Extremely low frequency (ELF) is the band of radio frequencies from 3 to 300 Hz.
ELF was used by the US Navy to communicate with submerged submarines. 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 more deeply. The low transmission rate of most ELF communications limits their use as communications channels; generally an ELF signal serves to request that a submarine surface and initiate some other form of contact. For details see: communications with submarines
One of the difficulties posed when broadcasting in the ELF frequency range is antenna size. In order to transmit internationally using ELF frequencies, an extremely large antenna is required. The US maintained two sites, in the Chequamegon National Forest, Wisconsin and the Escanaba State Forest , Michigan, until dismantling them beginning in late September 2004. Both sites used long power lines as antennae, in multiple strands ranging from 14 to 28 miles (22.5 to 45 kilometers) long. Considerable amounts of electrical power are generated and emitted by ELF.
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 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 pigs, used in the maintenance of pipelines. The transmitted signal is often used to track the pig when it becomes stuck in the pipeline.
Some amateur radio aficionados record ELF (or even lower) signals from very large homemade antennas, and play them back at higher speeds in order to catch the Earth's natural fluctuations in its electromagnetic field. Increasing the speed of the magnetic tape increases the pitch, so that it is brought into the audio frequency range.
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Last updated: 10-20-2005 13:43:35
Last updated: 10-29-2005 02:13:46
