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Low frequency
Frequency Range 30 to 300 kHz

ITU Radio Band Numbers

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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 ten to one kilometres.

Contents

Propagation of LF signals

Low frequency radio signals can 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 reflection can take place at the D layer (50–90 km) or the E layer (90–150 km). These waves, called skywaves, can be detected at distances exceeding 300 km from the transmitting antenna.[1]

Standard time signals

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

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

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 probably more navies communicate with submarines on these frequencies.

In the USA, 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

A 2.1 kHz allocation, the 136 kHz band (135.7 kHz to 137.8 kHz), is available to amateur radio operators in some countries in Europe [3] , New Zealand and French overseas dependencies. The world record distance for a two-way contact is over 10,000 km from near Vladivostok to New Zealand.[4] As well as conventional Morse code many operators use very slow computer controlled Morse code (QRSS) or specialized digital communications modes. A proposal at the WRC-07 World Radiocommunication Conference aims to make this 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.[5] A 1-watt transmission of very slow Morse Code between G3AQC (in the UK) and W1TAG (in the USA) 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, except where interference would occur to ten licensed location service stations located along the coasts. 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. 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

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 [6].

Radio navigation signals

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.[7]

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

Radio broadcasting

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

Other applications

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

Low cost LF time signal receiver

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 shorter than one quarter of the radiated wavelength. The only longwave transmission antenna realized with a height corresponding to a half radiated wavelength was Warsaw Radio Mast. Low height antennas need loading coils of high inductance. These coils have high power losses due to ohmic heating of the coil wire. The addition of a horizontal section ("top hat") 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 often directional antennas are 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.

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

See also

References

  1. ^ Alan Melia, G3NYK. "Understanding LF Propagation". Radcom (Bedford, UK: Radio Society of Great Britain) 85 (9): 32.  
  2. ^ "Very Low Frequency (VLF) - United States Nuclear Forces". 1998. http://www.fas.org/nuke/guide/usa/c3i/vlf.htm. Retrieved 2008-01-09.  
  3. ^ CEPT/ERC Recommendation 62-01 E (Mainz 1997): Use of the band 135.7-137.8 kHz by the Amateur Service.
  4. ^ "QSO ZL/UA0 on 136 kHz". The World of LF. http://www.wireless.org.uk/newspic92.htm.  
  5. ^ "UK Spectrum Strategy 2002". Ofcom. http://www.ofcom.org.uk/static/archive/ra/topics/spectrum-strat/future/strat02/strategy02app_a.doc.  
  6. ^ "DWD Sendeplan". http://www.dwd.de/de/wir/Geschaeftsfelder/Seeschifffahrt/Sendeplaene/Sendeplaene.htm. Retrieved 2008-01-08.  
  7. ^ Alan Gale, G4TMV (2008). "World DGPS database for DXers" (PDF). http://www.beaconworld.org.uk/files/worldDGPSfreqorder.pdf. Retrieved 2008-01-14.  

Further reading








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