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Extremely low frequency (ELF) is the designation given by the International Telecommunication Union (ITU) for radio waves or electromagnetic radiation with frequencies that range from just 3 Hz to 30 Hz and with corresponding wavelengths. But in atmospheric science, ELF is alternatively given a range of 3 Hz to 3,000 Hz. ELF signals are usually generated by naturally occurring phenomena such as lightning strikes and natural disturbances in the earth. Electric power lines also give out ELF waves.
Extremely low frequency, as the name suggests, are EM waves with very low frequencies, which mean that their wavelengths are extremely long. This also requires very wide antennas, about 2,175 to 3,700 miles (3500 to 6000 km) wide, to create the signals. Techniques such as electrical lengthening help to create radio stations with smaller sizes but the power requirement to run the system is still quite considerable, partly because the systems are inefficient. The U.S. has made the only two ELF stations ever created, but were decommissioned in September 2004.
These sort of waves travel less than the speed of light and are able to penetrate the Earth's surface and even salt water. Because of this, the best use for ELF has been for underwater communication with submarines since higher frequencies are unable to penetrate the conductive properties of salt water. However the major downside is that the communication is one directional, only from the station to the submarine, this is largely because an ELF antenna would be too large for a submarine. The ELF signal was only used to instruct the submarine to rise to a shallower depth where regular communications channels work.
Scientists and biologists believe that ELF signals can have negative effects on the human body, but so far research has been inconclusive and there have been no reported incidents of illness brought about by power lines that also emit ELF.