Extremely low frequency 366253 225271937 2008-07-12T20:34:11Z Dual Freq 578219 rv, extraordinary claims require extraordinary proof. [[Image:Clam Lake ELF.jpg|thumb|300px|1982 aerial view of the [[Clam Lake]], [[Wisconsin]] ELF facility.]] '''Extremely low frequency (ELF)''' is the [[band (radio)|band]] of [[radio frequencies]] from 3 to 30 [[Hertz|Hz]]. ELF was used by the [[United States|US Navy]] and [[Soviet Navy|Soviet]]/[[Russian Navy]] to [[Communication with submarines|communicate with submerged submarines]]. ==Explanation== Because of the electrical [[electrical conductivity|conductivity]] of [[salt]] [[water]], submarines are shielded from most [[electromagnetic]] [[Telecommunication|communication]]s. Signals in the ELF frequency range, however, can penetrate much deeper. Two factors limit the usefulness of ELF communications channels; the low data transmission rate of a few characters per minute, and to a lesser extent the one-way nature due to the impracticality of installing a huge transmitter on a submarine. Generally ELF signals were used to order a submarine to rise to a shallow depth where it could receive some other form of communication. == Difficulties of ELF communication == One of the difficulties posed when broadcasting in the ELF frequency range is [[antenna (electronics)|antenna]] size. This is because the antenna must be at least a substantial fraction of the size (in at least one dimension) of the wavelength of the frequency of EM waves you wish to create. Simply put, a 1 [[hertz|Hz]] (cycle per second) signal would have a wavelength equal to the distance EM waves travel through your chosen medium in 1 second. For ELF, this is very slightly slower than the [[speed of light]] (in a vacuum). Though ELF is defined as 3-30Hz the Russian and American Navies actually used aprox. 50-85 Hz. Therefore, for this purpose the wavelength would be ~299 792 (kilometers per second) divided by 50-85, which is 3,450km to 5,996 km long. The earth's diameter varies from 12,715 km (pole to pole) to 12,756 km (equatorial). Because of this huge size requirement, and in order to transmit internationally using ELF frequencies, the earth ''itself'' must be used as an antenna, with extremely long leads going into the ground. The US maintained two sites, in the [[Chequamegon-Nicolet National Forest]], [[Wisconsin]] and the [[Escanaba River State Forest]], [[Michigan]] (originally named Project Sanguine, then downsized and rechristened Project ELF prior to construction), until they were dismantled, beginning in late September [[2004]]. Both sites used long [[electric power transmission|power line]]s, so-called [[ground dipole]]s, as leads. These leads were in multiple strands ranging from 22.5 to 45 [[kilometer]]s long. Because of the inefficiency of this method, considerable amounts of [[electric power|electrical power]] were required to operate the system. == Ecological impact of ELF signals == There have been some concerns over the possible [[ecology|ecological]] impact of ELF signals. In 1984 a federal judge halted construction requiring more environmental and health studies. This judgment was overruled by a federal appeals court on the basis that the [[US Navy]] claimed to have spent over 25 million dollars studying the effects of the electromagnetic fields with results indicating that they were similar to the effect produced by standard power distribution lines. The judgement was not accepted by everyone and during the time ELF was in use, some Wisconsin politicians such as Senators [[Herb Kohl]], [[Russ Feingold]] and Congressman [[Dave Obey]] called for its closure (see for similar concerns [[electromagnetic radiation and health]]). == Other uses == Transmitters in the 20 Hz range are also found in [[pipeline inspection gauge]]s, also known as "PIGs". The transmitted signal is often used to track the pig should it become stuck in the pipeline. Some [[amateur radio|radio hams]] record ELF (or even lower) [[Signalling (telecommunication)|signal]]s from very large homemade antennas, and play them back at higher [[speed]]s in order to catch the natural fluctuations in the [[Earth]]'s [[electromagnetic field]]. Increasing the speed of the [[magnetic tape]] increases the [[pitch (music)|pitch]], so that it is brought into the [[audio frequency]] range. ==Natural ELF waves== Naturally-occurring ELF waves are present on Earth, resonating in the region between [[ionosphere]] and surface. They are initiated by [[lightning]] strikes that make electrons in the atmosphere oscillate<ref name="Titan ELF">{{cite news| url=http://saturn.jpl.nasa.gov/news/features/feature20070601c.cfm| title=Titan's Mysterious Radio Wave| date=2007-06-01| publisher=Jet Propulsion Laboratory| accessdate=2007-06-02}}</ref>. The fundamental mode of the Earth-ionosphere cavity has the wavelength equal to the circumference of the Earth, which gives a resonance frequency of 7.8 Hz. This frequency (and higher resonance modes: 14, 20, 26 and 32 Hz) appear as peaks in the ELF spectrum and are called [[Schumann resonance]]. They have also been tentatively identified on [[Saturn (planet)|Saturn]]'s [[natural satellite|moon]] [[Titan (moon)|Titan]]. Titan's surface is thought to be a poor reflector of ELF waves, so they may be reflecting instead off of the liquid-ice boundary of a subsurface ocean of water and ammonia predicted by some theoretical models. Titan's ionosphere is also more complex than Earth's, with the main ionosphere at an altitude of 1200 km but with an additional layer of charged particles at 63 km. This splits Titan's atmosphere to some extent into two separate resonating chambers. The source of natural ELF waves on Titan is unclear as there doesn't appear to be extensive lightning activity.<ref name="Titan ELF"/> Finally, huge ELF radiation power outputs of 100,000 times the Sun's output in visible light may be radiated by [[magnetars]]. The [[pulsar]] in the [[Crab nebula]] radiates powers of this order at the frequency 30 hertz [http://www.cv.nrao.edu/course/astr534/Pulsars.html] . Radiation of this frequency is below the [[plasma frequency]] of the interstellar medium, thus this medium is opaque to it, and it cannot be observed from Earth. ==Patents== * Tanner, R. L., ''{{US patent|3215937}}'', "''Extremely low-frequency antenna''". 1965. * Hansell, Clarence W., ''{{US patent|2389432}}'', "''Communication system by pulses through the Earth''". * Altshuler, {{US patent|4051479}}, ''ELF vertical dipole antenna suspended from aircraft '' ==See also== *[[Communication with submarines]] *[[Electromagnetic spectrum]] *[[Schumann resonance]] *[[TACAMO]] *[[List of initialisms]] *[[Skin effect]] *[[Ground dipole]] ==References== ===Citations and notes=== {{reflist}} ===General information=== * [[Non-ionizing radiation]], Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields (2002)] by the [[International Agency for Research on Cancer|IARC]]. ([http://monographs.iarc.fr/htdocs/monographs/vol80/80.html Non-Ionizing Radiation]) ===Websites=== * Tomislav Stimac, "''[http://www.vlf.it/frequency/bands.html Definition of frequency bands (VLF, ELF... etc.)]''". IK1QFK Home Page (vlf.it). * [http://www.inchem.org/documents/ehc/ehc/ehc35.htm Extremely low frequency (ELF) fields] (EHC 35, 1984) * "''[http://www.vlf.it/ Radio waves below 22kHz]: Nature's signals and strange emission at very low frequency''" - a site specialising in low-frequency signals . * Jacobsen, Trond, "''[http://www.vlf.it/zevs/zevs.htm ZEVS, the Russian 82 Hz ELF transmitter]: An Extrem Low Frequency transmission-system, using the real longwaves''" ALFLAB, Halden, Norway. * [http://spaceweather.com/glossary/inspire.html NASA live streaming ELF -> VLF Receiver] * [http://www.miklagaard.com/longitudinal-electric-waves Longitudinal Electric Waves] {{radio_spectrum}} {{EMSpectrum}} [[Category:Radio spectrum]] [[ca:ELF]] [[da:Extremely Low Frequency]] [[de:Extremely Low Frequency]] [[fr:Extrêmement basse fréquence]] [[hi:अत्यधिक निम्न आवृत्ति (ELF)]] [[id:ELF]] [[it:Extremely low frequency]] [[nl:Extremely Low Frequency]] [[no:Ekstremt lav frekvens]] [[pl:Extremely low frequency]] [[su:Extremely low frequency]] [[zh:極低頻]]