Frequency range | 30–300 kHz |
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Wavelength range | 10–1 km |
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Low frequency (LF) is theITU designation[1][2] forradio frequencies (RF) in the range of 30–300 kHz. Since its wavelengths range from 10–1 km, respectively, it is also known as thekilometre band orkilometre waves.
LF radio waves exhibit low signalattenuation, making them suitable for long-distance communications. In Europe and areas ofNorthern Africa and Asia, part of the LF spectrum is used forAM broadcasting as the "longwave" band. In the western hemisphere, its main use is for aircraft beacons, navigation (LORAN, mostly defunct), information, and weather systems. A number oftime signal broadcasts also use this band. The main mode of transmission used in this band isground waves, in which LF radio waves travel just above the Earth's surface, following the terrain. LF ground waves can travel over hills, and can travel far beyond the horizon, up to several hundred kilometers from the transmitter.
Because of their longwavelength, low frequencyradio waves candiffract over obstacles like mountain ranges and travel beyond the horizon, following the contour of the Earth. This mode of propagation, calledground wave, with the radio waves traveling horizontally through the atmosphere just above the surface of the Earth, is the main mode in the LF band.[3] Ground waves are absorbed by the Earth as they travel, so the signal strength (power density) decreases exponentially with distance from the transmitting antenna, limiting transmission distance. The attenuation of signal strength with distance is lower than at higher frequencies. Low frequency ground waves can be received up to 2,000 kilometres (1,200 mi) from the transmitting antenna. Ground waves must bevertically polarized (theelectric field is vertical while themagnetic field is horizontal), so verticalmonopole antennas are used for transmitting.
Low frequency waves can also occasionally travel long distances by reflecting from theionosphere (the actual mechanism is one ofrefraction), although this method, calledskywave or "skip" propagation, is not as common as at higher frequencies. Reflection occurs at the ionosphericE layer orF layers. Skywave signals can be detected at distances exceeding 300 kilometres (190 mi) from the transmitting antenna.[4]
AM broadcasting is authorized in thelongwave band on frequencies between 148.5 and 283.5 kHz in Europe and parts of Asia.
In Europe and Japan, many low-cost consumer devices have since the late 1980s containedradio clocks with an LF receiver for these signals. Since these frequencies propagate byground 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 ofWWVB was increased in 1997 and 1999.
JJY transmitting broadcast on the exact same frequency, and has a similartimecode.
Radio signals below 50 kHz are capable of penetrating ocean depths to approximately 200 metres (660 ft); the longer the wavelength, the deeper they go. The British, German, Indian, Russian, Swedish, United States,[5]and possibly othernavies communicate withsubmarines on these frequencies.
In addition,Royal Navy nuclear submarines carrying ballistic missiles are allegedly under standing orders to monitor theBBC 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 programmeToday, as an indicator that the UK is under attack, whereafter theirsealed orders take effect.[6]
The United States has four LF stations maintaining contact with its submarine force:Aguada, Puerto Rico,Keflavik, Iceland,Awase, Okinawa, andSigonella, Italy, using AN/FRT-95 solid state transmitters.
In the U.S., theGround 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.
The 2007 World Radiocommunication Conference (WRC-07) made a worldwide amateur radio allocation in this band. An international 2.1 kHz allocation, the2200 meter band (135.7–137.9 kHz) is available toamateur radio operators in several countries in Europe,[7]New Zealand, Canada, US,[8]and French overseas dependencies.
The world record distance for a two-way contact is over 10,000 km from nearVladivostok toNew Zealand.[9]As well as conventionalMorse code many operators use very slow computer-controlled Morse code (so-called"QRSS") or specialized digital communications modes.
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 Watt ERP. This was withdrawn on 30 June 2003 after a number of extensions in favor of the cross-European standard 136 kHz band.[10]Very slow Morse Code from G3AQC in the UK was received 3,275 miles (5,271 km) away, across theAtlantic Ocean, by W1TAG in the US on 21-22 November 2001 on 72.401 kHz.[a]
In the United States, there is an exemption within FCC Part 15 regulations permitting unlicensed transmissions in the frequency range of 160–190 kHz. Longwave radio hobbyists refer to this as the 'LowFER' band, and experimenters, and their transmitters are called 'LowFERs'. This frequency range between 160 kHz and 190 kHz is also referred to as the1750 meter band. Requirements[12] include:
Many experimenters in this band are amateur radio operators.[13]
Low-frequency transmissions have long been used for distributing marine meteorological information because LF signals propagate over long distances and remain reliable under poor atmospheric conditions.[4]
Low-frequency bands were historically important for maritime meteorology because ground-wave propagation allows coverage across coastal regions and the open sea at ranges difficult to achieve with higher frequencies. Before the adoption of systems such asNAVTEX andsatellite distribution under theGlobal Maritime Distress and Safety System (GMDSS), several national meteorological agencies used LF radiotelegraphy or radioteletype to transmit routine forecasts and observations to ships. Many of these services were gradually discontinued from the late twentieth century onward as GMDSS technologies became widely adopted.
In Germany, the German Meteorological Service (Deutscher Wetterdienst orDWD). operates DDH47 on 147.3 kHz, transmittingSYNOP weather reports and marine forecasts in RTTY format.[14]
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In parts of the world where there is no longwave broadcasting service,Non-directional beacons 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.
TheLORAN-C radio navigation system operated on 100 kHz.
In the past, theDecca 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.[15]
The commercial "Datatrak" radio navigation system operates on a number of frequencies, varying by country, between 120–148 kHz.
Some radio frequency identification (RFID) tags utilize LF. These tags are commonly known as LFIDs or LowFIDs (low frequency identification). The LF RFID tags arenear-field devices, interacting with theinductive near field, rather than with radiated waves (radio waves) that are the only part of the electromagnetic field that persists into the far field. As such, they are technicallynot radio devices nor radio antennas, even though they do operate at radio frequencies, and arecalled "antennas" in the RFID trade, but not inradio engineering. It is more proper, and technically more informative to think of them as secondary coils of very loosely coupledtransformers.
Since the ground waves used in this band requirevertical polarization, vertical antennas are used for transmission.Mast radiators are most common, either insulated from the ground and fed at the bottom, or occasionally fed through guy-wires.T-antennas andinverted L-antennas are used when antenna height is an issue.
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 withhuman exposure to radio waves.
Antenna requirements for LF reception are much more modest than for transmission. Although non-resonant long wire antennas are sometimes used, ferriteloop antennas are far more popular because of their small size.
Amateur radio operators have achieved good LF reception usingactive antennas: A short whip with a built-inpre-amplifier.
Due to the long wavelengths in the band, nearly all LF antennas areelectrically short, shorter than one quarter of the radiated wavelength, so their low radiation resistance makes them inefficient, requiring very low resistance grounds and conductors to avoid dissipating transmitter power.These electrically short antennas needloading coils at the base of the antenna to bring them into resonance. Many antenna types, such as theumbrella antenna and L- and T-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. Thecapacitance improves the efficiency of the antenna by increasing the current, without increasing its height.
The height of antennas differ by usage. For somenon-directional beacons (NDBs) the height can be as low as 10 meters, while for more powerful navigation transmitters such asDECCA, masts with a height around 100 meters are used.T-antennas have a height between 50–200 meters, while mast aerials are usually taller than 150 meters.
The height of mast antennas forLORAN-C is around 190 meters for transmitters with radiated power below 500 kW, and around 400 meters for transmitters greater than1000 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 orT-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 bytransmitter Orlunda in Sweden.
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