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Extremely low frequencyExtremely low frequency

Frequency range 3 to 30 Hz

ITU Radio Band Numbers

1 2 3 4 5 6 7 8 9 10 11 12

ITU Radio Band Symbols

ELF SLF ULF VLF LF MF HF VHF UHF SHFEHF THF

NATO Radio bands

A B C D E F G H I J K L M

IEEE Radar bands

HF VHF UHF L S C X Ku K Ka V W mm

1982 aerial view of the U.S. Navy Clam Lake, Wisconsin ELF transmitter facility,used to communicate with deeply submerged submarines.

Extremely low frequency (ELF) waves areelectromagnetic radiation (radio waves) withfrequencies from 3 to 30 Hz, andcorresponding wavelengths from 100,000 to10,000 kilometers. In atmosphere science,an alternative definition is usually given,from 3 Hz to 3 kHz.[1][2] In the relatedmagnetosphere science, the lower frequencyelectromagnetic oscillations (pulsationsoccurring below ~3 Hz) are considered to liein the ULF range, which is thus also defineddifferently from the ITU Radio Bands.

ELF radio waves are generated by lightningand natural disturbances in Earth's magneticfield, so they are a subject of research byatmospheric scientists. Because of thedifficulty of building antennas that canradiate such long waves, ELF frequencies have been used in only a very few man-made communication systems.ELF waves can penetrate seawater, which makes it useful to communicate with submarines. The US and Russia arethe only nations known to have constructed ELF communication facilities, with India being in the process ofconstructing one. The frequency of alternating current flowing in electric power grids, 50 or 60 Hz, falls within thisfrequency band, and power grids are an unintentional source of ELF radiation.

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Alternate definitionsELF is a subradio frequency.[3] Some medical peer reviewed journal articles refer to ELF in the context of"extremely low frequency (ELF) magnetic fields (MF)" with frequencies of 50 Hz and 50–80 Hz.[4] United StatesGovernment agencies, such as NASA, describe ELF as non-ionizing radiation with frequencies between 0 and300 Hz. The World Health Organization (WHO) have used ELF to refer to the concept of "extremely low frequency(ELF) electric and magnetic fields (EMF)"[5] and have also referred to "ELF electric and magnetic fields in thefrequency range >0 to 100,000 Hz (100 kHz)." The WHO also stated that at frequencies between 0 and 300 Hz, "thewavelengths in air are very long (6000 km at 50 Hz and 5000 km at 60 Hz), and, in practical situations, the electricand magnetic fields act independently of one another and are measured separately."

Military communicationsThe United States Navy utilized extremely low frequencies (ELFs) as radio band and radio communications. TheSubmarine Integrated Antenna System (SIAS) was a research and development effort to communicate withsubmerged submarines.[6] The Soviet/Russian Navy also utilized ELFs for submarine communications system,ZEVS.[7] The Indian Navy is in the process of constructing ELF communication facility to communicate with itsArihant class and Akula class submarines. This facility is expected to be operational by 2015.[8]

ExplanationBecause of its electrical conductivity, seawater shields submarines from most higher frequency radio waves, makingradio communication with submerged submarines at ordinary frequencies impossible. Signals in the ELF frequencyrange, however, can penetrate much deeper. Two factors limit the usefulness of ELF communications channels: thelow data transmission rate of a few characters per minute and, to a lesser extent, the one-way nature due to theimpracticality of installing an antenna of the required size on a submarine (antennas need to be of exceptional sizefor the users to achieve successful communication). Generally, ELF signals were used to order a submarine to rise toa shallow depth where it could receive some other form of communication.

Difficulties of ELF communicationOne of the difficulties posed when broadcasting in the ELF frequency range is antenna size because the length of theantenna must be at least a substantial fraction of the length of the waves. Simply put, a 3 Hz (cycle per second)signal would have a wavelength equal to the distance EM waves travel through a given medium in one third of asecond. Taking account of refractive index, ELF waves propagate slightly slower than the speed of light in avacuum. As used in military applications, the wavelength is 299,792 km (186,282 mi) per second divided by50–85 Hz, which equals around 3,500 to 6,000 km (2,200 to 3,700 mi) long. This is comparable to the Earth'sdiameter of around 12,742 km (7,918 mi). Because of this huge size requirement, to transmit internationally usingELF frequencies, the Earth itself forms a significant part of the antenna, and extremely long leads are necessary intothe ground. Various means, such as electrical lengthening, are taken to construct practical radio stations with smallersizes.The US maintained two sites, in the Chequamegon-Nicolet National Forest, Wisconsin and the Escanaba River StateForest, Michigan (originally named Project Sanguine, then downsized and rechristened Project ELF prior toconstruction), until they were dismantled, beginning in late September 2004. Both sites used long power lines,so-called ground dipoles, as leads. These leads were in multiple strands ranging from 22.5 to 45 kilometres (14.0 to28.0 mi) long. Because of the inefficiency of this method, considerable amounts of electrical power were required tooperate the system.

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Ecological impactThere have been some concerns over the possible ecological impact of ELF signals. In 1984 a federal judge haltedconstruction requiring more environmental and health studies. This judgment was overruled by a federal appealscourt on the basis that the US Navy claimed to have spent over 25 million dollars studying the effects of theelectromagnetic fields with results indicating that they were similar to the effect produced by standard powerdistribution lines. The judgment was not accepted by everyone and during the time ELF was in use, some Wisconsinpoliticians such as Senators Herb Kohl, Russ Feingold and Congressman Dave Obey called for its closure. Similarconcerns have in the past been raised about electromagnetic radiation and health.

Other usesTransmitters in the 20 Hz range are also found in pipeline inspection gauges, also known as "PIGs".Some radio hams record ELF (or even lower) signals from very large homemade antennas, and play them back athigher speeds to catch natural fluctuations in the Earth's electromagnetic field. Increasing the playback increases thepitch, so that it is brought into the audio frequency range. [citation needed]

Natural sourcesNaturally occurring ELF waves are present on Earth, resonating in the region between ionosphere and surface. Theyare initiated by lightning strikes that make electrons in the atmosphere oscillate.[9] Though VLF signals werepredominantly generated from lightning discharges, it was found that an observable ELF component (slow tail)followed the VLF component in almost all cases.[10] The fundamental mode of the Earth-ionosphere cavity has thewavelength equal to the circumference of the Earth, which gives a resonance frequency of 7.8 Hz. This frequency,and higher resonance modes of 14, 20, 26 and 32 Hz appear as peaks in the ELF spectrum and are called Schumannresonance.They have also been tentatively identified on Saturn's moon Titan. Titan's surface is thought to be a poor reflector ofELF waves, so the waves may instead be reflecting off the liquid-ice boundary of a subsurface ocean of water andammonia, the existence of which is predicted by some theoretical models. Titan's ionosphere is also more complexthan Earth's, with the main ionosphere at an altitude of 1,200 km (750 mi) but with an additional layer of chargedparticles at 63 km (39 mi). This splits Titan's atmosphere into two separate resonating chambers. The source ofnatural ELF waves on Titan is unclear as there doesn't appear to be extensive lightning activity.Finally, huge ELF radiation power outputs of 100,000 times the Sun's output in visible light may be radiated bymagnetars. The pulsar in the Crab nebula radiates powers of this order at the frequency 30 hertz.[11] Radiation of thisfrequency is below the plasma frequency of the interstellar medium, thus this medium is opaque to it, and it cannotbe observed from Earth.

ExposureIn electromagnetic therapy and electromagnetic radiation and health research, electromagnetic spectrum frequencies between 0 and 100 hertz are considered extremely low-frequency fields. Since the late 1970s, questions have been raised whether exposure to ELF electric and magnetic fields (EMF) within this range of frequencies produces adverse health consequences.[] In October 2005, WHO convened a Task Group of scientific experts to assess any risks to health that might exist from "exposure to ELF electric and magnetic fields in the frequency range >0 to 100,000 Hz (100 kHz) in regards to childhood leukaemia." There are established biological effects from acute exposure at high levels (well above 100 µT) that are explained by recognized biophysical mechanisms. External ELF magnetic fields induce electric fields and currents in the body which, at very high field strengths, cause nerve and muscle stimulation and changes in nerve cell excitability in the central nervous system. Health effects related to short-term, high-level exposure have been established and form the basis of two international exposure limit

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guidelines (ICNIRP, 1998; IEEE, 2002). At present, these bodies consider the scientific evidence related to possiblehealth effects from long-term, low-level exposure to ELF fields insufficient to justify lowering these quantitativeexposure limits. The long-term, low-level exposure is evaluated as average exposure to residential power-frequencymagnetic field above 0.3 to 0.4 µT, and it is estimated that only between 1% and 4% of children live in suchconditions. A common source of ELF fields in the United States is 60 Hz electric and magnetic fields fromhigh-voltage electric power transmission lines and secondary distribution lines, such as those found in residentialneighborhoods.[] The International Agency for Research on Cancer (IARC) has evaluated the scientific data and hasclassified ELF magnetic fields as being "possibly carcinogenic" to humans. IARC based this classification on thefollowing:

Human health population studies showing weak evidence of an association with childhood leukemia;and a large database of laboratory study results showing inadequate evidence of an association withcancer in animals.

In summary, when all of the studies are evaluated together, the evidence suggesting that EMFs may contribute to anincreased risk of cancer is very weak.[12] Epidemiological studies suggest a possible association between long termoccupational exposure to ELF and Alzheimer's disease.

ELF Effects on human nervous systemA study by Reilly in 1999 showed that the threshold for direct perception of exposure to ELF RF by humanvolunteer subjects started at around 2 to 5 kV/m at 60 Hz with 10% of volunteers detecting the ELF expose at thislevel.[citation needed] The percent of detection raised to 50% of volunteers when the ELF level was rasised to 7 to 20kV/m. 5% of all test subjects considered the perception of ELF at these thresholds as annoying.[citation needed]

ELF at human perceivable kV/m levels is said to create an annoying tingling sensation in the areas of the body incontact with clothing, particularly the arms due to the induction of a surface charge by the ELF. 7% of volunteersdescribed the spark discharges as painful where the subject was well-insulated and touched a grounded object withina 5 kV/m field. 50% of volunteers described a similar spark discharge as painful in a 10 kV/m field.[13]

Patents• Tanner, R. L., U.S. Patent 3,215,937 [14], "Extremely low-frequency antenna", 1965• Hansell, Clarence W., U.S. Patent 2,389,432 [15], "Communication system by pulses through the Earth"• Altshuler, U.S. Patent 4,051,479 [16], ELF vertical dipole antenna suspended from aircraft

References

Notes[1] Liemohn, Michael W. and A. A. CHAN, " Unraveling the Causes of Radiation Belt Enhancements (http:/ / lws-trt. gsfc. nasa. gov/

trt_liemohn05eos. pdf)". EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION, Volume 88, Number 42, 16 October 2007, pages427-440. Republished by NASA and accessed online, 8 February 2010. Adobe File, page 2.

[2][2] R. Barr, D. Llanwyn Jones, C. J. Rodger, "ELF and VLF radio waves", Journal of Atmospheric and Solar-Terrestrial Physics, Volume 62,Issues 17-18, November 2000, Pages 1689-1718, ISSN 1364-6826, .

[3] NASA.gov (http:/ / smad-ext. grc. nasa. gov/ shed/ pub/ ohpm/ ohpm10-non-ion. pdf), page 8. ">0 to 300 Hz ... Extremely low frequency(ELF)"

[4] ESTECIO, Marcos Roberto Higino and SILVA, Ana Elizabete. Alterações cromossômicas causadas pela radiação dos monitores de vídeo decomputadores (http:/ / www. docguide. com/ news/ content. nsf/ PaperFrameSet?OpenForm& refid=2&id=48dde4a73e09a969852568880078c249& c=& newsid=8525697700573E1885256C000059C721& u=http:/ / www. scielo. br/ scielo.php?script=sci_abstract& pid=S0034-89102002000300012& lng=en& nrm=iso& tlng=en& ref=/ news/ content. nsf/SearchResults?openform& Query=Extremely low frequency& so=date& id=48dde4a73e09a969852568880078c249& Search_Box=All DG).Rev. Saúde Pública [online]. 2002, vol.36, n.3, pp. 330-336. ISSN 0034-8910. Republished by docguide.com. Accessed 8 February 2010.

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[5] " Electromagnetic Fields and Public HealthL - Extremely Low Frequency (ELF) (http:/ / www. who. int/ docstore/ peh-emf/ publications/facts_press/ efact/ efs205. html)". Fact Sheet N205. November 1998. World Health Organization. Accessed 12 February 2010. "ELF fields aredefined as those having frequencies up to 300 Hz. ... the electric and magnetic fields act independently of one another and are measuredseparately."

[6] " U.S. Navy: Vision...Presence...Power (http:/ / www. navy. mil/ navydata/ policy/ vision/ vis98/ vis-p10. html)." SENSORS - SubsurfaceSensors. US Navy. Accessed 7 February 2010.

[7] http:/ / www. vlf. it/ zevs/ zevs. htm ZEVS, the Russian 82 Hz ELF transmitter[8] http:/ / www. janes. com/ article/ 11147/ india-makes-headway-with-elf-site-construction[9] Republished as " Casini - Unlocking Saturn's Secrets - Titan's mysterious radio wave (http:/ / www. nasa. gov/ mission_pages/ cassini/

whycassini/ cassinif-20070601-02. html)". 22 November 2007. NASA. Accessed 7 February 2010.[10] Tepley, Lee R. " A Comparison of Sferics as Observed in the Very Low Frequency and Extremely Low Frequency Bands (http:/ / www.

agu. org/ pubs/ crossref/ 1959/ JZ064i012p02315. shtml)". Stanford Research Institute Menlo Park, California. 10 August 1959. 64(12),2315–2329. Summary republished by American Geophysical Union. Accessed 13 February 2010

[11] http:/ / www. cv. nrao. edu/ course/ astr534/ Pulsars. html[12] GC.ca (http:/ / www. hc-sc. gc. ca/ hl-vs/ iyh-vsv/ environ/ magnet-eng. php#ty)[13] Extremely Low Frequency Fields Environmental Health Criteria Monograph No.238 (http:/ / www. who. int/ peh-emf/ publications/ elf_ehc/

en/ index. html), chapter 5, page 121, WHO[14] http:/ / www. google. com/ patents/ US3215937[15] http:/ / www. google. com/ patents/ US2389432[16] http:/ / www. google. com/ patents/ US4051479

General information• Non-ionizing radiation, Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields (2002)

by the IARC. ( Non-Ionizing Radiation (http:/ / monographs. iarc. fr/ ENG/ Monographs/ vol80/ mono80. pdf))

External links• Tomislav Stimac, " Definition of frequency bands (VLF, ELF... etc.) (http:/ / www. vlf. it/ frequency/ bands.

html)". IK1QFK Home Page (vlf.it).• Extremely low frequency (ELF) fields (http:/ / www. inchem. org/ documents/ ehc/ ehc/ ehc35. htm) (EHC 35,

1984)• " Radio waves below 22kHz (http:/ / www. vlf. it/ ): Nature's signals and strange emission at very low frequency"

- a site specialising in low-frequency signals .• Jacobsen, Trond, " ZEVS, the Russian 82 Hz ELF transmitter (http:/ / www. vlf. it/ zevs/ zevs. htm): An Extrem

Low Frequency transmission-system, using the real longwaves" ALFLAB, Halden, Norway.• NASA live streaming ELF -> VLF Receiver (http:/ / spaceweather. com/ glossary/ inspire. html)• Longitudinal Electric Waves (http:/ / www. miklagaard. com/ longitudinal-electric-waves)

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Article Sources and ContributorsExtremely low frequency  Source: http://en.wikipedia.org/w/index.php?oldid=594926781  Contributors: 1archie99, 2over0, 84user, 8r455, Adam bink, Adamantios, Aeusoes1, Aido2002,Amilnerwhite, Andy Christ, Ashishbhatnagar72, Avocade, BD2412, Before My Ken, BenBurch, Betacommand, Binksternet, BioTube, Bluap, Bryan Derksen, Cacycle, Cburnett, Ce garcon,Cexycy, Chetvorno, Chris the speller, ChrisGualtieri, Cleduc, Congruence, Conversion script, CyclePat, DJGB, Dachshund, Darrien, Deville, Dinsdagskind, Dougshuck, Dreish, Dual Freq,Dying, Edcolins, Edison, Edward, Ejay, Feedmecereal, Gaius Cornelius, Gene Nygaard, Giftlite, GilCahana, GoingBatty, Graham87, Grodno100, Gurch, Guyonthesubway, Harrisonpappas,Helium4, Heron, Hertz1888, Hqb, Hydrogen Iodide, Ily1145, Incnis Mrsi, Jaydee2584, Jeffrey Mall, Jerzy, Jll, Jonverve, Jorge Stolfi, Karl-Henner, Kjkolb, Kolbasz, Kurykh, Kyrsjo, LMB, Lightcurrent, LilHelpa, Longbyte1, LorenzoB, Lotje, Lowellian, Magari17, Maximus Rex, Maxistheman, Meggar, Meltonkt, Mercy11, Merovingian, Michael Hardy, Mulad, NickBush24, Nigelj,Nikai, Nutriveg, Omegatron, Omnipaedista, PDH, PatVanHove, Pinethicket, Plop, PrestonH, Prickus, Pvpoodle, Radiojon, Ralian, Rcooley, Reddi, Relmeligy, Richard W.M. Jones, Rijkbenik,Rjwilmsi, Roricka, SAE1962, SEWilco, Sam Hocevar, Sanders muc, Scott S, Sjc, Soakologist, Spra, StephanieM, Tbonnie, Tebloraf, Template namespace initialisation script, The Anome, TheBrain, The Epopt, Timc, Timo Honkasalo, TwoOneTwo, Verbal, Viriditas, William Avery, 133 anonymous edits

Image Sources, Licenses and ContributorsFile:Clam Lake ELF.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Clam_Lake_ELF.jpg  License: Public Domain  Contributors: Service Depicted: Navy

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