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Extremely high frequency (EHF) is the
International Telecommunication Union The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
(ITU) designation for the band of radio frequencies in the
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging fro ...
from 30 to 300
gigahertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...
(GHz). It lies between the
super high frequency 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 t ...
band and the
far infrared Far infrared (FIR) is a region in the infrared spectrum of electromagnetic radiation. Far infrared is often defined as any radiation with a wavelength of 15 micrometers (μm) to 1 mm (corresponding to a range of about 20  THz to ...
band, the lower part of which is the terahertz band.
Radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (sho ...
s in this band have
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
s 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 mmWave. Millimetre-length electromagnetic waves were first investigated by Indian physicist
Jagadish Chandra Bose Sir Jagadish Chandra Bose (;, ; 30 November 1858 – 23 November 1937) was a biologist, physicist, botanist and an early writer of science fiction. He was a pioneer in the investigation of radio microwave optics, made significant contribution ...
, who generated waves of
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
up to 60GHz during experiments in 18941896. Compared to lower bands, radio waves in this band have high
atmospheric An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...
attenuation In physics, attenuation (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 var ...
: they are absorbed by the gases in the atmosphere. Absorption increases with frequency until at the top end of the band the waves are attenuated to zero within a few meters. Absorption by humidity in the atmosphere is significant except in desert environments, and attenuation by rain (
rain fade Rain fade refers primarily to the absorption of a microwave radio frequency (RF) signal by atmospheric rain, snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, ...
) is a serious problem even over short distances. However the short propagation range allows smaller
frequency reuse A cellular network or mobile network is a communication network where the link to and from end nodes is wireless. The network is distributed over land areas called "cells", each served by at least one fixed-location transceiver (typically thr ...
distances than lower frequencies. The short wavelength allows modest size antennas to have a small
beam width The beam diameter or beam width of an electromagnetic beam is the diameter along any specified line that is perpendicular to the beam axis and intersects it. Since beams typically do not have sharp edges, the diameter can be defined in many differ ...
, further increasing frequency reuse potential. Millimeter waves are used for military
fire-control radar A fire-control radar (FCR) is a radar that is designed specifically to provide information (mainly target azimuth, elevation, range and range rate) to a fire-control system in order to direct weapons such that they hit a target. They are someti ...
, airport security scanners, short range
wireless network A wireless network is a computer network that uses wireless data connections between network nodes. Wireless networking is a method by which homes, telecommunications networks and business installations avoid the costly process of introducing ...
s, and scientific research. In a major new application of millimeter waves, certain frequency ranges near the bottom of the band are being used in the newest generation of
cell phone A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive telephone call, calls over a radio freq ...
networks, 5G networks. The design of millimeter-wave circuit and subsystems (such as antennas, power amplifiers, mixers and oscillators) also presents severe challenges to engineers due to semiconductor and process limitations, model limitations and poor ''Q'' factors of passive devices.


Propagation

Millimeter waves propagate solely by line-of-sight paths. They are not reflected by the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...
nor do they travel along the Earth as ground waves as lower frequency radio waves do. At typical power densities they are blocked by building walls and suffer significant attenuation passing through foliage. Absorption by atmospheric gases is a significant factor throughout the band and increases with frequency. However, this absorption is maximum at a few specific absorption lines, mainly those of
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
at 60 GHz and
water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous p ...
at 24 GHz and 184 GHz. 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
raindrop A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant d ...
s, so precipitation causes additional attenuation due to
scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
(
rain fade Rain fade refers primarily to the absorption of a microwave radio frequency (RF) signal by atmospheric rain, snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, ...
) as well as absorption. The high free space loss and atmospheric absorption limit useful propagation to a few kilometers. Thus, they are useful for densely packed communications networks such as
personal area network A personal area network (PAN) is a computer network for interconnecting electronic devices within an individual person's workspace. A PAN provides data transmission among devices such as computers, smartphones, tablets and personal digital ...
s that improve spectrum utilization through
frequency reuse A cellular network or mobile network is a communication network where the link to and from end nodes is wireless. The network is distributed over land areas called "cells", each served by at least one fixed-location transceiver (typically thr ...
. Millimeter waves show "optical" propagation characteristics and can be reflected and focused by small metal surfaces and
dielectric lens A Luneburg lens (original German ''Lüneburg lens'', sometimes incorrectly spelled ''Luneberg lens'') is a spherically symmetric gradient-index lens. A typical Luneburg lens's refractive index ''n'' decreases radially from the center to the outer ...
es 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 Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is ca ...
can be used.
Diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
is less than at lower frequencies, although millimeter waves can be diffracted by building edges. At millimeter wavelengths, surfaces appear rougher so
diffuse reflection 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'' ...
increases.
Multipath propagation In radio communication, 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 refle ...
, particularly reflection from indoor walls and surfaces, causes serious fading.
Doppler shift The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who ...
of frequency can be significant even at pedestrian speeds. 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 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 ...
s for airport security scanning.


Applications


Scientific research

This band is commonly used in
radio astronomy Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comin ...
and
remote sensing Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Ear ...
. Ground-based radio
astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
is limited to high altitude sites such as
Kitt Peak Kitt Peak ( ood, Ioligam) is a mountain in the U.S. state of Arizona, and at is the highest point in the Quinlan Mountains. It is the location of the Kitt Peak National Observatory. The radio telescope at the observatory is one of ten dishes co ...
and Atacama Large Millimeter Array ( ALMA) due to atmospheric absorption issues. Satellite-based
remote sensing Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Ear ...
near 60 GHz can determine temperature in the
upper atmosphere Upper atmosphere is a collective term that refers to various layers of the atmosphere of the Earth above the troposphere and corresponding regions of the atmospheres of other planets, and includes: * The mesosphere, which on Earth lies between th ...
by measuring radiation emitted from oxygen molecules that is a function of temperature and pressure. The
ITU The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Union ...
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 The Special Sensor Microwave/Imager (SSM/I) is a seven-channel, four-frequency, linearly polarized passive microwave radiometer system.Hollinger, J.P. 1989: ''DMSP Special Sensor Microwave/Imager Calibration/Validation''. Final Report, Vol. I., Spac ...
(SSMI/S) on Department of Defense satellite F-16 make use of this frequency range.FCC.gov
Comments of IEEE Geoscience and Remote Sensing Society, FCC RM-11104, 10/17/07


Telecommunications

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 The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represented ...
/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 The Federal Communications Commission (FCC) is an independent agency of the United States federal government that regulates communications by radio, television, wire, satellite, and cable across the United States. The FCC maintains jurisdicti ...
(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. 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 Internet access is the ability of individuals and organizations to connect to the Internet using computer terminals, computers, and other devices; and to access services such as email and the World Wide Web. Internet access is sold by Interne ...
.
WirelessHD WirelessHD, also known as UltraGig, is a proprietary standard owned by Silicon Image (originally SiBeam) for wireless transmission of high-definition video content for consumer electronics products. The consortium currently has over 40 adopters; ke ...
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 Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor v ...
systems with very high resolution. The
Wi-Fi Wi-Fi () is a family of wireless network protocols, based on the IEEE 802.11 family of standards, which are commonly used for local area networking of devices and Internet access, allowing nearby digital devices to exchange data by radio waves ...
standards
IEEE 802.11ad IEEE 802.11ad is an amendment to the IEEE 802.11 wireless networking standard, developed to provide a Multiple Gigabit Wireless System (MGWS) standard at 60 GHz frequency, and is a networking standard for WiGig networks. Because it uses the ...
and
IEEE 802.11ay IEEE 802.11ay, ''Enhanced Throughput for Operation in License-exempt Bands above 45 GHz'', is a follow-up to IEEE 802.11ad WiGig standard which quadruples the bandwidth and adds MIMO up to 8 streams. Development started in 2015 and the final standa ...
operate in the 60 GHz ( V band) spectrum to achieve data transfer rates as high as 7
Gbit/s In telecommunications, data-transfer rate is the average number of bits ( bitrate), characters or symbols ( baudrate), or data blocks per unit time passing through a communication link in a data-transmission system. Common data rate units are mu ...
and at least 20
Gbit/s In telecommunications, data-transfer rate is the average number of bits ( bitrate), characters or symbols ( baudrate), or data blocks per unit time passing through a communication link in a data-transmission system. Common data rate units are mu ...
, respectively. Uses of the millimeter wave bands include point-to-point communications, intersatellite links, and point-to-multipoint communications. In 2013 it was speculated that there were plans to use millimeter waves in future 5G mobile phones. In addition, use of millimeter wave bands for vehicular communication is also emerging as an attractive solution to support (semi-)autonomous vehicular communications. 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 A cellular network or mobile network is a communication network where the link to and from end nodes is wireless. The network is distributed over land areas called "cells", each served by at least one fixed-location transceiver (typically thr ...
, 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 Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pr ...
or very short links such as for the interconnect of circuit boards.


Weapons systems

Millimeter wave
radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor v ...
is used in short-range
fire-control radar A fire-control radar (FCR) is a radar that is designed specifically to provide information (mainly target azimuth, elevation, range and range rate) to a fire-control system in order to direct weapons such that they hit a target. They are someti ...
in tanks and aircraft, and automated guns (
CIWS A close-in weapon system (CIWS ) is a point-defense weapon system for detecting and destroying short-range incoming missiles and enemy aircraft which have penetrated the outer defenses, typically mounted on a naval ship. Nearly all classes of l ...
) 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. With
Raytheon Raytheon Technologies Corporation is an American multinational aerospace and defense conglomerate headquartered in Arlington, Virginia. It is one of the largest aerospace and defense manufacturers in the world by revenue and market capitali ...
the U.S. Air Force has developed a nonlethal antipersonnel weapon system called
Active Denial System The Active Denial System (ADS) is a non-lethal directed-energy weapon developed by the U.S. military, designed for area denial, perimeter security and crowd control. Informally, the weapon is also called the heat ray since it works by heating th ...
(ADS) which emits a beam of millimeter radio waves with a wavelength of 3 mm (frequency of 95 GHz). The weapon causes a person in the beam to feel an intense burning pain, as if their skin is going to catch fire. The military version had an output power of 100
kilowatts The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James W ...
(kW), and a smaller law enforcement version, called Silent Guardian that was developed by Raytheon later, had an output power of 30 kW.


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. 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 The Transportation Security Administration (TSA) is an agency of the United States Department of Homeland Security (DHS) that has authority over the security of transportation systems within, and connecting to the United States. It was created ...
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. 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. Passengers can decline scanning and be screened via a metal detector and patted down. Three
security scan A full-body scanner is a device that detects objects on or inside a person's body for security screening purposes, without physically removing clothes or making physical contact. Unlike metal detectors, full-body scanners can detect non-metal o ...
ners using millimeter waves were put into use at
Schiphol Airport Amsterdam Airport Schiphol , known informally as Schiphol Airport ( nl, Luchthaven Schiphol, ), is the main international airport of the Netherlands. It is located southwest of Amsterdam, in the municipality of Haarlemmermeer in the province ...
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.


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 Glass production involves two main methods – the float glass process that produces sheet glass, and glassblowing that produces bottles and other containers. It has been done in a variety of ways during the history of glass. Glass container ...
and mineral wool production.


Medicine

Low intensity (usually 10 mW/cm2 or less) electromagnetic radiation of extremely high frequency may be used in human
medicine Medicine is the science and practice of caring for a patient, managing the diagnosis, prognosis, prevention, treatment, palliation of their injury or disease, and promoting their health. Medicine encompasses a variety of health care pr ...
for the treatment of
disease A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
s. For example, "A brief, low-intensity MMW exposure can change
cell growth Cell growth refers to an increase in the total mass of a cell, including both cytoplasmic, nuclear and organelle volume. Cell growth occurs when the overall rate of cellular biosynthesis (production of biomolecules or anabolism) is greater th ...
and proliferation rates, activity of
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s, state of cell genetic apparatus, function of excitable membranes and peripheral receptors." This treatment is particularly associated with the range of 40–70 GHz. This type of treatment may be called ''millimeter wave (MMW) therapy'' or ''extremely high frequency (EHF) therapy''. This treatment is associated with
eastern Europe Eastern Europe is a subregion of the European continent. As a largely ambiguous term, it has a wide range of geopolitical, geographical, ethnic, cultural, and socio-economic connotations. The vast majority of the region is covered by Russia, whi ...
an nations (e.g., former
USSR The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nati ...
nations). The Russian Journal ''Millimeter waves in biology and medicine'' studies the scientific basis and clinical applications of millimeter wave therapy.


Police speed radar

Traffic police use speed-detecting
radar gun A radar speed gun (also radar gun and speed trap gun) is a device used to measure the speed of moving objects. It is used in law-enforcement to measure the speed of moving vehicles and is often used in professional spectator sport, for things su ...
s in the Ka-band (33.4–36.0 GHz).


See also

*
Electromagnetic shielding In electrical engineering, electromagnetic shielding is the practice of reducing or blocking the electromagnetic field (EMF) in a space with barriers made of conductive or magnetic materials. It is typically applied to enclosures, for isolatin ...
* '' Journal of Infrared, Millimeter, and Terahertz Waves'' *
Knife-edge effect Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
*
Microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...
*
Terahertz radiation Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of fr ...


References


External links


FCC bulletin on MMW propagation

FCC 70/80/90 GHz overview.






{{Authority control Radio spectrum Bangladeshi inventions Indian inventions