Editing Microwave (section)

== 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]].