Extremely high frequency

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Extremely high frequency
Extremely high frequency (ITU)
Frequency range
30 to 300 GHz
Wavelength range
1 cm to 1 mm
Related bands
Millimetre band (IEEE)
Frequency range
110 to 300 GHz
Wavelength range
2.73 to 1 mm
Related bands

Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It lies between the super high frequency band, and the far infrared band, the lower part of which is also referred to as the terahertz gap. Radio waves in this band have wavelengths from ten to one millimetre, so it is also called the millimetre band and radiation in this band is called millimetre waves, sometimes abbreviated MMW or mmW or mmWave. Millimetre-length electromagnetic waves were first investigated in the 1890s by Indian scientist Jagadish Chandra Bose.

International Telecommunication Union Specialised agency of the United Nations

The International Telecommunication Union (ITU), originally the International Telegraph Union, is a specialised agency of the United Nations that is responsible for issues that concern information and communication technologies. It is the oldest among all the 15 specialised agencies of UN.

Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around twenty thousand times per second to around three hundred billion times per second. This is roughly between the upper limit of audio frequencies and the lower limit of infrared frequencies; these are the frequencies at which energy from an oscillating current can radiate off a conductor into space as radio waves. Different sources specify different upper and lower bounds for the frequency range.

The electromagnetic spectrum is the range of frequencies of electromagnetic radiation and their respective wavelengths and photon energies.


Compared to lower bands, radio waves in this band have high atmospheric attenuation: they are absorbed by the gases in the atmosphere. Therefore, they have a short range and can only be used for terrestrial communication over about a kilometer. Absorption by humidity in the atmosphere is significant except in desert environments, and attenuation by rain (rain fade) is a serious problem even over short distances. However the short propagation range allows smaller frequency reuse distances than lower frequencies. The short wavelength allows modest size antennas to have a small beam width, further increasing frequency reuse potential.

In physics, attenuation or, in some contexts, extinction is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at variable attenuation rates.

Rain fade refers primarily to the absorption of a microwave radio frequency (RF) signal by atmospheric rain, snow, or ice, and losses which 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 storm front. Rain fade can be caused by precipitation at the uplink or downlink location. It does not need to be raining at a location for it to be affected by rain fade, as the signal may pass through precipitation many miles away, especially if the satellite dish has a low look angle. From 5% to 20% of rain fade or satellite signal attenuation may also be caused by rain, snow, or ice on the uplink or downlink antenna reflector, radome or feed horn. Rain fade is not limited to satellite uplinks or downlinks, as it can also affect terrestrial point-to-point microwave links.


Atmospheric attenuation in dB/km as a function of frequency over the EHF band. Peaks in absorption at specific frequencies are a problem, due to atmosphere constituents such as water vapour (H2O) and molecular oxygen (O2). The vertical scale is exponential. Micrwavattrp.png
Atmospheric attenuation in dB/km as a function of frequency over the EHF band. Peaks in absorption at specific frequencies are a problem, due to atmosphere constituents such as water vapour (H2O) and molecular oxygen (O2). The vertical scale is exponential.

Millimeter waves propagate solely by line-of-sight paths. They are not reflected by the ionosphere nor do they travel along the Earth as ground waves as lower frequency radio waves do. [1] At typical power densities they are blocked by building walls and suffer significant attenuation passing through foliage. [1] [2] [3] Absorption by atmospheric gases is a significant factor throughout the band and increases with frequency. However, it is maximum at a few specific absorption lines, mainly those of oxygen at 60 GHz and water vapor at 24 GHz and 184 GHz. [2] At frequencies in the "windows" between these absorption peaks, millimeter waves have much less atmospheric attenuation and greater range, so many applications use these frequencies. Millimeter wavelengths are the same order of size as raindrops, so precipitation causes additional attenuation due to scattering (rain fade) as well as absorption. [2] [3] The high free space loss and atmospheric absorption limits useful propagation to a few kilometers. [1] Thus, they are useful for densely packed communications networks such as personal area networks that improve spectrum utilization through frequency reuse. [1]

Line-of-sight propagation characteristic of electromagnetic radiation or acoustic wave propagation which means waves which travel in a direct path from the source to the receiver

Line-of-sight propagation is a characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver. Electromagnetic transmission includes light emissions traveling in a straight line. The rays or waves may be diffracted, refracted, reflected, or absorbed by the atmosphere and obstructions with material and generally cannot travel over the horizon or behind obstacles.

Ionosphere The ionized part of Earths upper atmosphere

The ionosphere is the ionized part of Earth's upper atmosphere, from about 60 km (37 mi) to 1,000 km (620 mi) altitude, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an important role in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. The region below the ionosphere is called neutral atmosphere, or neutrosphere.

Spectral line optical phenomenon

A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to identify atoms and molecules. These "fingerprints" can be compared to the previously collected "fingerprints" of atoms and molecules, and are thus used to identify the atomic and molecular components of stars and planets, which would otherwise be impossible.

Millimeter waves show "optical" propagation characteristics and can be reflected and focused by small metal surfaces and dielectric lenses around 5 to 30 cm (2 inches to 1 foot) diameter. Because their wavelengths are often much smaller than the equipment that manipulates them, the techniques of geometric optics can be used. Diffraction is less than at lower frequencies, although they can be diffracted by building edges. At millimeter wavelengths, surfaces appear rougher so diffuse reflection increases. [1] Multipath propagation, particularly reflection from indoor walls and surfaces, causes serious fading. [3] [4] Doppler shift of frequency can be significant even at pedestrian speeds. [1] In portable devices, shadowing due to the human body is a problem. Since the waves penetrate clothing and their small wavelength allows them to reflect from small metal objects they are used in millimeter wave scanners for airport security scanning.

Diffraction refers to various phenomena that occur when a wave encounters an obstacle or a slit

Diffraction refers to various phenomena that occur when a wave encounters an obstacle or a slit. It is defined as the 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 secondary source of the propagating wave. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1660.

Diffuse reflection Reflection with light scattered at random angles

Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered at many angles rather than at just one angle as in the case of specular reflection. An ideal diffuse reflecting surface is said to exhibit Lambertian reflection, meaning that there is equal luminance when viewed from all directions lying in the half-space adjacent to the surface.

In wireless telecommunications, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies and terrestrial objects such as mountains and buildings.


Scientific research

Part of the Atacama Large Millimeter Array (ALMA), a millimeter wave radio telescope The Atacama Compact Array.jpg
Part of the Atacama Large Millimeter Array (ALMA), a millimeter wave radio telescope

This band is commonly used in radio astronomy and remote sensing. Ground-based radio astronomy is limited to high altitude sites such as Kitt Peak and Atacama Large Millimeter Array (ALMA) due to atmospheric absorption issues.

Radio astronomy subfield of astronomy that studies celestial objects at radio frequencies

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 1932, when Karl Jansky at Bell Telephone Laboratories observed radiation coming from the Milky Way. Subsequent observations have identified a number of different sources of radio emission. These include stars and galaxies, as well as entirely new classes of objects, such as radio galaxies, quasars, pulsars, and masers. The discovery of the cosmic microwave background radiation, regarded as evidence for the Big Bang theory, was made through radio astronomy.

Remote sensing Acquisition of information at a significant distance from the subject

Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object and thus in contrast to on-site observation, especially the Earth. Remote sensing is used in numerous fields, including geography, land surveying and most Earth Science disciplines ; it also has military, intelligence, commercial, economic, planning, and humanitarian applications.

Astronomy Universe events since the Big Bang 13.8 billion years ago

Astronomy is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry to try and explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, and comets. Relevant phenomena include supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, astronomy studies everything that originates outside Earth's atmosphere. Cosmology is a branch of astronomy. It studies the Universe as a whole.

Satellite-based remote sensing near 60 GHz can determine temperature in the upper atmosphere by measuring radiation emitted from oxygen molecules that is a function of temperature and pressure. The ITU non-exclusive passive frequency allocation at 57–59.3 GHz is used for atmospheric monitoring in meteorological and climate sensing applications and is important for these purposes due to the properties of oxygen absorption and emission in Earth's atmosphere. Currently operational U.S. satellite sensors such as the Advanced Microwave Sounding Unit (AMSU) on one NASA satellite (Aqua) and four NOAA (15–18) satellites and the special sensor microwave/imager (SSMI/S) on Department of Defense satellite F-16 make use of this frequency range. [5]

The Special Sensor Microwave/Imager (SSM/I) is a seven-channel, four-frequency, linearly polarized passive microwave radiometer system. It is flown on board the United States Air Force Defense Meteorological Satellite Program (DMSP) Block 5D-2 satellites. The instrument measures surface/atmospheric microwave brightness temperatures (TBs) at 19.35, 22.235, 37.0 and 85.5 GHz. The four frequencies are sampled in both horizontal and vertical polarizations, except the 22 GHz which is sampled in the vertical only.


In the United States, the band 36.0 – 40.0 GHz is used for licensed high-speed microwave data links, and the 60 GHz band can be used for unlicensed short range (1.7 km) data links with data throughputs up to 2.5 Gbit/s. It is used commonly in flat terrain.

The 71–76, 81–86 and 92–95 GHz bands are also used for point-to-point high-bandwidth communication links. These higher frequencies do not suffer from oxygen absorption, but require a transmitting license in the US from the Federal Communications Commission (FCC). There are plans for 10 Gbit/s links using these frequencies as well. In the case of the 92–95 GHz band, a small 100 MHz range has been reserved for space-borne radios, limiting this reserved range to a transmission rate of under a few gigabits per second. [6]

A CableFree MMW link installed in the UAE installed for Safe City applications, providing 1Gbit/s capacity between sites. The links are fast to deploy and have a lower cost than fibre optics. CableFree MMW Link installed in UAE.jpg
A CableFree MMW link installed in the UAE installed for Safe City applications, providing 1Gbit/s capacity between sites. The links are fast to deploy and have a lower cost than fibre optics.

The band is essentially undeveloped and available for use in a broad range of new products and services, including high-speed, point-to-point wireless local area networks and broadband Internet access. WirelessHD is another recent technology that operates near the 60 GHz range. Highly directional, "pencil-beam" signal characteristics permit different systems to operate close to one another without causing interference. Potential applications include radar systems with very high resolution.

The Wi-Fi standard IEEE 802.11ad operates in the 60 GHz (V band) spectrum to achieve data transfer rates as high as 7 Gbit/s.

Uses of the millimeter wave bands include point-to-point communications, intersatellite links, and point-to-multipoint communications. There are tentative plans to use millimeter waves in future 5G mobile phones. [7] In addition, use of millimeter wave bands for vehicular communication is also emerging as an attractive solution to support (semi-)autonomous vehicular communications. [8]

Shorter wavelengths in this band permit the use of smaller antennas to achieve the same high directivity and high gain as larger ones in lower bands. The immediate consequence of this high directivity, coupled with the high free space loss at these frequencies, is the possibility of a more efficient use of frequencies for point-to-multipoint applications. Since a greater number of highly directive antennas can be placed in a given area, the net result is greater frequency reuse, and higher density of users. The high usable channel capacity in this band might allow it to serve some applications that would otherwise use fiber-optic communication.

Weapons systems

Millimeter wave fire control radar for CIWS gun on Russian aircraft carrier Minsk Minsk port bow AK-630 CIWS gun fire control radar.JPG
Millimeter wave fire control radar for CIWS gun on Russian aircraft carrier Minsk

Millimeter wave radar is used in short-range fire-control radar in tanks and aircraft, and automated guns (CIWS) on naval ships to shoot down incoming missiles. The small wavelength of millimeter waves allows them to track the stream of outgoing bullets as well as the target, allowing the computer fire control system to change the aim to bring them together. [ citation needed ]

With Raytheon the U.S. Air Force has developed a nonlethal antipersonnel weapon system called Active Denial System (ADS) which emits a beam of radiation with a wavelength of 3 mm. [9] The weapon causes a person in the beam to feel an intense burning pain, as if their skin is going to catch fire.

Security screening

Clothing and other organic materials are transparent to millimeter waves of certain frequencies, so a recent application has been scanners to detect weapons and other dangerous objects carried under clothing, for applications such as airport security. [10] Privacy advocates are concerned about the use of this technology because, in some cases, it allows screeners to see airport passengers as if without clothing.

The TSA has deployed millimeter wave scanners to many major airports.

Prior to a software upgrade the technology did not mask any part of the bodies of the people who were being scanned. However, passengers' faces were deliberately masked by the system. The photos were screened by technicians in a closed room, then deleted immediately upon search completion. Privacy advocates are concerned. "We're getting closer and closer to a required strip-search to board an airplane," said Barry Steinhardt of the American Civil Liberties Union. [11] To address this issue, upgrades have eliminated the need for an officer in a separate viewing area. The new software generates a generic image of a human. There is no anatomical differentiation between male and female on the image, and if an object is detected, the software only presents a yellow box in the area. If the device does not detect anything of interest, no image is presented. [12] Passengers can decline scanning and be screened via a metal detector and patted down.[ citation needed ]

Three security scanners using millimeter waves were put into use at Schiphol Airport in Amsterdam on 15 May 2007, with more expected to be installed later. The passenger's head is masked from the view of the security personnel.

According to Farran Technologies, a manufacturer of one model of the millimeter wave scanner, the technology exists to extend the search area to as far as 50 meters beyond the scanning area which would allow security workers to scan a large number of people without their awareness that they are being scanned. [13]

Thickness gauging

Recent studies at the University of Leuven have proven that millimeter waves can also be used as a non-nuclear thickness gauge in various industries. Millimeter waves provide a clean and contact-free way of detecting variations in thickness. Practical applications for the technology focus on plastics extrusion, paper manufacturing, glass production and mineral wool production.


Low intensity (usually 10 mW/cm2 or less) electromagnetic radiation of extremely high frequency may be used in human medicine for the treatment of diseases. For example, "A brief, low-intensity MMW exposure can change cell growth and proliferation rates, activity of enzymes, state of cell genetic apparatus, function of excitable membranes and peripheral receptors." [14] This treatment is particularly associated with the range of 40 – 70 GHz. [15] This type of treatment may be called millimeter wave (MMW) therapy or extremely high frequency (EHF) therapy. [16] This treatment is associated with eastern European nations (e.g., former USSR nations). [14] The Russian Journal Millimeter waves in biology and medicine studies the scientific basis and clinical applications of millimeter wave therapy. [17]

Police speed radar

Traffic police use speed-detecting radar gun s in the Ka-band (33.4 – 36.0 GHz). [18]

See also

Related Research Articles

Microwave form of electromagnetic radiation

Microwaves are a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter; with frequencies between 300 MHz (1 m) and 300 GHz (1 mm). Different sources define different frequency ranges as microwaves; the above broad definition includes both UHF and EHF bands. A more common definition in radio engineering is the range between 1 and 100 GHz. In all cases, microwaves include the entire SHF band 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 Ku band is the portion of the electromagnetic spectrum in the microwave range of frequencies from 12 to 18 gigahertz (GHz). The symbol is short for "K-under", because it is the lower part of the original NATO K band, which was split into three bands because of the presence of the atmospheric water vapor resonance peak at 22.24 GHz, (1.35 cm) which made the center unusable for long range transmission. In radar applications, it ranges from 12-18 GHz according to the formal definition of radar frequency band nomenclature in IEEE Standard 521-2002.

Ultra high frequency The range 300-3000 MHz of the electromagnetic spectrum

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 waves with frequencies above the UHF band fall into the super-high frequency (SHF) or microwave frequency range. Lower frequency signals fall into the VHF or lower bands. UHF radio waves propagate mainly by line of sight; they are blocked by hills and large buildings although the transmission through building walls is strong enough for indoor reception. They are used for television broadcasting, cell phones, satellite communication including GPS, personal radio services including Wi-Fi and Bluetooth, walkie-talkies, cordless phones, and numerous other applications.

Microwave radiometer

A microwave radiometer (MWR) is a radiometer that measures energy emitted at millimetre-to-centimetre wavelengths known as microwaves. Microwave radiometers are very sensitive receivers designed to measure thermal electromagnetic radiation emitted by atmospheric gases. They are usually equipped with multiple receiving channels in order to derive the characteristic emission spectrum of the atmosphere or extraterrestrial objects. Microwave radiometers are utilized in a variety of environmental and engineering applications, including weather forecasting, climate monitoring, radio astronomy and radio propagation studies.

Terahertz radiation The range 300-3000 GHz of the electromagnetic spectrum

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 frequencies from 0.1 to 30 terahertz (THz). One terahertz is 1012 Hz or 1000 GHz. Wavelengths of radiation in the terahertz band correspondingly range from 1 mm to 0.1 mm (or 100 μm). Because terahertz radiation begins at a wavelength of one millimeter and proceeds into shorter wavelengths, it is sometimes known as the submillimeter band, and its radiation as submillimeter waves, especially in astronomy.

Super high frequency (SHF) is the ITU designation for radio frequencies (RF) in the range between 3 and 30 gigahertz (GHz). This band of frequencies is also known as the centimetre band or centimetre wave as the wavelengths range from one to ten centimetres. These frequencies fall within the microwave band, so radio waves with these frequencies are called microwaves. The small wavelength of microwaves allows them to be directed in narrow beams by aperture antennas such as parabolic dishes and horn antennas, so they are used for point-to-point communication and data links and for radar. This frequency range is used for most radar transmitters, wireless LANs, satellite communication, microwave radio relay links, and numerous short range terrestrial data links. They are also used for heating in industrial microwave heating, medical diathermy, microwave hyperthermy to treat cancer, and to cook food in microwave ovens.

Indium phosphide chemical compound

Indium phosphide (InP) is a binary semiconductor composed of indium and phosphorus. It has a face-centered cubic ("zincblende") crystal structure, identical to that of GaAs and most of the III-V semiconductors.

The V band ("vee-band") is a standard designation by the Institute of Electrical and Electronics Engineers (IEEE) for a band of frequencies in the microwave portion of the electromagnetic spectrum ranging from 40 to 75 gigahertz (GHz). The V band is not heavily used, except for millimeter wave radar research and other kinds of scientific research. It should not be confused with the 600–1000 MHz range of Band V of the UHF frequency range.

Radio spectrum part of the electromagnetic spectrum from 3 Hz to 3000 GHz (3 THz)

The radio spectrum is the part of the electromagnetic spectrum with frequencies from 30 hertz to 300 GHz. Electromagnetic waves in this frequency range, called radio waves, are widely used in modern technology, particularly in telecommunication. To prevent interference between different users, the generation and transmission of radio waves is strictly regulated by national laws, coordinated by an international body, the International Telecommunication Union (ITU).

Absorption (electromagnetic radiation) way in which the energy of a photon is taken up by matter; physical process of absorbing light, while absorbance does not always measure absorption: it measures attenuation (of transmitted radiant power)

In physics, absorption of electromagnetic radiation is how matter takes up a photon's energy — and so transforms electromagnetic energy into internal energy of the absorber. A notable effect (attenuation) is to gradually reduce the intensity of light waves as they propagate through a medium. Although the absorption of waves does not usually depend on their intensity, in certain conditions (optics) the medium's transparency changes by a factor that varies as a function of wave intensity, and saturable absorption occurs.

The W band of the microwave part of the electromagnetic spectrum ranges from 75 to 110 GHz, wavelength ≈2.7–4 mm. It sits above the U.S. IEEE-designated V band (40–75 GHz) in frequency, and overlaps the NATO designated M band (60–100 GHz). The W band is used for satellite communications, millimeter-wave radar research, military radar targeting and tracking applications, and some non-military applications.

Microwave transmission technology of transmitting information or energy by the use of microwaves

Microwave transmission is the transmission of information by microwave radio waves. Although an experimental 40-mile (64 km) microwave telecommunication link across the English Channel was demonstrated in 1931, the development of radar in World War II provided the technology for practical exploitation of microwave communication. In the 1950s, large transcontinental microwave relay networks, consisting of chains of repeater stations linked by line-of-sight beams of microwaves were built in Europe and America to relay long distance telephone traffic and television programs between cities. Communication satellites which transferred data between ground stations by microwaves took over much long distance traffic in the 1960s. In recent years, there has been an explosive increase in use of the microwave spectrum by new telecommunication technologies such as wireless networks, and direct-broadcast satellites which broadcast television and radio directly into consumers' homes.

Millimeter wave scanner

A millimeter wave scanner is a whole-body imaging device used for detecting objects concealed underneath a person’s clothing using a form of electromagnetic radiation. Typical uses for this technology include detection of items for commercial loss prevention, smuggling and screening at government buildings and airport security checkpoints. Several countries employ the scanners for security screening.

The Q band is a range of frequencies contained in the microwave region of the electromagnetic spectrum. Common usage places this range between 33 and 50 GHz, but may vary depending on the source using the term. The foregoing range corresponds to the recommended frequency band of operation of WR22 waveguides. These frequencies are equivalent to wavelengths between 6 mm and 9.1 mm in air/vacuum. The Q band is in the EHF range of the radio spectrum.

Terahertz metamaterial

A terahertz metamaterial is a class of composite metamaterials designed to interact at terahertz (THz) frequencies. The terahertz frequency range used in materials research is usually defined as 0.1 to 10 THz.

Non-ionizing radiation electromagnetic radiation that does not carry enough energy per quantum to ionize atoms or molecules

Non-ionizingradiation refers to any type of electromagnetic radiation that does not carry enough energy per quantum to ionize atoms or molecules—that is, to completely remove an electron from an atom or molecule. Instead of producing charged ions when passing through matter, non-ionizing electromagnetic radiation has sufficient energy only for excitation, the movement of an electron to a higher energy state. In contrast, ionizing radiation has a higher frequency and shorter wavelength than nonionizing radiation, and can be a serious health hazard; exposure to it can cause burns, radiation sickness, cancer, and genetic damage. Using ionizing radiation requires elaborate radiological protection measures, which in general are not required with nonionizing radiation.

The waveguide E band is the range of radio frequencies from 60 GHz to 90 GHz in the electromagnetic spectrum, corresponding to the recommended frequency band of operation of WR12 waveguides. These frequencies are equivalent to wave lengths between 5 mm and 3.333 mm. The E band is in the EHF range of the radio spectrum.

The IEEE K band is a portion of the radio spectrum in the microwave range of frequencies from 18 to 27 gigahertz (GHz). The range of frequencies in the center of the K band between 18 and 26.5 GHz is absorbed by water vapor in the atmosphere due to its resonance peak at 22.24 GHz, 1.35 cm. Therefore these frequencies experience high atmospheric attenuation and cannot be used for long distance applications. For this reason the original K band has been split into three bands, Ka band, K-band, and Ku band as detailed below.


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