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Extremely low frequency (ELF)
Frequency: 3 Hz to 30 Hz Wavelength: 100 000 km to 10 000 km

Extremely low frequency (ELF) is the band of radio frequencies from 3 to 30 Hz. ELF was used by the US Navy and Soviet/Russian Navy to communicate with submerged submarines.

Contents 1 Explanation 2 Difficulties of ELF communication 3 Ecological impact of ELF signals 4 Other uses 5 Patents 6 See also 7 External links and references

Because of the electrical conductivity of salt water, submarines are shielded from most electromagnetic communications. 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.

One of the difficulties posed when broadcasting in the ELF frequency range is antenna size. This is because the antenna must be at least 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 antenna needs to be ~186,282 (miles per second) divided by 50-85, which is 2,144 to 3,726 miles long. The earth's diameter varies from 3,959 (pole to pole) to 3,963 (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 National Forest, Wisconsin and the Escanaba State Forest, Michigan (originally named Project Sanguine, then downsized and rechristened Project ELF prior to construction), until dismantling them began in late September 2004. Both sites used long power lines, such called ground dipoles, as leads. These leads were in multiple strands ranging from 14 to 28 miles (22.5 to 45 kilometers) long. Because of the inefficiency of this method, considerable amounts of electrical power were required to operate the system.

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 judgement 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 Herb Kohl, Russ Feingold and Dave Obey called for its closure.

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 tape increases the pitch, so that it is brought into the audio frequency range.

• Tanner, R. L., U.S. Patent 3215937 , "Extremely low-frequency antenna". 1965.
• Hansell, Clarence W., U.S. Patent 2389432 , "Communication system by pulses through the Earth".
• Altshuler, U.S. Patent 4051479 , ELF vertical dipole antenna suspended from aircraft

• Communication with submarines
• Electromagnetic spectrum
• Schumann resonance
• TACAMO
• List of initialisms
• Skin effect
• Ground dipole

• Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).
• Extremely low frequency (ELF) fields (EHC 35, 1984)
• "Radio waves below 22kHz: 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: An Extrem Low Frequency transmission-system, using the real longwaves" ALFLAB, Halden, Norway.
• Non-Ionizing Radiation, Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields (2002) by the IARC.
• A summary of the previous report by GreenFacts.
• NASA live streaming ELF -> VLF Receiver

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Radio spectrum
ELF	SLF	ULF	VLF	LF	MF	HF	VHF	UHF	SHF	EHF
3 Hz	30 Hz	300 Hz	3 kHz	30 kHz	300 kHz	3 MHz	30 MHz	300 MHz	3 GHz	30 GHz
30 Hz	300 Hz	3 kHz	30 kHz	300 kHz	3 MHz	30 MHz	300 MHz	3 GHz	30 GHz	300 GHz

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