Extremely low frequency

ELF radio waves are generated by lightning and natural disturbances in Earth's magnetic field, so they are a subject of research by atmospheric scientists.

Because of the difficulty of building antennas that can radiate such long waves, ELF have been used in only a very few human-made communication systems.

ELF waves can penetrate seawater, which makes them useful in communication with submarines, and a few nations have built military ELF transmitters to transmit signals to their submerged submarines, consisting of huge grounded wire antennas (ground dipoles) 15–60 km (9–37 mi) long driven by transmitters producing megawatts of power.

[16] United States Government agencies, such as NASA, describe ELF as non-ionizing radiation with frequencies between 0 and 300 Hz.

[17] Due to their extremely long wavelength, ELF waves can diffract around large obstacles, are not blocked by mountain ranges or the horizon, and can travel around the curvature of the Earth.

ELF waves have extremely low attenuation of 1–2 dB per 1,000 km (620 mi),[18][19] giving a single transmitter the potential to communicate worldwide.

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 an antenna of the required size on a submarine (the antenna needs to be of an exceptional size in order to achieve successful communication).

Generally, ELF signals have been used to order a submarine to rise to a shallow depth where it could receive some other form of communication.

When the refractive index of the medium is greater than one, ELF waves propagate slower than the speed of light in vacuum.

Because of this huge size requirement, to transmit internationally using ELF frequencies, the Earth itself forms a significant part of the antenna, and extremely long leads into the ground are necessary.

The signal is generated as an alternating magnetic field, and the transmitter is mounted to, or to part of, the "pig", the cleaning device inserted into the pipe.

They then replay them at higher speeds to more easily observe natural low-frequency fluctuations in the Earth's electromagnetic field.

[33] Also, 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.

[38] There is high uncertainty regarding correlations between long-term, low-level exposure to ELF fields and a number of health effects, including leukemia in children.

[36] Subsequently, in 2010, a pooled analysis of epidemiological evidence supported the hypothesis that exposure to power-frequency magnetic fields is related to childhood leukemia.

[39] No other study has found any evidence to support the hypothesis that ELF exposure is a contributing factor to leukemia in children.

In summary, when all of the studies are evaluated together, the evidence suggesting that EMFs may contribute to an increased risk of cancer is non-existent.

However, during the time that ELF was in use, some Wisconsin politicians, such as Democratic Senators Herb Kohl, Russ Feingold and Congressman Dave Obey, continued to call for its closure.

1982 aerial view of the U.S. Navy Clam Lake, Wisconsin , ELF transmitter facility, used to communicate with deeply submerged submarines. The rights of way of the two perpendicular 14 mile (23 km) overhead transmission lines that constituted the ground dipole antenna which radiated the ELF waves can be seen at lower left.
Typical spectrum of ELF electromagnetic waves in the Earth's atmosphere, showing peaks caused by the Schumann resonances . The Schumann resonances are the resonant frequencies of the spherical Earth–ionosphere cavity. Lightning strikes cause the cavity to "ring" like a bell, causing peaks in the noise spectrum. The sharp power peak at 50 Hz is caused by radiation from global electric power grids . The rise of the noise at low frequencies (left side) is radio noise caused by slow processes in the Earth's magnetosphere .
A ground dipole antenna used for transmitting ELF waves, similar to the U.S. Navy Clam Lake antennas, showing how it works. It functions as a huge loop antenna , with the alternating current I from the transmitter P passing through an overhead transmission line, then deep in the earth from one ground connection G to the other, then through another transmission line back to the transmitter. This creates an alternating magnetic field H , which radiates ELF waves. The alternating current is shown flowing in one direction only through the loop for clarity.