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| Extremely low frequency (ELF) |
|---|
| Cycles per second: 3Hz to 30Hz | Wavelength: 100000km to 10000 km
Applications
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. 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 caused by the impracticality of installing the 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.
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 began 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. Because of the inefficiency of the antenna, considerable amounts of electrical power were required to operate the system.
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 pipeline inspection gauges, also known as "pigs". The transmitted signal is often used to track the pig when it becomes 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.
Patents
- Tanner, R. L., , "Extremely low-frequency antenna". 1965.
- Hansell, Clarence W., , "Communication system by pulses through the Earth".
- Altshuler, , ELF vertical dipole antenna suspended from aircraft
See also
- Communication with submarines
- Electromagnetic spectrum
- Schumann resonance
- TACAMO
- List of initialisms
- Skin effect
External links and references
- Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).
- Extremely low frequency (ELF) fields (EHC 35, 1984)
- "Radio waves below 22kHz: Nature's signals and strange emission at very low frequency" - a site specialising in low-frequency signals .
- Jacobsen, Trond, "ZEVS, the Russian 82 Hz ELF transmitter: An Extrem Low Frequency transmission-system, using the real longwaves" ALFLAB, Halden, Norway.
- Non-Ionizing Radiation, Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields (2002) by the IARC.
- A summary of the previous report by GreenFacts.
- NASA live streaming ELF -> VLF Receiver
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