extremely low frequency

1982 aerial view of the Clam Lake, Wisconsin ELF facility.

Extremely low frequency (ELF) is the band of radio frequencies from 3 to 30 Hz, at one time used by the US Navy and Soviet/Russian Navy to communicate with submerged submarines.

Explanation

Because of the electrical conductivity of seawater, 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 transmitter of the required size on a submarine (transmitters need to be of exceptional size for the users to achieve successful communication). 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 the EM waves. 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. As used in military applications, the wavelength is ~299,792 km(~187,370 mi) per second divided by 50-85 Hz, which equals around 3,450 to 5,996 km (2,140 to 3,730 mi) long; by comparison, Earth's diameter varies from 12,715 km (7,901 mi), pole to pole to 12,756 km (7,926 mi), equatorial. Because of this huge size requirement and, to transmit internationally using ELF frequencies, the earth itself must be used as an antenna, with extremely long leads going into the ground. Various other means are taken to construct radio stations with substantially smaller sizes, such as electrical lengthening.

The US maintained two sites, in the Chequamegon-Nicolet National Forest, Wisconsin and the Escanaba River 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 kilometres (14.0 to 28.0 mi) 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 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, some Wisconsin politicians such as Senators Herb Kohl, Russ Feingold and Congressman Dave Obey called for its closure. (For similar concerns, see electromagnetic radiation and health.)

There are also concerns that very strong electromagnetic fields emanated by high power radio transmitters could lead to interference with electronic devices and may be linked with aircraft systems failure. ^[1]

Other uses

Transmitters in the 20 Hz range are also found in pipeline inspection gauges, also known as "PIGs".

Some radio hams record ELF (or even lower) signals from very large homemade antennas, and play them back at higher speeds to catch natural fluctuations in the Earth's electromagnetic field. Increasing the playback 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^[2]. 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 of 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 the waves may instead be reflecting off the liquid-ice boundary of a subsurface ocean of water and ammonia, the existence of which is predicted by some theoretical models. Titan's ionosphere is also more complex than Earth's, with the main ionosphere at an altitude of 1,200 km (750 mi) but with an additional layer of charged particles at 63 km (39 mi). This splits Titan's atmosphere to some extent^{[weasel words]} 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.^[2]

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[1]. 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.

Patents

• Tanner, R. L., U.S. Patent 3,215,937, "Extremely low-frequency antenna". 1965.
• Hansell, Clarence W., U.S. Patent 2,389,432, "Communication system by pulses through the Earth".
• Altshuler, U.S. Patent 4,051,479, ELF vertical dipole antenna suspended from aircraft

See also

• Communication with submarines
• Electromagnetic spectrum
• Schumann resonance
• TACAMO
• List of initialisms
• Skin effect
• Ground dipole
• Wardenclyffe Tower

References

Notes

General information

• Non-ionizing radiation, Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields (2002)] by the IARC. (Non-Ionizing Radiation)

External links

• 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.
• NASA live streaming ELF -> VLF Receiver
• Longitudinal Electric Waves