- Extremely low frequency
Because of the electrical conductivity of
salt water, submarines are shielded from most electromagneticcommunications. 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 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 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. Various other means are taken to construct radio stations with substantially smaller sizes, such as electrical lengthening. The US maintained two sites, in the Chequamegon-Nicolet National Forest, Wisconsinand 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 power lines, so-called ground dipoles, as leads. These leads were in multiple strands ranging from 22.5 to 45 kilometers long. Because of the inefficiency of this method, considerable amounts of electrical power were required to operate the system.
Ecological impact of ELF signals
There have been some concerns over the possible 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 Navyclaimed 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 Feingoldand Congressman Dave Obeycalled for its closure. (For similar concerns, see electromagnetic radiation and health.)
Transmitters in the 20 Hz range are also found in
pipeline inspection gauges, also known as "PIGs". The transmitted signal is often used to track the pig should it become stuck in the pipeline.
Some radio hams record ELF (or even lower) signals from very large homemade antennas, and play them back at higher
speeds in order to catch the natural fluctuations in the Earth's electromagnetic field. Increasing the speed of the magnetic tapeincreases the pitch, so that it is brought into the audio frequencyrange.
Natural ELF waves
Naturally-occurring ELF waves are present on Earth, resonating in the region between
ionosphereand surface. They are initiated by lightningstrikes that make electrons in the atmosphere oscillatecite 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] . 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's 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.
Finally, huge ELF radiation power outputs of 100,000 times the Sun's output in visible light may be radiated by
magnetars. The pulsarin the Crab nebularadiates 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 frequencyof the interstellar medium, thus this medium is opaque to it, and it cannot be observed from Earth.
* 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 "
Communication with submarines
List of initialisms
Non-ionizing radiation, Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields (2002)] by the IARC. ( [http://monographs.iarc.fr/ENG/Monographs/vol80/mono80.pdf Non-Ionizing Radiation] )
* 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]
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См. также в других словарях:
Extremely Low Frequency — (kurz ELF, dt. „Niederfrequenzbereich“) bzw. extrem niedrige Frequenz bezeichnet elektromagnetische Felder oder Wellen mit extrem niedrigen Frequenzen von 3 bis 30 Hz (Wellenlänge: 100.000 km bis 10.000 km; nicht zu verwechseln mit … Deutsch Wikipedia
extremely low frequency — noun below 3 kilohertz • Syn: ↑ELF • Hypernyms: ↑radio frequency • Part Holonyms: ↑electromagnetic spectrum * * * Radio. any frequency between 30 and 300 hertz. Abbr.: ELF, elf [ … Useful english dictionary
extremely low frequency — ypač žemas dažnis statusas T sritis radioelektronika atitikmenys: angl. extremely low frequency vok. Tiefstfrequenz, f rus. сверхнизкая частота, f pranc. fréquence ultra basse, f … Radioelektronikos terminų žodynas
Extremely Low Frequency — emission of low frequency electromagnetic radiation from electrical appliances … English contemporary dictionary
extremely low frequency — extreme′ly low′ fre′quency n. elo rtv any radio frequency between 30 and 300 hertz Abbr. : ELF • Etymology: 1965–70 … From formal English to slang
extremely low frequency — noun Date: 1966 a radio frequency in the lowest range of the radio spectrum see radio frequency table … New Collegiate Dictionary
extremely low frequency — Radio. any frequency between 30 and 300 hertz. Abbr.: ELF, elf [1965 70] * * * … Universalium
extremely low frequency — noun any radiofrequency between 3 and 30 hertz. Abbrev.: ELF … Australian-English dictionary
extremely low frequency band — ypač žemų dažnių diapazonas statusas T sritis radioelektronika atitikmenys: angl. extra low frequency band; extremely low frequency band vok. Kleinstfrequenzband, n rus. диапазон крайне низких частот, m pranc. fréquences extrêmement basses, f … Radioelektronikos terminų žodynas
extremely low-frequency emission — Abbreviated ELF. Radiation emitted by a computer monitor and other common electrical appliances. ELF emissions fall into the range of 5 to 2000 hertz and decline with the square of the distance from the source. Emissions are not constant… … Dictionary of networking