Microwave

Microwave

Overview

Microwaves, a subset of radio waves, have wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

s ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 between 300 MHz (0.3 GHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

) and 300 GHz. This broad definition includes both UHF and EHF
Extremely high frequency
Extremely high frequency is the highest radio frequency band. EHF runs the range of frequencies from 30 to 300 gigahertz, above which electromagnetic radiation is considered to be low infrared light, also referred to as terahertz radiation...

 (millimeter waves), and various sources use different boundaries. In all cases, microwave includes the entire SHF
Super high frequency
Super high frequency refers to radio frequencies in the range of 3 GHz and 30 GHz. This band of frequencies is also known as the centimetre band or centimetre wave as the wavelengths range from ten to one centimetres....

 band (3 to 30 GHz, or 10 to 1 cm) at minimum, with RF engineering
RF engineering
Radio frequency engineering is a subset of electrical engineering that deals with devices that are designed to operate in the Radio Frequency spectrum...

 often putting the lower boundary at 1 GHz (30 cm), and the upper around 100 GHz (3 mm).

Apparatus and techniques may be described qualitatively as "microwave" when the wavelengths of signals are roughly the same as the dimensions of the equipment, so that lumped-element circuit theory
Lumped element model
The lumped element model simplifies the description of the behaviour of spatially distributed physical systems into a topology consisting of discrete entities that approximate the behaviour of the distributed system under certain assumptions...

 is inaccurate.
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Encyclopedia

Microwaves, a subset of radio waves, have wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

s ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 between 300 MHz (0.3 GHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

) and 300 GHz. This broad definition includes both UHF and EHF
Extremely high frequency
Extremely high frequency is the highest radio frequency band. EHF runs the range of frequencies from 30 to 300 gigahertz, above which electromagnetic radiation is considered to be low infrared light, also referred to as terahertz radiation...

 (millimeter waves), and various sources use different boundaries. In all cases, microwave includes the entire SHF
Super high frequency
Super high frequency refers to radio frequencies in the range of 3 GHz and 30 GHz. This band of frequencies is also known as the centimetre band or centimetre wave as the wavelengths range from ten to one centimetres....

 band (3 to 30 GHz, or 10 to 1 cm) at minimum, with RF engineering
RF engineering
Radio frequency engineering is a subset of electrical engineering that deals with devices that are designed to operate in the Radio Frequency spectrum...

 often putting the lower boundary at 1 GHz (30 cm), and the upper around 100 GHz (3 mm).

Apparatus and techniques may be described qualitatively as "microwave" when the wavelengths of signals are roughly the same as the dimensions of the equipment, so that lumped-element circuit theory
Lumped element model
The lumped element model simplifies the description of the behaviour of spatially distributed physical systems into a topology consisting of discrete entities that approximate the behaviour of the distributed system under certain assumptions...

 is inaccurate. As a consequence, practical microwave technique tends to move away from the discrete resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...

s, capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

s, and inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...

s used with lower-frequency radio waves
Radio waves
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves,...

. Instead, distributed circuit element
Distributed element model
In electrical engineering, the distributed element model or transmission line model of electrical circuits assumes that the attributes of the circuit are distributed continuously throughout the material of the circuit...

s and transmission-line theory are more useful methods for design and analysis. Open-wire and coaxial transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...

s give way to waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...

s and stripline
Stripline
Stripline is a transverse electromagnetic transmission line medium, that was invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s.- Description :...

, and lumped-element tuned circuits are replaced by cavity resonator
Resonator
A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either electromagnetic or mechanical...

s or resonant lines. Effects of reflection
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

, polarization, scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...

, diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

, and atmospheric absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...

 usually associated with visible light are of practical significance in the study of microwave propagation
Radio propagation
Radio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere...

. The same equations
Maxwell's equations
Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...

 of electromagnetic theory apply at all frequencies.

The prefix "micro-" in "microwave" is not meant to suggest a wavelength in the micrometer range. It indicates that microwaves are "small" compared to waves used in typical radio broadcasting, in that they have shorter wavelengths. The boundaries between far infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 light, terahertz radiation
Terahertz radiation
In physics, terahertz radiation refers to electromagnetic waves propagating at frequencies in the terahertz range. It is synonymously termed submillimeter radiation, terahertz waves, terahertz light, T-rays, T-waves, T-light, T-lux, THz...

, microwaves, and ultra-high-frequency radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

 wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...

s are fairly arbitrary and are used variously between different fields of study.

Electromagnetic waves longer (lower frequency) than microwaves are called "radio waves". Electromagnetic radiation with shorter wavelengths may be called "millimeter waves", terahertz radiation
Terahertz radiation
In physics, terahertz radiation refers to electromagnetic waves propagating at frequencies in the terahertz range. It is synonymously termed submillimeter radiation, terahertz waves, terahertz light, T-rays, T-waves, T-light, T-lux, THz...

 or even T-rays. Definitions differ for millimeter wave band, which the IEEE defines as 110 GHz to 300 GHz.

Above 300 GHz, the absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...

 of electromagnetic radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

 by Earth's atmosphere is so great that it is in effect opaque
Opacity (optics)
Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...

, until the atmosphere becomes transparent again in the so-called infrared and optical window
Optical window
The meaning of this term depends on the context:* In astronomy, the optical window is the optical portion of the electromagnetic spectrum that passes through the atmosphere all the way to the ground...

 frequency ranges.

Microwave sources


Vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

 devices operate on the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the magnetron, klystron
Klystron
A klystron is a specialized linear-beam vacuum tube . Klystrons are used as amplifiers at microwave and radio frequencies to produce both low-power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for modern...

, traveling-wave tube (TWT), and gyrotron
Gyrotron
Gyrotrons are high powered vacuum tubes which emit millimeter-wave beams by bunching electrons with cyclotron motion in a strong magnetic field. Output frequencies range from about 20 to 250 GHz, covering wavelengths from microwave to the edge of the terahertz gap. Typical output powers range from...

. These devices work in the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 modulated mode, rather than the current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

 modulated mode. This means that they work on the basis of clumps of electrons flying ballistically through them, rather than using a continuous stream.
Low-power microwave sources use solid-state devices such as the field-effect transistor
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...

 (at least at lower frequencies), tunnel diode
Tunnel diode
A tunnel diode or Esaki diode is a type of semiconductor diode which is capable of very fast operation, well into the microwave frequency region, by using quantum mechanical effects....

s, Gunn diode
Gunn diode
A Gunn diode, also known as a transferred electron device , is a form of diode used in high-frequency electronics. It is somewhat unusual in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions...

s, and IMPATT diode
IMPATT diode
An IMPATT diode is a form of high power diode used in high-frequency electronics and microwave devices. They are typically made with silicon carbide owing to their high breakdown fields....

s.

A maser
Maser
A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. Historically, “maser” derives from the original, upper-case acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation"...

 is a device similar to a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

, which amplifies light energy by stimulating photons. The maser, rather than amplifying light energy, amplifies the lower frequency, longer wavelength microwaves and radio frequency emissions.

The sun also emits microwave radiation, and most of it is blocked by Earth's atmosphere.

The Cosmic Microwave Background Radiation
Cosmic microwave background radiation
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....

 (CMBR) is a source of microwaves that supports the science of cosmology's
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...

 Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

 theory of the origin of the Universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

.

Communication


Before the advent of fiber-optic transmission, most long-distance telephone call
Telephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...

s were carried via networks of microwave radio relay links run by carriers such as AT&T Long Lines. Starting in the early 1950s, frequency division multiplex 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
Wireless LAN
A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...

 protocols, such as Bluetooth
Bluetooth
Bluetooth is a proprietary open wireless technology standard for exchanging data over short distances from fixed and mobile devices, creating personal area networks with high levels of security...

 and the IEEE 802.11 specifications, also use microwaves in the 2.4 GHz ISM band
ISM band
The industrial, scientific and medical radio bands are radio bands reserved internationally for the use of radio frequency energy for industrial, scientific and medical purposes other than communications....

, although 802.11a uses ISM band
ISM band
The industrial, scientific and medical radio bands are radio bands reserved internationally for the use of radio frequency energy for industrial, scientific and medical purposes other than communications....

 and U-NII frequencies in the 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services have been used for almost a decade in many countries in the 3.5–4.0 GHz range. The FCC recently carved out spectrum for carriers that wish to offer services in this range in the U.S. — with emphasis on 3.65 GHz. Dozens of service providers across the country are securing or have already received licenses from the FCC to operate in this band. The WIMAX service offerings that can be carried on the 3.65 GHz band will give business customers another option for connectivity.

Metropolitan area network
Metropolitan area network
A metropolitan area network is a computer network that usually spans a city or a large campus. A MAN usually interconnects a number of local area networks using a high-capacity backbone technology, such as fiber-optical links, and provides up-link services to wide area networks and the...

 (MAN) protocols, such as WiMAX
WiMAX
WiMAX is a communication technology for wirelessly delivering high-speed Internet service to large geographical areas. The 2005 WiMAX revision provided bit rates up to 40 Mbit/s with the 2011 update up to 1 Gbit/s for fixed stations...

 (Worldwide Interoperability for Microwave Access) are based on standards such as IEEE 802.16
IEEE 802.16
IEEE 802.16 is a series of Wireless Broadband standards authored by the Institute of Electrical and Electronics Engineers . The IEEE Standards Board in established a working group in 1999 to develop standards for broadband Wireless Metropolitan Area Networks...

, designed to operate between 2 to 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
Mobile broadband is the marketing term for wireless Internet access through a portable modem, mobile phone or other mobile device.-Description:...

 Wireless Access (MBWA) protocols based on standards specifications such as IEEE 802.20
IEEE 802.20
IEEE 802.20 or Mobile Broadband Wireless Access was a specification by the standard association of the Institute of Electrical and Electronics Engineers for mobile wireless Internet access networks...

 or ATIS/ANSI HC-SDMA (such as iBurst
IBurst
iBurst is a wireless broadband technology originally developed by ArrayComm. It optimizes the use of its bandwidth with the help of smart antennas...

) 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
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

 networks, like GSM, use the low-microwave/high-UHF frequencies around 1.8 and 1.9 GHz in the Americas
Americas
The Americas, or America , are lands in the Western hemisphere, also known as the New World. In English, the plural form the Americas is often used to refer to the landmasses of North America and South America with their associated islands and regions, while the singular form America is primarily...

 and elsewhere, respectively. DVB-SH
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 downlink and for example a GPRS uplink...

 and S-DMB
S-DMB
S-DMB is a hybrid version of the Digital Multimedia Broadcasting. The S-DMB uses the S band of IMT-2000. and delivers around 18 channels at 128 kbps in 15 MHz. It incorporates a high power geostationary satellite, the MBSat 1...

 use 1.452 to 1.492 GHz, while proprietary/incompatible satellite radio
Satellite radio
Satellite radio is an analogue or digital radio signal that is relayed through one or more satellites and thus can be received in a much wider geographical area than terrestrial FM radio stations...

 in the U.S. uses around 2.3 GHz for DARS
Digital Audio Radio Service
Digital Audio Radio Service or DARS refers to any type of digital radio service. In the United States it is the official FCC term for digital radio services....

.

Microwave radio is used in broadcasting
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...

 and telecommunication
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

 transmissions because, due to their short wavelength, highly directional antenna
Directional antenna
A directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources....

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 to transmit a signal from a remote location to a television station from a specially equipped van. See broadcast auxiliary service
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...

 (BAS), remote pickup unit
Remote pickup unit
A remote pickup unit or RPU is a radio system using special radio frequencies set aside for electronic news gathering and remote broadcasting. It can also be used for other types of point-to-point radio links....

 (RPU), and studio/transmitter link
Studio/transmitter link
A studio-transmitter link sends a radio station's or television station's audio and video from the broadcast studio to a radio transmitter or television transmitter in another location....

 (STL).

Most satellite communications systems operate in the C, X, Ka, or Ku bands of the microwave spectrum. These frequencies allow large bandwidth while avoiding the crowded UHF frequencies and staying below the atmospheric absorption of EHF frequencies. Satellite TV either operates in the C band for the traditional 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.

Radar


Radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 uses microwave radiation to detect the range, speed, and other characteristics of remote objects. Development of radar was accelerated during World War II due to its great military utility. Now radar is widely used for applications such as air traffic control
Air traffic control
Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...

, weather forecasting, navigation of ships, and speed limit
Speed limit
Road speed limits are used in most countries to regulate the speed of road vehicles. Speed limits may define maximum , minimum or no speed limit and are normally indicated using a traffic sign...

 enforcement.

A Gunn diode
Gunn diode
A Gunn diode, also known as a transferred electron device , is a form of diode used in high-frequency electronics. It is somewhat unusual in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions...

 oscillator and waveguide are used as a motion detector for automatic door openers
Swing door operator
A swing-door operator is a device that operates a swing door for pedestrian use...

.

Radio astronomy


Most radio astronomy
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...

 uses microwaves. Usually the naturally-occurring microwave radiation is observed, but active radar experiments have also been done with objects in the solar system, such as determining the distance to the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 or mapping the invisible surface of Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 through cloud cover.

Navigation


Global Navigation Satellite System
Global Navigation Satellite System
A satellite navigation or SAT NAV system is a system of satellites that provide autonomous geo-spatial positioning with global coverage. It allows small electronic receivers to determine their location to within a few metres using time signals transmitted along a line-of-sight by radio from...

s (GNSS) including the Chinese Beidou
Beidou navigation system
The BeiDou Navigation System or BeiDou Navigation Satellite System is a project by China to develop an independent satellite navigation system...

, the American Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

 (GPS) and the Russian GLONASS
GLONASS
GLONASS , acronym for Globalnaya navigatsionnaya sputnikovaya sistema or Global Navigation Satellite System, is a radio-based satellite navigation system operated for the Russian government by the Russian Space Forces...

 broadcast navigational signals in various bands between about 1.2 GHz and 1.6 GHz.

Power


A microwave oven
Microwave oven
A microwave oven is a kitchen appliance that heats food by dielectric heating, using microwave radiation to heat polarized molecules within the food...

 passes (non-ionizing) microwave radiation (at a frequency near 2.45 GHz) through food, causing dielectric heating
Dielectric heating
Dielectric heating, also known as electronic heating, RF heating, high-frequency heating and diathermy, is the process in which a high-frequency alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is...

 by absorption of energy in the water, fats, and sugar contained in the food. Microwave ovens became common kitchen appliances in Western countries in the late 1970s, following development of inexpensive cavity magnetron
Cavity magnetron
The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field. The 'resonant' cavity magnetron variant of the earlier magnetron tube was invented by John Randall and Harry Boot in 1940 at the University of...

s. Water in the liquid state possesses many molecular interactions that broaden the absorption peak. In the vapor phase, isolated water molecules absorb at around 22 GHz, almost ten times the frequency of the microwave oven.

Microwave heating is used in industrial processes for drying and curing products.

Many semiconductor processing techniques use microwaves to generate plasma for such purposes as reactive ion etching
Reactive ion etching
Reactive-ion etching is an etching technology used in microfabrication. It uses chemically reactive plasma to remove material deposited on wafers. The plasma is generated under low pressure by an electromagnetic field...

 and plasma-enhanced chemical vapor deposition
Chemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...

 (PECVD).

Microwave frequencies typically ranging from 110 – 140 GHz are used in stellarator
Stellarator
A stellarator is a device used to confine a hot plasma with magnetic fields in order to sustain a controlled nuclear fusion reaction. It is one of the earliest controlled fusion devices, first invented by Lyman Spitzer in 1950 and built the next year at what later became the Princeton Plasma...

s and more notably in tokamak experimental fusion reactors to help heat the fuel into a plasma state. The upcoming ITER
ITER
ITER is an international nuclear fusion research and engineering project, which is currently building the world's largest and most advanced experimental tokamak nuclear fusion reactor at Cadarache in the south of France...

 Thermonuclear Reactor is expected to range from 110–170 GHz and will employ Electron Cyclotron Resonance Heating (ECRH).

Microwaves can be used to transmit power over long distances, and post-World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 research was done to examine possibilities. NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 worked in the 1970s and early 1980s to research the possibilities of using solar power satellite (SPS) systems with large solar array
Photovoltaic module
A solar panel is a packaged, connected assembly of solar cells, also known as photovoltaic cells...

s that would beam power down to the Earth's surface via microwaves.

Less-than-lethal weaponry exists that uses millimeter waves to heat a thin layer of human skin to an intolerable temperature so as to make the targeted person move away. A two-second burst of the 95 GHz focused beam heats the skin to a temperature of 130 °F (54 °C) at a depth of 1/64th of an inch (0.4 mm). The United States Air Force
United States Air Force
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...

 and Marines
United States Marine Corps
The United States Marine Corps is a branch of the United States Armed Forces responsible for providing power projection from the sea, using the mobility of the United States Navy to deliver combined-arms task forces rapidly. It is one of seven uniformed services of the United States...

 are currently using this type of active denial system
Active Denial System
The Active Denial System is a non-lethal, directed-energy weapon developed by the U.S. military. It is a strong millimeter-wave transmitter primarily used for crowd control . Some ADS such as HPEM ADS are also used to disable vehicles. Informally, the weapon is also called the heat ray...

.

Spectroscopy


Microwave radiation is used in electron paramagnetic resonance
Electron paramagnetic resonance
Electron paramagnetic resonance or electron spin resonance spectroscopyis a technique for studying chemical species that have one or more unpaired electrons, such as organic and inorganic free radicals or inorganic complexes possessing a transition metal ion...

 (EPR or ESR) spectroscopy, typically in the X-band region (~9 GHz) in conjunction typically with magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

s of 0.3 T. This technique provides information on unpaired electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s in chemical systems, such as free radicals or transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...

 ions such as Cu(II). The microwave radiation can also be combined with electrochemistry
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...

 as in microwave enhanced electrochemistry
Microwave enhanced electrochemistry
Microwave radiation was applied in electrochemical methods in 1998 when Frank Marken and Richard G. Compton in Oxford placed a piece of platinum wire inside microwave cavity in small electrochemical cell....

.

Microwave frequency bands


The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain
Radio Society of Great Britain
First founded in 1913 as the London Wireless Club, the Radio Society of Great Britain is the United Kingdom's recognised national society for amateur radio operators. The society's patron is Prince Philip, Duke of Edinburgh and it represents the interests of the UK’s 60,000 licensed radio amateurs...

 (RSGB), are shown in the table below:
Microwave frequency bands
Letter DesignationFrequency range
L band
L band
L band refers to four different bands of the electromagnetic spectrum: 40 to 60 GHz , 1 to 2 GHz , 1565 nm to 1625 nm , and around 3.5 micrometres .-NATO L band:...

1 to 2 GHz
S band
S band
The S band is defined by an IEEE standard for radio waves with frequencies that range from 2 to 4 GHz, crossing the conventional boundary between UHF and SHF at 3.0 GHz. It is part of the microwave band of the electromagnetic spectrum...

2 to 4 GHz
C band
C band
The C band is a name given to certain portions of the electromagnetic spectrum, including wavelengths of microwaves that are used for long-distance radio telecommunications. The IEEE C-band - and its slight variations - contains frequency ranges that are used for many satellite communications...

4 to 8 GHz
X band
X band
The X band is a segment of the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of X band is rather indefinitely set at approximately 7.0 to 11.2 gigahertz . In radar engineering, the frequency range is specified...

8 to 12 GHz
Ku band
Ku band
The Kμ band is a portion of the electromagnetic spectrum in the microwave range of frequencies. This symbol refers to —in other words, the band directly below the K-band...

12 to 18 GHz
K band
K band
K band designates certain portions of the electromagnetic spectrum, in either the microwave domain or in the infrared domain. The microwave K bands are used primarily for radar and satellite communications while the infrared K band is used for astronomical observations.-NATO K band:The NATO K band...

18 to 26.5 GHz
Ka band
Ka band
The Ka band covers the frequencies of 26.5–40 GHz. The Ka band is part of the K band of the microwave band of the electromagnetic spectrum. This symbol refers to "K-above" — in other words, the band directly above the K-band...

26.5 to 40 GHz
Q band
Q band
The Q band of the microwave part of the electromagnetic spectrum and ranges from 33 to 50 GHz. It sits above, and partly overlaps with, the U.S. IEEE designated Ka band . It sits below the U.S...

33 to 50 GHz
U band 40 to 60 GHz
V band
V band
The V band of the electromagnetic spectrum ranges from 50 to 75 GHz. The V band is not heavily used, except for millimeter wave radar research and other kinds of scientific research...

50 to 75 GHz
E band
E band
The NATO E band is the range of radio frequencies from 2 GHz to 3 GHz in the electromagnetic spectrum. This is equal to wave lengths between 15 cm and 10 cm. The E band is in the upper UHF range of the radio spectrum...

60 to 90 GHz
W band
W band
The W band of the microwave part of the electromagnetic spectrum ranges from 75 to 110 GHz. It sits above the U.S. IEEE designated V band in frequency, yet overlaps the NATO designated M band...

75 to 110 GHz
F band
F band
The F band is the range of radio frequencies from 90 GHz to 140 GHz in the electromagnetic spectrum. This is equal to wave lengths between 2.1 mm and 3.3 mm...

90 to 140 GHz
D band
D band
IEEE D band is the range of radio frequencies from 110 GHz to 170 GHz in the electromagnetic spectrum. This is equal to wave lengths between 1.8 mm and 2.7 mm...

110 to 170 GHz


Footnote (1): P band is sometimes incorrectly used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521, see and. For other definitions see Letter Designations of Microwave Bands.

Footnote (2): When radars were first developed at K band during World War II, it was not realized that there was a nearby absorption band (due to water vapor and oxygen at the atmosphere). To avoid this problem, the original K band was split into a lower band, Ku, and upper band, Ka see.

Microwave frequency measurement


Microwave frequency can be measured by either electronic or mechanical techniques.

Frequency counter
Frequency counter
A frequency counter is an electronic instrument, or component of one, that is used for measuring frequency. Frequency is defined as the number of events of a particular sort occurring in a set period of time. Frequency counters usually measure the number of oscillations or pulses per second in a...

s or high frequency heterodyne
Heterodyne
Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden where high frequency signals are converted to lower frequencies by combining two frequencies. Heterodyning is useful for frequency shifting information of interest into a useful...

 systems can be used. Here the unknown frequency is compared with harmonics of a known lower frequency by use of a low frequency generator, a harmonic generator and a mixer. Accuracy of the measurement is limited by the accuracy and stability of the reference source.

Mechanical methods require a tunable resonator such as an absorption wavemeter
Absorption wavemeter
The Absorption wavemeter is a simple device for the measurement of radio frequency energy at different frequencies.The device can be used by the users of radio equipment to check the approximate frequency of a strong signal source, and also to check the output of a transmitter for harmonics. Many...

, which has a known relation between a physical dimension and frequency.
In a laboratory setting, Lecher lines
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 UHF and microwave frequencies. They form a short length of balanced transmission line. When attached to a source of radio frequency power such as 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
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...

 or slotted coaxial line to directly measure the wavelength. These devices consist of a probe introduced into the line through a longitudinal slot, so that the probe is free to travel up and down the line. Slotted lines are primarily intended for measurement of the voltage standing wave ratio on the line. However, provided a standing wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...

 is present, they may also be used to measure the distance between the nodes
Node (physics)
A node is a point along a standing wave where the wave has minimal amplitude. For instance, in a vibrating guitar string, the ends of the string are nodes. By changing the position of the end node through frets, the guitarist changes the effective length of the vibrating string and thereby the...

, which is equal to half the wavelength. Precision of this method is limited by the determination of the nodal locations.

Health effects


Microwaves do not contain sufficient energy to chemically change substances by ionization, and so are an example of nonionizing radiation. The word "radiation" refers to the fact that energy can radiate. The term in this context is not to be confused with radioactivity. It has not been shown conclusively that microwaves (or other nonionizing electromagnetic radiation) have significant adverse biological effects at low levels. Some, but not all, studies suggest that long-term exposure may have a carcinogen
Carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...

ic effect. This is separate from the risks associated with very high intensity exposure, which can cause heating and burns like any heat source, and not a unique property of microwaves specifically.

During World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, it was observed that individuals in the radiation path of radar installations experienced clicks and buzzing sounds in response to microwave radiation. This microwave auditory effect
Microwave auditory effect
The microwave auditory effect, also known as the microwave hearing effect or the Frey effect, consists of audible clicks induced by pulsed/modulated microwave frequencies. The clicks are generated directly inside the human head without the need of any receiving electronic device...

 was thought to be caused by the microwaves inducing
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....

 an electric current in the hearing centers of the brain. Research by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 in the 1970s has shown this to be caused by thermal expansion in parts of the inner ear.

When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in the body. Exposure to microwave radiation can produce cataract
Cataract
A cataract is a clouding that develops in the crystalline lens of the eye or in its envelope, varying in degree from slight to complete opacity and obstructing the passage of light...

s by this mechanism, because the microwave heating denatures protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s in the crystalline lens
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...

 of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

 (in the same way that heat turns egg white
Egg white
Egg white is the common name for the clear liquid 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 around either fertilized or unfertilized egg yolks...

s white and opaque) faster than the lens can be cooled by surrounding structures. The lens and cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

 of the eye are especially vulnerable because they contain no blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

s that can carry away heat. Exposure to heavy doses of microwave radiation (as from an oven that has been tampered with to allow operation even with the door open) can produce heat damage in other tissues as well, up to and including serious burn
Burn
A burn is an injury to flesh caused by heat, electricity, chemicals, light, radiation, or friction.Burn may also refer to:*Combustion*Burn , type of watercourses so named in Scotland and north-eastern England...

s that may not be immediately evident because of the tendency for microwaves to heat deeper tissues with higher moisture content.

History and research


The existence of electromagnetic waves was predicted by James Clerk Maxwell
James Clerk Maxwell
James Clerk Maxwell of Glenlair was a Scottish physicist and mathematician. His most prominent achievement was formulating classical electromagnetic theory. This united all previously unrelated observations, experiments and equations of electricity, magnetism and optics into a consistent theory...

 in 1864 from his equations
Maxwell's equations
Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...

. In 1888, Heinrich Hertz was the first to demonstrate the existence of electromagnetic waves by building an apparatus that produced and detected microwaves in the UHF region. The design necessarily used horse-and-buggy materials, including a horse trough, a wrought iron point spark, Leyden jar
Leyden jar
A Leyden jar, or Leiden jar, is a device that "stores" static electricity between two electrodes on the inside and outside of a jar. It was invented independently by German cleric Ewald Georg von Kleist on 11 October 1745 and by Dutch scientist Pieter van Musschenbroek of Leiden in 1745–1746. The...

s, and a length of zinc gutter whose parabolic cross-section worked as a reflection antenna. In 1894 J. C. Bose
Jagdish Chandra Bose
Acharya Sir Jagadish Chandra Bose, CSI, CIE, FRS was a Bengali polymath: a physicist, biologist, botanist, archaeologist, as well as an early writer of science fiction...

 publicly demonstrated radio control of a bell using millimeter wavelengths, and conducted research into the propagation of microwaves.

Perhaps the first, documented, formal use of the term microwave occurred in 1931:
"When trials with wavelengths as low as 18 cm were made known, there was undisguised surprise that the problem of the micro-wave had been solved so soon." Telegraph & Telephone Journal XVII. 179/1


In 1943, the Hungarian engineer Zoltán Bay sent ultra-short radio waves to the moon, which, reflected from there, worked as a radar, and could be used to measure distance, as well as to study the moon.

Perhaps the first use of the word microwave in an astronomical context occurred in 1946 in an article "Microwave Radiation from the Sun and Moon" by Robert Dicke and Robert Beringer. This same article also made a showing in the New York Times issued in 1951.

In the history of electromagnetic theory
History of electromagnetic theory
The history of electromagnetism dates back over several thousand years. In the history of electromagnetic theory, the ancients would have been acquainted with the effects of atmospheric electricity, in particular lightning as thunderstorms in most southern latitudes are common, and they also knew...

, significant work specifically in the area of microwaves and their applications was carried out by researchers including:
Specific work on microwaves
Work carried out byArea of work
Barkhausen
Heinrich Barkhausen
Heinrich Georg Barkhausen , born at Bremen, was a German physicist.Born into a patrician family in Bremen, he showed interest in natural sciences from an early age...

 and Kurz
Positive grid oscillators
Hull Smooth bore magnetron
Varian Brothers Velocity modulated electron beam → klystron
Klystron
A klystron is a specialized linear-beam vacuum tube . Klystrons are used as amplifiers at microwave and radio frequencies to produce both low-power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for modern...

 tube
Randall
John Randall (physicist)
Sir John Turton Randall, FRS, FRSE, was a British physicist and biophysicist, credited with radical improvement of the cavity magnetron, an essential component of centimetric wavelength radar, which was one of the keys to the Allied victory in the Second World War. It is also the key component of...

 and Boot
Harry Boot
Henry Albert Howard "Harry" Boot 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.-Biography:...

Cavity magnetron
Cavity magnetron
The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field. The 'resonant' cavity magnetron variant of the earlier magnetron tube was invented by John Randall and Harry Boot in 1940 at the University of...



See also

  • Block upconverter
    Block upconverter
    A block upconverter is used in the transmission of satellite signals. It converts a band of frequencies from a lower frequency to a higher frequency. Modern BUCs convert from the L band to Ku band, C band and Ka band...

     (BUC)
  • Cosmic microwave background radiation
    Cosmic microwave background radiation
    In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....

  • Electron cyclotron resonance
    Electron cyclotron resonance
    Electron cyclotron resonance is a phenomenon observed both in plasma physics and condensed matter physics. An electron in a static and uniform magnetic field will move in a circle due to the Lorentz force...

  • International Microwave Power Institute
    International Microwave Power Institute
    The International Microwave Power Institute is an organization devoted to microwave energy and its usage. Founded in Canada in 1966, it is now headquartered in Mechanicsville, Virginia....

  • Low-noise block converter
    Low-noise block converter
    A low-noise block downconverter is the receiving device of a parabolic satellite dish antenna of the type commonly used for satellite TV reception...

     (LNB)
  • Maser
    Maser
    A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. Historically, “maser” derives from the original, upper-case acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation"...

  • Microwave transmission
    Microwave transmission
    Microwave transmission refers to the technology of transmitting information or power by the use of radio waves whose wavelengths are conveniently measured in small numbers of centimeters; these are called microwaves. This part of the radio spectrum ranges across frequencies of roughly...

  • Microwave chemistry
    Microwave chemistry
    Microwave chemistry is the science of applying microwave irradiation to chemical reactions. Microwaves act as high frequency electric fields and will generally heat any material containing mobile electric charges, such as polar molecules in a solvent or conducting ions in a solid...

  • Microwave auditory effect
    Microwave auditory effect
    The microwave auditory effect, also known as the microwave hearing effect or the Frey effect, consists of audible clicks induced by pulsed/modulated microwave frequencies. The clicks are generated directly inside the human head without the need of any receiving electronic device...

  • Microwave cavity
    Microwave cavity
    A microwave cavity is a closed metal structure that confines electromagnetic fields in the microwave region of the spectrum. Such cavities act as resonant circuits with extremely low loss at their frequency of operation...

  • Microwave radio relay
  • Orthomode transducer
    Orthomode transducer
    An orthomode transducer is a microwave duct component of the class of microwave circulators. It is commonly referred to as an OMT, and commonly referred as a polarisation duplexer. Such device may be part of a VSAT antenna feed Orthomode transducers serve either to combine or to separate two...

     (OMT)
  • Plasma-enhanced chemical vapour deposition
  • Rain fade
    Rain fade
    Rain fade refers primarily to the absorption of a microwave radio frequency signal by atmospheric rain, snow or ice, and losses are especially prevalent at frequencies above 11 GHz. It also refers to the degradation of a signal caused by the electromagnetic interference of the leading edge of a...

  • RF switch matrix
    RF Switch Matrix
    RF Switch Matrix or Microwave Switch Matrix or Switch MatrixAn RF/Microwave Switch Matrix is used in test systems, in both design verification and manufacturing test, to route high frequency signals between the device under test and the test and measurement equipment...

  • Thing (listening device)
    Thing (listening device)
    The Thing, also known as the Great Seal bug, was one of the first covert listening devices to use passive techniques to transmit an audio signal...

  • Tropospheric scatter
    Tropospheric scatter
    Tropospheric scatter is a method of transmitting and receiving microwave radio signals over considerable distances – often up to 300 km...


External links