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Extremely low frequency
Additional recommended knowledge
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.
Difficulties of ELF 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 a substantial fraction of 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 ~299 792 (kilometers per second) divided by 50-85, which is 3,450km to 5,996 km long. The earth's diameter varies from 12,715 km (pole to pole) to 12,756 km (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-Nicolet National Forest, Wisconsin and the Escanaba State Forest, Michigan (originally named Project Sanguine, then downsized and rechristened Project ELF prior to construction), until they were dismantled, beginning in late September 2004. Both sites used long power lines, so-called ground dipoles, as leads. These leads were in multiple strands ranging from 22.5 to 45 kilometers long. Because of the inefficiency of this method, considerable amounts of electrical power were required to operate the system.
Ecological impact of ELF signals
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 judgment 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.
There are similar concerns about the health effects of electromagnetic radiation at other frequencies. electromagnetic radiation and health
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.
Natural ELF waves
Naturally-occurring ELF waves are present on Earth, resonating in the region between ionosphere and surface. They are initiated by lightning strikes that make electrons in the atmosphere oscillate. 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. This frequency (and higher resonance modes: 14, 20, 26 and 32 Hz) appear as peaks in the ELF spectrum and are called Schumann resonance.
They have also been tentatively identified on Saturn's moon Titan. Titan's surface is thought to be a poor reflector of ELF waves, so they may be reflecting instead off of the liquid-ice boundary of a subsurface ocean of water and ammonia predicted by some theoretical models. Titan's ionosphere is also more complex than Earth's, with the main ionosphere at an altitude of 1200 km but with an additional layer of charged particles at 63 km. This splits Titan's atmosphere to some extent into two separate resonating chambers. The source of natural ELF waves on Titan is unclear as there doesn't appear to be extensive lightning activity.
Finally, huge ELF radiation power outputs of 100,000 times the Sun's output in visible light may be radiated by magnetars. The pulsar in the Crab nebula radiates powers of this order at the frequency 30 Hertz  . Radiation of this frequency is below the plasma frequency of the interstellar medium, thus this medium is opaque to it, and it cannot be observed from Earth.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Extremely_low_frequency". A list of authors is available in Wikipedia.|