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Microwaves are a form of electromagnetic radiation with ranging from about one meter to one millimeter; with between and .

(2019). 9781931504553, American Industrial Hygiene Assn.. .
(2019). 9788120349353, PHI Learning Pvt. Ltd. .
(2019). 9781136034107, Taylor & Francis. .
Pozar, David M. (1993). Microwave Engineering Addison–Wesley Publishing Company. .Sorrentino, R. and Bianchi, Giovanni (2010) Microwave and RF Engineering, John Wiley & Sons, p. 4, . Different sources define different frequency ranges as microwaves; the above broad definition includes both UHF and EHF () bands. A more common definition in radio engineering is the range between 1 and 100 GHz (wavelengths between 0.3 m and 3 mm). In all cases, microwaves include the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum. Frequencies in the microwave range are often referred to by their IEEE radar band designations: , C, , , K, or , or by similar NATO or EU designations.

The in microwave is not meant to suggest a wavelength in the range. Rather, it indicates that microwaves are "small" (having shorter wavelengths), compared to the used prior to microwave technology. The boundaries between , terahertz radiation, microwaves, and ultra-high-frequency are fairly arbitrary and are used variously between different fields of study.

Microwaves travel by line-of-sight; unlike lower frequency radio waves they do not diffract around hills, follow the earth's surface as , or reflect from the , 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. Microwaves are widely used in modern technology, for example in point-to-point communication links, , microwave radio relay networks, , satellite and spacecraft communication, medical and cancer treatment, , , particle accelerators, , industrial heating, collision avoidance systems, garage door openers and keyless entry systems, and for cooking food in .

Electromagnetic spectrum
Microwaves occupy a place in the electromagnetic spectrum with frequency above ordinary , and below light:
> 62.1
124 keV – 124 eV
124 eV – 3 eV
3.2 eV – 1.7 eV
1.7 eV – 1.24
1.24 meV – 1.24

1.24 – 12.4
In descriptions of the electromagnetic 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. This is an arbitrary distinction.

Microwaves travel solely by line-of-sight paths; unlike lower frequency radio waves, they do not travel as which follow the contour of the Earth, or reflect off the ().
(2019). 9780471743682, John Wiley and Sons. .
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 first are required. Therefore, on the surface of the Earth, microwave communication links are limited by the visual horizon to about . Microwaves are absorbed by moisture in the atmosphere, and the attenuation increases with frequency, becoming a significant factor () 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 great that it is in effect opaque, until the atmosphere becomes transparent again in the so-called and 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 the . 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 in tropospheric scatter (troposcatter) communication systems to communicate beyond the horizon, at distances up to 300 km.

The short of microwaves allow omnidirectional antennas for portable devices to be made very small, from 1 to 20 centimeters long, so microwave frequencies are widely used for such as , , and (Wi-Fi) access for , and earphones. Antennas used include short , rubber ducky antennas, sleeve , , and increasingly the printed circuit inverted F antenna (PIFA) used in cell phones.

Their short also allows narrow beams of microwaves to be produced by conveniently small antennas from a half meter to 5 meters in diameter. Therefore, beams of microwaves are used for point-to-point communication links, and for . An advantage of narrow beams is that they don't interfere with nearby equipment using the same frequency, allowing by nearby transmitters. Parabolic ("dish") antennas are the most widely used directive antennas at microwave frequencies, but , and antennas are also used. Flat microstrip antennas are being increasingly used in consumer devices. Another directive antenna practical at microwave frequencies is the , a computer-controlled array of antennas which produces a beam which can be electronically steered in different directions.

At microwave frequencies, the transmission lines which are used to carry lower frequency radio waves to and from antennas, such as and , have excessive power losses, so when low attenuation is required microwaves are carried by metal pipes called waveguides. Due to the high cost and maintenance requirements of waveguide runs, in many microwave antennas the output stage of the or the RF front end of the is located at the antenna.

Design and analysis
The term microwave also has a more technical meaning in and .
(2019). 142000672X, CRC Press. . 142000672X
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 that lumped-element circuit theory is inaccurate, and instead distributed circuit elements and transmission-line theory are more useful methods for design and analysis.

As a consequence, practical microwave circuits tend to move away from the discrete , , and used with lower-frequency . Open-wire and coaxial transmission lines used at lower frequencies are replaced by and , and lumped-element tuned circuits are replaced by cavity or . 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 of are used.

Microwave sources
High-power microwave sources use specialized 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 the (used in ), , traveling-wave tube (TWT), and . These devices work in the modulated mode, rather than the 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.

Low-power microwave sources use solid-state devices such as the field-effect transistor (at least at lower frequencies), , , and . Microwave Oscillator notes by Herley General Microwave Low-power sources are available as benchtop instruments, rackmount instruments, embeddable modules and in card-level formats. A is a solid state device which amplifies microwaves using similar principles to the , which amplifies higher frequency light waves.

All warm objects emit low level microwave black-body radiation, depending on their , so in meteorology and microwave radiometers are used to measure the temperature of objects or terrain.

(2019). 9788122413380, New Age International. .
The sun
(2019). 9780124514515, Academic Press. .
and other astronomical radio sources such as emit low level microwave radiation which carries information about their makeup, which is studied by using receivers called . The cosmic microwave background radiation (CMBR), for example, is a weak microwave noise filling empty space which is a major source of information on cosmology's theory of the origin of the .

Microwave uses
Microwave technology is extensively used for point-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, allowing ; their comparatively higher frequencies allow broad bandwidth and high data transmission rates, and antenna sizes are smaller than at lower frequencies because antenna size is inversely proportional to 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 and communications satellites. Microwaves are also employed in and in technology.

Before the advent of transmission, most long-distance 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.

protocols, such as and the IEEE 802.11 specifications used for Wi-Fi, also use microwaves in the 2.4 GHz , although 802.11a uses and 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 network (MAN) protocols, such as (Worldwide Interoperability for Microwave Access) are based on standards such as IEEE 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.

Wireless Access (MBWA) protocols based on standards specifications such as IEEE 802.20 or ATIS/ANSI (such as ) 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 networks, like GSM, use the low-microwave/high-UHF frequencies around 1.8 and 1.9 GHz in the Americas and elsewhere, respectively. and use 1.452 to 1.492 GHz, while proprietary/incompatible in the U.S. uses around 2.3 GHz for DARS.

Microwave radio is used in 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 to transmit a signal from a remote location to a television station from a specially equipped van. See broadcast auxiliary service (BAS), remote pickup unit (RPU), and studio/transmitter link (STL).

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. either operates in the C band for the traditional 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 .

Global Navigation Satellite Systems (GNSS) including the Chinese Beidou, the American Global Positioning System (introduced in 1978) and the Russian broadcast navigational signals in various bands between about 1.2 GHz and 1.6 GHz.

is a 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 as parabolic 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 as air traffic control, weather forecasting, navigation of ships, and speed limit enforcement. Long distance radars use the lower microwave frequencies since at the upper end of the band atmospheric absorption limits the range, but are used for short range radar such as collision avoidance systems.

Radio astronomy
Microwaves emitted by astronomical radio sources; planets, stars, , and are studied in with large dish antennas called . In addition to receiving naturally occurring microwave radiation, radio telescopes have been used in active radar experiments to bounce microwaves off planets in the solar system, to determine the distance to the or map the invisible surface of through cloud cover.

A recently completed microwave radio telescope is the Atacama Large Millimeter Array, located at more than 5,000 meters (16,597 ft) altitude in Chile, observes the in the millimetre and submillimetre 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.

A major recent focus of microwave radio astronomy has been mapping the cosmic microwave background radiation (CMBR) discovered in 1964 by radio astronomers and Robert Wilson. This faint background radiation, which fills the universe and is almost the same in all directions, is "relic radiation" from the , 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 sensitive can detected the CMBR as a faint signal that is not associated with any star, galaxy, or other object.

(2019). 052175576X, Cambridge University Press. 052175576X

Heating and power application
A passes microwave radiation at a frequency near through food, causing dielectric 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 expensive . 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 and curing products.

Many semiconductor processing techniques use microwaves to generate 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 and tokamak experimental fusion reactors to help heat the fuel into a plasma state. The upcoming 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. 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.

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 at a depth of . The United States Air Force and Marines are currently using this type of active denial system in fixed installations. Silent Guardian Protection System. Less-than-Lethal Directed Energy Protection.

Microwave radiation is used in electron paramagnetic resonance (EPR or ESR) spectroscopy, typically in the X-band region (~9 GHz) in conjunction typically with of 0.3 T. This technique provides information on unpaired in chemical systems, such as or ions such as Cu(II). Microwave radiation is also used to perform rotational spectroscopy and can be combined with as in microwave enhanced electrochemistry.

Microwave 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.
(2019). 9781420006711, CRC Press. .
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 the Radio Society of Great Britain (RSGB), is tabulated below:

+ Microwave frequency bands ! Designation !! Frequency range !! Wavelength range !! Typical uses
15 cm to 30 cmmilitary telemetry, GPS, mobile phones (GSM), amateur radio
7.5 cm to 15 cmweather radar, surface ship radar, and some communications satellites (microwave ovens, microwave devices/communications, radio astronomy, mobile phones, wireless LAN, Bluetooth, ZigBee, GPS, amateur radio)
3.75 cm to 7.5 cmlong-distance radio telecommunications
25 mm to 37.5 mmsatellite communications, radar, terrestrial broadband, space communications, amateur radio, molecular rotational spectroscopy
16.7 mm to 25 mmsatellite communications, molecular rotational spectroscopy
11.3 mm to 16.7 mmradar, satellite communications, astronomical observations, automotive radar, molecular rotational spectroscopy
5.0 mm to 11.3 mmsatellite communications, molecular rotational spectroscopy
6.0 mm to 9.0 mmsatellite communications, terrestrial microwave communications, radio astronomy, automotive radar, molecular rotational spectroscopy
5.0 mm to 7.5 mm
4.0 mm to 6.0 mmmillimeter wave radar research, molecular rotational spectroscopy and other kinds of scientific research
2.7 mm to 4.0 mmsatellite communications, millimeter-wave radar research, military radar targeting and tracking applications, and some non-military applications, automotive radar
2.1 mm to 3.3 mmSHF transmissions: Radio astronomy, microwave devices/communications, wireless LAN, most modern radars, communications satellites, satellite television broadcasting, DBS, amateur radio
1.8 mm to 2.7 mmEHF transmissions: Radio astronomy, high-frequency microwave radio relay, microwave remote sensing, amateur radio, directed-energy weapon, millimeter wave scanner

P band is sometimes 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.For other definitions see Letter Designations of Microwave Bands.

When radars were first developed at K band during World War II, 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.Skolnik, Merrill I. (2001) Introduction to Radar Systems, Third Ed., p. 522, McGraw Hill. 1962 Edition full text

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

Frequency counters or high frequency 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.

In a laboratory setting, 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 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 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.

Effects on health
Microwaves do not contain sufficient energy to chemically change substances by ionization, and so are an example of radiation. The word "radiation" refers to energy radiating from a source and not to . It has not been shown conclusively that microwaves (or other electromagnetic radiation) have significant adverse biological effects at low levels. Some, but not all, studies suggest that long-term exposure may have a 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.Philip L. Stocklin, , December 19, 1983 Research by 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 0-1°C using microwave diathermy.

When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in the body. Exposure to microwave radiation can produce by this mechanism, because the microwave heating denatures in the crystalline lens of the (in the same way that heat turns white and opaque). The lens and of the eye are especially vulnerable because they contain no 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 that may not be immediately evident because of the tendency for microwaves to heat deeper tissues with higher moisture content.

Eleanor R. Adair conducted microwave health research by exposing herself, animals and humans to microwave levels that made them feel warm or even start to sweat and feel quite uncomfortable. She found no adverse health effects other than heat.


Hertzian optics
Microwaves were first generated in the 1880s and 1890s in some of the earliest experiments by physicists who thought of them as a form of "invisible light".
(2019). 9780262082983, MIT Press. .
James Clerk Maxwell in his 1873 theory of , now called Maxwell's equations, had predicted the existence of electromagnetic waves and proposed that light was composed of these waves. In 1888, German physicist was the first to demonstrate the existence of using a primitive spark gap radio transmitter.
(2019). 9781461525004, Springer Science and Business Media. .
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 the UHF and microwave ranges, with which they could duplicate classic experiments, using components such as and lenses made of , and pitch and wire diffraction gratings, to refract and diffract radio waves like light rays.
(2019). 9780471783015, John Wiley and Sons. .
Hertz produced waves up to 450 MHz; his directional 450 MHz transmitter consisted of a 26 cm brass rod with a spark gap between the ends suspended at the focal line of a parabolic antenna made of a curved zinc sheet, powered by high voltage pulses from an . His historic experiments demonstrated that radio waves like light exhibited , , polarization, interference and , proving that radio waves and light waves were both forms of Maxwell's electromagnetic waves. Image:Hertz spark gap transmitter and parabolic antenna.png|'s 450 MHz spark transmitter, 1888, consisting of 23 cm dipole and spark gap at focus of parabolic reflector Image:Microwave Apparatus - Jagadish Chandra Bose Museum - Bose Institute - Kolkata 2011-07-26 4051.JPG|Jagadish Chandra Bose in 1894 was the first person to produce ; his spark oscillator (in box, right) generated 60 GHz (5 mm) waves using 3 mm metal ball resonators. Image:Refraction of Hertzian waves by paraffin prism.png|Experiment by John Ambrose Fleming in 1897 showing refraction of 1.4 GHz microwaves by paraffin prism. Image:Marconi parabolic xmtr and rcvr 1895.jpg|1.2 GHz microwave spark transmitter (left) and receiver (right) used by Guglielmo Marconi during his 1895 experiments had a range of In 1894, and generated 1.5 and 12 GHz microwaves respectively with small metal ball spark resonators. The same year Indian physicist Jagadish Chandra Bose was the first person to produce , generating 60 GHz (5 millimeter) microwaves using a 3 mm metal ball spark oscillator. Bose also invented waveguide and for use in his experiments. Russian physicist in 1895 generated 50 GHz millimeter waves. In 1897 solved the mathematical boundary-value problem of electromagnetic waves propagating through conducting tubes and dielectric rods of arbitrary shape.
(2019). 9781118636800, John Wiley and Sons. .
(2019). 9780521835268, Cambridge University Press. .
which gave the modes and of microwaves propagating through a waveguide.

However, since microwaves were limited to line of sight paths, they could not communicate beyond the visual horizon. The subsequent development of radio communication after 1896 employed lower frequencies, which could travel beyond the horizon as and by reflecting off the as , 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 the (valve) electronic oscillator used in radio transmitters could not produce frequencies above a few hundred due to excessive electron transit time and interelectrode capacitance. By the 1930s, the first low power microwave vacuum tubes had been developed using new principles; the Barkhausen-Kurz tube and the split-anode magnetron. 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. Image:English Channel microwave relay antennas 1931.jpg|Antennas of 1931 experimental 1.7 GHz microwave relay link across the English Channel. Image:Westinghouse experimental 700 MHz transmitter 1932.jpg|Experimental 700 MHz transmitter 1932 at Westinghouse labs transmits voice over a mile. In 1931 an Anglo-French consortium demonstrated the first experimental link, across the between , UK and , France. The system transmitted telephony, telegraph and data over bidirectional 1.7 GHz beams with a power of one-half watt, produced by miniature Barkhausen-Kurz tubes at the focus of metal dishes.

A word was needed to distinguish these new shorter wavelengths, which had previously been lumped into the "" band, which meant all waves shorter than 200 meters. The terms quasi-optical waves and ultrashort waves were used briefly, but didn't catch on. The first usage of the word microwave apparently occurred in 1931.

(2019). 9787560028743 .

The development of , mainly in secrecy, before and during World War 2, resulted in the technological advances which made microwaves practical. Radar antennas small enough to fit on aircraft which had a narrow enough beamwidth to localize enemy aircraft required wavelengths in the centimeter range. It was found that conventional transmission lines used to carry radio waves had excessive power losses at microwave frequencies, and George Southworth at and at MIT independently invented waveguide in 1936. Barrow invented the in 1938 as a means to efficiently radiate microwaves into or out of a waveguide. In a microwave , a component was needed that would act as a detector and at these frequencies, as vacuum tubes had too much capacitance. To fill this need researchers resurrected an obsolete technology, the point contact (cat whisker detector) which was used as a in around the turn of the century before vacuum tube receivers.
(1988). 9780393318517, W. W. Norton & Company. .
The low capacitance of semiconductor junctions allowed them to function at microwave frequencies. The first modern and were developed as microwave detectors in the 1930s, and the principles of semiconductor physics learned during their development led to semiconductor electronics after the war.

Image:Southworth demonstrating waveguide.jpg|Southworth (at left) demonstrating waveguide at IRE meeting in 1938, showing 1.5 GHz microwaves passing through the 7.5 m flexible metal hose registering on a diode detector. Image:Wilmer Barrow & horn antenna 1938.jpg|The first modern horn antenna in 1938 with inventor Wilmer L. Barrow Image:An-APS-4 side view.jpg|AN/APS-4 10 GHz air intercept radar used on US and British warplanes in World War 2 Image:US Army Signal Corps AN-TRC-1, 5, 6, & 8 microwave relay station 1945.jpg|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 2: the tube by Russell and Sigurd Varian at Stanford University in 1937, and the tube by John Randall and at Birmingham University, UK in 1940. Britain's 1940 decision to share its microwave technology with the US (the ) significantly influenced the outcome of the war. The MIT Radiation Laboratory established secretly at Massachusetts Institute of Technology in 1940 to research radar, produced much of the theoretical knowledge necessary to use microwaves. By 1943, 10 centimeter (3 GHz) radar was in use on British and American warplanes. The first microwave relay systems were developed by the Allied military near the end of the war and used for secure battlefield communication networks in the European theater.

Post World War 2
After World War 2, microwaves were rapidly exploited commercially. 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 transcontinental microwave relay networks were built in the US and Europe to exchange telephone calls between cities and distribute television programs. In the new television broadcasting industry, from the 1940s microwave dishes were used to transmit backhaul video feed from mobile back to the studio, allowing the first . The first communications satellites were launched in the 1960s, which relayed telephone calls and television between widely separated points on Earth using microwave beams. In 1964, and Robert Woodrow Wilson while investigating noise in a satellite horn antenna at , Holmdel, New Jersey discovered cosmic microwave background radiation.

Microwave radar became the central technology used in air traffic control, maritime , anti-aircraft defense, ballistic missile detection, and later many other uses. Radar and satellite communication motivated the development of modern microwave antennas; the parabolic antenna (the most common type), cassegrain antenna, , , and .

The ability of to quickly heat materials and cook food had been investigated in the 1930s by I. F. Mouromtseff at Westinghouse, and at the 1933 Chicago World's Fair demonstrated cooking meals with a 60 MHz radio transmitter. In 1945 , an engineer working on radar at , noticed that microwave radiation from a magnetron oscillator melted a candy bar in his pocket. He investigated cooking with microwaves and invented the , consisting of a magnetron feeding microwaves into a closed metal cavity containing food, which was patented by Raytheon on 8 October 1945. Microwave heating became widely used as an industrial process in industries such as plastics fabrication, and as a medical therapy to kill cancer cells in .

The traveling wave tube (TWT) developed in 1943 by and John 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). The tube family developed in Russia could produce megawatts of power up into frequencies, and is used in industrial heating and plasma research, and to power particle accelerators and nuclear .

Solid state microwave devices
The development of semiconductor electronics in the 1950s led to the first solid state microwave devices which worked by a new principle; negative resistance (some of the prewar microwave tubes had also used negative resistance). The feedback oscillator and amplifiers which were used at lower frequencies became unstable at microwave frequencies, and negative resistance oscillators and amplifiers based on devices like worked better.

The invented in 1957 by Japanese physicist 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 the in 1956 by W.T. Read and Ralph L. Johnston and the in 1962 by J. B. Gunn. Diodes are the most widely used microwave sources today. Two low-noise solid state negative resistance microwave were developed; the ruby invented in 1953 by Charles H. Townes, James P. Gordon, and H. J. Zeiger, and the parametric amplifier developed in 1956 by Marion Hines. These were used for low noise microwave receivers in radio telescopes and satellite ground stations. The maser led to the development of , which keep time using a precise microwave frequency emitted by atoms undergoing an electron transition between two energy levels. Negative resistance amplifier circuits required the invention of new nonreciprocal waveguide components, such as , isolators, and directional couplers. In 1969 Kurokawa derived mathematical conditions for stability in negative resistance circuits which formed the basis of microwave oscillator design.

Microwave integrated circuits
Prior to the 1970s microwave devices and circuits were bulky and expensive, so microwave frequencies were generally limited to the output stage of transmitters and the RF front end of receivers, and signals were to a lower intermediate 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 significant signal processing at microwave frequencies. This has made possible satellite television, , devices, and modern wireless devices, such as , , and which connect to networks using microwaves.

, a type of transmission line usable at microwave frequencies, was invented with in the 1950s. The ability to cheaply fabricate a wide range of shapes on printed circuit boards allowed microstrip versions of , , resonant stubs, splitters, directional couplers, , filters and antennas to be made, thus allowing compact microwave circuits to be constructed.

that operated at microwave frequencies were developed in the 1970s. The semiconductor (GaAs) has a much higher electron mobility than silicon, so devices fabricated with this material can operate at 4 times the frequency of similar devices of silicon. Beginning in the 1970s GaAs was used to make the first microwave transistors, and it has dominated microwave semiconductors ever since. MESFETs (metal-semiconductor field-effect transistors), fast GaAs field effect transistors using for the gate, were developed starting in 1968 and have reached cutoff frequencies of 100 GHz, and are now the most widely used active microwave devices. Another family of transistors with a higher frequency limit is the HEMT (high electron mobility transistor), a field effect transistor made with two different semiconductors, AlGaAs and GaAs, using technology, and the similar HBT (heterojunction bipolar transistor).

GaAs can be made semi-insulating, allowing it to be used as a substrate on which circuits containing passive components as well as transistors can be fabricated by lithography. By 1976 this led to the first integrated circuits (ICs) which functioned at microwave frequencies, called monolithic microwave integrated circuits (MMIC). 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, broadband , , and .

See also
  • Block upconverter (BUC)
  • Cosmic microwave background
  • Electron cyclotron resonance
  • International Microwave Power Institute
  • Low-noise block converter (LNB)
  • Microwave auditory effect
  • Microwave chemistry
  • Microwave radio relay
  • Microwave transmission
  • RF switch matrix
  • The Thing (listening device)

External links

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