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Microwave is a form of
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

electromagnetic radiation
with
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

wavelength
s ranging from about one meter to one millimeter corresponding to
frequencies Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparent ...

frequencies
between 300 MHz and 300 GHz respectively. Different sources define different frequency ranges as microwaves; the above broad definition includes both
UHF Ultra high frequency (UHF) is the ITU 260px, ITU Monument, Bern The International Telecommunication Union is a specialized agency of the United Nations responsible for all matters related to information and communication technologies ...

UHF
and EHF (
millimeter wave Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It lies between the super high frequency band and ...
) bands. A more common definition in
radio-frequency engineering Radio-frequency (RF) engineering is a subset of electronic engineering Printed circuit board Electronic engineering (also called electronics and communications engineering) is an electrical engineering discipline which utilizes nonlinear and ...
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: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations. The
prefix A prefix is an affix In linguistics Linguistics is the scientific study of language A language is a structured system of communication used by humans, including speech (spoken language), gestures (Signed language, sign language) ...
' in ''microwave'' is not meant to suggest a wavelength in the
micrometerMicrometer can mean: * Micrometer (device) A micrometer, sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw widely used for Accuracy and precision, accurate measurement of components in mechanical engineeri ...
range. Rather, it indicates that microwaves are "small" (having shorter wavelengths), compared to the
radio wave Radio waves are a type of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies ma ...
s used prior to microwave technology. The boundaries between
far infrared 300px, Diagram of part of the electromagnetic spectrum Far infrared (FIR) is a region in the infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. I ...
,
terahertz radiation Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic wave Electromagnetism is a branch of physic ...
, microwaves, and
ultra-high-frequency Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz The hertz (symbol: Hz) is the SI derived unit, derived unit of frequency in the International System of Units (SI) and is defin ...
radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device ...

radio
wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

wave
s 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
ground wave A diving thumb The thumb is the first digit of the hand A hand is a prehensile, multi- fingered appendage located at the end of the forearm The forearm is the region of the upper limb between the Elbow-joint, elbow and the wrist. The ter ...
s, or reflect from the
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere The thermosphere is the layer in the directly above the and below the . Within this layer of the atmosphere, ...
, so terrestrial microwave communication links are limited by the visual horizon to about . 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,
wireless network A wireless network is a computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of technologies, ...

wireless network
s,
microwave radio relay Microwave transmission is the by with s in the range (1 m - 1 mm) of the . Microwave signals are normally limited to the , so long-distance transmission using these signals requires a series of repeaters forming a microwave relay. It is poss ...
networks,
radar Radar (radio detection and ranging) is a detection system that uses radio waves to determine the distance (''ranging''), angle, or velocity of objects. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor ...

radar
, satellite and spacecraft communication, medical
diathermy Diathermy is electrically induced heat or the use of high-frequency electromagnetic currents as a form of physical therapy and in surgical procedures. The earliest observations on the reactions of high-frequency electromagnetic currents upon the ...
and cancer treatment,
remote sensing image of Death Valley colored using polarimetry. Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied e ...

remote sensing
,
radio astronomy Radio astronomy is a subfield of astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial ...
,
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies, and to contain them in well-defined particle beam, beams. Large accelerators are used for funda ...
s,
spectroscopy Spectroscopy is the study of the interaction Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way ...

spectroscopy
, industrial heating,
collision avoidance system A collision avoidance system (CAS), also known as a pre-crash system, forward collision warning system, or collision mitigation system, is an advanced driver-assistance system Advanced driver-assistance systems (ADAS) are electronic syst ...
s,
garage door opener A garage door opener is a motorized device that opens and closes a garage door A garage door is a large door on a Garage (house), garage that opens either manually or by an electric motor (a garage door opener). Garage doors are frequently la ...

garage door opener
s and
keyless entry system A smart entry system is an electronic lock An electronic lock (or electric lock) is a locking device which operates by means of electric current. Electric locks are sometimes stand-alone with an electronic control assembly mounted directly to ...
s, and for cooking food in
microwave oven A microwave oven (commonly referred to as a microwave) is an electric oven upA double oven A ceramic oven An oven is a tool A tool is an object that can extend an individual's ability to modify features of the surrounding environment. ...

microwave oven
s.


Electromagnetic spectrum

Microwaves occupy a place in the
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existe ...

electromagnetic spectrum
with frequency above ordinary
radio wave Radio waves are a type of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies ma ...
s, and below
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

infrared
light: In descriptions of the
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existe ...

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.


Propagation

Microwaves travel solely by line-of-sight paths; unlike lower frequency radio waves, they do not travel as
ground wave A diving thumb The thumb is the first digit of the hand A hand is a prehensile, multi- fingered appendage located at the end of the forearm The forearm is the region of the upper limb between the Elbow-joint, elbow and the wrist. The ter ...
s which follow the contour of the Earth, or reflect off the
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere The thermosphere is the layer in the directly above the and below the . Within this layer of the atmosphere, ...
(
skywave off the ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere and parts of the mesosphere upright=0.5, Diagram showing the five primary layers of the Ea ...

skywave
s). 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
Fresnel zone A Fresnel zone ( ), named after physicist Augustin-Jean Fresnel, is one of a series of confocal prolate ellipsoid An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional Scaling (geometry), scalings ...

Fresnel zone
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 (
rain fadeRain fade refers primarily to the absorption Absorption may refer to: Chemistry and biology *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which substances enter the b ...
) 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 Opacity or opaque may refer to: * Impediments to (especially, visible) light: ** Opacities, absorption coefficients ** Opacity (optics), property or degree of blocking the transmission of light * Metaphors derived from literal optics: ** Opaque con ...
, until the atmosphere becomes transparent again in the so-called
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

infrared
and
optical window The optical window is a range of wavelengths that are not blocked by the Earth, earth's atmosphere. The window runs from around 300 nanometers (Ultraviolet, ultraviolet-B) up into the range the human eye can detect, roughly 400–700 nm and continu ...
frequency ranges.


Troposcatter

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
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere Planetary means relating to a planet A planet is an astronomical body orbiting a star or Stellar evo ...
. 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 Valdez-Cordova Census Area, Alaska, Boswell Bay, Alaska White Alice Site, Tropospheric scatter antenna and feeder. Tropospheric scatter, also known as troposcatter, is a method of communicating with microwave radio signals over considerable dista ...

tropospheric scatter
(troposcatter) communication systems to communicate beyond the horizon, at distances up to 300 km.


Antennas

The short
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

wavelength
s of microwaves allow
omnidirectional antenna on a walkie-talkie A walkie-talkie, more formally known as a handheld transceiver (HT), is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald Hings, radio enginee ...
s for portable devices to be made very small, from 1 to 20 centimeters long, so microwave frequencies are widely used for
wireless device Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points that do not use an electrical conductor as a medium by which to perform the transfer. The most common wireless technolo ...
s such as
cell phone A mobile phone, cellular phone, cell phone, cellphone, handphone, or hand phone, sometimes shortened to simply mobile, cell or just phone, is a portable telephone A telephone is a telecommunication Telecommunication is the tra ...

cell phone
s,
cordless phone The term cordless is generally used to refer to electrical or electronic devices that are powered by a battery (electricity), battery or battery pack and can operate without a power cord or cable attached to an electrical outlet to provide mains po ...
s, and
wireless LAN A wireless LAN (WLAN) is a wireless computer network that links two or more devices using wireless communication Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more point ...
s (Wi-Fi) access for
laptop A laptop, laptop computer, or notebook computer is a small, portable personal computer File:Crystal Project computer.png, upright=0.9, An artist's depiction of a 2000s-era desktop-style personal computer, which includes a metal case with ...

laptop
s, and
Bluetooth Bluetooth is a short-range wireless Wireless communication (or just wireless, when the context allows) is the telecommunication, transfer of information between two or more points that do not use an electrical conductor as a medium by which ...

Bluetooth
earphones. Antennas used include short
whip antenna A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver radio in the 1940s. During the golden age of radio, 1925–1955, families gathered to listen to the ho ...
s,
rubber ducky antenna The rubber ducky antenna (or rubber duck aerial) is an electrically short monopole antenna monopole antenna of an AM radio station in Chapel Hill, North Carolina. The mast itself is connected to the transmitter and radiates the radio waves. ...
s, sleeve
dipole In electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is c ...

dipole
s,
patch antenna A patch antenna is a type of antenna with a low profile, which can be mounted on a surface. It consists of a planar rectangular, circular, triangular, or any geometrical sheet or "patch" of metal, mounted over a larger sheet of metal called a gr ...
s, and increasingly the printed circuit inverted F antenna (PIFA) used in cell phones. Their short
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

wavelength
also allows narrow beams of microwaves to be produced by conveniently small high gain
antenna Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio) In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal con ...
s from a half meter to 5 meters in diameter. Therefore, beams of microwaves are used for point-to-point communication links, and for
radar Radar (radio detection and ranging) is a detection system that uses radio waves to determine the distance (''ranging''), angle, or velocity of objects. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor ...

radar
. An advantage of narrow beams is that they do not interfere with nearby equipment using the same frequency, allowing
frequency reuse A cellular network or mobile network is a communication network where the last link is wireless Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points that do not use an ...

frequency reuse
by nearby transmitters. are the most widely used directive antennas at microwave frequencies, but
horn antenna. . A horn antenna or microwave horn is an antenna (radio), antenna that consists of a flaring metal waveguide shaped like a horn (acoustic), horn to direct radio waves in a beam. Horns are widely used as antennas at Ultrahigh frequency, UHF an ...
s,
slot antenna A slot antenna consists of a metal A metal (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in South ...
s and dielectric lens antennas are also used. Flat
microstrip antenna receiver. Image:Antenna flat panel.png, Diagram of the feed structure of a microstrip antenna array. In telecommunication, a microstrip antenna (also known as a printed antenna) usually means an antenna (radio), antenna fabricated using photolitho ...
s are being increasingly used in consumer devices. Another directive antenna practical at microwave frequencies is the
phased array In antenna Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the ...
, a computer-controlled array of antennas that produces a beam that can be electronically steered in different directions. At microwave frequencies, the
transmission line In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics The field of electronics is a branch o ...

transmission line
s which are used to carry lower frequency radio waves to and from antennas, such as
coaxial cable Coaxial cable, or coax (pronounced ) is a type of consisting of an inner surrounded by a concentric conducting , with the two separated by a ( material); many coaxial cables also have a protective outer sheath or jacket. The term ' refers t ...
and parallel wire lines, have excessive power losses, so when low attenuation is required microwaves are carried by metal pipes called
waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities dec ...
s. Due to the high cost and maintenance requirements of waveguide runs, in many microwave antennas the output stage of the
transmitter In electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electron is a subatomic particle In physical sciences, subatomic particle ...
or the
RF front end upright=1.5, Block diagram of a superheterodyne receiver. The RF front end consists of the components on the left colored red. In a radio receiver circuit, the RF front end is a generic term for all the Electrical circuit, circuitry between a rec ...
of the is located at the antenna.


Design and analysis

The term ''microwave'' also has a more technical meaning in
electromagnetics Electromagnetism is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics ...
and
circuit theoryCircuit may refer to: Science and technology Electrical engineering * Electrical circuit An electrical network is an interconnection of electronic component, electrical components (e.g., battery (electricity), batteries, resistors, induct ...
. 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
resistor A resistor is a passive Passive may refer to: * Passive voice, a grammatical voice common in many languages, see also Pseudopassive (disambiguation), Pseudopassive * Passive language, a language from which an interpreter works * Passivity (b ...

resistor
s,
capacitor A capacitor is a device that stores electric charge in an electric field An electric field (sometimes E-field) is the physical field that surrounds electrically-charged particle In physics Physics (from grc, φυσική (ἐπ ...

capacitor
s, and
inductor An inductor, also called a coil, choke, or reactor, is a passive Passive may refer to: * Passive voice, a grammatical voice common in many languages, see also Pseudopassive (disambiguation), Pseudopassive * Passive language, a language from whi ...

inductor
s used with lower-frequency
radio waves Radio waves are a type of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

radio waves
. Open-wire and coaxial
transmission line In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics The field of electronics is a branch o ...

transmission line
s used at lower frequencies are replaced by
waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities dec ...

waveguide
s and
stripline Stripline is a transverse electromagnetic (TEM) transmission line In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems whic ...

stripline
, and lumped-element tuned circuits are replaced by cavity
resonator A resonator is a device or system that exhibits resonance Resonance describes the phenomenon of increased amplitude The amplitude of a Periodic function, periodic Variable (mathematics), variable is a measure of its change in a single Pe ...
s or resonant stubs. 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 Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...

optics
are used.


Microwave sources

High-power microwave sources use specialized
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...
s 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
magnetron Image:Magnetron2.jpg, Magnetron with section removed to exhibit the cavities. The cathode in the center is not visible. The antenna emitting microwaves is at the left. The magnets producing a field parallel to the long axis of the device are not ...

magnetron
(used in
microwave oven A microwave oven (commonly referred to as a microwave) is an electric oven upA double oven A ceramic oven An oven is a tool A tool is an object that can extend an individual's ability to modify features of the surrounding environment. ...

microwave oven
s),
klystron A klystron is a specialized linear-beam vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current An electric current is a stream of charged particles, such as e ...

klystron
,
traveling-wave tube Ruselectronics TWT from the 1980s used in the Russian Gorizont communication satellites ">Gorizont.html" ;"title="Ruselectronics TWT from the 1980s used in the Russian Gorizont">Ruselectronics TWT from the 1980s used in the Russian Gorizont commun ...

traveling-wave tube
(TWT), and
gyrotron file:W7-X gyrotron.jpg, High-power 140 GHz gyrotron for plasma heating in the Wendelstein 7-X fusion experiment, Germany. A gyrotron is a class of high-power linear-beam vacuum tubes which generates millimeter wave, millimeter-wave electromagnetic ...

gyrotron
. These devices work in the
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

density
modulated mode, rather than the
current Currents or The Current may refer to: Science and technology * Current (fluid) A current in a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. ...
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 The field-effect transistor (FET) is a type of transistor upright=1.4, gate Candi bentar, a typical Indonesian gate that is often found on the islands of Java">Indonesia.html" ;"title="Candi bentar, a typical Indonesia">Candi bentar, ...
(at least at lower frequencies),
tunnel diode A tunnel diode or Esaki diode is a type of semiconductor diode Various semiconductor diodes. Bottom: A bridge rectifier. In most diodes, a white or black painted band identifies the cathode into which electrons will flow when the diode is co ...

tunnel diode
s,
Gunn diode A Gunn diode, also known as a transferred electron device (TED), is a form of diode A diode is a two- that conducts primarily in one direction (asymmetric ); it has low (ideally zero) in one direction, and high (ideally infinite) in the ...
s, and
IMPATT diode An IMPATT diode (impact ionization avalanche transit-time diode) is a form of high-power semiconductor A semiconductor material has an electrical conductivity Electrical resistivity (also called specific electrical resistance or volume res ...
s. Low-power sources are available as benchtop instruments, rackmount instruments, embeddable modules and in card-level formats. A
maser (see description below) A maser (, an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherence (physics), coherent electromagnetic waves through amplification by stimulated emission. The firs ...

maser
is a solid state device which amplifies microwaves using similar principles to the
laser A laser is a device that emits light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as h ...

laser
, which amplifies higher frequency light waves. All warm objects emit low level microwave
black-body radiation Black-body radiation is the within or surrounding a body in with its environment, emitted by a (an idealized opaque, non-reflective body). It has a specific spectrum of wavelengths, inversely related to intensity that depend only on the bod ...
, depending on their
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

temperature
, so in meteorology and
remote sensing image of Death Valley colored using polarimetry. Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied e ...

remote sensing
,
microwave radiometer A microwave radiometer (MWR) is a radiometer that measures energy emitted at millimetre-to-centimetre wavelengths (frequencies of 1–1000 GHz) known as microwaves. Microwave radiometers are very sensitive receivers designed to measure thermally-e ...
s are used to measure the temperature of objects or terrain. The sun and other astronomical radio sources such as
Cassiopeia A Cassiopeia A (Cas A) () is a supernova remnant A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected mat ...

Cassiopeia A
emit low level microwave radiation which carries information about their makeup, which is studied by
radio astronomer Radio astronomy is a subfield of astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial ...
s using receivers called
radio telescope A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the r ...

radio telescope
s. The
cosmic microwave background radiation The cosmic microwave background (CMB, CMBR), in Big Bang The Big Bang Scientific theory, theory is the prevailing cosmological model explaining the existence of the observable universe from the Planck units#Cosmology, earliest known perio ...
(CMBR), for example, is a weak microwave noise filling empty space which is a major source of information on
cosmology Cosmology (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...
's
Big Bang The Big Bang theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity of consciously making sense of things, applying logic ...

Big Bang
theory of the origin of the
Universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

Universe
.


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
frequency reuse A cellular network or mobile network is a communication network where the last link is wireless Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points that do not use an ...

frequency reuse
; their comparatively higher frequencies allow broad
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
and high data 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 and
communications satellite A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a Transponder (satellite communications), transponder; it creates a communication channel between a source transmitter and a Radi ...
s. Microwaves are also employed in
microwave oven A microwave oven (commonly referred to as a microwave) is an electric oven upA double oven A ceramic oven An oven is a tool A tool is an object that can extend an individual's ability to modify features of the surrounding environment. ...

microwave oven
s and in
radar Radar (radio detection and ranging) is a detection system that uses radio waves to determine the distance (''ranging''), angle, or velocity of objects. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor ...

radar
technology.


Communication

Before the advent of
fiber-optic An optical fiber (or fibre in British English British English (BrE) is the standard dialect of the English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, ea ...
transmission, most long-distance
telephone call A telephone call is a connection over a telephone network A telephone network is a telecommunications network that connects Telephone A telephone is a telecommunications Appliance (disambiguation), device that permits two or more users ...
s were carried via networks of
microwave radio relay Microwave transmission is the by with s in the range (1 m - 1 mm) of the . Microwave signals are normally limited to the , so long-distance transmission using these signals requires a series of repeaters forming a microwave relay. It is poss ...
links run by carriers such as
AT&T Long Lines AT&T Communications, Inc., was a division of the AT&T Corporation AT&T Corporation, originally the American Telephone and Telegraph Company, is the subsidiary of AT&T Inc. that provides voice, video, data, and Internet The Internet ...
. Starting in the early 1950s,
frequency-division multiplexing In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separat ...
was used to send up to 5,400 telephone channels on each microwave radio channel, with as many as ten radio channels combined into one antenna for the ''hop'' to the next site, up to 70 km away.
Wireless LAN A wireless LAN (WLAN) is a wireless computer network that links two or more devices using wireless communication Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more point ...
protocol Protocol may refer to: Sociology and politics * Protocol (politics) Protocol originally (in Late Middle English, c. 15th century) meant the minutes or logbook taken at a meeting, upon which an agreement was based. The term now commonly refers to a ...
s, such as
Bluetooth Bluetooth is a short-range wireless Wireless communication (or just wireless, when the context allows) is the telecommunication, transfer of information between two or more points that do not use an electrical conductor as a medium by which ...

Bluetooth
and the
IEEE The Institute of Electrical and Electronics Engineers (IEEE) is a professional association for electronic engineering and electrical engineering (and associated disciplines) with its corporate office in New York City and its operations center i ...
802.11 IEEE 802.11 is part of the IEEE 802 set of local area network A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, school, laboratory, university campus or office buildi ...
specifications used for Wi-Fi, also use microwaves in the 2.4 GHz
ISM band The ISM radio bands are portions of the radio spectrum The radio spectrum is the part of the electromagnetic spectrum with Frequency, frequencies from 3 hertz, Hz to 300 Hertz, GHz. Electromagnetic waves in this frequency range, call ...
, although
802.11a IEEE 802.11a-1999 or 802.11a was an amendment to the IEEE 802.11 (legacy mode), IEEE 802.11 wireless local network specifications that defined requirements for an orthogonal frequency division multiplexing (OFDM) communication system. It was origina ...
uses
ISM band The ISM radio bands are portions of the radio spectrum The radio spectrum is the part of the electromagnetic spectrum with Frequency, frequencies from 3 hertz, Hz to 300 Hertz, GHz. Electromagnetic waves in this frequency range, call ...
and
U-NII The Unlicensed National Information Infrastructure (U-NII) radio band The radio spectrum is the part of the electromagnetic spectrum with Frequency, frequencies from 3 hertz, Hz to 300 Hertz, GHz. Electromagnetic waves in this frequenc ...
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 A metropolitan area network (MAN) is a computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up o ...
(MAN) protocols, such as
WiMAX Worldwide Interoperability for Microwave Access (WiMAX) is a family of wireless broadband Wireless broadband is telecommunications technology that provides high-speed wireless Wireless communication (or just wireless, when the context al ...

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 Mobile broadband is the marketing term for wireless Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points that do not use an electrical conductor as a medium by which to ...
Wireless Access (MBWA) protocols based on standards specifications such as IEEE 802.20 or ATIS/ANSI HC-SDMA (such as
iBurst IEEE 802.20 or Mobile Broadband Wireless Access (MBWA) was a specification by the standard association of the Institute of Electrical and Electronics Engineers The Institute of Electrical and Electronics Engineers (IEEE) is a professional ...
) 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 A mobile phone, cellular phone, cell phone, cellphone, handphone, or hand phone, sometimes shortened to simply mobile, cell or just phone, is a portable telephone A telephone is a telecommunication Telecommunication is the tra ...

mobile phone
networks, like
GSM GSM (Global System for Mobile communications) is a European standard for mobile devices. GSM may also refer to: Education * GSM London, a higher education provider * Guildhall School of Music and Drama The Guildhall School of Music and Drama i ...
, use the low-microwave/high-UHF frequencies around 1.8 and 1.9 GHz in the Americas and elsewhere, respectively.
DVB-SH DVB-SH ("Digital Video Broadcasting - Satellite services to Handhelds") is a physical layer standard for delivering IP based media content and data to handheld terminals such as mobile phones or PDAs, based on a hybrid satellite/terrestrial dow ...
and
S-DMB S-DMB (Satellite-DMB) was a hybrid version of the Digital Multimedia Broadcasting#REDIRECT Digital multimedia broadcasting {{rcat shell, {{R from other capitalization {{R from move .... The S-DMB used the S band The S band is a designation b ...
use 1.452 to 1.492 GHz, while proprietary/incompatible
satellite radio Satellite radio is defined by the International Telecommunication Union 260px, ITU Monument, Bern The International Telecommunication Union is a specialized agency of the United Nations United Nations Specialized Agencies are autono ...
in the U.S. uses around 2.3 GHz for
DARSDARS may refer to: * DARS (gene), a human gene * DARS (radar) Deployable Air operations centre, Recognized air picture production centre, Sensor Fusion Post, a NATO mobile deployable Command & Control radar system * DoD Architecture Registry System ...
. Microwave radio is used in
broadcasting Broadcasting is the distributionDistribution may refer to: Mathematics *Distribution (mathematics) Distributions, also known as Schwartz distributions or generalized functions, are objects that generalize the classical notion of functi ...

broadcasting
and
telecommunication Telecommunication is the transmission of information by various types of technologies over wire A wire is a single usually cylindrical A cylinder (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Gr ...
transmissions because, due to their short wavelength, highly
directional antenna A directional antenna or beam antenna is an antenna Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV o ...
s are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies). There is also more
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
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 News broadcasting is the medium of broadcasting Broadcasting is the distribution of sound, audio or video content to a dispersed audience via any electronic medium (communication), mass communications medium, but typically one using th ...
to transmit a signal from a remote location to a television station from a specially equipped van. See broadcast auxiliary service (BAS),
remote pickup unitA remote pickup unit or RPU is a radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They ...
(RPU), and
studio/transmitter link A studio transmitter link (or STL) sends a radio station's or television station's audio signal, audio and video from the Broadcasting, broadcast studio or origination facility to a radio transmitter, television transmitter or Ground station, uplink ...
(STL). Most
satellite communications A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a Transponder (satellite communications), transponder; it creates a communication channel between a source transmitter and a Radio ...
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 Satellite television is a service that delivers television programming to viewers by relaying it from a communications satellite A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a ...
either operates in the C band for the traditional large dish
fixed satellite service Fixed-satellite service (short: FSS | also: fixed-satellite radiocommunication service) is – according to ''article 1.21'' of the International Telecommunication Union's (ITU) Radio Regulations (RR) – defined as ''A radiocommunication se ...
or Ku band for
direct-broadcast satellite Satellite television is a service that delivers television programming to viewers by relaying it from a communications satellite A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a ...
. Military communications run primarily over X or Ku-band links, with Ka band being used for
Milstar Milstar (Military Strategic and Tactical Relay)https://www.globalsecurity.org/military/library/budget/fy2000/dot-e/airforce/00milstar.html is a constellation A constellation is an area on the celestial sphere in which a group of visible st ...

Milstar
.


Navigation

Global Navigation Satellite System A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows small electronics, electronic receivers to determine their location (longitude, latitude, and altitude/elevation) t ...
s (GNSS) including the Chinese
Beidou The BeiDou Navigation Satellite System (BDS) ( ) is a Chinese satellite navigation A satellite navigation or satnav system is a system that uses satellite In the context of spaceflight Spaceflight (or space flight) is an appli ...
, the American
Global Positioning System The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government The federal government of the United States (U.S. federal government or U.S. governme ...
(introduced in 1978) and the Russian
GLONASS GLONASS (russian: ГЛОНАСС, ; , Global Navigation Satellite System) is a Russian space-based satellite navigation A satellite navigation or satnav system is a system that uses satellite File:ERS 2.jpg, alt=, A full-size model of ...
broadcast navigational signals in various bands between about 1.2 GHz and 1.6 GHz.


Radar

Radar Radar (radio detection and ranging) is a detection system that uses radio waves to determine the distance (''ranging''), angle, or velocity of objects. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor ...

Radar
is a
radiolocation Radiolocation, also known as radiolocating or radiopositioning, is the process of finding the location In geography Geography (from Ancient Greek, Greek: , ''geographia'', literally "earth description") is a field of science devoted t ...
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 antenna antenna at Erdfunkstelle Raisting, the biggest facility for satellite communication in the world, in Raisting, Bavaria Bavaria (; German language, German and Bavarian language, Bavarian: ''Bayern'' ), officially the Free State of Bavaria (German ...

parabolic antenna
s 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 Air traffic control (ATC) is a service provided by ground-based air traffic controller Air traffic control specialists, abbreviated ATCS, are personnel responsible for the safe, orderly, and expeditious flow of air traffic in the global ...

air traffic control
, weather forecasting, navigation of ships, and
speed limit enforcement Speed limits are enforced on most public roadways by authorities, with the purpose to improve driver compliance with Speed limit, speed limits. Methods used include roadside speed traps set up and operated by the police and automated roadside 's ...
. Long-distance radars use the lower microwave frequencies since at the upper end of the band atmospheric absorption limits the range, but
millimeter wave Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It lies between the super high frequency band and ...
s are used for short-range radar such as
collision avoidance system A collision avoidance system (CAS), also known as a pre-crash system, forward collision warning system, or collision mitigation system, is an advanced driver-assistance system Advanced driver-assistance systems (ADAS) are electronic syst ...
s.


Radio astronomy

Microwaves emitted by
astronomical radio sourceAn astronomical radio source is an object in outer space that emits strong radio waves. Radio emission comes from a wide variety of sources. Such objects are among the most extreme and energetic physical processes in the universe. History In 1932, A ...
s; planets, stars,
galaxies A galaxy is a gravitationally bound system of star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to ...

galaxies
, and
nebula A nebula (Latin for 'cloud' or 'fog'; pl. nebulae, nebulæ or nebulas) is a distinct body of interstellar clouds (which can consist of cosmic dust, hydrogen, helium, molecular clouds; possibly as Plasma (physics), ionized gases). Originally, th ...

nebula
s are studied in
radio astronomy Radio astronomy is a subfield of astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial ...
with large dish antennas called
radio telescope A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the r ...

radio telescope
s. 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
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
or map the invisible surface of
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

Venus
through cloud cover. A recently completed microwave radio telescope is the
Atacama Large Millimeter Array The Atacama Large Millimeter/submillimeter Array (ALMA) is an astronomical interferometer An astronomical interferometer is an array of separate telescopes, mirror segments, or radio telescope antenna (radio), antennas that work together as a sing ...
, located at more than 5,000 meters (16,597 ft) altitude in Chile, observes the
universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

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 The cosmic microwave background (CMB, CMBR), in Big Bang The Big Bang Scientific theory, theory is the prevailing cosmological model explaining the existence of the observable universe from the Planck units#Cosmology, earliest known perio ...
(CMBR) discovered in 1964 by radio astronomers
Arno Penzias Arno Allan Penzias (; born April 26, 1933) is an United States, American physicist, radio astronomer and Nobel laureate in Nobel Prize in physics, physics. Along with Robert Woodrow Wilson, he discovered the cosmic microwave background radiation, ...

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 The Big Bang theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity of consciously making sense of things, applying logic ...

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
radio telescope A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the r ...

radio telescope
s can detect the CMBR as a faint signal that is not associated with any star, galaxy, or other object.


Heating and power application

A
microwave oven A microwave oven (commonly referred to as a microwave) is an electric oven upA double oven A ceramic oven An oven is a tool A tool is an object that can extend an individual's ability to modify features of the surrounding environment. ...

microwave oven
passes microwave radiation at a frequency near through food, causing
dielectric heating Dielectric heating, also known as electronic heating, radio frequency heating, and high-frequency heating, is the process in which a radio frequency Radio frequency (RF) is the oscillation rate of an Alternating_current, alternating electric cur ...
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
cavity magnetron The cavity magnetron is a high-power vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric volt ...
s. 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
plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood plasma ...
for such purposes as
reactive ion etching{{short description, Method used to relatively precisely remove material in a controlled and fine fashion Reactive-ion etching (RIE) is an etching (microfabrication), etching technology used in microfabrication. RIE is a type of dry etching which h ...
and plasma-enhanced
chemical vapor deposition Chemical vapor deposition (CVD) is a vacuum deposition Vacuum deposition is a family of processes used to deposit layers of material atom-by-atom or molecule-by-molecule on a solid surface. These processes operate at pressures well below atmo ...
(PECVD). Microwaves are used in
stellarator A stellarator is a Plasma (physics), plasma device that relies primarily on external magnets to confine a plasma. Scientists researching magnetic confinement fusion aim to use stellarator devices as a vessel for nuclear fusion reactions. The na ...
s and
tokamak A tokamak (; russian: токамáк) is a device which uses a powerful magnetic field to confine plasma (physics), plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement fusion, magnetic confinement devices ...
experimental fusion reactors to help break down the gas into a plasma, and heat it to very high temperatures. The frequency is tuned to the
cyclotron resonance Cyclotron resonance describes the interaction of external forces with charged particles experiencing a magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and ma ...
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 upcoming
ITER ITER (initially the International Thermonuclear Experimental Reactor, "iter" meaning "the way" or "the path" in Latin) is an international nuclear fusion research and engineering megaproject aimed at replicating the fusion processes of the Su ...
thermonuclear reactor will use up to 20 MW of 170 GHz microwaves. Microwaves can be used to transmit power over long distances, and post-
World War 2 World War II or the Second World War, often abbreviated as WWII or WW2, was a World war, global war that lasted from 1939 to 1945. It involved World War II by country, the vast majority of the world's countries—including all the great ...
research was done to examine possibilities.
NASA The National Aeronautics and Space Administration (NASA; ) is an independent agency A regulatory agency or regulatory authority, is a Public benefit corporation Public-benefit corporation is a term that has different meanings in differen ...

NASA
worked in the 1970s and early 1980s to research the possibilities of using
solar power satellite Space-based solar power (SBSP) is the concept of collecting solar power in outer space and distributing it to Earth. Potential advantages of collecting solar energy in space include a higher collection rate and a longer collection period due to t ...
(SPS) systems with large
solar array A photovoltaic system, also PV system or solar power system, is an electric power system An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of a power system is the e ...
s that would beam power down to the Earth's surface via microwaves.
Less-than-lethal Non-lethal weapons, also called less-lethal weapons, less-than-lethal weapons, non-deadly weapons, compliance weapons, or pain-inducing weapons are weapons intended to be lethality, less likely to kill a living target than conventional weapons suc ...
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 The United States Air Force (USAF) is the air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmosph ...

United States Air Force
and
Marines Marines, or naval infantry, are typically a military force trained to operate in littoral zones The littoral zone or nearshore is the part of a sea, lake, or river that is close to the shore. In coastal environments, the littoral zone exten ...
are currently using this type of
active denial system The Active Denial System (ADS), is a non-lethal Non-lethal weapons, also called less-lethal weapons, less-than-lethal weapons, non-deadly weapons, compliance weapons, or pain-inducing weapons are weapon A weapon, arm or armament is any imp ...
in fixed installations.


Spectroscopy

Microwave radiation is used in
electron paramagnetic resonance Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy Spectroscopy is the study of the interaction Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of ...
(EPR or ESR) spectroscopy, typically in the X-band region (~9 GHz) in conjunction typically with
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

magnetic field
s of 0.3 T. This technique provides information on unpaired
electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are general ...

electron
s in chemical systems, such as
free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Ailments of unknown cause Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabo ...
s or
transition metal In chemistry, the term transition metal (or transition element) has three possible definitions: * The IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations tha ...
ions such as Cu(II). Microwave radiation is also used to perform
rotational spectroscopy Rotational spectroscopy is concerned with the measurement of the energies of transitions between quantized rotational states of molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five ca ...
and can be combined with
electrochemistry Electrochemistry is the branch of physical chemistry Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying ...

electrochemistry
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. 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: Other definitions exist. The term P band is sometimes used for
UHF Ultra high frequency (UHF) is the ITU 260px, ITU Monument, Bern The International Telecommunication Union is a specialized agency of the United Nations responsible for all matters related to information and communication technologies ...

UHF
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.Skolnik, Merrill I. (2001) ''Introduction to Radar Systems'', Third Ed., p. 522, McGraw Hill
1962 Edition full text
/ref>


Microwave frequency measurement

Microwave frequency can be measured by either electronic or mechanical techniques. Frequency counters or high frequency heterodyne systems can be used. Here the unknown frequency is compared with harmonics of a known lower frequency by use of a low-frequency generator, a harmonic generator and a mixer. The 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 A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities dec ...

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 node (physics), nodes, which is equal to half the wavelength. The precision of this method is limited by the determination of the nodal locations.


Effects on health

Microwaves are non-ionizing radiation, which means that microwave photons do not contain sufficient energy to ionize molecules or break chemical bonds, or cause DNA damage, as ionizing radiation such as x-rays or ultraviolet can. The word "radiation" refers to energy radiating from a source and not to radioactivity. 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 other non-ionizing electromagnetic radiation) have significant adverse biological effects at low levels. Some, but not all, studies suggest that long-term exposure may have a carcinogenic effect. During
World War 2 World War II or the Second World War, often abbreviated as WWII or WW2, was a World war, global war that lasted from 1939 to 1945. It involved World War II by country, the vast majority of the world's countries—including all the great ...
, it was observed that individuals in the radiation path of radar installations experienced clicks and buzzing sounds in response to microwave radiation. Research by
NASA The National Aeronautics and Space Administration (NASA; ) is an independent agency A regulatory agency or regulatory authority, is a Public benefit corporation Public-benefit corporation is a term that has different meanings in differen ...

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 cataracts by this mechanism, because the microwave heating denaturation (biochemistry), denatures proteins in the Lens (anatomy), crystalline lens of the Human eye, eye (in the same way that heat turns egg whites white and opaque). The lens and cornea of the eye are especially vulnerable because they contain no blood vessels that can carry away heat. Exposure to heavy doses of microwave radiation (as from an oven that has been tampered with to allow operation even with the door open) can produce heat damage in other tissues as well, up to and including serious burns 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.


History


Hertzian optics

Microwaves were first generated in the 1890s in some of the earliest
radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device ...

radio
experiments by physicists who thought of them as a form of "invisible light". James Clerk Maxwell in his 1873 theory of electromagnetism, now called Maxwell's equations, had predicted that a coupled electric field and
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

magnetic field
could travel through space as an electromagnetic wave, and proposed that light consisted of electromagnetic waves of short wavelength. In 1888, German physicist Heinrich Hertz was the first to demonstrate the existence of
radio wave Radio waves are a type of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies ma ...
s using a primitive spark gap transmitter, spark gap radio transmitter. 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 ultrahigh frequency, UHF and microwave ranges, with which they could duplicate classic
optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...

optics
experiments in their laboratories, using quasioptics, quasioptical components such as prisms and lens (optics), lenses made of paraffin wax, paraffin, sulfur and pitch (resin), pitch and wire diffraction gratings, to refract and diffract radio waves like light rays. Hertz produced waves up to 450 MHz; his directional 450 MHz transmitter consisted of a 26 cm brass rod dipole antenna with a spark gap between the ends, suspended at the focal line of a
parabolic antenna antenna at Erdfunkstelle Raisting, the biggest facility for satellite communication in the world, in Raisting, Bavaria Bavaria (; German language, German and Bavarian language, Bavarian: ''Bayern'' ), officially the Free State of Bavaria (German ...

parabolic antenna
made of a curved zinc sheet, powered by high voltage pulses from an induction coil. His historic experiments demonstrated that radio waves like light exhibited refraction, diffraction, polarization (waves), polarization, interference (wave motion), interference and standing waves, proving that radio waves and light waves were both forms of Maxwell's electromagnetic waves. Image:Hertz spark gap transmitter and parabolic antenna.png, Heinrich Hertz'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
millimeter wave Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It lies between the super high frequency band and ...
s; 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, Microwave spectroscopy experiment by John Ambrose Fleming in 1897 showing refraction of 1.4 GHz microwaves by paraffin prism, duplicating earlier experiments by Bose and Righi. Image:Oscillatore di Righi con riflettore parabolico - Museo scienza tecnologia Milano 08757 1.jpg, Augusto Righi's 12 GHz spark oscillator and receiver, 1895
Beginning in 1894 Indian physicist Jagadish Chandra Bose performed the first experiments with microwaves. He was the first person to produce
millimeter wave Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It lies between the super high frequency band and ...
s, generating frequencies up to 60 GHz (5 millimeter) using a 3 mm metal ball spark oscillator. Bose also invented
waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities dec ...
,
horn antenna. . A horn antenna or microwave horn is an antenna (radio), antenna that consists of a flaring metal waveguide shaped like a horn (acoustic), horn to direct radio waves in a beam. Horns are widely used as antennas at Ultrahigh frequency, UHF an ...
s, and semiconductor crystal detectors for use in his experiments. Independently in 1894, Oliver Lodge and Augusto Righi experimented with 1.5 and 12 GHz microwaves respectively, generated by small metal ball spark resonators. Russian physicist Pyotr Lebedev in 1895 generated 50 GHz millimeter waves. In 1897 Lord Rayleigh solved the mathematical boundary-value problem of electromagnetic waves propagating through conducting tubes and dielectric rods of arbitrary shape. which gave the modes and cutoff frequency of microwaves propagating through a
waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities dec ...
. However, since microwaves were limited to line-of-sight propagation, line 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 as
ground wave A diving thumb The thumb is the first digit of the hand A hand is a prehensile, multi- fingered appendage located at the end of the forearm The forearm is the region of the upper limb between the Elbow-joint, elbow and the wrist. The ter ...
s and by reflecting off the
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere The thermosphere is the layer in the directly above the and below the . Within this layer of the atmosphere, ...
as
skywave off the ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere and parts of the mesosphere upright=0.5, Diagram showing the five primary layers of the Ea ...

skywave
s, 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 triode
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...
(valve) electronic oscillator used in radio transmitters could not produce frequencies above a few hundred megahertz 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. Image:Southworth demonstrating waveguide.jpg, Southworth ''(at left)'' demonstrating waveguide at Institute of Radio Engineers, 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 In 1931 an Anglo-French consortium headed by Andre C. Clavier demonstrated the first experimental microwave relay link, across the English Channel between Dover, UK and Calais, France. The system transmitted telephony, telegraph and facsimile 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 "short wave" band, which meant all waves shorter than 200 meters. The terms ''quasi-optical waves'' and ''ultrashort waves'' were used briefly, but did not catch on. The first usage of the word ''micro-wave'' apparently occurred in 1931.


Radar

The development of
radar Radar (radio detection and ranging) is a detection system that uses radio waves to determine the distance (''ranging''), angle, or velocity of objects. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor ...

radar
, mainly in secrecy, before and during
World War 2 World War II or the Second World War, often abbreviated as WWII or WW2, was a World war, global war that lasted from 1939 to 1945. It involved World War II by country, the vast majority of the world's countries—including all the great ...
, resulted in the technological advances which made microwaves practical. Wavelengths in the centimeter range were required to give the small radar antennas which were compact enough to fit on aircraft a narrow enough beamwidth to localize enemy aircraft. It was found that conventional
transmission line In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics The field of electronics is a branch o ...

transmission line
s used to carry radio waves had excessive power losses at microwave frequencies, and George Southworth at Bell Labs and Wilmer Barrow at Massachusetts Institute of Technology, MIT independently invented
waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities dec ...
in 1936. Barrow invented the
horn antenna. . A horn antenna or microwave horn is an antenna (radio), antenna that consists of a flaring metal waveguide shaped like a horn (acoustic), horn to direct radio waves in a beam. Horns are widely used as antennas at Ultrahigh frequency, UHF an ...
in 1938 as a means to efficiently radiate microwaves into or out of a waveguide. In a microwave , a linear circuit, nonlinear component was needed that would act as a detector (radio), detector and frequency mixer, mixer at these frequencies, as vacuum tubes had too much capacitance. To fill this need researchers resurrected an obsolete technology, the point contact diode, point contact crystal detector (cat whisker detector) which was used as a demodulator in crystal radios around the turn of the century before vacuum tube receivers. The low capacitance of semiconductor junctions allowed them to function at microwave frequencies. The first modern silicon and germanium diodes 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:R&B Magnetron.jpg, John Randall (physicist), Randall and Harry Boot, Boot's prototype cavity magnetron tube at the University of Birmingham, 1940. In use the tube was installed between the poles of an electromagnet Image:Prototype klystron cutaway.jpg, First commercial klystron tube, by General Electric, 1940, sectioned to show internal construction Image:AI Mk. VIIIA radar in Bristol Beaufighter VIF CH16665.jpg, AI Mk. VIII radar, British Mk. VIII, the first microwave air intercept radar, in nose of British fighter. Microwave radar, powered by the new magnetron, magnetron tube, significantly shortened 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
klystron A klystron is a specialized linear-beam vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current An electric current is a stream of charged particles, such as e ...

klystron
tube by Russell and Sigurd Varian at Stanford University in 1937, and the
cavity magnetron The cavity magnetron is a high-power vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric volt ...
tube by John Randall (physicist), John Randall and Harry Boot at Birmingham University, UK in 1940. Ten centimeter (3 GHz) microwave radar 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 (the Tizard Mission) significantly shortened 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. 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 Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
); a single microwave beam could carry tens of thousands of phone calls. In the 1950s and 60s transcontinental microwave transmission, 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 (broadcasting), backhaul video feeds from mobile production trucks back to the studio, allowing the first remote broadcast, remote TV broadcasts. The first
communications satellite A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a Transponder (satellite communications), transponder; it creates a communication channel between a source transmitter and a Radi ...
s were launched in the 1960s, which relayed telephone calls and television between widely separated points on Earth using microwave beams. In 1964,
Arno Penzias Arno Allan Penzias (; born April 26, 1933) is an United States, American physicist, radio astronomer and Nobel laureate in Nobel Prize in physics, physics. Along with Robert Woodrow Wilson, he discovered the cosmic microwave background radiation, ...

Arno Penzias
and Robert Woodrow Wilson while investigating noise in a satellite horn antenna at Bell Labs, Holmdel, New Jersey discovered
cosmic microwave background radiation The cosmic microwave background (CMB, CMBR), in Big Bang The Big Bang Scientific theory, theory is the prevailing cosmological model explaining the existence of the observable universe from the Planck units#Cosmology, earliest known perio ...
. Microwave radar became the central technology used in
air traffic control Air traffic control (ATC) is a service provided by ground-based air traffic controller Air traffic control specialists, abbreviated ATCS, are personnel responsible for the safe, orderly, and expeditious flow of air traffic in the global ...

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 antenna at Erdfunkstelle Raisting, the biggest facility for satellite communication in the world, in Raisting, Bavaria Bavaria (; German language, German and Bavarian language, Bavarian: ''Bayern'' ), officially the Free State of Bavaria (German ...

parabolic antenna
(the most common type), cassegrain antenna, lens antenna,
slot antenna A slot antenna consists of a metal A metal (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in South ...
, and
phased array In antenna Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the ...
. The ability of short waves 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 A microwave oven (commonly referred to as a microwave) is an electric oven upA double oven A ceramic oven An oven is a tool A tool is an object that can extend an individual's ability to modify features of the surrounding environment. ...

microwave 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 in hyperthermy, microwave hyperthermy. The traveling wave tube (TWT) developed in 1943 by Rudolph Kompfner and John R. Pierce, 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 file:W7-X gyrotron.jpg, High-power 140 GHz gyrotron for plasma heating in the Wendelstein 7-X fusion experiment, Germany. A gyrotron is a class of high-power linear-beam vacuum tubes which generates millimeter wave, millimeter-wave electromagnetic ...

gyrotron
tube family developed in Russia could produce megawatts of power up into
millimeter wave Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It lies between the super high frequency band and ...
frequencies and is used in industrial heating and plasma (physics), plasma research, and to power
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies, and to contain them in well-defined particle beam, beams. Large accelerators are used for funda ...
s and nuclear fusion reactors.


Solid state microwave devices

The development of semiconductor electronics in the 1950s led to the first solid state electronics, solid state microwave devices which worked by a new principle; negative resistance (some of the prewar microwave tubes had also used negative resistance). The electronic oscillator, feedback oscillator and two-port amplifiers which were used at lower frequencies became unstable at microwave frequencies, and negative resistance oscillators and amplifiers based on one-port devices like diodes worked better. The
tunnel diode A tunnel diode or Esaki diode is a type of semiconductor diode Various semiconductor diodes. Bottom: A bridge rectifier. In most diodes, a white or black painted band identifies the cathode into which electrons will flow when the diode is co ...

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 An IMPATT diode (impact ionization avalanche transit-time diode) is a form of high-power semiconductor A semiconductor material has an electrical conductivity Electrical resistivity (also called specific electrical resistance or volume res ...
in 1956 by W.T. Read and Ralph L. Johnston and the
Gunn diode A Gunn diode, also known as a transferred electron device (TED), is a form of diode A diode is a two- that conducts primarily in one direction (asymmetric ); it has low (ideally zero) in one direction, and high (ideally infinite) in the ...
in 1962 by J. B. Gunn. Diodes are the most widely used microwave sources today. Two low-noise Solid-state electronics, solid state negative resistance microwave amplifiers were developed; the ruby
maser (see description below) A maser (, an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherence (physics), coherent electromagnetic waves through amplification by stimulated emission. The firs ...

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 atomic clocks, 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 Reciprocity (electrical networks), nonreciprocal waveguide components, such as circulators, isolator (microwave), 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 upright=1.5, Block diagram of a superheterodyne receiver. The RF front end consists of the components on the left colored red. In a radio receiver circuit, the RF front end is a generic term for all the Electrical circuit, circuitry between a rec ...
of receivers, and signals were heterodyning, heterodyned 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, cable television, GPS devices, and modern wireless devices, such as smartphones, Wi-Fi, and
Bluetooth Bluetooth is a short-range wireless Wireless communication (or just wireless, when the context allows) is the telecommunication, transfer of information between two or more points that do not use an electrical conductor as a medium by which ...

Bluetooth
which connect to networks using microwaves. Microstrip, a type of
transmission line In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics The field of electronics is a branch o ...

transmission line
usable at microwave frequencies, was invented with printed circuits in the 1950s. The ability to cheaply fabricate a wide range of shapes on printed circuit boards allowed microstrip versions of
capacitor A capacitor is a device that stores electric charge in an electric field An electric field (sometimes E-field) is the physical field that surrounds electrically-charged particle In physics Physics (from grc, φυσική (ἐπ ...

capacitor
s,
inductor An inductor, also called a coil, choke, or reactor, is a passive Passive may refer to: * Passive voice, a grammatical voice common in many languages, see also Pseudopassive (disambiguation), Pseudopassive * Passive language, a language from whi ...

inductor
s, Stub (electronics), resonant stubs, Power dividers and directional couplers, splitters, directional couplers, diplexers, electronic filter, filters and antennas to be made, thus allowing compact microwave circuits to be constructed. Transistors 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, Schottky junctions 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 wafer (electronics), 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 amplifiers, modems, and microprocessors.


See also

* Block upconverter, Block upconverter (BUC) * Cosmic microwave background * Electron cyclotron resonance * International Microwave Power Institute * Low-noise block downconverter, Low-noise block converter (LNB) * Maser * Microwave auditory effect * Microwave cavity * Microwave chemistry * Microwave radio relay * Microwave transmission * Rain fade * RF switch matrix * The Thing (listening device)


References


External links


EM Talk, Microwave Engineering Tutorials and Tools

Millimeter Wave
and Microwave Waveguide dimension chart. {{Authority control Microwave technology, Electromagnetic spectrum Radio technology