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1982 aerial view of the U.S. Navy Clam Lake, Wisconsin ELF transmitter facility, used to communicate with deeply submerged submarines.

Extremely low frequency (ELF) is a term used to describe electromagnetic radiation (radio waves) with frequencies from 3 to 300 Hz, and corresponding wavelengths from 100,000 to 1000 kilometers.^[1] In atmosphere science, an alternative definition is usually given, from 3 Hz to 3 kHz.^[2][3] In the related magnetosphere science, the lower frequency electromagnetic oscillations (pulsations occurring below ~3 Hz) are considered to lie in the ULF range, which is thus also defined differently from the ITU Radio Bands.

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 transmitters that can generate such long waves, ELF frequencies have been used in only a very few manmade communication systems. ELF waves can penetrate seawater to a depth of several hundred feet, so the US and Russian military have used ELF transmission facilities to communicate with their submerged submarines. The frequency of alternating current flowing in electric power grids, 50 or 60 Hz, falls within this frequency band, and power grids are an unintentional source of ELF radiation.

Alternate definitions[link]

ELF is a subradio frequency.^[4] Some medical peer reviewed journal articles refer to ELF in the context of "extremely low frequency (ELF) magnetic fields (MF)" with frequencies of 50 Hz^[5] and 50–80 Hz.^[6] United States Government agencies, such as NASA, describe ELF as non-ionizing radiation with frequencies between 0 and 300 Hz.^[4] The World Health Organization (WHO) have used ELF to refer to the concept of "extremely low frequency (ELF) electric and magnetic fields (EMF)"^[7] and have also referred to "ELF electric and magnetic fields in the frequency range >0 to 100,000 Hz (100 kHz)."^[8] The WHO also stated that at frequencies between 0 and 300 Hz, "the wavelengths in air are very long (6000 km at 50 Hz and 5000 km at 60 Hz), and, in practical situations, the electric and magnetic fields act independently of one another and are measured separately."^[7]

Military communications[link]

The United States Navy utilized extremely low frequencies (ELFs) as radio band and radio communications. The Submarine Integrated Antena System (SIAS) was a research and development effort to communicate with submerged submarines.^[9] The Soviet/Russian Navy also utilized ELFs for submarine communications system, ZEVS.^[10]

Explanation[link]

Because of its electrical conductivity, seawater shields submarines from most higher frequency radio waves, making radio communication with submerged submarines at ordinary frequencies impossible. 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^{[dubious – discuss]} an antenna of the required size on a submarine (antennas 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[link]

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 a given 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 (~186,282 mi) per second divided by 50–85 Hz, which equals around 3,527 to 5,996 km (2,192 to 3,726 mi) long; by comparison, Earth's diameter is around 12,742 km (7,918 mi). 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 mi) long. Because of the inefficiency of this method, considerable amounts of electrical power were required to operate the system.

Ecological impact[link]

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 judgment 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. Similar concerns have in the past been raised about electromagnetic radiation and health.

Other uses[link]

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 sources[link]

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.^[11] Though VLF signals were predominantly generated from lightning discharges, it was found that an observable ELF component (slow tail) followed the VLF component in almost all cases.^[12] 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.^[11]

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.

Exposure[link]

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In electromagnetic therapy and electromagnetic radiation and health research, electromagnetic spectrum frequencies between 0 and 100 hertz are considered extremely low-frequency fields.^[13] Since the late 1970s, questions have been raised whether exposure to ELF electric and magnetic fields (EMF) within this range of frequencies produces adverse health consequences.^[8] In October 2005, WHO convened a Task Group of scientific experts to assess any risks to health that might exist from "exposure to ELF electric and magnetic fields in the frequency range >0 to 100,000 Hz (100 kHz) in regards to childhood leukaemia."^[8] There are established biological effects from acute exposure at high levels (well above 100 µT) that are explained by recognized biophysical mechanisms. External ELF magnetic fields induce electric fields and currents in the body which, at very high field strengths, cause nerve and muscle stimulation and changes in nerve cell excitability in the central nervous system. Health effects related to short-term, high-level exposure have been established and form the basis of two international exposure limit guidelines (ICNIRP, 1998; IEEE, 2002). At present, these bodies consider the scientific evidence related to possible health effects from long-term, low-level exposure to ELF fields insufficient to justify lowering these quantitative exposure limits. The long-term, low-level exposure is evaluated as average exposure to residential power-frequency magnetic field above 0.3 to 0.4 µT. And it is estimated that only between 1% and 4% of children live in such conditions"^[8] A common source of ELF fields in the United States is 60 Hz electric and magnetic fields from high-voltage electric power transmission lines and secondary distribution lines, such as those found in residential neighborhoods.^[7][8][13] The International Agency for Research on Cancer (IARC) has evaluated the scientific data and has classified ELF magnetic fields as being "possibly carcinogenic" to humans. IARC based this classification on the following:

Human health population studies showing weak evidence of an association with childhood leukemia; and a large database of laboratory study results showing inadequate evidence of an association with cancer in animals.

To put this into context, it is important to understand that the "possibly carcinogenic" classification is also applied to coffee, gasoline engine exhaust and pickled vegetables, and is often used for agents that require further study. In summary, when all of the studies are evaluated together, the evidence suggesting that EMFs may contribute to an increased risk of cancer is very weak.^[14][15] Epidemiological studies suggest a possible association between long term occupational exposure to ELF and Alzheimer's disease.^[16][17]

Patents[link]

• 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[link]

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

References[link]

Notes[link]

General information[link]

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

External links[link]

• 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

Low frequency or low freq or LF refers to radio frequencies (RF) in the range of 30 kHz–300 kHz. In Europe, and parts of Northern Africa and of Asia, part of the LF spectrum is used for AM broadcasting as the longwave band. In the western hemisphere, its main use is for aircraft beacon, navigation (LORAN), information, and weather systems. Time signal stations MSF, HBG, DCF77, JJY and WWVB are found in this band. Also known as the kilometre band or kilometre wave as the wavelengths range from one to ten kilometres.

Propagation of LF signals[link]

Because of their long wavelength, low frequency radio signals can refract over obstacles like mountain ranges and follow the curvature of the Earth. Radio waves reaching the receiver by this route are called ground waves. Their strength is not reduced by absorption as much as in higher frequencies. Ground wave can cover an area with a radius of 2000 km about the transmitting antenna.

Propagation by reflection (the actual mechanism is one of refraction) from the ionosphere is also possible. The refraction can take place at the E layer or F layers. These waves, called skywaves, can be detected at distances exceeding 300 km from the transmitting antenna.^[1]

Standard time signals[link]

An LF Radio clock

In the frequency range 40 kHz–80 kHz, there are several standard time and frequency stations, such as

• JJY in Japan (40 kHz and 60 kHz)
• MSF in Anthorn, England (60 kHz)
• WWVB in Fort Collins, Colorado, US (60 kHz)
• DCF77 in Mainflingen near Frankfurt am Main, Germany (77.5 kHz)
• HBG in Prangins, Switzerland (75 kHz) (closed down on 1 January 2012)

In Europe and Japan, many low-cost consumer devices have since the late 1980s contained radio clocks with an LF receiver for these signals. Since these frequencies propagate by ground wave only, the precision of time signals is not affected by varying propagation paths between the transmitter, the ionosphere, and the receiver. In the United States, such devices became feasible for the mass market only after the output power of WWVB was increased in 1997 and 1999.

Military[link]

Radio signals below 50 kHz are capable of penetrating ocean depths to approximately 200 metres, the longer the wavelength, the deeper. The British, German, Indian, Russian, Swedish, United States ^[2] and possibly other navies communicate with submarines on these frequencies.

In addition, Royal Navy nuclear submarines carrying ballistic missiles are allegedly under standing orders to monitor the BBC Radio 4 transmission on 198 kHz in waters near the UK. It is rumoured that they are to construe a sudden halt in transmission, particularly of the morning news programme Today, as an indicator that the UK is under attack, whereafter their sealed orders take effect.^[3]

In the US, the Ground Wave Emergency Network or GWEN operated between 150 and 175 kHz, until replaced by satellite communications systems in 1999. GWEN was a land based military radio communications system which could survive and continue to operate even in the case of a nuclear attack.

Experimental and amateur[link]

An international 2.1 kHz allocation, the 2200-meter band (135.7 kHz to 137.8 kHz), is available to amateur radio operators in several countries in Europe,^[4] New Zealand, Canada and French overseas dependencies. The world record distance for a two-way contact is over 10,000 km from near Vladivostok to New Zealand.^[5] As well as conventional Morse code many operators use very slow computer controlled Morse code (QRSS) or specialized digital communications modes. The 2007 World Radiocommunication Conference (WRC-07) made this band a worldwide amateur radio allocation.

The UK allocated a 2.8 kHz sliver of spectrum from 71.6 kHz to 74.4 kHz beginning in April 1996 to UK amateurs who applied for a Notice of Variation to use the band on a noninterference basis with a maximum output power of 1 W ERP (effective radiated power). This was withdrawn on 30 June 2003 after a number of extensions in favor of the European-harmonized 136 kHz band.^[6] A 1-watt transmission of very slow Morse Code between G3AQC (in the UK) and W1TAG (in the US) spanned the Atlantic Ocean for 3,275 miles (5,271 km) on November 21–22, 2001.

In the United States, there is a special license free allocation in the longwave range called LowFER. This experimental allocation between 160 kHz and 190 kHz is sometimes called the "Lost Band". Unlicensed operation by the public is permitted south of 60 degrees north latitude,^{[citation needed]} except where interference would occur to ten licensed location service stations located along the coasts.^{[citation needed]} Regulations for use include a power output of no more than 1 watt, a combined antenna/ground-lead length of no more than 15 meters, and a field strength of no more than 4.9 microvolts/meter.^{[citation needed]} Also, emissions outside of the 160 kHz–190 kHz band must be attenuated by at least 20 dB below the level of the unmodulated carrier. Many experimenters in this band are amateur radio operators.

Meteorological information broadcasts[link]

This section requires expansion.

A regular service transmitting RTTY marine meteorological information on LF is the German Meteorological Service (Deutscher Wetterdienst or DWD). The DWD operates station DDH47 on 147.3 kHz using standard ITA-2 alphabet with a transmission speed of 50 baud and FSK modulation with 85 Hz shift.^[7]

Radio navigation signals[link]

This section requires expansion.

In parts of the world where there is no longwave broadcasting service, Non-directional beacons or NDB's used for aeronavigation operate on 190–300 kHz (and beyond into the MW band). In Europe, Asia and Africa, the NDB allocation starts on 283.5 kHz.

The LORAN-C radio navigation system operates on 100 kHz.

In the past, the Decca Navigator System operated between 70 kHz and 129 kHz. The last Decca chains were closed down in 2000.

Differential GPS telemetry transmitters operate between 283.5 and 325  kHz.^[8]

The commercial "DATATRAK" radio navigation system operates on a number of frequencies, varying by country, between 120 and 148 kHz.

Radio broadcasting[link]

The longwave radio broadcasting service operates on frequencies between 148.5 and 283.5 kHz in Europe and parts of Asia.

Other applications[link]

Some radio frequency identification (RFID) tags utilize LF. These tags are commonly known as LFID's or LowFID's (Low Frequency Identification). The LF RFID tags are near field devices.

Antennas[link]

Low cost LF time signal receiver

Transmitting antennas (aerials) used at these low frequencies are usually mast radiators, which are fed at the bottom and which are insulated from ground, or mast antennas fed by the guy ropes (such masts are usually grounded). T-antennas and L-antennas are used when antenna height is an issue. Long wire antennas are also used in rare cases. Nearly all LF antennas are electrically short, shorter than one quarter of the radiated wavelength. The only longwave transmission antenna realized with a height corresponding to a half radiated wavelength was the Warsaw Radio Mast. These electrically short antennas need loading coils of high inductance to bring them into resonance. These coils have high power losses due to ohmic heating of the coil wire, making the antenna inefficient. Many antenna types, such as the umbrella antenna, use capacitive top-loading (a "top hat"), in the form of a network of horizontal wires attached to the top of the vertical radiator. The capacitance improves the efficiency of electrically short LF antennas without increasing the height of the antenna or its supporting structures.

The height of antennas differ by usage.

For some non-directional beacons (NDBs) the height can be as low as 10 meters, while for more powerful navigation transmitters such as DECCA, masts with a height around 100 meters are used. T-antennas have a height between 50 and 200 meters, while mast aerials are usually taller than 150 meters.

The height of mast antennas for LORAN-C is around 190 meters for transmitters with radiated power below 500 kW, and around 400 meters for transmitters greater than 1000 kilowatts. The main type of LORAN-C antenna is insulated from ground.

LF (longwave) broadcasting stations use mast antennas with heights of more than 150 meters or T-aerials. The mast antennas can be ground-fed insulated masts or upper-fed grounded masts. It is also possible to use cage antennas on grounded masts.

For broadcasting stations, directional antennas are often required. They consist of multiple masts, which often have the same height. Some longwave antennas consist of multiple mast antennas arranged in a circle with or without a mast antenna in the center. Such antennas focus the transmitted power toward ground and give a large zone of fade-free reception. This type of antenna is rarely used, because they are very expensive and require much space and because fading occurs on longwave much more rarely than in the medium wave range. One antenna of this kind was used by transmitter Orlunda in Sweden.

For reception, long wire antennas are used, or more often ferrite loop antennas because of their small size.

LF transmitting antennas for high power transmitters require large amounts of space, and have been the cause of controversy in Europe and the United States due to concerns about possible health hazards associated with exposure to high-power radio waves.

See also[link]

• High frequency
• Long wave
• LowFER
• 2200-meter band
• Time signal
• WGU-20
• LowFID passive RFID tags

References[link]

Further reading[link]

• Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)".
• IK1QFK Home Page.
• Klawitter, G.; Oexner, M., Herold, K. (2000) (in German). Langwelle und Längstwelle. Meckenheim: Siebel Verlag GmbH. ISBN 3-89632-043-2. 
• Marten, M. (2007) (in German). Spezial-Frequenzliste 2007/08. Meckenheim: Siebel Verlag GmbH. pp. 36–39. ISBN 978-3-88180-665-7. 
• Mike Dennison, G3XDV and Jim Moritz, M0BMU (2007). LF Today. Radio Society of Great Britain. ISBN 978-1-905086-36-8. 

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