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Microwave is a form of
electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
with
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
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. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is e ...
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 designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (on ...
and EHF ( millimeter wave) bands. A more common definition in
radio-frequency engineering Radio-frequency (RF) engineering is a subset of electronic engineering involving the application of transmission line, waveguide, antenna and electromagnetic field principles to the design and application of devices that produce or use sign ...
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 which is placed before the stem of a word. Adding it to the beginning of one word changes it into another word. For example, when the prefix ''un-'' is added to the word ''happy'', it creates the word ''unhappy''. Particul ...
' in ''microwave'' is not meant to suggest a wavelength in the
micrometer Micrometer can mean: * Micrometer (device), used for accurate measurements by means of a calibrated screw * American spelling of micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...
range. Rather, it indicates that microwaves are "small" (having shorter wavelengths), compared to the
radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (sho ...
s used prior to microwave technology. The boundaries between far infrared,
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 waves within the ITU-designated band of fr ...
, microwaves, and
ultra-high-frequency Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300  megahertz (MHz) and 3  gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter ...
radio Radio is the technology of signaling and 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 called a tr ...
wave In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (re ...
s are fairly arbitrary and are used variously between different fields of study. Microwaves travel by line-of-sight; unlike lower frequency
radio waves Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (s ...
they do not diffract around hills, follow the earth's surface as ground waves, or reflect from the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...
, 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 that uses wireless data connections between network nodes. Wireless networking is a method by which homes, telecommunications networks and business installations avoid the costly process of introducing ...
s,
microwave radio relay Microwave transmission is the transmission of information by electromagnetic waves with wavelengths in the microwave frequency range of 300MHz to 300GHz(1 m - 1 mm wavelength) of the electromagnetic spectrum. Microwave signals are normally lim ...
networks,
radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor v ...
, 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 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 especially to acquiring information about Ear ...
,
radio astronomy Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comin ...
,
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams. Large accelerators are used for fundamental research in particle ...
s,
spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter ...
, 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 designed to prevent or reduce the severity of a collision. In its ...
s,
garage door opener A garage door opener is a motorized device that opens and closes a garage door controlled by switches on the garage wall. Most also include a handheld radio remote control carried by the owner, which can be used to open and close the door from a ...
s and
keyless entry system A remote keyless system (RKS), also known as keyless entry or remote central locking, is an electronic lock that controls access to a building or vehicle by using an electronic remote control (activated by a handheld device or automatically b ...
s, and for cooking food in
microwave oven A microwave oven (commonly referred to as a microwave) is an electric oven that heats and cooks food by exposing it to electromagnetic radiation in the microwave frequency range. This induces polar molecules in the food to rotate and produce ...
s.


Electromagnetic spectrum

Microwaves occupy a place in the
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging fro ...
with frequency above ordinary
radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (sho ...
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 ...
light: In descriptions of the
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging fro ...
, 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 waves which follow the contour of the Earth, or reflect off the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...
(
skywave In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere. Since it is not limited by the curvature o ...
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 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 fade Rain fade refers primarily to the absorption of a microwave radio frequency (RF) signal by atmospheric rain, snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, ...
) 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: ** In lingui ...
, 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 ...
and
optical window The optical window is a range of wavelengths that are not blocked by the earth's atmosphere. The window runs from around 300 nanometers (ultraviolet-B) up into the range the human eye can detect, roughly 400–700 nm and continues up to approxim ...
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, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. Fro ...
. A sensitive receiver beyond the horizon with a high gain antenna focused on that area of the troposphere can pick up the signal. This technique has been used at frequencies between 0.45 and 5 GHz in tropospheric scatter (troposcatter) communication systems to communicate beyond the horizon, at distances up to 300 km.


Antennas

The short
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
s of microwaves allow
omnidirectional antenna In radio communication, an omnidirectional antenna is a class of antenna which radiates equal radio power in all directions perpendicular to an axis (azimuthal directions), with power varying with angle to the axis ( elevation angle), declining ...
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 without the use of an electrical conductor, optical fiber or other continuous guided medium for the transfer. The most ...
s such as
cell phone A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive telephone call, calls over a radio freq ...
s, cordless phones, and
wireless LAN A wireless LAN (WLAN) is a wireless computer network that links two or more devices using wireless communication to form a local area network (LAN) within a limited area such as a home, school, computer laboratory, campus, or office buildi ...
s (Wi-Fi) access for
laptop A laptop, laptop computer, or notebook computer is a small, portable personal computer (PC) with a screen and alphanumeric keyboard. Laptops typically have a clam shell form factor with the screen mounted on the inside of the upper li ...
s, and
Bluetooth Bluetooth is a short-range wireless technology standard that is used for exchanging data between fixed and mobile devices over short distances and building personal area networks (PANs). In the most widely used mode, transmission power is limi ...
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 or transmitter. A whip antenna is a form of monopole antenna. The antenna is designed to be flexible so ...
s, rubber ducky antennas, sleeve dipoles, patch antennas, and increasingly the printed circuit
inverted F antenna An inverted-F antenna is a type of antenna used in wireless communication, mainly at UHF and microwave frequencies. It consists of a monopole antenna running parallel to a ground plane and grounded at one end. The antenna is fed from an inte ...
(PIFA) used in cell phones. Their short
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
also allows narrow beams of microwaves to be produced by conveniently small high gain
antenna Antenna ( 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 name of two collid ...
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 is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor v ...
. 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 link to and from end nodes is wireless. The network is distributed over land areas called "cells", each served by at least one fixed-location transceiver (typically thr ...
by nearby transmitters. Parabolic ("dish") antennas are the most widely used directive antennas at microwave frequencies, but
horn antenna A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz. They are ...
s,
slot antenna A slot antenna consists of a metal surface, usually a flat plate, with one or more holes or slots cut out. When the plate is driven as an antenna by an applied radio frequency current, the slot radiates electromagnetic waves in a way similar to ...
s and lens antennas are also used. Flat microstrip antennas are being increasingly used in consumer devices. Another directive antenna practical at microwave frequencies is the
phased array In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
, 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, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...
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 electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ( insulating material); many coaxial cables also have a p ...
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 de ...
s. Due to the high cost and maintenance requirements of waveguide runs, in many microwave antennas the output stage of the
transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
or the RF front end of the receiver is located at the antenna.


Design and analysis

The term ''microwave'' also has a more technical meaning in electromagnetics and circuit theory. 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 two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active e ...
s,
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...
s, and
inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
s used with lower-frequency
radio waves Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (s ...
. Open-wire and coaxial
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...
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 de ...
s and
stripline Stripline is a transverse electromagnetic (TEM) transmission line medium invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s. Stripline is the earliest form of planar transmission line. Description A stri ...
, and lumped-element tuned circuits are replaced by cavity
resonator A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a resonator ...
s or
resonant stub In microwave and radio-frequency engineering, a stub or resonant stub is a length of transmission line or waveguide that is connected at one end only. The free end of the stub is either left open-circuit, or short-circuited (as is always the c ...
s. 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 instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...
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 potential difference has been applied. The type known as ...
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 A microwave oven (commonly referred to as a microwave) is an electric oven that heats and cooks food by exposing it to electromagnetic radiation in the microwave frequency range. This induces polar molecules in the food to rotate and produce ...
s), klystron, traveling-wave tube (TWT), and gyrotron. These devices work in the
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
modulated mode, rather than the 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. Low-power microwave sources use solid-state devices such as the
field-effect transistor The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs co ...
(at least at lower frequencies),
tunnel diode A tunnel diode or Esaki diode is a type of semiconductor diode that has effectively " negative resistance" due to the quantum mechanical effect called tunneling. It was invented in August 1957 by Leo Esaki, Yuriko Kurose, and Takashi Suz ...
s, Gunn diodes, and IMPATT diodes. Low-power sources are available as benchtop instruments, rackmount instruments, embeddable modules and in card-level formats. A
maser A maser (, an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The first maser was built by Charles H. Townes, James ...
is a solid state device which amplifies microwaves using similar principles to the
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The ...
, which amplifies higher frequency light waves. All warm objects emit low level microwave
black-body radiation Black-body radiation is the thermal electromagnetic radiation within, or surrounding, a body in thermodynamic equilibrium with its environment, emitted by a black body (an idealized opaque, non-reflective body). It has a specific, continuous spe ...
, depending on their
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
, so in meteorology and
remote sensing 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 especially to acquiring information about Ear ...
, microwave radiometers 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 (SNR) in the constellation Cassiopeia and the brightest extrasolar radio source in the sky at frequencies above 1 GHz. The supernova occurred approximately away within the Milky Way ...
emit low level microwave radiation which carries information about their makeup, which is studied by
radio astronomer Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation coming ...
s using receivers called
radio telescope A radio telescope is a specialized 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 radio frequency ...
s. The cosmic microwave background radiation (CMBR), for example, is a weak microwave noise filling empty space which is a major source of information on
cosmology Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosopher ...
's
Big Bang The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
theory of the origin of the
Universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the univers ...
.


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 link to and from end nodes is wireless. The network is distributed over land areas called "cells", each served by at least one fixed-location transceiver (typically thr ...
; their comparatively higher frequencies allow broad bandwidth and high data transmission rates, and antenna sizes are smaller than at lower frequencies because antenna size is inversely proportional to 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; it creates a communication channel between a source transmitter and a receiver at different locations on Earth ...
s. Microwaves are also employed in
microwave oven A microwave oven (commonly referred to as a microwave) is an electric oven that heats and cooks food by exposing it to electromagnetic radiation in the microwave frequency range. This induces polar molecules in the food to rotate and produce ...
s and in
radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor v ...
technology.


Communication

Before the advent of fiber-optic transmission, most long-distance
telephone call A telephone call is a connection over a telephone network between the called party and the calling party. First telephone call The first telephone call was made on March 10, 1876, by Alexander Graham Bell. Bell demonstrated his ability to "ta ...
s were carried via networks of
microwave radio relay Microwave transmission is the transmission of information by electromagnetic waves with wavelengths in the microwave frequency range of 300MHz to 300GHz(1 m - 1 mm wavelength) of the electromagnetic spectrum. Microwave signals are normally lim ...
links run by carriers such as AT&T Long Lines. 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 to form a local area network (LAN) within a limited area such as a home, school, computer laboratory, campus, or office buildi ...
protocol Protocol may refer to: Sociology and politics * Protocol (politics), a formal agreement between nation states * Protocol (diplomacy), the etiquette of diplomacy and affairs of state * Etiquette, a code of personal behavior Science and technology ...
s, such as
Bluetooth Bluetooth is a short-range wireless technology standard that is used for exchanging data between fixed and mobile devices over short distances and building personal area networks (PANs). In the most widely used mode, transmission power is limi ...
and the
IEEE The Institute of Electrical and Electronics Engineers (IEEE) is a 501(c)(3) professional association for electronic engineering and electrical engineering (and associated disciplines) with its corporate office in New York City and its operati ...
802.11 IEEE 802.11 is part of the IEEE 802 set of local area network (LAN) technical standards, and specifies the set of media access control (MAC) and physical layer (PHY) protocols for implementing wireless local area network (WLAN) computer com ...
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 reserved internationally for industrial, scientific and medical (ISM) purposes, excluding applications in telecommunications. Examples of applications for the use of radio frequency (RF) ener ...
, although 802.11a uses
ISM band The ISM radio bands are portions of the radio spectrum reserved internationally for industrial, scientific and medical (ISM) purposes, excluding applications in telecommunications. Examples of applications for the use of radio frequency (RF) ener ...
and
U-NII The Unlicensed National Information Infrastructure (U-NII) radio band, as defined by the United States Federal Communications Commission, is part of the radio frequency spectrum used by WLAN devices and by many wireless ISPs. As of March 2021, ...
frequencies in the 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services have been used for almost a decade in many countries in the 3.5–4.0 GHz range. The FCC recently carved out spectrum for carriers that wish to offer services in this range in the U.S. — with emphasis on 3.65 GHz. Dozens of service providers across the country are securing or have already received licenses from the FCC to operate in this band. The WIMAX service offerings that can be carried on the 3.65 GHz band will give business customers another option for connectivity. Metropolitan area network (MAN) protocols, such as
WiMAX Worldwide Interoperability for Microwave Access (WiMAX) is a family of wireless broadband communication standards based on the IEEE 802.16 set of standards, which provide physical layer (PHY) and media access control (MAC) options. The WiMAX ...
(Worldwide Interoperability for Microwave Access) are based on standards such as
IEEE 802.16 IEEE 802.16 is a series of wireless broadband standards written by the Institute of Electrical and Electronics Engineers (IEEE). The IEEE Standards Board established a working group in 1999 to develop standards for broadband for wireless metrop ...
, 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 Internet access via mobile networks. Access to the network can be made through a portable modem, wireless modem, or a tablet/ smartphone (possibly tethered) or other mobile device. The fir ...
Wireless Access (MBWA) protocols based on standards specifications such as
IEEE 802.20 IEEE 802.20 or Mobile Broadband Wireless Access (MBWA) was a specification by the standard association of the Institute of Electrical and Electronics Engineers (IEEE) for mobile broadband networks. The main standard was published in 2008. MBWA ...
or ATIS/ANSI
HC-SDMA IEEE 802.20 or Mobile Broadband Wireless Access (MBWA) was a specification by the standard association of the Institute of Electrical and Electronics Engineers (IEEE) for mobile broadband networks. The main standard was published in 2008. MBWA ...
(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 (IEEE) for mobile broadband networks. The main standard was published in 2008. MBWA ...
) 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, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link whi ...
networks, like
GSM The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation ( 2G) digital cellular networks used by mobile devices such ...
, 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. The S-DMB used the S band (2170-2200 MHz) of IMT-2000. and delivered around 18 channels at 128 kbit/s in 15 MHz. It incorporated a high power geostati ...
use 1.452 to 1.492 GHz, while proprietary/incompatible
satellite radio Satellite radio is defined by the International Telecommunication Union (ITU)'s ITU Radio Regulations (RR) as a '' broadcasting-satellite service''. The satellite's signals are broadcast nationwide, across a much wider geographical area than t ...
in the U.S. uses around 2.3 GHz for DARS. Microwave radio is used in
broadcasting Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum (radio waves), in a one-to-many model. Broadcasting began wi ...
and
telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that ...
transmissions because, due to their short wavelength, highly
directional antenna A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources. Directional antennas provide increased performanc ...
s are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies). There is also more bandwidth in the microwave spectrum than in the rest of the radio spectrum; the usable bandwidth below 300 MHz is less than 300 MHz while many GHz can be used above 300 MHz. Typically, microwaves are used in
television news News broadcasting is the medium of broadcasting various news events and other information via television, radio, or the internet in the field of broadcast journalism. The content is usually either produced locally in a radio studio or tel ...
to transmit a signal from a remote location to a television station from a specially equipped van. See
broadcast auxiliary service A broadcast auxiliary service or BAS is any radio frequency system used by a radio station or TV station, which is not part of its direct broadcast to listeners or viewers. These are essentially internal-use backhaul channels not intended for a ...
(BAS), remote pickup unit (RPU), and
studio/transmitter link A studio transmitter link (or STL) sends a radio station's or television station's audio and video from the broadcast studio or origination facility to a radio transmitter, television transmitter or uplink facility in another location. This is ...
(STL). Most
satellite communications A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. ...
systems operate in the C, X, Ka, or Ku bands of the microwave spectrum. These frequencies allow large bandwidth while avoiding the crowded UHF frequencies and staying below the atmospheric absorption of EHF frequencies. Satellite TV either operates in the C band for the traditional large dish fixed satellite service or Ku band for direct-broadcast satellite. Military communications run primarily over X or Ku-band links, with Ka band being used for Milstar.


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 satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pr ...
s (GNSS) including the Chinese
Beidou The BeiDou Navigation Satellite System (BDS; ) is a Chinese satellite navigation system. It consists of two separate satellite constellations. The first BeiDou system, officially called the BeiDou Satellite Navigation Experimental System a ...
, 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 and operated by the United States Space Force. It is one of the global navigation satellite ...
(introduced in 1978) and the Russian
GLONASS GLONASS (russian: ГЛОНАСС, label=none, ; rus, links=no, Глобальная навигационная спутниковая система, r=Global'naya Navigatsionnaya Sputnikovaya Sistema, t=Global Navigation Satellite System) is ...
broadcast navigational signals in various bands between about 1.2 GHz and 1.6 GHz.


Radar

Radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor v ...
is a
radiolocation Radiolocation, also known as radiolocating or radiopositioning, is the process of finding the location of something through the use of radio waves. It generally refers to passive uses, particularly radar—as well as detecting buried cables, ...
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 A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or par ...
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 controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airsp ...
, 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 waves 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 designed to prevent or reduce the severity of a collision. In its ...
s.


Radio astronomy

Microwaves emitted by astronomical radio sources; planets, stars,
galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System ...
, and
nebula A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regio ...
s are studied in
radio astronomy Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comin ...
with large dish antennas called
radio telescope A radio telescope is a specialized 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 radio frequency ...
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. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
or map the invisible surface of
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...
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 of 66 radio telescopes in the Atacama Desert of northern Chile, which observe electromagnetic radiation at millimeter and submillimeter wavelengths. The ...
, located at more than 5,000 meters (16,597 ft) altitude in Chile, observes the
universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the univers ...
in the millimetre and submillimetre wavelength ranges. The world's largest ground-based astronomy project to date, it consists of more than 66 dishes and was built in an international collaboration by Europe, North America, East Asia and Chile. A major recent focus of microwave radio astronomy has been mapping the cosmic microwave background radiation (CMBR) discovered in 1964 by radio astronomers Arno Penzias and Robert Wilson. This faint background radiation, which fills the universe and is almost the same in all directions, is "relic radiation" from the
Big Bang The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
, and is one of the few sources of information about conditions in the early universe. Due to the expansion and thus cooling of the Universe, the originally high-energy radiation has been shifted into the microwave region of the radio spectrum. Sufficiently sensitive
radio telescope A radio telescope is a specialized 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 radio frequency ...
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 that heats and cooks food by exposing it to electromagnetic radiation in the microwave frequency range. This induces polar molecules in the food to rotate and produce ...
passes microwave radiation at a frequency near through food, causing dielectric heating primarily by absorption of the energy in water. Microwave ovens became common kitchen appliances in Western countries in the late 1970s, following the development of less expensive cavity magnetrons. 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 for such purposes as
reactive ion etching Reactive-ion etching (RIE) is an etching technology used in microfabrication. RIE is a type of dry etching which has different characteristics than wet etching. RIE uses chemically reactive plasma to remove material deposited on wafers. The p ...
and plasma-enhanced
chemical vapor deposition Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (subst ...
(PECVD). Microwaves are used in stellarators and 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 cyclotron resonance 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 creating energy by replicating, on Ear ...
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 research was done to examine possibilities.
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...
worked in the 1970s and early 1980s to research the possibilities of using
solar power satellite Space-based solar power (SBSP, SSP) is the concept of collecting solar power in outer space by solar power satellites (SPS) and distributing it to Earth. Its advantages include a higher collection of energy due to the lack of reflection and a ...
(SPS) systems with large
solar array A photovoltaic system, also PV system or solar power system, is an electric power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to absorb and ...
s that would beam power down to the Earth's surface via microwaves.
Less-than-lethal Non-lethal weapons, also called nonlethal weapons, less-lethal weapons, less-than-lethal weapons, non-deadly weapons, compliance weapons, or pain-inducing weapons are weapons intended to be less likely to kill a living target than conventional ...
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 service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part of the United States Army Si ...
and
Marines Marines, or naval infantry, are typically a military force trained to operate in littoral zones in support of naval operations. Historically, tasks undertaken by marines have included helping maintain discipline and order aboard the ship (refl ...
are currently using this type of
active denial system The Active Denial System (ADS) is a non-lethal directed-energy weapon developed by the U.S. military, designed for area denial, perimeter security and crowd control. Informally, the weapon is also called the heat ray since it works by heating th ...
in fixed installations.


Spectroscopy

Microwave radiation is used in
electron paramagnetic resonance Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials that have unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but the spin ...
(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 that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
s of 0.3 T. This technique provides information on unpaired
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
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, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
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 molecules in the gas phase. The spectra of polar molecules can be measured in absorption or emission by microwave s ...
and can be combined with
electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an out ...
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 designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (on ...
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 A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called ''heterodyning'', which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is u ...
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 In electronics, a Lecher line or Lecher wires is a pair of parallel wires or rods that were used to measure the wavelength of radio waves, mainly at VHF, UHF and microwave frequencies. They form a short length of balanced transmission line (a ...
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 de ...
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 In radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the tran ...
on the line. However, provided a
standing wave In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect ...
is present, they may also be used to measure the distance between the 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 Non-ionizing (or non-ionising) radiation refers to any type of electromagnetic radiation that does not carry enough energy per quantum (photon energy) to ionize atoms or molecules—that is, to completely remove an electron from an atom or m ...
radiation, which means that microwave
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
s do not contain sufficient energy to ionize molecules or break chemical bonds, or cause DNA damage, as ionizing radiation such as
x-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
s or
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
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 Non-ionizing (or non-ionising) radiation refers to any type of electromagnetic radiation that does not carry enough energy per quantum (photon energy) to ionize atoms or molecules—that is, to completely remove an electron from an atom or m ...
electromagnetic radiation) have significant adverse biological effects at low levels. Some, but not all, studies suggest that long-term exposure may have a
carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive sub ...
ic effect. During
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
, 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 of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...
in the 1970s has shown this to be caused by thermal expansion in parts of the inner ear. In 1955 Dr.
James Lovelock James Ephraim Lovelock (26 July 1919 – 26 July 2022) was an English independent scientist, environmentalist and futurist. He is best known for proposing the Gaia hypothesis, which postulates that the Earth functions as a self-regulating sy ...
was able to reanimate rats chilled to using microwave diathermy. When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in the body. The lens and
cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical ...
of the eye are especially vulnerable because they contain no
blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide awa ...
s that can carry away heat. Exposure to microwave radiation can produce cataracts by this mechanism, because the microwave heating denatures
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s in the crystalline lens of the eye (in the same way that heat turns
egg white Egg white is the clear liquid (also called the albumen or the glair/glaire) contained within an egg. In chickens it is formed from the layers of secretions of the anterior section of the hen's oviduct during the passage of the egg. It forms ar ...
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 A burn is an injury to skin, or other tissues, caused by heat, cold, electricity, chemicals, friction, or ultraviolet radiation (like sunburn). Most burns are due to heat from hot liquids (called scalding), solids, or fire. Burns occur ma ...
s that may not be immediately evident because of the tendency for microwaves to heat deeper tissues with higher moisture content.


History


Hertzian optics

Microwaves were first generated in the 1890s in some of the earliest
radio Radio is the technology of signaling and 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 called a tr ...
experiments by physicists who thought of them as a form of "invisible light".
James Clerk Maxwell James Clerk Maxwell (13 June 1831 – 5 November 1879) was a Scottish mathematician and scientist responsible for the classical theory of electromagnetic radiation, which was the first theory to describe electricity, magnetism and ligh ...
in his 1873 theory of
electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...
, now called
Maxwell's equations Maxwell's equations, or Maxwell–Heaviside equations, are a set of coupled partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric circuits ...
, had predicted that a coupled
electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field ...
and
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
could travel through space as an
electromagnetic wave In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) ...
, and proposed that light consisted of electromagnetic waves of short wavelength. In 1888, German physicist
Heinrich Hertz Heinrich Rudolf Hertz ( ; ; 22 February 1857 – 1 January 1894) was a German physicist who first conclusively proved the existence of the electromagnetic waves predicted by James Clerk Maxwell's equations of electromagnetism. The uni ...
was the first to demonstrate the existence of electromagnetic waves, generating
radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (sho ...
s using a primitive 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
UHF Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (on ...
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 instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...
experiments in their laboratories, using quasioptical components such as prisms and
lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...
es made of paraffin,
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...
and pitch and wire
diffraction grating In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structur ...
s, 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 In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole wi ...
with a spark gap between the ends, suspended at the focal line of a
parabolic antenna A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or par ...
made of a curved zinc sheet, powered by high voltage pulses from an
induction coil An induction coil or "spark coil" ( archaically known as an inductorium or Ruhmkorff coil after Heinrich Rühmkorff) is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current (DC) supply. p.98 ...
. His historic experiments demonstrated that radio waves like light exhibited
refraction In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomen ...
,
diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
, polarization, interference and
standing wave In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect ...
s, proving that radio waves and light waves were both forms of Maxwell's
electromagnetic wave In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) ...
s. Image:Hertz spark gap transmitter and parabolic antenna.png,
Heinrich Hertz Heinrich Rudolf Hertz ( ; ; 22 February 1857 – 1 January 1894) was a German physicist who first conclusively proved the existence of the electromagnetic waves predicted by James Clerk Maxwell's equations of electromagnetism. The uni ...
'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 Sir Jagadish Chandra Bose (;, ; 30 November 1858 – 23 November 1937) was a biologist, physicist, botanist and an early writer of science fiction. He was a pioneer in the investigation of radio microwave optics, made significant contribution ...
in 1894 was the first person to produce millimeter waves; 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 Sir John Ambrose Fleming FRS (29 November 1849 – 18 April 1945) was an English electrical engineer and physicist who invented the first thermionic valve or vacuum tube, designed the radio transmitter with which the first transatlantic r ...
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 Sir Jagadish Chandra Bose (;, ; 30 November 1858 – 23 November 1937) was a biologist, physicist, botanist and an early writer of science fiction. He was a pioneer in the investigation of radio microwave optics, made significant contribution ...
performed the first experiments with microwaves. He was the first person to produce millimeter waves, 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 de ...
,
horn antenna A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz. They are ...
s, and
semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way ...
crystal detector A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal. It was employed as a detector (dem ...
s for use in his experiments. Independently in 1894,
Oliver Lodge Sir Oliver Joseph Lodge, (12 June 1851 – 22 August 1940) was a British physicist and writer involved in the development of, and holder of key patents for, radio. He identified electromagnetic radiation independent of Hertz's proof and at his ...
and Augusto Righi experimented with 1.5 and 12 GHz microwaves respectively, generated by small metal ball spark resonators. Russian physicist
Pyotr Lebedev Pyotr Nikolaevich Lebedev (; 24 February 1866 – 1 March 1912) was a Russian physicist. His name was also transliterated as Peter Lebedew and Peter Lebedev. Lebedev was a creator of first scientific school in Russia. Career Lebedev made his doc ...
in 1895 generated 50 GHz millimeter waves. In 1897
Lord Rayleigh John William Strutt, 3rd Baron Rayleigh, (; 12 November 1842 – 30 June 1919) was an English mathematician and physicist who made extensive contributions to science. He spent all of his academic career at the University of Cambridge. A ...
solved the mathematical
boundary-value problem In mathematics, in the field of differential equations, a boundary value problem is a differential equation together with a set of additional constraints, called the boundary conditions. A solution to a boundary value problem is a solution to t ...
of electromagnetic waves propagating through conducting tubes and dielectric rods of arbitrary shape. which gave the modes and
cutoff frequency In physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced ( attenuated or reflected) rather tha ...
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 de ...
. However, since microwaves were limited to 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 waves and by reflecting off the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...
as
skywave In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere. Since it is not limited by the curvature o ...
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 A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's ...
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 potential difference has been applied. The type known as ...
(valve)
electronic oscillator An electronic oscillator is an electronic circuit that produces a periodic, oscillating electronic signal, often a sine wave or a square wave or a triangle wave. Oscillators convert direct current (DC) from a power supply to an alternating ...
used in radio transmitters could not produce frequencies above a few hundred
megahertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one ...
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 The Barkhausen–Kurz tube, also called the retarding-field tube, reflex triode, B–K oscillator, and Barkhausen oscillator was a high frequency vacuum tube electronic oscillator invented in 1920 by German physicists Heinrich Georg Barkhau ...
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
IRE Ire or IRE may refer to: Ire * Extreme anger; intense fury * Irē, the Livonian name for Mazirbe, Latvia * A town in Oye, Nigeria * ''Ire'' (album), a 2015 album by the Australian metalcore band Parkway Drive * Ire (Iliad), a town mentioned in ...
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 The English Channel, "The Sleeve"; nrf, la Maunche, "The Sleeve" ( Cotentinais) or ( Jèrriais), ( Guernésiais), "The Channel"; br, Mor Breizh, "Sea of Brittany"; cy, Môr Udd, "Lord's Sea"; kw, Mor Bretannek, "British Sea"; nl, Het Ka ...
between
Dover Dover () is a town and major ferry port in Kent, South East England. It faces France across the Strait of Dover, the narrowest part of the English Channel at from Cap Gris Nez in France. It lies south-east of Canterbury and east of Maids ...
, UK and
Calais Calais ( , , traditionally , ) is a port city in the Pas-de-Calais department, of which it is a subprefecture. Although Calais is by far the largest city in Pas-de-Calais, the department's prefecture is its third-largest city of Arras. Th ...
, France. The system transmitted telephony, telegraph and
facsimile A facsimile (from Latin ''fac simile'', "to make alike") is a copy or reproduction of an old book, manuscript, map, art print, or other item of historical value that is as true to the original source as possible. It differs from other forms of ...
data over bidirectional 1.7 GHz beams with a power of one-half watt, produced by miniature
Barkhausen–Kurz tube The Barkhausen–Kurz tube, also called the retarding-field tube, reflex triode, B–K oscillator, and Barkhausen oscillator was a high frequency vacuum tube electronic oscillator invented in 1920 by German physicists Heinrich Georg Barkhau ...
s 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 Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 m ...
" 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 is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor v ...
, mainly in secrecy, before and during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
, 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 The beam diameter or beam width of an electromagnetic beam is the diameter along any specified line that is perpendicular to the beam axis and intersects it. Since beams typically do not have sharp edges, the diameter can be defined in many differ ...
to localize enemy aircraft. It was found that conventional
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...
s used to carry radio waves had excessive power losses at microwave frequencies, and George Southworth at
Bell Labs Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial research and scientific development company owned by mul ...
and
Wilmer Barrow Wilmer Lanier Barrow (July 26, 1903 – August 29, 1975) was an American electrical engineer, inventor, teacher, industrial manager, and a counselor to government agencies. He obtained a BSEE degree in 1926 from Louisiana State University, and a ...
at 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 de ...
in 1936. Barrow invented the
horn antenna A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz. They are ...
in 1938 as a means to efficiently radiate microwaves into or out of a waveguide. In a microwave receiver, a
nonlinear In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many oth ...
component was needed that would act as a detector and
mixer Mixer may refer to: Electronics * DJ mixer, a type of audio mixing console used by disc jockeys * Electronic mixer, electrical circuit for adding signal voltages * Frequency mixer, electrical circuit that creates new frequencies from two signals ...
at these frequencies, as vacuum tubes had too much capacitance. To fill this need researchers resurrected an obsolete technology, the point contact
crystal detector A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal. It was employed as a detector (dem ...
(cat whisker detector) which was used as a
demodulator Demodulation is extracting the original information-bearing signal from a carrier wave. A demodulator is an electronic circuit (or computer program in a software-defined radio) that is used to recover the information content from the modulated ...
in
crystal radio A crystal radio receiver, also called a crystal set, is a simple radio receiver, popular in the early days of radio. It uses only the power of the received radio signal to produce sound, needing no external power. It is named for its most imp ...
s around the turn of the century before vacuum tube receivers. The low capacitance of
semiconductor junction A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
s allowed them to function at microwave frequencies. The first modern
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...
and
germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors ...
diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diod ...
s were developed as microwave detectors in the 1930s, and the principles of
semiconductor physics A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
learned during their development led to semiconductor electronics after the war. Image:R&B Magnetron.jpg,
Randall Randall may refer to the following: Places United States *Randall, California, former name of White Hall, California, an unincorporated community * Randall, Indiana, a former town *Randall, Iowa, a city *Randall, Kansas, a city *Randall, Minnesot ...
and
Boot A boot is a type of footwear. Most boots mainly cover the foot and the ankle, while some also cover some part of the lower calf. Some boots extend up the leg, sometimes as far as the knee or even the hip. Most boots have a heel that is c ...
's prototype cavity magnetron tube at the
University of Birmingham , mottoeng = Through efforts to heights , established = 1825 – Birmingham School of Medicine and Surgery1836 – Birmingham Royal School of Medicine and Surgery1843 – Queen's College1875 – Mason Science College1898 – Mason Univers ...
, 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, British Mk. VIII, the first microwave air intercept radar, in nose of British fighter. Microwave radar, powered by the new magnetron tube, significantly shortened World War II. 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 II: the klystron tube by Russell and Sigurd Varian at
Stanford University Stanford University, officially Leland Stanford Junior University, is a private research university in Stanford, California. The campus occupies , among the largest in the United States, and enrolls over 17,000 students. Stanford is conside ...
in 1937, and the cavity magnetron tube by John Randall and
Harry Boot Henry Albert Howard Boot (29 July 1917 – 8 February 1983) was an English physicist who with Sir John Randall and James Sayers developed the cavity magnetron, which was one of the keys to the Allied victory in the Second World War. Biograph ...
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 The Tizard Mission, officially the British Technical and Scientific Mission, was a British delegation that visited the United States during WWII to obtain the industrial resources to exploit the military potential of the research and development ( ...
) significantly shortened the war. The MIT Radiation Laboratory established secretly at
Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of th ...
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 II

After World War II, microwaves were rapidly exploited commercially. Due to their high frequency they had a very large information-carrying capacity ( bandwidth); a single microwave beam could carry tens of thousands of phone calls. In the 1950s and 60s transcontinental microwave relay networks were built in the US and Europe to exchange telephone calls between cities and distribute television programs. In the new
television broadcasting A television network or television broadcaster is a telecommunications network for distribution of television program content, where a central operation provides programming to many television stations or pay television providers. Until the mid- ...
industry, from the 1940s microwave dishes were used to transmit backhaul video feeds from mobile production trucks back to the studio, allowing the first remote TV broadcasts. The first
communications satellite A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth ...
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 Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial research and scientific development company owned by mul ...
, Holmdel, New Jersey discovered cosmic microwave background radiation. Microwave radar became the central technology used in
air traffic control Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airsp ...
, maritime
navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation ...
, anti-aircraft defense,
ballistic missile A ballistic missile is a type of missile that uses projectile motion to deliver warheads on a target. These weapons are guided only during relatively brief periods—most of the flight is unpowered. Short-range ballistic missiles stay within t ...
detection, and later many other uses. Radar and satellite communication motivated the development of modern microwave antennas; the
parabolic antenna A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or par ...
(the most common type), cassegrain antenna, lens antenna,
slot antenna A slot antenna consists of a metal surface, usually a flat plate, with one or more holes or slots cut out. When the plate is driven as an antenna by an applied radio frequency current, the slot radiates electromagnetic waves in a way similar to ...
, and
phased array In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
. The ability of
short wave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 m ...
s 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 Raytheon Technologies Corporation is an American multinational aerospace and defense conglomerate headquartered in Arlington, Virginia. It is one of the largest aerospace and defense manufacturers in the world by revenue and market capitali ...
, 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 that heats and cooks food by exposing it to electromagnetic radiation in the microwave frequency range. This induces polar molecules in the food to rotate and produce ...
, 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 microwave hyperthermy. The traveling wave tube (TWT) developed in 1943 by Rudolph Kompfner and John Pierce provided a high-power tunable source of microwaves up to 50 GHz, and became the most widely used microwave tube (besides the ubiquitous magnetron used in microwave ovens). The gyrotron tube family developed in Russia could produce megawatts of power up into millimeter wave frequencies and is used in industrial heating and plasma research, and to power
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams. Large accelerators are used for fundamental research in particle ...
s and nuclear fusion reactors.


Solid state microwave devices

The development of semiconductor electronics in the 1950s led to the first solid state microwave devices which worked by a new principle; negative resistance (some of the prewar microwave tubes had also used negative resistance). The feedback oscillator and 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 A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diod ...
s worked better. The
tunnel diode A tunnel diode or Esaki diode is a type of semiconductor diode that has effectively " negative resistance" due to the quantum mechanical effect called tunneling. It was invented in August 1957 by Leo Esaki, Yuriko Kurose, and Takashi Suz ...
invented in 1957 by Japanese physicist
Leo Esaki Reona Esaki (江崎 玲於奈 ''Esaki Reona'', born March 12, 1925), also known as Leo Esaki, is a Japanese physicist who shared the Nobel Prize in Physics in 1973 with Ivar Giaever and Brian David Josephson for his work in electron tunneling i ...
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 WT or Wt may refer to: * Weight People * Watertender, a former petty officer rating in the United States Navy * Weapons technician, a former petty officer rating in the United States Navy * Robert Wight (1796–1872), in botanical taxonomy, a ...
and Ralph L. Johnston and the Gunn diode in 1962 by
J. B. Gunn John Battiscombe "J. B." Gunn (13 May 1928 – 2 December 2008), known as Ian or Iain, was a British physicist, who spent most of his career in the United States. He discovered the Gunn effect, which led to the invention of the Gunn diode, t ...
. Diodes are the most widely used microwave sources today. Two low-noise solid state negative resistance microwave
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost t ...
s were developed; the ruby
maser A maser (, an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The first maser was built by Charles H. Townes, James ...
invented in 1953 by
Charles H. Townes Charles Hard Townes (July 28, 1915 – January 27, 2015) was an American physicist. Townes worked on the theory and application of the maser, for which he obtained the fundamental patent, and other work in quantum electronics associated wit ...
,
James P. Gordon James Power Gordon (March 20, 1928 – June 21, 2013) was an American physicist known for his work in the fields of optics and quantum electronics. His contributions include the design, analysis and construction of the first maser in 1954 ...
, 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 clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwe ...
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 nonreciprocal waveguide components, such as
circulator A circulator is a passive, non-reciprocal three- or four- port device that only allows a microwave or radio-frequency signal to exit through the port directly after the one it entered. Optical circulators have similar behavior. Ports are where ...
s, isolators, and directional couplers. In 1969 Kurokawa derived mathematical conditions for stability in negative resistance circuits which formed the basis of microwave oscillator design.


Microwave integrated circuits

Prior to the 1970s microwave devices and circuits were bulky and expensive, so microwave frequencies were generally limited to the output stage of transmitters and the RF front end of receivers, and signals were heterodyned to a lower
intermediate frequency In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier wave is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier sig ...
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 Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing '' signals'', such as sound, images, and scientific measurements. Signal processing techniques are used to optimize transmissions, ...
at microwave frequencies. This has made possible
satellite television Satellite television is a service that delivers television programming to viewers by relaying it from a communications satellite orbiting the Earth directly to the viewer's location. The signals are received via an outdoor parabolic antenna com ...
,
cable television Cable television is a system of delivering television programming to consumers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fibre-optic cables. This contrasts with bro ...
, GPS devices, and modern wireless devices, such as
smartphone A smartphone is a portable computer device that combines mobile telephone and computing functions into one unit. They are distinguished from feature phones by their stronger hardware capabilities and extensive mobile operating systems, whi ...
s,
Wi-Fi Wi-Fi () is a family of wireless network protocols, based on the IEEE 802.11 family of standards, which are commonly used for local area networking of devices and Internet access, allowing nearby digital devices to exchange data by radio waves ...
, and
Bluetooth Bluetooth is a short-range wireless technology standard that is used for exchanging data between fixed and mobile devices over short distances and building personal area networks (PANs). In the most widely used mode, transmission power is limi ...
which connect to networks using microwaves.
Microstrip Microstrip is a type of electrical transmission line which can be fabricated with any technology where a conductor is separated from a ground plane by a dielectric layer known as the substrate. Microstrip lines are used to convey microwave-freque ...
, a type of
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...
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 board A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a laminated sandwich str ...
s allowed microstrip versions of
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...
s,
inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
s, resonant stubs, splitters, directional couplers,
diplexer A diplexer is a passive device that implements frequency-domain multiplexing. Two ports (e.g., L and H) are multiplexed onto a third port (e.g., S). The signals on ports L and H occupy disjoint frequency bands. Consequently, the signals on L and ...
s,
filters Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component that ...
and antennas to be made, thus allowing compact microwave circuits to be constructed.
Transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
s 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
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost t ...
s, 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