original K band was split into a lower band, Ku, and upper band, Ka.[10]
The atmosphericattenuation of microwaves and far infrared radiation 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. This graph includes a range of frequencies from 0 to 1 THz; the microwaves are the subset in the range between 0.3 and 300 gigahertz.
Microwaves travel solely byline-of-sight paths; unlike lower frequency radio waves, they do not travel asground waves which follow the contour of the Earth, or reflect off theionosphere (skywaves).[11] Although at the low end of the band, they can pass through building walls enough for useful reception, usually rights of way cleared to the firstFresnel zone are required. Therefore, on the surface of the Earth, microwave communication links are limited by the visual horizon to about 30–40 miles (48–64 km). Microwaves are absorbed by moisture in the atmosphere, and the attenuation increases with frequency, becoming a significant factor (rain fade) at the high end of the band. Beginning at about 40 GHz, atmospheric gases also begin to absorb microwaves, so above this frequency microwave transmission is limited to a few kilometers. A spectral band structure causes absorption peaks at specific frequencies (see graph at right). Above 100 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so effective that it is in effectopaque, until the atmosphere becomes transparent again in the so-calledinfrared andoptical window frequency ranges.
In a microwave beam directed at an angle into the sky, a small amount of the power will be randomly scattered as the beam passes through thetroposphere.[11] A sensitive receiver beyond the horizon with a high gain antenna focused on that area of the troposphere can pick up the signal. This technique has been used at frequencies between 0.45 and 5 GHz intropospheric scatter (troposcatter) communication systems to communicate beyond the horizon, at distances up to 300 km.
Their shortwavelength also allows narrow beams of microwaves to be produced by conveniently smallhigh gainantennas from a half meter to 5 meters in diameter. Therefore, beams of microwaves are used forpoint-to-point communication links, and forradar. An advantage of narrow beams is that they do not interfere with nearby equipment using the same frequency, allowingfrequency reuse by nearby transmitters.Parabolic ("dish") antennas are the most widely used directive antennas at microwave frequencies, buthorn antennas,slot antennas andlens antennas are also used. Flatmicrostrip antennas are being increasingly used in consumer devices. Another directive antenna practical at microwave frequencies is thephased array, a computer-controlled array of antennas that produces a beam that can be electronically steered in different directions.
At microwave frequencies, thetransmission lines which are used to carry lower frequency radio waves to and from antennas, such ascoaxial cable andparallel wire lines, have excessive power losses, so when low attenuation is required, microwaves are carried by metal pipes calledwaveguides. Due to the high cost and maintenance requirements of waveguide runs, in many microwave antennas the output stage of thetransmitter or theRF front end of thereceiver is located at the antenna.
Design and analysis
The termmicrowave also has a more technical meaning inelectromagnetics andcircuit theory.[12][13] Apparatus and techniques may be described qualitatively as "microwave" when the wavelengths of signals are roughly the same as the dimensions of the circuit, so thatlumped-element circuit theory is inaccurate, and insteaddistributed circuit elements and transmission-line theory are more useful methods for design and analysis.
As a consequence, practical microwave circuits tend not to use the discreteresistors,capacitors, andinductors used with lower-frequencyradio waves. Open-wire and coaxialtransmission lines used at lower frequencies are replaced bywaveguides andstripline, and lumped-element tuned circuits are replaced by cavityresonators orresonant stubs.[12] In turn, at even higher frequencies, where the wavelength of the electromagnetic waves becomes small in comparison to the size of the structures used to process them, microwave techniques become inadequate, and the methods ofoptics are used.
Sources
Cutaway view inside acavity magnetron as used in amicrowave oven(left). Antenna splitter:microstrip techniques become increasingly necessary at higher frequencies(right).
Disassembledradar speed gun. The grey assembly attached to the end of the copper-coloredhorn antenna is theGunn diode which generates the microwaves.
High-power microwave sources use specializedvacuum tubes to generate microwaves. These devices operate on different principles from low-frequency vacuum tubes, using the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include themagnetron (used inmicrowave ovens),klystron,traveling-wave tube (TWT),crossed-field amplifier, andgyrotron. These devices work in thedensity modulated mode, rather than thecurrent modulated mode. This means that they work on the basis of clumps of electrons flying ballistically through them, rather than using a continuous stream of electrons.[14]
Low-power microwave sources use solid-state devices such as thefield-effect transistor (at least at lower frequencies),tunnel diodes,Gunn diodes, andIMPATT diodes.[15] Low-power sources are available as benchtop instruments, rackmount instruments, embeddable modules and in card-level formats. Amaser is a solid-state device that amplifies microwaves using similar principles to thelaser, which amplifies higher-frequency light waves.
Microwave technology is extensively used forpoint-to-point telecommunications (i.e., non-broadcast uses). Microwaves are especially suitable for this use since they are more easily focused into narrower beams than radio waves, allowingfrequency reuse; their comparatively higher frequencies allow broadbandwidth and highdata transmission rates, and antenna sizes are smaller than at lower frequencies because antenna size is inversely proportional to the transmitted frequency. Microwaves are used in spacecraft communication, and much of the world's data, TV, and telephone communications are transmitted long distances by microwaves between ground stations andcommunications satellites. Microwaves are also employed inmicrowave ovens and inradar technology.
Wireless LANprotocols, such asBluetooth and theIEEE802.11 specifications used for Wi-Fi, also use microwaves in the 2.4 GHzISM band, although802.11a usesISM band andU-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[when?] 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 network (MAN) protocols, such asWiMAX (Worldwide Interoperability for Microwave Access) are based on standards such asIEEE 802.16, designed to operate between 2 and 11 GHz. Commercial implementations are in the 2.3 GHz, 2.5 GHz, 3.5 GHz and 5.8 GHz ranges.
Mobile Broadband Wireless Access (MBWA) protocols based on standards specifications such asIEEE 802.20 or ATIS/ANSIHC-SDMA (such asiBurst) 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.[18]
Somemobile phone networks, likeGSM, use the low-microwave/high-UHF frequencies around 1.8 and 1.9 GHz in the Americas and elsewhere, respectively.DVB-SH andS-DMB use 1.452 to 1.492 GHz, while proprietary/incompatiblesatellite radio in the U.S. uses around 2.3 GHz forDARS.
Microwave radio is used inpoint-to-pointtelecommunications transmissions because, due to their short wavelength, highlydirectional antennas are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies). There is also morebandwidth 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 inremote broadcasting of news or sports events as thebackhaul link to transmit a signal from a remote location to a television station from a specially equipped van. Seebroadcast auxiliary service (BAS),remote pickup unit (RPU), andstudio/transmitter link (STL).
Mostsatellite 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 traditionallarge dishfixed satellite service or Ku band fordirect-broadcast satellite. Military communications run primarily over X or Ku-band links, with Ka band being used forMilstar.
Theparabolic antenna (lower curved surface) of an ASR-9airport surveillance radar which radiates a narrow vertical fan-shaped beam of 2.7–2.9 GHz (S band) microwaves to locate aircraft in the airspace surrounding an airport
Radar is aradiolocation technique in which a beam of radio waves emitted by a transmitter bounces off an object and returns to a receiver, allowing the location, range, speed, and other characteristics of the object to be determined. The short wavelength of microwaves causes large reflections from objects the size of motor vehicles, ships and aircraft. Also, at these wavelengths, the high gain antennas such asparabolic antennas which are required to produce the narrow beamwidths needed to accurately locate objects are conveniently small, allowing them to be rapidly turned to scan for objects. Therefore, microwave frequencies are the main frequencies used in radar. Microwave radar is widely used for applications such asair traffic control, weather forecasting, navigation of ships, andspeed limit enforcement. Long-distance radars use the lower microwave frequencies since at the upper end of the band atmospheric absorption limits the range, butmillimeter waves are used for short-range radar such ascollision avoidance systems.
Some of the dish antennas of theAtacama Large Millimeter Array (ALMA) a radio telescope located in northern Chile. It receives microwaves in themillimeter wave range, 31 – 1000 GHz.
Maps of thecosmic microwave background radiation (CMBR), showing the improved resolution which has been achieved with better microwave radio telescopes
Microwaves emitted byastronomical radio sources; planets, stars,galaxies, andnebulas are studied inradio astronomy with large dish antennas calledradio telescopes. In addition to receiving naturally occurring microwave radiation, radio telescopes have been used in active radar experiments to bounce microwaves off planets in theSolar System, to determine the distance to theMoon or map the invisible surface ofVenus through cloud cover.
A recently[when?] completed microwave radio telescope is theAtacama Large Millimeter Array, located at more than 5,000 meters (16,597 ft) altitude in Chile, which observes theuniverse in themillimeter and submillimeter wavelength ranges. The world's largest ground-based astronomy project to date, it consists of more than 66 dishes and was built in an international collaboration by Europe, North America, East Asia and Chile.[19][20]
A major recent[when?] focus of microwave radio astronomy has been mapping thecosmic microwave background radiation (CMBR) discovered in 1964 by radio astronomersArno Penzias andRobert Wilson. This faint background radiation, which fills the universe and is almost the same in all directions, is "relic radiation" from theBig Bang, and is one of the few sources of information about conditions in the early universe. Due to the expansion and thus cooling of the Universe, the originally high-energy radiation has been shifted into the microwave region of the radio spectrum. Sufficiently sensitiveradio telescopes can detect the CMBR as a faint signal that is not associated with any star, galaxy, or other object.[21]
Heating and power application
Smallmicrowave oven on a kitchen counterMicrowaves are widely used for heating in industrial processes. A microwave tunnel oven for softening plastic rods prior to extrusion.
Amicrowave oven passes microwave radiation at a frequency near2.45 GHz (12 cm) through food, causingdielectric heating primarily by absorption of the energy in water. Microwave ovens became common kitchen appliances in Western countries in the late 1970s, following the development of less expensivecavity magnetrons. Water in the liquid state possesses many molecular interactions that 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 andcuring products.
Microwaves are used instellarators andtokamak experimental fusion reactors to help break down the gas into a plasma and heat it to very high temperatures. The frequency is tuned to thecyclotron resonance of the electrons in the magnetic field, anywhere between 2–200 GHz, hence it is often referred to as Electron Cyclotron Resonance Heating (ECRH). The upcomingITER thermonuclear reactor[22] will use up to 20 MW of 170 GHz microwaves.
Microwaves can be used totransmit power over long distances, and post-World War 2 research was done to examine possibilities.NASA worked in the 1970s and early 1980s to research the possibilities of usingsolar power satellite (SPS) systems with largesolar 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 two-second burst of the 95 GHz focused beam heats the skin to a temperature of 54 °C (129 °F) at a depth of 0.4 millimetres (1⁄64 in). TheUnited States Air Force andMarines are currently using this type ofactive denial system in fixed installations.[23]
Microwave frequency can be measured by either electronic or mechanical techniques.
Frequency counters or high frequencyheterodyne 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. The accuracy of the measurement is limited by the accuracy and stability of the reference source.
Mechanical methods require a tunable resonator such as anabsorption wavemeter, which has a known relation between a physical dimension and frequency.
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 slottedwaveguide 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 thevoltage standing wave ratio on the line. However, provided astanding wave is present, they may also be used to measure the distance between thenodes, which is equal to half the wavelength. The precision of this method is limited by the determination of the nodal locations.
Microwaves arenon-ionizing radiation, which means that microwavephotons do not contain sufficient energy toionize molecules or break chemical bonds, or cause DNA damage, as ionizing radiation such asx-rays orultraviolet can.[24] The word "radiation" refers to energy radiating from a source and not toradioactivity. The main effect of absorption of microwaves is to heat materials; the electromagnetic fields cause polar molecules to vibrate. It has not been shown conclusively that microwaves (or othernon-ionizing electromagnetic radiation) have significant adverse biological effects at low levels. Some, but not all, studies suggest that long-term exposure may have acarcinogenic effect.[25]
DuringWorld War II, it was observed that individuals in the radiation path of radar installations experienced clicks and buzzing sounds in response to microwave radiation. Research byNASA in the 1970s has shown this to be caused by thermal expansion in parts of the inner ear. In 1955, Dr.James Lovelock was able to reanimate rats chilled to 0 and 1 °C (32 and 34 °F) using microwave diathermy.[26]
When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in the body. The lens andcornea of the eye are especially vulnerable because they contain noblood vessels that can carry away heat. Exposure to microwave radiation can producecataracts by this mechanism, because the microwave heatingdenaturesproteins in thecrystalline lens of theeye[27] (in the same way that heat turnsegg whites white and opaque). 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 seriousburns that may not be immediately evident because of the tendency for microwaves to heat deeper tissues with higher moisture content.
History
The motivation for exploiting the microwave frequencies was the increasing congestion in the lower frequency bands, and the ability to use smaller antennas at higher frequencies[28]: 830
Hertz and the other early radio researchers were interested in exploring the similarities between radio waves and light waves, to test Maxwell's theory. They concentrated on producing short wavelength radio waves in theUHF and microwave ranges, with which they could duplicate classicoptics experiments in their laboratories, usingquasioptical components such asprisms andlenses made ofparaffin,sulfur andpitch and wirediffraction gratings, to refract and diffract radio waves like light rays.[31] Hertz used frequencies at the threshold of the microwave region: 50, 100, and 430 MHz.[28]: 831 His directional 430 MHz transmitter consisted of a 26 cm brass roddipole antenna with a spark gap between the ends, suspended at the focal line of aparabolic antenna made of a curved zinc sheet, powered by high voltage pulses from aninduction coil.[28]: 845 [30] His historic experiments demonstrated that radio waves like light exhibitedrefraction,diffraction,polarization,interference andstanding waves,[28]: 845-846 [31] proving that radio waves and light waves were both forms of Maxwell'selectromagnetic waves. He also experimented with open wire and coaxial transmission lines.[28]: 841
Heinrich Hertz's 430 MHz spark transmitter, 1888, consisting of 23 cm dipole and spark gap at the focus of a parabolic reflector
Jagadish Chandra Bose in 1894 was the first person to producemillimeter waves; his spark oscillator(in box, right) generated 60 GHz (5 mm) waves using 3 mm metal ball resonators.
3 mm spark ball oscillator Bose used to generate 60 GHz waves
Microwave spectroscopy experiment byJohn Ambrose Fleming in 1897 showing refraction of 1.4 GHz microwaves by paraffin prism, duplicating earlier experiments by Bose and Righi.
Augusto Righi's 12 GHz spark oscillator and receiver, 1895
Oliver Lodge's 5 inch oscillator ball he used to generate 1.2 GHz microwaves in 1894
1.2 GHz microwave spark transmitter(left) andcoherer receiver(right) used byGuglielmo Marconi during his 1895 experiments had a range of 6.5 km (4.0 mi)
However, since microwaves were limited toline-of-sight paths, they could not communicate beyond the visual horizon, and the low power of the spark transmitters then in use limited their practical range to a few miles. The subsequent development of radio communication after 1896 employed lower frequencies, which could travel beyond the horizon asground waves and by reflecting off theionosphere asskywaves, and microwave frequencies were not further explored at this time.
First microwave communication experiments
Practical use of microwave frequencies did not occur until the 1940s and 1950s due to a lack of adequate sources, since thetriodevacuum tube (valve)electronic oscillator used in radio transmitters could not produce frequencies above a few hundredmegahertz due to excessive electron transit time and interelectrode capacitance.[30] By the 1930s, the first low-power microwave vacuum tubes had been developed using new principles; theBarkhausen–Kurz tube and thesplit-anode magnetron.[28]: 849-850 [37][30] These could generate a few watts of power at frequencies up to a few gigahertz and were used in the first experiments in communication with microwaves.
Antennas of 1931 experimental 1.7 GHz microwave relay link across the English Channel
Experimental 3.3 GHz (9 cm) transmitter 1933 using split-anode magnetron, at Westinghouse labs[38]
Southworth(at left) demonstrating waveguide atIRE meeting in 1938, showing 1.5 GHz microwaves passing through the 7.5 m flexible metal hose registering on a diode detector
The first modern horn antenna in 1938 with inventorWilmer L. Barrow
In 1931 an Anglo-French consortium headed byAndre C. Clavier demonstrated the first experimentalmicrowave relay link, across theEnglish Channel 40 miles (64 km) betweenDover, UK andCalais, France.[39][40] The system transmitted telephony, telegraph andfacsimile data over bidirectional 1.7 GHz beams with a power of one-half watt, produced by miniatureBarkhausen–Kurz tubes at the focus of 10-foot (3 m) metal dishes.
A word was needed to distinguish these new shorter wavelengths, which had previously been lumped into the "short wave" band, which meant all waves shorter than 200 meters. The termsquasi-optical waves andultrashort waves were used briefly[37][41][38] but did not catch on. The first usage of the wordmicro-wave occurred in 1931[40][42] in reporting of the Clavier Anglo-French microwave link.[28]: 849
Radar development
The development ofradar, mainly in secrecy, before and duringWorld War II, resulted in the technological advances which made microwaves practical.[28]: 851 [30] Microwave wavelengths in the centimeter range were required to give the small radar antennas which were compact enough to fit on aircraft a narrow enoughbeamwidth to localize enemy aircraft. It was found that conventionaltransmission lines used to carry radio waves had excessive power losses at microwave frequencies, andGeorge Southworth atBell Labs andWilmer Barrow atMIT independently inventedwaveguide in 1936.[33] Barrow invented thehorn antenna in 1938 as a means to efficiently radiate microwaves into or out of a waveguide. In a microwavereceiver, anonlinear component was needed that would act as adetector andmixer at these frequencies, as vacuum tubes had too much capacitance. To fill this need researchers resurrected an obsolete technology, thepoint contactcrystal detector (cat whisker detector) which was used as ademodulator incrystal radios around the turn of the century before vacuum tube receivers.[30][43] The low capacitance ofsemiconductor junctions allowed them to function at microwave frequencies. The first modernsilicon andgermaniumdiodes were developed as microwave detectors in the 1930s, and the principles ofsemiconductor physics learned during their development led tosemiconductor electronics after the war.[30]
Randall andBoot's prototype cavity magnetron tube at theUniversity of Birmingham, 1940. In use the tube was installed between the poles of an electromagnet
First commercial klystron tube, by General Electric, 1940, sectioned to show internal construction
British Mk. VIII, the first microwave air intercept radar, in nose of British fighter
Mobile US Army microwave relay station 1945 demonstrating relay systems using frequencies from 100 MHz to 4.9 GHz which could transmit up to 8 phone calls on a beam
The first powerful sources of microwaves were invented at the beginning of World War II: theklystron tube byRussell and Sigurd Varian atStanford University in 1937,[28]: 852-853 and thecavity magnetron tube byJohn Randall andHarry Boot at Birmingham University, UK in 1940.[28]: 853-855 [30] Ten centimeter (3 GHz) microwave radar powered by the magnetron tube was in use on British warplanes in late 1941 and proved to be a game changer. Britain's 1940 decision to share its microwave technology with its US ally (theTizard Mission)[28]: 852 significantly shortened the war. TheMIT Radiation Laboratory established secretly atMassachusetts Institute of Technology in 1940 to research radar, produced much of the theoretical knowledge necessary to use microwaves.[28]: 852 Microwave relay systems were developed by the Allied military in the war and used for secure battlefield communication networks in the European theater. The first was the British Wireless Set No. 10, a 5 GHz, 8 telephone line,time division multiplex system developed by the UK'sSignals Research and Development Establishment in 1942.[28]: 855
Post World War II exploitation
After World War II, microwaves were rapidly exploited commercially.[30] Due to their high frequency they had a very large information-carrying capacity (bandwidth); a single microwave beam could carry tens of thousands of phone calls. In the 1950s and 60s transcontinentalmicrowave relay networks were built in the US and Europe to exchange telephone calls between cities and distribute television programs. In the newtelevision broadcasting industry, from the 1940s microwave dishes were used to transmitbackhaul video feeds from mobileproduction trucks back to the studio, allowing the firstremote TV broadcasts. The firstcommunications satellites were launched in the 1960s, which relayed telephone calls and television between widely separated points on Earth using microwave beams. In 1964,Arno Penzias andRobert Woodrow Wilson while investigating noise in a satellite horn antenna atBell Labs, Holmdel, New Jersey discoveredcosmic microwave background radiation.
C-bandhorn antennas at a telephone switching center in Seattle, belonging to AT&T's Long Lines microwave relay network built in the 1960s.
The ability ofshort waves to quickly heat materials and cook food had been investigated in the 1930s by Ilia E. Mouromtseff at Westinghouse, and at the1933 Chicago World's Fair demonstrated cooking meals with a 60 MHz radio transmitter.[44] In 1945Percy Spencer, an engineer working on radar atRaytheon, noticed that microwave radiation from a magnetron oscillator melted a candy bar in his pocket. He investigated cooking with microwaves and invented themicrowave oven, consisting of a magnetron feeding microwaves into a closed metal cavity containing food, which was patented by Raytheon on 8 October 1945. Due to their expense microwave ovens were initially used in institutional kitchens, but by 1986 roughly 25% of households in the U.S. owned one. Microwave heating became widely used as an industrial process in industries such as plastics fabrication, and as a medical therapy to kill cancer cells inmicrowave hyperthermy.
Thetraveling wave tube (TWT) developed in 1943 byRudolph Kompfner andJohn Pierce provided a high-power tunable source of microwaves up to 50 GHz and became the most widely used microwave tube (besides the ubiquitous magnetron used in microwave ovens). Thegyrotron tube family developed in Russia could produce megawatts of power up intomillimeter wave frequencies and is used in industrial heating andplasma research, and to powerparticle accelerators and nuclearfusion reactors.
Solid state microwave devices
First caesium atomic clock, and inventor Louis Essen(left), 1955
Thetunnel diode invented in 1957 by Japanese physicistLeo Esaki could produce a few milliwatts of microwave power. Its invention set off a search for better negative resistance semiconductor devices for use as microwave oscillators, resulting in the invention of theIMPATT diode in 1956 byW.T. Read and Ralph L. Johnston and theGunn diode in 1962 byJ. B. Gunn.[30] Diodes are the most widely used microwave sources today.
Prior to the 1980s microwave devices and circuits were bulky and expensive, so microwave frequencies were generally limited to the output stage of transmitters and theRF front end of receivers, and signals wereheterodyned to a lowerintermediate frequency for processing. The period from the 1970s to the present has seen the development of tiny inexpensive active solid-state microwave components which can be mounted on circuit boards, allowing circuits to perform significantsignal processing at microwave frequencies. This has made possiblesatellite television,cable television,GPS devices, and modern wireless devices, such assmartphones,Wi-Fi, andBluetooth which connect to networks using microwaves.
GaAs can be made semi-insulating, allowing it to be used as asubstrate on which circuits containingpassive components, as well as transistors, can be fabricated by lithography.[30] By 1976 this led to the firstintegrated circuits (ICs) which functioned at microwave frequencies, calledmonolithic microwave integrated circuits (MMIC).[30] The word "monolithic" was added to distinguish these from microstrip PCB circuits, which were called "microwave integrated circuits" (MIC). Since then, silicon MMICs have also been developed. Today MMICs have become the workhorses of both analog and digital high-frequency electronics, enabling the production of single-chip microwave receivers, broadbandamplifiers,modems, andmicroprocessors.
^"Frequency Letter bands".Microwave Encyclopedia. Microwaves101 website, Institute of Electrical and Electronics Engineers (IEEE). 14 May 2016. Retrieved1 July 2018.
^abEduard, Karplus (1932)."Communication with quasi-optical waves".Standards Yearbook, 1932. Washington D.C.: Bureau of Standards, US Dept. of Commerce: 15. Retrieved27 February 2025.
^abMouromtseff, Ilia A. (September 1933)."3 1/2 Inch Waves Now Practical"(PDF).Short Wave Craft.4 (5). New York: Popular Book Corp.:266–267.
^"Microwaves span the English Channel"(PDF).Short Wave Craft. Vol. 6, no. 5. New York: Popular Book Co. September 1935. pp. 262, 310. RetrievedMarch 24, 2015.
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