View source for Extremely high frequency

{{short description|The range 30–300 GHz of the electromagnetic spectrum}}
{{MWband
| name = Extremely high frequency
| freq = 30 to 300 [[Hertz|GHz]]
| wave = 10–1 [[Meter|mm]]
| bands = {{startplainlist|class=nowrap}}
* [[K band (NATO)|K]]{{\}}[[L band (NATO)|L]]{{\}}[[M band (NATO)|M]] (NATO)
* [[Ka band|K<sub>a</sub>]]{{\}}[[V band|V]]{{\}}[[W band|W]]{{\}}mm (IEEE)
{{endplainlist}}
| name2 = Millimetre band (IEEE)
| freq2 = 110 to 300 GHz
| wave2 = 2.73 to 1 mm
| bands2 = EHF (IEEE)
}}

'''Extremely high frequency''' (EHF) is the [[International Telecommunication Union]] designation for the band of radio frequencies in the [[electromagnetic spectrum]] from 30 to 300 gigahertz (GHz).<ref name="1037B">{{cite book | title = US Federal Standard 1037B: Telecommunications, Glossary of Telecommunications Terms | publisher = Office of Technology Standards, General Services Administration | date = 3 June 1991 | pages = S-18 | url = https://books.google.com/books?id=zYLXU4fkD34C&pg=RA15-PA18}}</ref><ref name="itu-2015-acts">{{cite conference |date=2015 |title=Final Acts WRC-15 |url=https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.12-2015-PDF-E.pdf |conference=World Radiocommunication Conference |location=Geneva, Switzerland |publisher=International Telecommunications Union |pages = 4|access-date=2025-01-12}}</ref> It is in the [[microwave]] part of the radio spectrum, between the [[super high frequency]] band and the [[terahertz band]]. [[Radio wave]]s in this band have [[wavelength]]s from ten to one millimeter, so it is also called the '''millimeter band''' and radiation in this band is called '''millimeter waves''', sometimes abbreviated '''MMW''' or '''mmWave'''.<ref name=Bose>{{cite web |title=Milestones: First Millimeter-wave Communication Experiments by J.C. Bose, 1894-96 |url=https://ethw.org/Milestones:First_Millimeter-wave_Communication_Experiments_by_J.C._Bose,_1894-96 |website=[[List of IEEE milestones]] |date=14 June 2022 |publisher=[[Institute of Electrical and Electronics Engineers]]}}</ref> 

Some define '''mmWaves''' as starting at 24 GHz, thus covering the entire [[5G NR frequency bands#Frequency Range 2|FR2]] band (24.25 to 71 GHz), among others.<ref>{{cite web |url=https://www.wired.com/story/wired-guide-5g/ |title=The WIRED Guide to 5G |date=Dec 31, 2022 |journal=Wired}}</ref><ref>{{cite web |url=https://www.5gradio.com/5g-technology/5g-nr-frequency-bands/ |title=5G NR frequency bands |date=2020 |publisher=CableFree Networks}}</ref>

Compared to lower bands, radio waves in this band have high [[atmospheric]] [[attenuation]]: 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]]) 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 [[Beamwidth|beam width]], further increasing frequency reuse potential. Millimeter waves are used for military [[fire-control radar]], [[millimeter wave scanner|airport security scanners]], short range [[wireless network]]s, and scientific research.

In a major new application of millimeter waves, certain [[5G NR frequency bands#Frequency Range 2|frequency ranges]] near the bottom of the band are being used in the newest generation of [[cell phone]] networks, [[5G]] networks.<ref>{{Cite tech report
| title = User Equipment (UE) radio transmission and reception; Part 3: Range 1 and Range 2 Interworking operation with other radios
| version = 3GPP TS 38.101-3 version 15.2.0 Release 15
| institution = ETSI
| type = Technical Specification
| date = July 2018
| access-date = 5 December 2019
| url = https://www.etsi.org/deliver/etsi_ts/138100_138199/13810103/15.02.00_60/ts_13810103v150200p.pdf
| page = 11
}}</ref> 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 factor|''Q'' factors]] of passive devices.<ref name="duPreez">{{cite book
 | last1  = du Preez
 | first1 = Jaco
 | last2  = Sinha
 | first2 = Saurabh
 | title  = Millimeter-Wave Power Amplifiers
 | publisher = Springer
 | date   = 2017
 | pages  = 1–35
 | url    = https://books.google.com/books?id=Z7M4DwAAQBAJ
 | isbn   = 978-3-319-62166-1
 }}</ref>

== Propagation ==
[[File:Micrwavattrp.png|thumb|upright=1.7|Atmospheric attenuation in dB/km as a function of frequency over the extremely high frequency band. Peaks in absorption at specific frequencies are a problem, due to atmosphere constituents such as [[water]] vapour ({{chem2|H2O}}) and molecular [[oxygen]] ({{chem2|O2}}). The vertical scale is double logarithmic, as dB are themselves logarithmic.]] 
Millimeter waves propagate solely by [[line-of-sight propagation|line-of-sight]] paths. They are not refracted by the [[ionosphere]] nor do they travel along the Earth as [[ground wave]]s as lower frequency radio waves do.<ref name="Huang">{{cite book
 | last = Huang
 | first = Kao-Cheng
 | author2 = Zhaocheng Wang
 | title = Millimeter Wave Communication Systems
 | publisher = John Wiley & Sons
 | year = 2011
 | pages = Sections 1.1.1–1.2
 | url = https://books.google.com/books?id=kCmyPdxme1AC&pg=PP17
 | isbn = 978-1-118-10275-6}}</ref> At typical power densities they are blocked by building walls and suffer significant attenuation passing through foliage.<ref name="Huang" /><ref name="FCCbulletin70" /><ref name="Preez">{{cite book
 | last1  = du Preez
 | first1 = Jaco
 | last2  = Sinha
 | first2 = Saurabh
 | title  = Millimeter-Wave Antennas: Configurations and Applications
 | publisher = Springer
 | date   = 2016
 | pages  = 13–14
 | url    = https://books.google.com/books?id=8_B6DAAAQBAJ&pg=PA13
 | isbn   = 978-3-319-35068-4
 }}</ref> Absorption by atmospheric gases is a significant factor throughout the band and increases with frequency. However, this absorption is maximum at a few specific [[spectral line|absorption lines]], mainly those of [[oxygen]] at 60&nbsp;GHz and [[water vapor]] at 24&nbsp;GHz and 184&nbsp;GHz.<ref name="FCCbulletin70">{{cite web
  | title = Millimeter Wave Propagation: Spectrum Management Implications
  | version = Office of Engineering and Technology, Bulletin No. 70
  | publisher = Federal Communications Commission (FCC), US Dept. of Commerce
  | date = July 1997
  | url = https://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet70/oet70a.pdf
  | access-date = May 20, 2017}}</ref> 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]]s, so precipitation causes additional attenuation due to [[scattering (optics)|scattering]] ([[rain fade]]) as well as absorption.<ref name="FCCbulletin70" /><ref name="Preez" /> The high [[free space loss]] and atmospheric absorption limit useful propagation to a few kilometers.<ref name="Huang" /> Thus, they are useful for densely packed communications networks such as [[personal area network]]s that improve spectrum utilization through [[frequency reuse]].<ref name="Huang" />

Millimeter waves show "optical" propagation characteristics and can be reflected and focused by small metal surfaces and [[dielectric lens]]es around 5 to 30&nbsp;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 millimeter waves can be diffracted by building edges. At millimeter wavelengths, surfaces appear rougher so [[diffuse reflection]] increases.<ref name="Huang" /> [[Multipath propagation]], particularly reflection from indoor walls and surfaces, causes serious fading.<ref name="Preez" /><ref name="Seybold">{{cite book
 | last1  = Seybold
 | first1 = John S. 
 | title  = Introduction to RF Propagation
 | publisher = John Wiley and Sons
 | date   = 2005
 | pages  = 55–58
 | url    = https://books.google.com/books?id=4LtmjGNwOPIC&q=cross+polarization+discrimination&pg=PA57
 | isbn   = 0-471-74368-2
 }}</ref> [[Doppler shift]] of frequency can be significant even at pedestrian speeds.<ref name="Huang" /> In portable devices, [[Fading|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]]s for airport security scanning.

== Applications ==

=== Scientific research ===
[[File:The Atacama Compact Array.jpg|thumb|Part of the [[Atacama Large Millimeter Array]] (ALMA), [[Chile]], [[America]], a millimeter wave [[radio telescope]]]]
This [[band (radio)|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 ([[Atacama Large Millimeter Array|ALMA]]) due to atmospheric absorption issues.

Satellite-based [[remote sensing]] near 60&nbsp;GHz can determine temperature in the [[upper atmosphere]] by measuring radiation emitted from oxygen molecules that is a function of temperature and pressure. The [[International Telecommunication Union]] non-exclusive passive frequency allocation at 57–59.3&nbsp;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.<ref name="Comments of IEEE Geoscience and Remote Sensing Society, FCC RM-11104, 10/17/07">[http://gullfoss2.fcc.gov/prod/ecfs/retrieve.cgi?native_or_pdf=pdf&id_document=6519741794 FCC.gov]{{Dead link|date=August 2019 |bot=InternetArchiveBot |fix-attempted=yes }}, Comments of IEEE Geoscience and Remote Sensing Society, FCC RM-11104, 10/17/07</ref>

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

The 71–76, 81–86 and 92–95&nbsp;GHz bands are also used for [[point-to-point (telecommunications)|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&nbsp;Gbit/s links using these frequencies as well. In the case of the 92–95&nbsp;GHz band, a small 100&nbsp;MHz range has been reserved for space-borne radios, limiting this reserved range to a transmission rate of under a few gigabits per second.<ref>[http://rfdesign.com/mag/605RFDF4.pdf Rfdesign.com] {{Webarchive|url=https://web.archive.org/web/20120716154135/http://rfdesign.com/mag/605RFDF4.pdf |date=2012-07-16 }}, Multigigabit wireless technology at 70&nbsp;GHz, 80&nbsp;GHz and 90&nbsp;GHz, [[RF Design]], May 2006</ref>

[[File:CableFree MMW Link installed in UAE.jpg|thumb|A CableFree MMW link installed in the UAE installed for [[Safe City]] applications, providing 1&nbsp;Gbit/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&nbsp;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]] standards [[IEEE 802.11ad]] and [[IEEE 802.11ay]] operate in the 60&nbsp;GHz ([[V band]]) spectrum to achieve data transfer rates as high as 7 [[Gigabit|Gbit/s]] and at least 20 [[Gigabit|Gbit/s]], respectively.

Uses of the millimeter wave bands include point-to-point communications, [[Inter-satellite service|intersatellite links]], and [[point-to-multipoint communication]]s. In 2013 it was speculated that there were plans to use millimeter waves in future [[5G]] mobile phones.<ref>{{Cite journal|title = Millimeter Wave Mobile Communications for 5G Cellular: It Will Work! |journal = IEEE Access |date = 2013-01-01 |issn = 2169-3536 |pages = 335–349 |volume = 1 |doi = 10.1109/ACCESS.2013.2260813 |first1 = T.S. |last1 = Rappaport |first2 = Shu |last2 = Sun |first3 = R. |last3 = Mayzus |first4 = Hang |last4 = Zhao |first5 = Y. |last5 = Azar |first6 = K. |last6 = Wang |first7 = G.N. |last7 = Wong |first8 = J.K. |last8 = Schulz |first9 = M. |last9 = Samimi|doi-access = free |bibcode = 2013IEEEA...1..335R }}</ref> In addition, use of millimeter wave bands for vehicular communication is also emerging as an attractive solution to support (semi-)autonomous vehicular communications.<ref>{{Cite journal|title = FML: Fast Machine Learning for 5G mmWave Vehicular Communications |url = https://www.researchgate.net/publication/324804467 |journal = IEEE Infocom'18 |date = 2018-04-15 |first1 = Arash |last1 = Asadi |first2 = Sabrina |last2 = Klos |first3 = Gek Hong |last3 = Sim |first4 = Anja |last4 = Klein |first5 = Matthias |last5 = Hollick}}</ref>

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]] or very short links such as for the interconnect of circuit boards.<ref>{{cite journal |author=Peter Smulders | title=The Road to 100 Gb/s Wireless and Beyond: Basic Issues and Key Directions | journal=IEEE Communications Magazine | volume=51 | issue=12 | pages=86–91 | year=2013 | doi=10.1109/MCOM.2013.6685762| s2cid=12358456 }}</ref>

=== Weapons systems ===
[[File:Minsk port bow AK-630 CIWS gun fire control radar.JPG|thumb|Millimeter wave fire control radar for CIWS gun on [[Soviet aircraft carrier Minsk|Soviet aircraft carrier ''Minsk'']], [[Russia]]]]
Millimeter wave [[radar]] is used in short-range [[fire-control radar]] in tanks and aircraft, and automated guns ([[close-in weapon system|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|date=March 2019}}

With [[Raytheon Technologies|Raytheon]] the [[U.S. Air Force]] has developed a nonlethal antipersonnel weapon system called [[Active Denial System]] (ADS) which emits a beam of millimeter radio waves with a wavelength of 3&nbsp;mm (frequency of 95&nbsp;GHz).<ref>{{cite magazine |url=https://www.wired.com/2007/01/72134/ |title=Slideshow: Say Hello to the Goodbye Weapon |access-date=16 August 2016 | magazine=Wired |date=5 December 2006}}</ref> 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 [[Watt#Kilowatt|kilowatts]] (kW),<ref>{{cite web|url= https://terasense.com/news/active-denial-system-a-terahertz-based-military-deterrent-for-safe-crowd-control/|title= Active Denial System: a terahertz based military deterrent for safe crowd control|publisher= Terasense Group Inc|date= 2019-05-29|access-date= 2020-05-03}}</ref> and a smaller law enforcement version, called [[Active Denial System#Silent Guardian|Silent Guardian]] that was developed by Raytheon later, had an output power of 30&nbsp;kW.<ref>{{cite web|url= https://www.wired.com/2009/08/pain-ray-first-commercial-sale-looms/|title= 'Pain ray' first commercial sale looms|first= David|last= Hambling|date= 2009-05-08|publisher= Wired|access-date= 2020-05-03}}</ref>

=== Security screening ===
{{Main|Millimeter wave scanner}}
[[Image:NRW-Verkehrsminister Hendrik Wüst - Vorstellung Easy Security-6274.jpg|thumb|Millimeter wave security scanner at Bonn airport]]
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.<ref>[http://www.newscientisttech.com/article.ns?id=dn10160&feedId=tech_rss20 Newscientisttech.com]  {{webarchive |url=https://web.archive.org/web/20070311160429/http://www.newscientisttech.com/article.ns?id=dn10160&feedId=tech_rss20 |date=March 11, 2007 }}</ref> 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 [[Transportation Security Administration|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.<ref name="USAToday">{{cite news |url=https://www.usatoday.com/travel/flights/2009-02-17-detectors_N.htm |title=Body scanners replace metal detectors in tryout at Tulsa airport. | work=USA Today | first=Thomas | last=Frank | date=18 February 2009 | access-date=2 May 2010}}</ref> 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.<ref name="RobertKane">{{cite web |url=http://www.tsa.gov/assets/pdf/110311_kane_tsa_technology.pdf |title=Statement of Robert Kane to House of Representatives |date=2011-11-03 |page=2 |archive-url=https://web.archive.org/web/20111125014911/http://www.tsa.gov/assets/pdf/110311_kane_tsa_technology.pdf |archive-date=2011-11-25 }}</ref> Passengers can decline scanning and be screened via a metal detector and patted down.<ref>{{cite web |last1=Cortez |first1=Joe |title=The Three Inspection Options at TSA Checkpoints |url=https://www.tripsavvy.com/inspection-options-at-tsa-checkpoints-3259855 |website=Trip Savvy |access-date=11 January 2024}}</ref>

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.<ref>{{cite web|url=http://www.esa.int/ESA_in_your_country/Ireland/Bat_inspires_space_tech_for_airport_security|title=Bat inspires space tech for airport security|last=esa|website=esa.int|access-date=7 April 2018}}</ref>

=== 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|mineral wool production]].

=== Medicine ===
{{Expand section|mmWave measuring of blood pressure and blood glucose|date=May 2023}}
Low [[intensity (physics)|intensity]] (usually 10&nbsp;mW/cm<sup>2</sup> or less) electromagnetic radiation of extremely high frequency may be used in human [[medicine]] for the treatment of [[disease]]s. For example, "A brief, low-intensity MMW exposure can change [[cell growth]] and proliferation rates, activity of [[enzyme]]s, state of cell genetic apparatus, function of excitable membranes and peripheral receptors."<ref name="pakhomov_murphy_IEEE" /> This treatment is particularly associated with the range of 40–70 [[GHz]].<ref>{{cite journal |author=Betskii, O. V. |author2=Devyatkov, N. D. |author3=Kislov, V. | title=Low Intensity Millimeter Waves in Medicine and Biology | journal=Critical Reviews in Biomedical Engineering | year=2000 | volume=28 | issue=1&2 | pages=247–268 | url=http://www.begellhouse.com/journals/4b27cbfc562e21b8.html | publisher=Begellhouse.com| doi=10.1615/CritRevBiomedEng.v28.i12.420 | pmid=10999395 | url-access=subscription }}</ref> This type of treatment may be called ''millimeter wave therapy'' or ''extremely high frequency therapy''.<ref>{{cite journal |author=M. Rojavin |author2=M. Ziskin | title=Medical application of millimetre waves | journal=QJM: An International Journal of Medicine | volume=91 | issue=1 | pages=57–66 | year=1998 | doi=10.1093/qjmed/91.1.57 | pmid=9519213 | doi-access=free }}</ref> This treatment is associated with [[eastern Europe]]an nations (e.g., former [[Soviet Union|USSR]] nations).<ref name="pakhomov_murphy_IEEE">{{cite journal |author=Pakhomov, A. G. |author2=Murphy, P. R. | title=Low-intensity millimeter waves as a novel therapeutic modality | journal=IEEE Transactions on Plasma Science | year =2000 | volume=28 | issue=1 | pages=34–40 | doi=10.1109/27.842821 | bibcode=2000ITPS...28...34P | s2cid=22730643 }}</ref> The Russian Journal ''Millimeter waves in biology and medicine'' studies the scientific basis and clinical applications of millimeter wave therapy.<ref>[http://www.benran.ru/Magazin/El/13/N71320.HTM Benran.ru] {{webarchive|url=https://web.archive.org/web/20110718035939/http://www.benran.ru/Magazin/El/13/N71320.HTM |date=2011-07-18 }}</ref>

=== Police speed radar ===
Traffic police use speed-detecting [[radar gun]]s in the Ka-band (33.4–36.0&nbsp;GHz).<ref>{{cite web|url=https://copradar.com/chapts/chapt7/ch7d1.html|title=Radio and Radar Frequency Bands|website=copradar.com|access-date=30 April 2020}}</ref>

== History ==
Millimeter-length electromagnetic waves were first investigated by [[Jagadish Chandra Bose]], who generated waves of [[frequency]] up to 60{{nbsp}}GHz during experiments in 1894{{ndash}}1896.<ref name=Bose/>

== See also ==
* [[Electromagnetic shielding]]
* ''[[Journal of Infrared, Millimeter, and Terahertz Waves]]''
* [[Knife-edge effect]]
* [[Microwave]]
* [[Terahertz radiation]]
* [[Microwave analog signal processing|Microwave Analog Signal Processing]]

== References ==
{{Reflist}}

== External links ==
* [http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet70/oet70a.pdf FCC bulletin on MMW propagation]
* [http://wireless.fcc.gov/services/millimeterwave FCC 70/80/90 GHz overview.] {{Webarchive|url=https://web.archive.org/web/20051219070746/http://wireless.fcc.gov/services/millimeterwave |date=2005-12-19 }}
* [http://edocket.access.gpo.gov/cfr_2007/octqtr/47cfr15.255.htm FCC 57–64&nbsp;GHz rules.]
* [http://www.vlf.it/frequency/bands.html Definition of frequency bands (VLF, ELF... etc.)]
* [https://web.archive.org/web/20110612202601/http://www.patentstorm.us/patents/7220488/description.html US Patent 7220488 – Deflecting magnetic field shield]

{{Radio spectrum}}
{{EMSpectrum}}

{{Authority control}}

[[Category:Radio spectrum]]
[[Category:Transportation Security Administration]]