View source for Microwave

{{short description|Electromagnetic radiation with wavelengths from 1 m to 1 mm}}
{{about|the electromagnetic wave|the cooking appliance|Microwave oven|other uses|Microwaves (disambiguation)}}
{{pp-vandalism|small=yes}}
[[File:Frazier Peak, tower and Honda Element.jpg|thumb|A telecommunications tower with a variety of dish antennas for [[microwave relay]] links  on [[Frazier Peak]], Ventura County, [[California]]. The apertures of the dishes are covered by plastic sheets ([[radome]]s) to keep out moisture.]]

'''Microwave''' is a form of [[electromagnetic radiation]] with [[wavelength]]s shorter than other [[radio wave]]s but longer than [[infrared]] waves. Its wavelength ranges from about one meter to one millimeter, corresponding to {{anchor|MICROWAVE_FREQUENCY}}[[frequency|frequencies]] between 300&nbsp;MHz and 300&nbsp;GHz, broadly construed.<ref name="Hitchcock">{{cite book
 | last1  = Hitchcock
 | first1 = R. Timothy
 | title  = Radio-frequency and Microwave Radiation
 | publisher = American Industrial Hygiene Assn.
 | date   = 2004
 | pages  = 1
 | url    = https://books.google.com/books?id=0TUIQ9-Ap5cC&q=microwave&pg=PA1
 | isbn   = 978-1931504553
 }}</ref><ref name="NAB">{{cite book
 | last1  = Jones
 | first1 = Graham A.
 | last2  = Layer
 | first2 = David H.
 | last3  = Osenkowsky
 | first3 = Thomas G.
 | title  = National Association of Broadcasters Engineering Handbook, 10th Ed.
 | publisher = Focal Press
 | date   = 2013
 | pages  = 6
 | url    = https://books.google.com/books?id=K9N1TVhf82YC&q=microwave&pg=PA6
 | isbn   = 978-1136034107
 }}</ref><ref>{{Cite web |title=Electromagnetic radiation - Microwaves, Wavelengths, Frequency {{!}} Britannica |url=https://www.britannica.com/science/electromagnetic-radiation/Microwaves |access-date=2023-08-15 |website=www.britannica.com |language=en}}</ref>  A more common definition in [[radio-frequency engineering]] is the range between 1 and 100&nbsp;GHz (wavelengths between 30&nbsp;cm and 3&nbsp;mm),<ref name="Kumar">{{cite book
 | last1  = Kumar
 | first1 = Sanjay
 | last2  = Shukla
 | first2 = Saurabh
 | title  = Concepts and Applications of Microwave Engineering
 | publisher = PHI Learning Pvt. Ltd
 | date   = 2014
 | url    = https://books.google.com/books?id=GY9eBAAAQBAJ&q=microwave&pg=PA3
 | isbn   = 978-8120349353
 }}</ref>{{rp|p=3}} or between 1 and 3000&nbsp;GHz (30&nbsp;cm and 0.1&nbsp;mm).<ref name="IEV-i391">{{cite web | title=Details for IEV number 713-06-03: "microwave" | website=International Electrotechnical Vocabulary | url=https://www.electropedia.org/iev/iev.nsf/display?openform&ievref=713-06-03 | language=ja | access-date=2024-03-27}}</ref><ref name="IEV-o981">{{cite web | title=Details for IEV number 701-02-12: "radio wave" | website=International Electrotechnical Vocabulary | url=https://www.electropedia.org/iev/iev.nsf/display?openform&ievref=701-02-12 | language=ja | access-date=2024-03-27}}</ref>  In all cases, microwaves include the entire [[super high frequency]] (SHF) band (3 to 30&nbsp;GHz, or 10 to 1&nbsp;cm) at minimum. The boundaries between [[far infrared]], [[terahertz radiation]], microwaves, and [[ultra-high-frequency]] (UHF) are fairly arbitrary and differ between different fields of study.<ref name="Kumar" />{{rp|p=3}}

The [[prefix]] ''{{linktext|micro-}}'' in ''microwave'' indicates that microwaves are small (having shorter wavelengths), compared to the [[radio wave]]s used in prior [[radio technology]].  Frequencies in the microwave range are often referred to by their [[Radio band#IEEE|IEEE radar band]] designations: [[S band|S]], [[C band (IEEE)|C]], [[X band|X]], [[Ku band|K<sub>u</sub>]], [[K band (IEEE)|K]], or [[Ka band|K<sub>a</sub> band]], or by similar NATO or EU designations.

Microwaves travel by [[line-of-sight propagation|line-of-sight]]; unlike lower frequency [[radio waves]], they do not diffract around hills, nor follow the Earth's surface as [[ground wave]]s, nor reflect from the [[ionosphere]], so terrestrial microwave communication links are limited by the visual horizon to about {{convert|40|mi|km}}. At the high end of the band of microwave frequencies, 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 (telecommunications)|point-to-point]] communication links, [[wireless network]]s, [[microwave radio relay]] networks, [[radar]], [[satellite communication|satellite and spacecraft communication]], medical [[diathermy]] and cancer treatment, [[remote sensing]], [[radio astronomy]], [[particle accelerator]]s, [[spectroscopy]], industrial heating, [[collision avoidance system]]s, [[garage door opener]]s and [[keyless entry system]]s, and for cooking food in [[microwave oven]]s.

== Electromagnetic spectrum ==
Microwaves occupy a place in the [[electromagnetic spectrum]] with frequency above ordinary [[radio wave]]s, and below [[infrared]] light:
{| class="wikitable nowrap" style="text-align:center;"
!colspan="5"| [[Electromagnetic spectrum]]
|-
! Name || Wavelength || [[Hertz#SI multiples|Frequency (Hz)]] || [[Photon]] [[energy]] ([[Electronvolt#Properties|eV]])
|-
| [[Gamma ray]]     || < 0.01&nbsp;nm        || > 30 [[Exa-|E]]Hz || > 124 [[kilo-|k]]eV     
|-
| [[X-ray]]         || 0.01&nbsp;nm – 10&nbsp;nm  ||  30 EHz – 30 [[Peta-|P]]Hz || 124 keV – 124 eV
|-
| [[Ultraviolet]]   ||   10&nbsp;nm – 400&nbsp;nm ||  30 PHz – 750 THz ||   124 eV – 3 eV          
|-
| [[Visible light]] ||  400&nbsp;nm – 750&nbsp;nm || 750 THz – 400 THz ||   3 eV – 1.7 eV        
|-
| [[Infrared]]      ||  750&nbsp;nm – 1&nbsp;mm   || 400 THz – 300&nbsp;GHz ||   1.7 eV – 1.24 [[milli|m]]eV 
|- style="background:#ccffcc;"
| {{strong|Microwave}}  ||    1&nbsp;mm – 1&nbsp;m    || 300&nbsp;GHz – 300&nbsp;MHz || 1.24 meV – 1.24 [[Micro-|μ]]eV
<!-- Radio waves _include_ microwaves, so following ranges overlap those above: -->
|-
| [[Radio waves|Radio]] || ≥ 1&nbsp;m || ≤ 300&nbsp;MHz || ≤ 1.24 μeV
|}
In descriptions of the [[electromagnetic spectrum]], some sources classify microwaves as radio waves, a subset of the radio wave band, while others classify microwaves and radio waves as distinct types of radiation.<ref name="Hitchcock" /> This is an arbitrary distinction.

=== Frequency bands ===
Bands of frequencies in the microwave spectrum are designated by letters.  Unfortunately, there are several incompatible band designation systems, and even within a system the frequency ranges corresponding to some of the letters vary somewhat between different application fields.<ref name="Microwaves101">{{cite encyclopedia
  | title = Frequency Letter bands
  | encyclopedia = Microwave Encyclopedia
  | publisher = Microwaves101 website, Institute of Electrical and Electronics Engineers (IEEE)
  | date = 14 May 2016
  | url = https://www.microwaves101.com/encyclopedia/588-frequency-letter-bands
  | access-date = 1 July 2018}}</ref><ref name="Golio2">{{cite book
 | last1  = Golio
 | first1 = Mike
 | last2  = Golio
 | first2 = Janet
 | title  = RF and Microwave Applications and Systems
 | publisher = CRC Press
 | date   = 2007
 | pages  = 1.9–1.11
 | url    = https://books.google.com/books?id=fNJLcL1LBpEC&q=Microwave+letter+bands&pg=SL9-PA9
 | isbn   = 978-1420006711
 }}</ref>  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:
{{MWband}}
{| class="wikitable nowrap"
|+ Microwave frequency bands
! Designation !! Frequency range !! Wavelength range !! Typical uses
|-
| [[L band]] || 1 to 2&nbsp;GHz || 15&nbsp;cm to 30&nbsp;cm
|style="white-space:normal;"| military telemetry, GPS, mobile phones (GSM), amateur radio
|-
| [[S band]] || 2 to 4&nbsp;GHz || 7.5&nbsp;cm to 15&nbsp;cm
|style="white-space:normal;"| weather radar, surface ship radar, some communications satellites, microwave ovens, microwave devices/communications, radio astronomy, mobile phones, wireless LAN, Bluetooth, ZigBee, GPS, amateur radio
|-
| [[C band (IEEE)|C band]] || 4 to 8&nbsp;GHz || 3.75&nbsp;cm to 7.5&nbsp;cm
|style="white-space:normal;"| long-distance radio telecommunications, wireless LAN, amateur radio
|-
| [[X band]] || 8 to 12&nbsp;GHz || 25&nbsp;mm to 37.5&nbsp;mm
|style="white-space:normal;"| satellite communications, radar, terrestrial broadband, space communications, amateur radio, molecular rotational spectroscopy
|-
| [[Ku band|K<sub>u</sub> band]] || 12 to 18&nbsp;GHz || 16.7&nbsp;mm to 25&nbsp;mm
|style="white-space:normal;"| satellite communications, molecular rotational spectroscopy
|-
| [[K band (IEEE)|K band]] || 18 to 26.5&nbsp;GHz || 11.3&nbsp;mm to 16.7&nbsp;mm
|style="white-space:normal;"| radar, satellite communications, astronomical observations, automotive radar, molecular rotational spectroscopy
|-
| [[Ka band|K<sub>a</sub> band]] || 26.5 to 40&nbsp;GHz || 5.0&nbsp;mm to 11.3&nbsp;mm
|style="white-space:normal;"| satellite communications, molecular rotational spectroscopy
|-
| [[Q band]] || 33 to 50&nbsp;GHz || 6.0&nbsp;mm to 9.0&nbsp;mm
|style="white-space:normal;"| satellite communications, terrestrial microwave communications, radio astronomy, automotive radar, molecular rotational spectroscopy
|-
| [[U band]] || 40 to 60&nbsp;GHz || 5.0&nbsp;mm to 7.5&nbsp;mm
|style="white-space:normal;"|
|-
| [[V band]] || 50 to 75&nbsp;GHz || 4.0&nbsp;mm to 6.0&nbsp;mm
|style="white-space:normal;"| millimeter wave radar research, molecular rotational spectroscopy and other kinds of scientific research
|-
| [[W band]] || 75 to 110&nbsp;GHz || 2.7&nbsp;mm to 4.0&nbsp;mm
|style="white-space:normal;"| satellite communications, millimeter-wave radar research, military radar targeting and tracking applications, and some non-military applications, automotive radar
|-
| [[F band (waveguide)|F band]] || 90 to 140&nbsp;GHz || 2.1&nbsp;mm to 3.3&nbsp;mm
|style="white-space:normal;"| SHF transmissions: Radio astronomy, microwave devices/communications, wireless LAN, most modern radars, communications satellites, satellite television broadcasting, [[Direct broadcast satellite|DBS]], amateur radio
|-
| [[D band (waveguide)|D band]] || 110 to 170&nbsp;GHz || 1.8&nbsp;mm to 2.7&nbsp;mm
|style="white-space:normal;"| EHF transmissions: Radio astronomy, high-frequency microwave radio relay, microwave remote sensing, amateur radio, directed-energy weapon, millimeter wave scanner
|}

Other definitions exist.<ref>See {{cite web |url=http://www.radioing.com/eengineer/bands.html |title=eEngineer – Radio Frequency Band Designations |publisher=Radioing.com |access-date=2011-11-08 }}, {{cite web |author=PC Mojo – Webs with MOJO from Cave Creek, AZ |url=http://www.microwaves101.com/encyclopedia/letterbands.cfm |title=Frequency Letter bands – Microwave Encyclopedia |publisher=Microwaves101.com |date=2008-04-25 |access-date=2011-11-08 |archive-url=https://web.archive.org/web/20140714171156/http://www.microwaves101.com/ENCYCLOPEDIA/letterbands.cfm |archive-date=2014-07-14 |url-status=dead }}, [http://www.jneuhaus.com/fccindex/letter.html Letter Designations of Microwave Bands].</ref>

{{anchor|P band}}The term P band is sometimes used for [[Ultra high frequency|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, K<sub>u</sub>, and upper band, K<sub>a</sub>.<ref name="test">Skolnik, Merrill I. (2001) ''Introduction to Radar Systems'', Third Ed., p. 522, McGraw Hill. [https://archive.org/details/IntroductionToRadarSystems 1962 Edition full text]</ref>

== Propagation ==
{{main|Radio propagation}}
[[File:Atmospheric Microwave Transmittance at Mauna Kea (simulated).svg|right|thumb|upright=1.3|The atmospheric [[attenuation]] of microwaves and far infrared radiation in dry air with a precipitable water vapor level of 0.001&nbsp;mm. The downward spikes in the graph correspond to frequencies at which microwaves are absorbed more strongly. This graph includes a range of frequencies from 0 to 1 THz; the microwaves are the subset in the range between 0.3 and 300 gigahertz.]]Microwaves travel solely by [[line-of-sight propagation|line-of-sight]] paths; unlike lower frequency radio waves, they do not travel as [[ground wave]]s which follow the contour of the Earth, or reflect off the [[ionosphere]] ([[skywave]]s).<ref name="Seybold">{{cite book | last1  = Seybold | first1 = John S. | title  = Introduction to RF Propagation | publisher = John Wiley and Sons | date = 2005 | pages = 55–58 | url = https://books.google.com/books?id=4LtmjGNwOPIC&q=cross+polarization+discrimination&pg=PA57 | isbn = 978-0471743682 }}</ref>  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]] are required.  Therefore, on the surface of the Earth, microwave communication links are limited by the visual horizon to about {{convert|30|-|40|miles}}. Microwaves are absorbed by moisture in the atmosphere, and the attenuation increases with frequency, becoming a significant factor ([[rain fade]]) at the high end of the band.   Beginning at about 40&nbsp;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&nbsp;GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so effective that it is in effect [[Opacity (optics)|opaque]], until the atmosphere becomes transparent again in the so-called [[infrared]] and [[optical window]] frequency ranges.

=== Troposcatter ===
{{main|Tropospheric scatter}}
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]].<ref name="Seybold" />  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&nbsp;GHz in [[tropospheric scatter]] (troposcatter) communication systems to communicate beyond the horizon, at distances up to 300&nbsp;km.

== Antennas ==
[[File:Diplexer1.jpg|thumb|upright|Waveguide is used to carry microwaves. Example of [[waveguide]]s and a [[diplexer]] in an [[air traffic control]] radar.]]

The short [[wavelength]]s of microwaves allow [[omnidirectional antenna]]s for portable devices to be made very small, from 1 to 20 centimeters long, so microwave frequencies are widely used for [[wireless device]]s such as [[cell phone]]s, [[cordless phone]]s, and [[wireless LAN]]s (Wi-Fi) access for [[laptop]]s, and [[Bluetooth]] earphones.  Antennas used include short [[whip antenna]]s, [[rubber ducky antenna]]s, sleeve [[dipole antenna|dipole]]s, [[patch antenna]]s, and increasingly the printed circuit [[inverted F antenna]] (PIFA) used in cell phones.

Their short [[wavelength]] also allows narrow beams of microwaves to be produced by conveniently small [[antenna gain|high gain]] [[antenna (radio)|antenna]]s from a half meter to 5 meters in diameter.  Therefore, beams of microwaves are used for [[point-to-point (telecommunications)|point-to-point]] communication links, and for [[radar]].  An advantage of narrow beams is that they do not interfere with nearby equipment using the same frequency, allowing [[frequency reuse]] by nearby transmitters.  [[Parabolic antenna|Parabolic ("dish") antennas]] are the most widely used directive antennas at microwave frequencies, but [[horn antenna]]s, [[slot antenna]]s and [[lens antenna]]s are also used.  Flat [[microstrip antenna]]s are being increasingly used in consumer devices.   Another directive antenna practical at microwave frequencies is the [[phased array]], a computer-controlled array of antennas that produces a beam that can be electronically steered in different directions.

At microwave frequencies, the [[transmission line]]s which are used to carry lower frequency radio waves to and from antennas, such as [[coaxial cable]] and [[twin lead|parallel wire lines]], have excessive power losses, so when low attenuation is required, microwaves are carried by metal pipes called [[waveguide (electromagnetism)|waveguide]]s.  Due to the high cost and maintenance requirements of waveguide runs, in many microwave antennas the output stage of the [[transmitter]] or the [[RF front end]] of the [[radio receiver|receiver]] is located at the antenna.

== Design and analysis ==
The term ''microwave'' also has a more technical meaning in [[electromagnetics]] and [[circuit theory]].<ref name="Golio1">{{cite book
 | last1  = Golio
 | first1 = Mike
 | last2  = Golio
 | first2 = Janet
 | title  = RF and Microwave Passive and Active Technologies
 | publisher = CRC Press
 | date   = 2007
 | pages  = I.2–I.4
 | url    = https://books.google.com/books?id=MCj9jxSVQKIC&q=lumped-element+distributed-element&pg=PR13
 | isbn   = 978-1420006728
 }}</ref><ref name="Karmel">{{cite book
 | last1  = Karmel
 | first1 = Paul R.
 | last2  = Colef
 | first2 = Gabriel D.
 | last3  = Camisa
 | first3 = Raymond L.
 | title  = Introduction to Electromagnetic and Microwave Engineering
 | publisher = John Wiley and Sons
 | date   = 1998
 | pages  = 1
 | url    = https://books.google.com/books?id=iruLnH941OEC&pg=PA1
 | doi    = 
 | id     = 
 | isbn   =  9780471177814
 }}</ref>  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 model|lumped-element circuit theory]] is inaccurate, and instead [[Distributed-element model|distributed circuit element]]s and transmission-line theory are more useful methods for design and analysis.

As a consequence, practical microwave circuits tend not to use the discrete [[resistor]]s, [[capacitor]]s, and [[inductor]]s used with lower-frequency [[radio waves]].  Open-wire and coaxial [[transmission line]]s used at lower frequencies are replaced by [[waveguide]]s and [[stripline]], and lumped-element tuned circuits are replaced by cavity [[resonator]]s or [[resonant stub]]s.<ref name="Golio1" />  In turn, at even higher frequencies, where the wavelength of the electromagnetic waves becomes small in comparison to the size of the structures used to process them, microwave techniques become inadequate, and the methods of [[optics]] are used.

== Sources ==
{{multiple image
 |direction = horizontal
 |align= right
 |width1= 225
 |width2= 100
 |image1=Magnetron section transverse to axis.JPG
 |image2=Antennenw1.jpg
 |footer=Cutaway view inside a [[cavity magnetron]] as used in a [[microwave oven]] ''(left)''. Antenna splitter: [[microstrip]] techniques become increasingly necessary at higher frequencies ''(right)''.
}}
[[File:Radar speed gun internal works.jpg|thumb|upright=1.2|Disassembled [[radar speed gun]]. The grey assembly attached to the end of the copper-colored [[horn antenna]] is the [[Gunn diode]] which generates the microwaves.]]

High-power microwave sources use specialized [[vacuum tube]]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]] (used in [[microwave oven]]s), [[klystron]], [[traveling-wave tube]] (TWT), [[crossed-field amplifier]], and [[gyrotron]]. These devices work in the [[density]] modulated mode, rather than the [[Electric current|current]] modulated mode.  This means that they work on the basis of clumps of electrons flying ballistically through them, rather than using a continuous stream of electrons.<ref name="asg">{{cite book |last1=Gilmour |first1=A.S. |title=Klystrons, Traveling Wave Tubes, Magnetrons, Crossed-Field Amplifiers, and Gyrotrons |date=2011 |publisher=Artech House |location=Boston |isbn=9781608071845 |pages=3–7}}</ref>

Low-power microwave sources use solid-state devices such as the [[field-effect transistor]] (at least at lower frequencies), [[tunnel diode]]s, [[Gunn diode]]s, and [[IMPATT diode]]s.<ref>[http://www.herley.com/index.cfm?act=app_notes&notes=oscillators Microwave Oscillator] {{Webarchive|url=https://web.archive.org/web/20131030115909/http://www.herley.com/index.cfm?act=app_notes&notes=oscillators |date=2013-10-30 }} notes by [[Herley Industries|Herley General Microwave]]</ref> Low-power sources are available as benchtop instruments, rackmount instruments, embeddable modules and in card-level formats.  A [[maser]] is a solid-state device that amplifies microwaves using similar principles to the [[laser]], which amplifies higher-frequency light waves.

All warm objects emit low level microwave [[black-body radiation]], depending on their [[temperature]], so in meteorology and [[remote sensing]], [[microwave radiometer]]s are used to measure the temperature of objects or terrain.<ref name="Sisodia">{{cite book |last=Sisodia |first=M. L. |title=Microwaves : Introduction To Circuits, Devices And Antennas |publisher=New Age International |date=2007 |pages=1.4–1.7 |url=https://books.google.com/books?id=iEvgmwH1esgC&q=microwaves&pg=SA1-PA4 |isbn=978-8122413380}}</ref>  The sun<ref name="Liou">{{cite book |last=Liou |first=Kuo-Nan |title=An introduction to atmospheric radiation |url=https://books.google.com/books?id=6xUpdPOPLckC&q=microwaves%20from%20Sun&pg=PR13 |page=2 |year=2002 |publisher=Academic Press |isbn=978-0-12-451451-5 |access-date=12 July 2010}}</ref> and other astronomical radio sources such as [[Cassiopeia A]] emit low level microwave radiation which carries information about their makeup, which is studied by [[radio astronomer]]s using receivers called  [[radio telescope]]s.<ref name="Sisodia" />  The [[cosmic microwave background radiation]] (CMBR), for example, is a weak microwave noise filling empty space which is a major source of information on [[Physical cosmology|cosmology]]'s [[Big Bang]] theory of the origin of the [[Universe]].

== Applications ==

Microwave technology is extensively used for [[Point-to-point (telecommunications)|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]]; their comparatively higher frequencies allow broad [[Bandwidth (signal processing)|bandwidth]] and high [[data transmission rate]]s, 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]]s. Microwaves are also employed in [[microwave oven]]s and in [[radar]] technology.

=== Communication ===
{{main|Point-to-point (telecommunications)|Microwave transmission|Satellite communications}}
[[File:SuperDISH121.jpg|thumb|A [[satellite dish]] on a residence, which receives [[satellite television]] over a [[Ku band|K<sub>u</sub> band]] 12–14&nbsp;GHz microwave beam from a direct broadcast [[communications satellite]] in a [[geostationary orbit]] 35,700 kilometres (22,000 miles) above the Earth]]

Before the advent of [[fiber-optic]] transmission, most [[long-distance call|long-distance]] [[telephone call]]s were carried via networks of [[microwave radio relay]] links run by carriers such as [[AT&T Long Lines]].  Starting in the early 1950s, [[frequency-division multiplexing]] 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&nbsp;km away.

[[Wireless LAN]] [[Protocol (computing)|protocol]]s, such as [[Bluetooth]] and the [[IEEE Standards Association|IEEE]] [[802.11]] specifications used for Wi-Fi, also use microwaves in the 2.4&nbsp;GHz [[ISM band]], although [[802.11a]] uses [[ISM band]] and [[U-NII]] frequencies in the 5&nbsp;GHz range.  Licensed long-range (up to about 25&nbsp;km) Wireless Internet Access services have been used for almost a decade in many countries in the 3.5–4.0&nbsp;GHz range.  The FCC recently{{when |date=August 2011}} carved out spectrum for carriers that wish to offer services in this range in the U.S. — with emphasis on 3.65&nbsp;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&nbsp;GHz band will give business customers another option for connectivity.

[[Metropolitan area network]] (MAN) protocols, such as [[WiMAX]] (Worldwide Interoperability for Microwave Access) are based on standards such as [[IEEE 802.16]], designed to operate between 2 and 11&nbsp;GHz. Commercial implementations are in the 2.3&nbsp;GHz, 2.5&nbsp;GHz, 3.5&nbsp;GHz and 5.8&nbsp;GHz ranges.

[[Mobile Broadband]] Wireless Access (MBWA) protocols based on standards specifications such as [[IEEE 802.20]] or ATIS/ANSI [[HC-SDMA]] (such as [[iBurst]]) operate between 1.6 and 2.3&nbsp;GHz to give mobility and in-building penetration characteristics similar to mobile phones but with vastly greater spectral efficiency.<ref>{{cite web |title= IEEE 802.20: Mobile Broadband Wireless Access (MBWA) |work= Official web site |url= https://grouper.ieee.org/groups/802/20/ |access-date= August 20, 2011 }}</ref>

Some [[mobile phone]] networks, like [[GSM frequency bands|GSM]], use the low-microwave/high-UHF frequencies around 1.8 and 1.9&nbsp;GHz in the Americas and elsewhere, respectively.  [[DVB-SH]] and [[S-DMB]] use 1.452 to 1.492&nbsp;GHz, while proprietary/incompatible [[satellite radio]] in the U.S. uses around 2.3&nbsp;GHz for [[Digital Audio Radio Service|DARS]].

Microwave radio is used in [[point-to-point (telecommunications)|point-to-point]] [[telecommunications]] transmissions because, due to their short wavelength, highly [[directional antenna]]s are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies).  There is also more [[bandwidth (signal processing)|bandwidth]] in the microwave spectrum than in the rest of the radio spectrum; the usable bandwidth below 300&nbsp;MHz is less than 300&nbsp;MHz while many GHz can be used above 300&nbsp;MHz. Typically, microwaves are used in [[remote broadcasting]] of news or sports events as the [[backhaul (broadcasting)|backhaul]] link to transmit a signal from a remote location to a television station from a specially equipped van.  See [[broadcast auxiliary service]] (BAS), [[remote pickup unit]] (RPU), and [[studio/transmitter link]] (STL).

Most [[satellite communications]] systems operate in the C, X, K<sub>a</sub>, or K<sub>u</sub> bands of the microwave spectrum. These frequencies allow large bandwidth while avoiding the crowded UHF frequencies and staying below the atmospheric absorption of EHF frequencies.  [[Satellite TV]] either operates in the C band for the traditional [[TVRO|large dish]] [[fixed satellite service]] or K<sub>u</sub> band for [[direct-broadcast satellite]].  Military communications run primarily over X or K<sub>u</sub>-band links, with K<sub>a</sub> band being used for [[Milstar]].

=== Navigation ===

{{further|Satellite navigation|Navigation|}}

[[Global Navigation Satellite System]]s (GNSS) including the Chinese [[Beidou navigation system|Beidou]], the American [[Global Positioning System]] (introduced in 1978) and the Russian [[GLONASS]] broadcast navigational signals in various bands between about 1.2&nbsp;GHz and 1.6&nbsp;GHz.

=== Radar ===
{{main|Radar}}
[[File:ASR-9 Radar Antenna.jpg|thumb|The [[parabolic antenna]] (lower curved surface) of an ASR-9 [[airport surveillance radar]] which radiates a narrow vertical fan-shaped beam of 2.7–2.9&nbsp;GHz ([[S band]]) microwaves to locate aircraft in the airspace surrounding an airport]]

[[Radar]] is a [[radiolocation]] 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]]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]], weather forecasting, navigation of ships, and [[speed limit enforcement]].  Long-distance radars use the lower microwave frequencies since at the upper end of the band atmospheric absorption limits the range, but [[millimeter wave]]s are used for short-range radar such as [[collision avoidance system]]s.

{{multiple image
 |direction = vertical
 |align = left
 |width = 270
 |image1=The Atacama Compact Array.jpg
 |caption1= Some of the dish antennas of the [[Atacama Large Millimeter Array]] (ALMA) a radio telescope located in northern Chile. It receives microwaves in the [[millimeter wave]] range, 31 – 1000&nbsp;GHz.
 |image2=BigBangNoise.jpg
 |caption2=Maps of the [[cosmic microwave background radiation]] (CMBR), showing the improved resolution which has been achieved with better microwave radio telescopes}}

=== Radio astronomy ===
{{main|radio astronomy}}

Microwaves emitted by [[astronomical radio source]]s; planets, stars, [[galaxy|galaxies]], and [[nebula]]s are studied in [[radio astronomy]] with large dish antennas called [[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]] or map the invisible surface of [[Venus]] through cloud cover.

A recently{{When|date=July 2024}} completed microwave radio telescope is the [[Atacama Large Millimeter Array]], located at more than 5,000 meters (16,597&nbsp;ft) altitude in Chile, which observes the [[universe]] in the [[Terahertz radiation|millimeter and submillimeter]] wavelength ranges. The world's largest ground-based astronomy project to date, it consists of more than 66 dishes and was built in an international collaboration by Europe, North America, East Asia and Chile.<ref>{{cite web|url=http://www.almaobservatory.org/en | title = ALMA website | access-date = 2011-09-21}}</ref><ref>{{cite web|url=http://www.eso.org/sci/facilities/alma/ | title = Welcome to ALMA! | access-date = 2011-05-25}}</ref>

A major recent{{When|date=July 2024}} focus of microwave radio astronomy has been mapping the [[cosmic microwave background radiation]] (CMBR) discovered in 1964 by radio astronomers [[Arno Penzias]] and [[Robert Woodrow Wilson|Robert Wilson]].  This faint background radiation, which fills the universe and is almost the same in all directions, is "relic radiation" from the [[Big Bang]], and is one of the few sources of information about conditions in the early universe.  Due to the expansion and thus cooling of the Universe, the originally high-energy radiation has been shifted into the microwave region of the radio spectrum. Sufficiently sensitive [[radio telescope]]s can detect the CMBR as a faint signal that is not associated with any star, galaxy, or other object.<ref name="Wright">
{{cite book|last=Wright|first=E.L.|date=2004|chapter=Theoretical Overview of Cosmic Microwave Background Anisotropy|editor=W. L. Freedman|title=Measuring and Modeling the Universe|series=Carnegie Observatories Astrophysics Series|publisher=[[Cambridge University Press]]|page=291|isbn=978-0-521-75576-4|arxiv=astro-ph/0305591 |bibcode=2004mmu..symp..291W}}</ref>

=== Heating and power application ===
[[File:Electrodomésticos de línea blanca 18.JPG|thumb|Small [[microwave oven]] on a kitchen counter]]
[[File:Microwave tunnel closeup.jpg|thumb|Microwaves are widely used for heating in industrial processes. A microwave tunnel oven for softening plastic rods prior to extrusion.]]
A [[microwave oven]] passes microwave radiation at a frequency near [[ISM band|{{convert|2.45|GHz|cm|abbr=on|sigfig=2}}]] through food, causing [[dielectric heating]] primarily by absorption of the energy in water. Microwave ovens became common kitchen appliances in Western countries in the late 1970s, following the development of less expensive [[cavity magnetron]]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&nbsp;GHz, almost ten times the frequency of the microwave oven.

Microwave heating is used in industrial processes for drying and [[curing (chemistry)|curing]] products.

Many [[Fabrication (semiconductor)|semiconductor processing]] techniques use microwaves to generate [[plasma physics|plasma]] for such purposes as [[reactive ion etching]] and plasma-enhanced [[chemical vapor deposition]] (PECVD).

Microwaves are used in [[stellarator]]s and [[Tokamak#Radio-frequency heating|tokamak]] 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 [[Electron cyclotron resonance|cyclotron resonance]] of the electrons in the magnetic field, anywhere between 2–200&nbsp;GHz, hence it is often referred to as Electron Cyclotron Resonance Heating (ECRH). The upcoming [[ITER]] thermonuclear reactor<ref>{{cite web |url=http://www.iter.org/default.aspx |title=The way to new energy |publisher=ITER |date=2011-11-04 |access-date=2011-11-08}}</ref> will use up to 20&nbsp;MW of 170&nbsp;GHz microwaves.

Microwaves can be used to [[microwave power transmission|transmit power]] over long distances, and post-[[World War 2]] research was done to examine possibilities.  [[NASA]] worked in the 1970s and early 1980s to research the possibilities of using [[solar power satellite]] (SPS) systems with large [[Photovoltaic module|solar array]]s that would beam power down to the Earth's surface via microwaves.

[[Less-than-lethal]] weaponry exists that uses millimeter waves to heat a thin layer of human skin to an intolerable temperature so as to make the targeted person move away. A two-second burst of the 95&nbsp;GHz focused beam heats the skin to a temperature of {{convert|54|C|F}} at a depth of {{convert|0.4|mm|in|frac=64}}. The [[United States Air Force]] and [[United States Marine Corps|Marines]] are currently using this type of [[active denial system]] in fixed installations<!-- can someone confirm this? -->.<ref>[https://web.archive.org/web/20070128014922/http://www.raytheon.com/products/stellent/groups/public/documents/content/cms04_017939.pdf Silent Guardian Protection System. Less-than-Lethal Directed Energy Protection]. raytheon.com</ref>

=== Spectroscopy ===

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

== Frequency measurement ==

[[File:Ondamtr.JPG|thumb|upright=0.75|[[Absorption wavemeter]] for measuring in the K<sub>u</sub> band]]
Microwave frequency can be measured by either electronic or mechanical techniques.

[[Frequency counter]]s 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]] 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 {{anchor|Health effects}} ==
{{further|Electromagnetic radiation and health|Microwave burn}}

Microwaves are [[non-ionizing]] radiation, which means that microwave [[photon]]s do not contain sufficient energy to [[ionize]] molecules or break chemical bonds, or cause DNA damage, as ionizing radiation such as [[x-ray]]s or [[ultraviolet]] can.<ref name="HyperPhysics Radiation Interaction">{{cite web |last1=Nave |first1=Rod |title=Interaction of Radiation with Matter |url=http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html |website=HyperPhysics |access-date=20 October 2014}}</ref> 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 [[carcinogen]]ic effect.<ref>{{cite journal |last=Goldsmith |first=JR |title=Epidemiologic evidence relevant to radar (microwave) effects |journal=Environmental Health Perspectives |volume=105 |issue=Suppl. 6 |pages=1579–1587 |date=December 1997 |pmid=9467086 |doi=10.2307/3433674 |pmc=1469943 |jstor=3433674}}</ref>

During [[World War II]], it was observed that individuals in the radiation path of radar installations experienced clicks and buzzing sounds in response to microwave radiation.  Research by [[NASA]] in the 1970s has shown this to be caused by thermal expansion in parts of the inner ear. In 1955, Dr. [[James Lovelock]] was able to reanimate rats chilled to {{convert|0|and|1|C}} using microwave diathermy.<ref>{{cite journal |pages=541–546 |journal=The Journal of Physiology |date=1955 |first1=R.K. |last1=Andjus |first2=J.E. |last2=Lovelock |volume=128 |issue=3 |title=Reanimation of rats from body temperatures between 0 and 1 °C by microwave diathermy |pmc=1365902 |pmid=13243347 |doi=10.1113/jphysiol.1955.sp005323}}</ref>

When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in the body. The lens and [[cornea]] of the eye are especially vulnerable because they contain no [[blood vessel]]s that can carry away heat.  Exposure to microwave radiation can produce [[cataract]]s by this mechanism, because the microwave heating [[denaturation (biochemistry)|denature]]s [[protein]]s in the [[Lens (anatomy)|crystalline lens]] of the [[Human eye|eye]]<ref>{{cite journal|title=Cataracts Induced by Microwave and Ionizing Radiation|last1=Lipman|first1=Richard M.|last2=Tripathi|first2=Brenda J.|last3=Tripathi|first3=Ramesh C.|journal=[[Survey of Ophthalmology]]|date=November–December 1988|volume=33|issue=3|pages=206–207|pmid=3068822|doi=10.1016/0039-6257(88)90088-4}}</ref> (in the same way that heat turns [[egg white]]s white and opaque).  Exposure to heavy doses of microwave radiation (as from an oven that has been tampered with to allow operation even with the door open) can produce heat damage in other tissues as well, up to and including serious [[burn]]s that may not be immediately evident because of the tendency for microwaves to heat deeper tissues with higher moisture content.

== History ==
The motivation for exploiting the microwave frequencies was the increasing congestion in the lower frequency bands, and the ability to use smaller antennas at higher frequencies<ref name="Bryant">{{cite journal
  | last = Bryant
  | first = John H.
  | title = The first century of microwaves - 1896 to 1996
  | journal = IEEE Transactions on Microwave Theory and Techniques
  | volume = 36
  | issue = 5
  | pages = 830–858
  | publisher = Institute of Electrical and Electronic Engineers
  | date = May 1988
  | language = 
  | url = https://cc.ee.ntu.edu.tw/~rbwu/rapid_content/course/AP/Hertz88.pdf
  | jstor = 
  | issn = 
  | doi = 10.1109/22.3602
  | bibcode = 1988ITMTT..36..830B
 | id = 
  | mr = 
  | zbl = 
  | jfm = 
  | access-date = 8 June 2025}}</ref>{{rp|p=830}}

===Hertzian optics===
Microwaves were first generated in the 1890s in some of the earliest [[radio wave]] experiments by physicists who thought of them as a form of "invisible light".<ref name="Hong1">{{cite book
 | last1  = Hong
 | first1 = Sungook
 | title  = Wireless: From Marconi's Black-box to the Audion
 | publisher = MIT Press
 | date   = 2001
 | pages  = 5–9, 22
 | isbn   = 978-0262082983
 }}</ref> [[James Clerk Maxwell]] in his 1873 theory of [[electromagnetism]], now called [[Maxwell's equations]], had predicted that a coupled [[electric field]] and [[magnetic field]] could travel through space as an [[electromagnetic wave]], and proposed that light consisted of electromagnetic waves of short wavelength.<ref name="Bryant" />{{rp|p=832}}  In 1888, physicist [[Heinrich Hertz]] was the first to demonstrate the existence of electromagnetic waves, generating [[radio wave]]s using a primitive [[spark gap transmitter|spark gap radio transmitter]].<ref name="Bryant" />{{rp|p=830}}<ref name="Roer">{{cite book
 | last  = Roer
 | first = T.G.
 | title  = Microwave Electronic Devices
 | publisher = Springer Science and Business Media
 | date   = 2012
 | pages  = 1–12
 | url    = https://books.google.com/books?id=deDvBwAAQBAJ&pg=PA1
 | isbn   = 978-1461525004
 }}</ref> 

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]] experiments in their laboratories, using [[quasioptics|quasioptical]] components such as [[Prism (optics)|prism]]s and [[lens (optics)|lens]]es made of [[paraffin wax|paraffin]], [[sulfur]] and [[pitch (resin)|pitch]] and wire [[diffraction grating]]s, to refract and diffract radio waves like light rays.<ref name="Sarkar1">{{cite book
 | last1  = Sarkar
 | first1 = T. K.
 | last2  = Mailloux
 | first2 = Robert
 | last3  = Oliner
 | first3 = Arthur A.
 | title  = History of Wireless
 | publisher = John Wiley and Sons
 | date   = 2006
 | pages  = 474–486
 | url    = https://archive.org/stream/HistoryOfWireless#page/n496/mode/2up
 | isbn   = 978-0471783015
 | author-link1=Tapan Sarkar
 | author-link3=Arthur A. Oliner
 }}</ref> Hertz used frequencies at the threshold of the microwave region: 50, 100, and 430&nbsp;MHz.<ref name="Bryant" />{{rp|p=831}} His directional 430&nbsp;MHz transmitter consisted of a 26&nbsp;cm brass rod [[dipole antenna]] with a spark gap between the ends, suspended at the focal line of a [[parabolic antenna]] made of a curved zinc sheet, powered by high voltage pulses from an [[induction coil]].<ref name="Bryant" />{{rp|p=845}}<ref name="Roer" /> His historic experiments demonstrated that radio waves like light exhibited [[refraction]], [[diffraction]], [[polarization (waves)|polarization]], [[interference (wave motion)|interference]] and [[standing wave]]s,<ref name="Bryant" />{{rp|p=845-846}}<ref name="Sarkar1" /> proving that radio waves and light waves were both forms of Maxwell's [[electromagnetic wave]]s.  He also experimented with open wire and coaxial transmission lines.<ref name="Bryant" />{{rp|p=841}}

<gallery mode="packed" heights="150">
File:Drawing of Heinrich Hertz spark radio transmitter and parabolic antenna 1888.jpg|[[Heinrich Hertz]]'s 430&nbsp;MHz spark transmitter, 1888, consisting of 23&nbsp;cm dipole and spark gap at the focus of a parabolic reflector
File: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]]s; his spark oscillator ''(in box, right)'' generated 60&nbsp;GHz (5&nbsp;mm) waves using 3&nbsp;mm metal ball resonators.
File:Bose 60 GHz microwave spark oscillator 1894.png|3&nbsp;mm spark ball oscillator Bose used to generate 60&nbsp;GHz waves
File:Refraction of Hertzian waves by paraffin prism.png|Microwave spectroscopy experiment by [[John Ambrose Fleming]] in 1897 showing refraction of 1.4&nbsp;GHz microwaves by paraffin prism, duplicating earlier experiments by Bose and Righi
File:Oscillatore di Righi con riflettore parabolico - Museo scienza tecnologia Milano 08757 1.jpg|[[Augusto Righi]]'s 12&nbsp;GHz spark oscillator and receiver, 1895
File:Lodge spark oscillator ball 1894.jpg|Oliver Lodge's 5 inch oscillator ball he used to generate 1.2&nbsp;GHz microwaves in 1894
File:Marconi parabolic xmtr and rcvr 1895.jpg|1.2&nbsp;GHz microwave spark transmitter ''(left)'' and [[coherer]] receiver ''(right)'' used by [[Guglielmo Marconi]] during his 1895 experiments had a range of {{convert|6.5|km|mi|abbr=on|sigfig=2}}
</gallery>
Beginning in 1894 [[Jagadish Chandra Bose]] performed the first experiments with microwaves. He was the first person to produce [[millimeter wave]]s, generating frequencies up to 60&nbsp;GHz (5&nbsp;millimeter) using a 3&nbsp;mm metal ball spark oscillator.<ref name="Emerson">{{cite web |url=http://www.tuc.nrao.edu/~demerson/bose/bose.html |title=The work of Jagdish Chandra Bose: 100 years of MM-wave research |publisher=National Radio Astronomy Observatory |date=February 1998 |author=Emerson, D.T.}}</ref><ref name="Sarkar1" /> Bose also invented [[waveguide (electromagnetism)|waveguide]], [[horn antenna]]s, and [[semiconductor]] [[crystal detector]]s for use in his experiments.  Independently in 1894, [[Oliver Lodge]] and [[Augusto Righi]] experimented with 1.5 and 12&nbsp;GHz microwaves respectively, generated by small metal ball spark resonators.<ref name="Sarkar1" /> Russian physicist [[Pyotr Lebedev]] in 1895 generated 50&nbsp;GHz millimeter waves.<ref name="Sarkar1" /> In 1897 [[Lord Rayleigh]] solved the mathematical [[boundary-value problem]] of electromagnetic waves propagating through conducting tubes and dielectric rods of arbitrary shape<ref name="Packard" /><ref name="Rayleigh">{{cite journal
  | last1  = Strutt
  | first1 = William (Lord Rayleigh)
  | title =  On the passage of electric waves through tubes, or the vibrations of dielectric cylinders
  | journal = Philosophical Magazine
  | volume = 43
  | issue = 261
  | pages = 125–132
  | date = February 1897
  | doi = 10.1080/14786449708620969
  | url = https://zenodo.org/record/1431225
  }}</ref><ref name="Kizer">{{cite book
 | last1  = Kizer
 | first1 = George
 | title  = Digital Microwave Communication: Engineering Point-to-Point Microwave Systems
 | publisher = John Wiley and Sons
 | date   = 2013
 | pages  = 7
 | url    = https://books.google.com/books?id=JVhGmjQ8TyoC&q=southworth+bose+lodge+waveguide
 | isbn   = 978-1118636800
 }}</ref><ref name="Lee3">{{cite book
 | last1  = Lee
 | first1 = Thomas H.
 | title  = Planar Microwave Engineering: A Practical Guide to Theory, Measurement, and Circuits, Vol. 1
 | publisher = Cambridge University Press
 | date   = 2004
 | pages  = 18, 118
 | url    = https://books.google.com/books?id=uoj3IWFxbVYC&pg=PA18
 | isbn   = 978-0521835268
 }}</ref> which gave the modes and [[cutoff frequency]] of microwaves propagating through a [[waveguide (electromagnetism)|waveguide]].<ref name="Roer" />

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]]s and by reflecting off the [[ionosphere]] as [[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]] (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.<ref name="Roer" /> 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]].<ref name="Bryant" />{{rp|p=849-850}}<ref name="Karplus" /><ref name="Roer" /> 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.

<gallery mode="packed" heights="150">
File:English Channel microwave relay antennas 1931.jpg|Antennas of 1931 experimental 1.7&nbsp;GHz microwave relay link across the English Channel
File:Experimental Westinghouse 3GHz microwave transmitter 1933.jpg|Experimental 3.3&nbsp;GHz (9&nbsp;cm) transmitter 1933 using split-anode magnetron, at Westinghouse labs<ref name="Mouromtseff"/>
File:Southworth demonstrating waveguide.jpg|Southworth ''(at left)'' demonstrating waveguide at [[Institute of Radio Engineers|IRE]] meeting in 1938, showing 1.5&nbsp;GHz microwaves passing through the 7.5 m flexible metal hose registering on a diode detector
File:Wilmer Barrow & horn antenna 1938.jpg|The first modern horn antenna in 1938 with inventor [[Wilmer L. Barrow]]
</gallery>
In 1931 an Anglo-French consortium headed by [[Andre C. Clavier]] demonstrated the first experimental [[microwave relay]] link, across the [[English Channel]] {{convert|40|mi|km}} between [[Dover]], UK and [[Calais]], France.<ref name="EC">{{cite magazine
  | title = Microwaves span the English Channel
  | magazine = Short Wave Craft
  | volume = 6
  | issue = 5
  | pages = 262, 310
  | publisher = Popular Book Co
  | location = New York
  | date = September 1935
  | url = https://www.worldradiohistory.com/Archive-Short-Wave-Television/30s/SW-TV-1935-09.pdf
  | access-date = March 24, 2015}}</ref><ref name="Free">{{cite magazine
  | last1  = Free
  | first1 = E.E.
  | title = Searchlight radio with the new 7 inch waves
  | magazine = Radio News
  | volume = 8
  | issue = 2
  | pages = 107–109
  | publisher = Radio Science Publications
  | location = New York
  | date = August 1931
  | url = https://www.worldradiohistory.com/Archive-Radio-News/30s/Radio-News-1931-08-R.pdf
  | access-date = March 24, 2015}}</ref> The system transmitted telephony, telegraph and [[facsimile]] data over bidirectional 1.7&nbsp;GHz beams with a power of one-half watt, produced by miniature [[Barkhausen–Kurz tube]]s at the focus of {{convert|10|ft|m|0|adj=on}} 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<ref name="Karplus">{{cite journal
  | last = Eduard
  | first = Karplus
  | title = Communication with quasi-optical waves
  | journal = Standards Yearbook, 1932
  | pages = 15
  | publisher = Bureau of Standards, US Dept. of Commerce
  | location = Washington D.C.
  | date = 1932
  | language = 
  | url = https://books.google.com/books?id=N-KSwF4ahGUC&pg=PA15
  | jstor = 
  | issn = 
  | doi = 
  | id = 
  | mr = 
  | zbl = 
  | jfm = 
  | access-date = 27 February 2025}}</ref><ref name="Loomis">{{cite book
  | last = Loomis
  | first = Mary Texanna 
  | title = Radio Theory and Operating: For the Radio Student and Practical Operator, 4th Ed.
  | publisher = Loomis Publishing Co.
  | date = 1928
  | location = 
  | pages = 603
  | language = 
  | url = https://books.google.com/books?id=B82nsBBEMCYC&pg=PA310
  | doi = 
  | id = 
  | isbn = 
  | mr = 
  | zbl = 
  | jfm =}}</ref><ref name="Mouromtseff">{{cite journal
  | last = Mouromtseff
  | first = Ilia A.
  | title = 3 1/2 Inch Waves Now Practical
  | journal = Short Wave Craft
  | volume = 4
  | issue = 5
  | pages = 266–267
  | publisher = Popular Book Corp.
  | location = New York
  | date = September 1933
  | language = 
  | url = https://www.worldradiohistory.com/Archive-Short-Wave-Television/30s/SW-TV-1933-09.pdf#search=%22mouromtseff%20ultra-short%20waves%22
  | jstor = 
  | issn = 
  | doi = 
  | id = 
  | mr = 
  | zbl = 
  | jfm = 
  }}</ref> but did not catch on. The first usage of the word ''micro-wave'' occurred in 1931<ref name="Free" /><ref name="Ayto">{{cite book
 | last1  = Ayto
 | first1 = John
 | title  = 20th century words
 | date   = 2002
 | pages  = 269
 | publisher = Foreign Language Teaching and Research Press
 | url    = https://books.google.com/books?id=p0h5AAAAIAAJ&q=%22When+trials+with+wavelengths+as+low+as+18+cm+were+made+known,+there+was+undisguised+surprise+that+the+problem+of+the+micro-wave+had+been+solved+so+soon.%22
 | isbn   = 978-7560028743
 }}</ref> in reporting of the Clavier Anglo-French microwave link.<ref name="Bryant" />{{rp|p=849}}

===Radar development===
The development of [[radar]], mainly in secrecy, before and during [[World War II]], resulted in the technological advances which made microwaves practical.<ref name="Bryant" />{{rp|p=851}}<ref name="Roer" /> Microwave 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]]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 (electromagnetism)|waveguide]] in 1936.<ref name="Packard">{{cite journal
  | last1  = Packard
  | first1 = Karle S.
  | title = The Origin of Waveguides: A Case of Multiple Rediscovery
  | journal = IEEE Transactions on Microwave Theory and Techniques
  | volume = MTT-32
  | issue = 9
  | pages = 961–969
  | date = September 1984
  | url = http://www.ieeeghn.org/wiki/images/8/86/MTT_Waveguide_History.pdf
  | doi = 10.1109/tmtt.1984.1132809
  | access-date = March 24, 2015|bibcode = 1984ITMTT..32..961P | citeseerx = 10.1.1.532.8921
  }}</ref> Barrow invented the [[horn antenna]] in 1938 as a means to efficiently radiate microwaves into or out of a waveguide. In a microwave [[radio receiver|receiver]], 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 radio]]s around the turn of the century before vacuum tube receivers.<ref name="Roer" /><ref name="Riordan">{{Cite book|author1-link=Michael Riordan (physicist) | last = Riordan | first = Michael |author2=Lillian Hoddeson |author2-link=Lillian Hoddeson| title = Crystal fire: the invention of the transistor and the birth of the information age | publisher = W. W. Norton & Company | year = 1988 | location = US | pages = 89–92 | url = https://books.google.com/books?id=SZ6wm5ZSUmsC&pg=PA89 | isbn = 978-0-393-31851-7 }}</ref> The low capacitance of [[semiconductor junction]]s allowed them to function at microwave frequencies. The first modern [[silicon]] and [[germanium]] [[diode]]s 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.<ref name="Roer" />

<gallery mode="packed" heights="150">
File: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.
File:Prototype klystron cutaway.jpg|First commercial klystron tube, by General Electric, 1940, sectioned to show internal construction
File: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
File: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&nbsp;MHz to 4.9&nbsp;GHz which could transmit up to 8 phone calls on a beam
</gallery>

The first powerful sources of microwaves were invented at the beginning of World War II: the [[klystron]] tube by [[Russell and Sigurd Varian]] at [[Stanford University]] in 1937,<ref name="Bryant" />{{rp|p=852-853}} and the [[cavity magnetron]] tube by [[John Randall (physicist)|John Randall]] and [[Harry Boot]] at Birmingham University, UK in 1940.<ref name="Bryant" />{{rp|p=853-855}}<ref name="Roer" /> Ten centimeter (3&nbsp;GHz) microwave radar powered by the magnetron tube was in use on British warplanes in late 1941 and proved to be a game changer.  Britain's 1940 decision to share its microwave technology with its US ally (the [[Tizard Mission]])<ref name="Bryant" />{{rp|p=852}} 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.<ref name="Bryant" />{{rp|p=852}}  Microwave relay systems were developed by the Allied military in the war and used for secure battlefield communication networks in the European theater.  The first was the British Wireless Set No. 10, a 5&nbsp;GHz, 8 telephone line, [[time division multiplex]] system developed by the UK's [[Signals Research and Development Establishment]] in 1942.<ref name="Bryant" />{{rp|p=855}}

===Post World War II exploitation===
After World War II, microwaves were rapidly exploited commercially.<ref name="Roer" /> Due to their high frequency they had a very large information-carrying capacity ([[bandwidth (signal processing)|bandwidth]]); 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 truck]]s back to the studio, allowing the first [[remote broadcast|remote TV broadcasts]]. The first [[communications satellite]]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]] and [[Robert Woodrow Wilson]] while investigating noise in a satellite horn antenna at [[Bell Labs]], Holmdel, New Jersey discovered [[cosmic microwave background radiation]].

{{multiple image
| align             = center
| direction         = horizontal
| header            = 
| image1            = Hogg horn antennas.jpg
| caption1          = C-band [[horn antenna]]s at a telephone switching center in Seattle, belonging to AT&T's Long Lines microwave relay network built in the 1960s
| width1            = 149
| image2            = Charles Townes and first maser.jpg
| caption2          = First [[maser]] and inventor [[Charles Townes]], 1955
| width2            = 194
| image3            = NIKE AJAX Anti-Aircraft Missile Radar3.jpg
| caption3          = Microwave lens antenna used in the radar for the 1954 [[Nike Ajax]] anti-aircraft missile
| width3            = 270
| image4            = NS Savannah microwave oven MD8.jpg
| caption4          = The first commercial microwave oven, Amana's [[Microwave ovens|Radarange]], installed in the kitchen of US merchant ship {{ship|NS|Savannah}} in 1961
| width4            = 119
| image5            = Telstar 1 replica.jpg
| caption5          = [[Telstar 1]] [[communications satellite]] launched July 10, 1962 the first satellite to relay television signals. The ring of [[microwave cavity]] antennas received the 6.39&nbsp;GHz [[uplink]], and transmitted the 4.17&nbsp;GHz [[downlink]] signal.
| width5            = 181
}}

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

The ability of [[short wave]]s to quickly heat materials and cook food had been investigated in the 1930s by Ilia E. Mouromtseff at Westinghouse, and at the [[1933 Chicago World's Fair]] demonstrated cooking meals with a 60&nbsp;MHz radio transmitter.<ref name="SWC">{{cite journal| title = Cooking with Short Waves| journal = Short Wave Craft| volume = 4| issue = 7| page = 394| date = November 1933| url = https://www.worldradiohistory.com/Archive-Short-Wave-Television/30s/SW-TV-1933-11.pdf| access-date = 23 March 2015}}</ref> In 1945 [[Percy Spencer]], an engineer working on radar at [[Raytheon]], noticed that microwave radiation from a magnetron oscillator melted a candy bar in his pocket. He investigated cooking with microwaves and invented the [[microwave oven]], consisting of a magnetron feeding microwaves into a closed metal cavity containing food, which was patented by Raytheon on 8 October 1945. 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&nbsp;GHz and became the most widely used microwave tube (besides the ubiquitous magnetron used in microwave ovens). The [[gyrotron]] tube family developed in Russia could produce megawatts of power up into [[millimeter wave]] frequencies and is used in industrial heating and [[plasma (physics)|plasma]] research, and to power [[particle accelerator]]s and nuclear [[fusion reactor]]s.

===Solid state microwave devices===
{{multiple image
| align             = right
| direction         = horizontal
| image1            = Atomic Clock-Louis Essen.jpg
| caption1          = First caesium atomic clock, and inventor Louis Essen ''(left)'', 1955
| width1            = 205
| image2            = Ruby maser amplifier 1961.jpg
| caption2          = Experimental ruby maser ''(lower end of rod)'', 1961
| width2            = 80
| image3            = Ganna gjenerators M31102-1.jpg
| caption3          = Microwave oscillator consisting of a [[Gunn diode]] inside a [[cavity resonator]], 1970s
| width3            = 120
| image4            = Radar Gun Electronics.jpg
| caption4          = Modern [[radar speed gun]].  At the right end of the copper [[horn antenna]] is the [[Gunn diode]] ''(grey assembly)'' which generates the microwaves.
| width4            = 180
}}

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).<ref name="Roer" /> 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 [[diode]]s worked better.

The [[tunnel diode]] invented in 1957 by Japanese physicist [[Leo Esaki]] could produce a few milliwatts of microwave power. Its invention set off a search for better negative resistance semiconductor devices for use as microwave oscillators, resulting in the invention of the [[IMPATT diode]] in 1956 by [[W.T. Read]] and Ralph L. Johnston and the [[Gunn diode]] in 1962 by [[J. B. Gunn]].<ref name="Roer" /> Diodes are the most widely used microwave sources today.

Two low-noise [[Solid-state electronics|solid state]] negative resistance microwave [[amplifier]]s were developed; the [[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.<ref name="Roer" /> The parametric amplifier and the [[maser|ruby maser]], invented in 1958 by a team at [[Bell Labs]] headed by [[H.E.D. Scovil]] were used for low noise microwave receivers in radio telescopes and [[satellite ground station]]s.  The maser led to the development of [[atomic clock]]s, 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 [[circulator]]s, [[isolator (microwave)|isolator]]s, and [[directional coupler]]s. In 1969 Kaneyuki Kurokawa derived mathematical conditions for stability in negative resistance circuits which formed the basis of microwave oscillator design.<ref name="Kurokawa">{{cite journal
  | last = Kurokawa
  | first = Kaneyuki
  | title = Some Basic Characteristics of Broadband Negative Resistance Oscillator Circuits
  | journal = Bell System Tech. J.
  | volume = 48
  | issue = 6
  | pages = 1937–1955
  | date = July 1969
  | url = https://archive.org/details/bstj48-6-1937
  | doi = 10.1002/j.1538-7305.1969.tb01158.x
  | bibcode = 1969BSTJ...48.1937K
 | access-date = December 8, 2012}}</ref>

===Microwave integrated circuits===
[[File:LNB dissassembled.JPG|thumb|upright=0.7|[[ku band|k<sub>u</sub> band]] [[microstrip]] circuit used in [[satellite television]] dish]]
Prior to the 1980s microwave devices and circuits were bulky and expensive, so microwave frequencies were generally limited to the output stage of transmitters and the [[RF front end]] 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 [[smartphone]]s, [[Wi-Fi]], and [[Bluetooth]] which connect to networks using microwaves.

[[Microstrip]], a type of [[transmission line]] usable at microwave frequencies, was invented with [[printed circuit]]s in the 1950s.<ref name="Roer" /> The ability to cheaply fabricate a wide range of shapes on [[printed circuit board]]s allowed microstrip versions of [[capacitor]]s, [[inductor]]s, [[Stub (electronics)|resonant stubs]], [[Power dividers and directional couplers|splitters]], [[directional coupler]]s, [[diplexer]]s, [[electronic filter|filters]] and antennas to be made, thus allowing compact microwave circuits to be constructed.<ref name="Roer" />

[[Transistor]]s that operated at microwave frequencies were developed in the 1970s.  The semiconductor [[gallium arsenide]] (GaAs) has a much higher [[electron mobility]] than silicon,<ref name="Roer" /> 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,<ref name="Roer" /> and it has dominated microwave semiconductors ever since. MESFETs ([[metal-semiconductor field-effect transistor]]s), fast GaAs [[field effect transistor]]s using [[Schottky diode|Schottky junctions]] for the gate, were developed starting in 1968 and have reached cutoff frequencies of 100&nbsp;GHz, and are now the most widely used active microwave devices.<ref name="Roer" /> 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]]).<ref name="Roer" />

GaAs can be made semi-insulating, allowing it to be used as a [[wafer (electronics)|substrate]] on which circuits containing [[passive component]]s, as well as transistors, can be fabricated by lithography.<ref name="Roer" /> By 1976 this led to the first [[integrated circuit]]s (ICs) which functioned at microwave frequencies, called [[monolithic microwave integrated circuit]]s (MMIC).<ref name="Roer" /> 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 [[amplifier]]s, [[modem]]s, and [[microprocessor]]s.

== See also ==
{{portal|Electronics|Telecommunication}}
{{Div col|colwidth=25em}}
* [[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)]]
{{Div col end}}

== References ==
{{reflist|35em}}

== External links ==
{{Commons category|Microwaves (radio)}}
* [http://www.emtalk.com EM Talk, Microwave Engineering Tutorials and Tools]
* [http://miwv.com/millimeter-wave-resources/waveguide-dimensions Millimeter Wave] {{Webarchive|url=https://web.archive.org/web/20130609213255/http://miwv.com/millimeter-wave-resources/waveguide-dimensions |date=2013-06-09 }} and Microwave Waveguide dimension chart.

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