Extremely low frequency

ELF is used by the US Navy to communicate with submerged submarines. Despite the extremely high electrical conductivity of salt water, the water's density shields submarines 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.

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 maintains two sites, in Wisconsin and Michigan. Both sites use 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, and there have been some concerns over the possible ecological impact of such signals.

Some amateur radio aficionados record ELF (or even lower) signals from very large homemade antennas, and play them back at higher speedss in order to catch the Earth's natural fluctuationss in its electromagnetic field. Increasing the speed of the magnetic tape increases the pitch, so that it is brought into the audio frequency range.

See also: electromagnetic spectrum, Schumann resonance, TACAMO, HAARP, list of initialisms, Serial Experiments Lain

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