Microwaves are a form of electromagnetic radiation with ranging from about one meter to one millimeter; with frequency between and .
The prefix in microwave is not meant to suggest a wavelength in the Micrometre range. Rather, it indicates that microwaves are "small" (having shorter wavelengths), compared to the used prior to microwave technology. The boundaries between far infrared, terahertz radiation, microwaves, and ultra-high-frequency radio 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 ionosphere, 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, radar, satellite and spacecraft communication, medical diathermy and cancer treatment, remote sensing, radio astronomy, particle accelerators, spectroscopy, industrial heating, collision avoidance systems, garage door openers and keyless entry systems, and for cooking food in .
|> 62.1 kilo-|
|124 keV – 124 eV|
|124 eV – 3 eV|
|3.2 eV – 1.7 eV|
|1.7 eV – 1.24 milli|
|1.24 meV – 1.24 Micro-|
|1.24 Micro- – 12.4 Femto-|
Their short wavelength also allows narrow beams of microwaves to be produced by conveniently small antenna gain antennas from a half meter to 5 meters in diameter. Therefore, beams of microwaves are used for point-to-point communication links, and for radar. An advantage of narrow beams is that they don't interfere with nearby equipment using the same frequency, allowing frequency reuse by nearby transmitters. Parabolic ("dish") antennas are the most widely used directive antennas at microwave frequencies, but , and dielectric lens antennas are also used. Flat microstrip antennas are being increasingly used in consumer devices. Another directive antenna practical at microwave frequencies is the phased array, 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 coaxial cable and twin lead, 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 transmitter or the RF front end of the radio receiver is located at the antenna.
As a consequence, practical microwave circuits tend to move away from the discrete , , and used with lower-frequency radio waves. Open-wire and coaxial transmission lines used at lower frequencies are replaced by and stripline, 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 optics are used.
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 maser is a solid state device which amplifies microwaves using similar principles to the laser, which amplifies higher frequency light waves.
All warm objects emit low level microwave black-body radiation, depending on their temperature, so in meteorology and remote sensing microwave radiometers are used to measure the temperature of objects or terrain.
Wireless LAN protocols, such as Bluetooth and the IEEE 802.11 specifications used for Wi-Fi, 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 network (MAN) protocols, such as WiMAX (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.
Mobile Broadband Wireless Access (MBWA) protocols based on standards specifications such as IEEE 802.20 or ATIS/ANSI HC-SDMA (such as iBurst) 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 S-DMB 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. 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. Satellite TV either operates in the C band for the traditional TVRO 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.
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 universe 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 Arno Penzias and Robert Wilson. This faint background radiation, which fills the universe and is almost the same in all directions, is "relic radiation" from the Big 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 sensitive can detected the CMBR as a faint signal that is not associated with any star, galaxy, or other object.
Microwave heating is used in industrial processes for drying and curing products.
Many semiconductor processing techniques use microwaves to generate plasma physics 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 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 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. raytheon.com
|+ Microwave frequency bands ! Designation !! Frequency range !! Wavelength range !! Typical uses|
|15 cm to 30 cm||military telemetry, GPS, mobile phones (GSM), amateur radio|
|7.5 cm to 15 cm||weather 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 cm||long-distance radio telecommunications|
|25 mm to 37.5 mm||satellite communications, radar, terrestrial broadband, space communications, amateur radio, molecular rotational spectroscopy|
|16.7 mm to 25 mm||satellite communications, molecular rotational spectroscopy|
|11.3 mm to 16.7 mm||radar, satellite communications, astronomical observations, automotive radar, molecular rotational spectroscopy|
|5.0 mm to 11.3 mm||satellite communications, molecular rotational spectroscopy|
|6.0 mm to 9.0 mm||satellite communications, terrestrial microwave communications, radio astronomy, automotive radar, molecular rotational spectroscopy|
|5.0 mm to 7.5 mm|
|4.0 mm to 6.0 mm||millimeter wave radar research, molecular rotational spectroscopy and other kinds of scientific research|
|2.7 mm to 4.0 mm||satellite communications, millimeter-wave radar research, military radar targeting and tracking applications, and some non-military applications, automotive radar|
|2.1 mm to 3.3 mm||SHF 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 mm||EHF 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
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.
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.
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 NASA 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 Human eye (in the same way that heat turns white and opaque). The lens and cornea 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.
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 ionosphere as , and microwave frequencies were not further explored at this time.
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 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.
The first powerful sources of microwaves were invented at the beginning of World War 2: the klystron tube by Russell and Sigurd Varian at Stanford University in 1937, and the cavity magnetron tube by John Randall and Harry Boot at Birmingham University, UK in 1940. Britain's 1940 decision to share its microwave technology with the US (the Tizard Mission) 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.
Microwave radar became the central technology used in air traffic control, maritime navigation, 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, lens antenna, slot antenna, and phased array.
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 Percy Spencer, an engineer working on radar at Raytheon, noticed that microwave radiation from a magnetron oscillator melted a candy bar in his pocket. He investigated cooking with microwaves and invented the microwave oven, 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 hyperthermy.
The traveling wave tube (TWT) developed in 1943 by Rudolph Kompfner 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 gyrotron tube family developed in Russia could produce megawatts of power up into millimeter wave frequencies, and is used in industrial heating and plasma research, and to power particle accelerators and nuclear .
The tunnel diode invented in 1957 by Japanese physicist Leo 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 the IMPATT diode in 1956 by W.T. Read and Ralph L. Johnston and the Gunn diode 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 maser invented in 1953 by Charles H. Townes, James P. Gordon, and H. J. Zeiger, and the varactor 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.
Microstrip, 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 gallium arsenide (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 Schottky diode 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 heterojunction 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 .