Low frequency or low freq or LF is the ITU designation for radio frequencies (RF) in the range of 30 kHz–300 kHz. Also known as the kilometre band or kilometre wave as the wavelengths range from one to ten kilometres, the low signal attenuation in this band allows long distance communication. 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.
Because of their long wavelength, low frequency radio waves can diffract over obstacles like mountain ranges and follow the curvature of the Earth. This mode of propagation, called ground wave, is the main mode in the LF band. The attenuation of signal strength with distance by absorption in the ground is lower than at higher frequencies. Low frequency ground waves can be received up to 2000 km from the transmitting antenna.
Low frequency waves can also travel long distances by reflecting from the ionosphere (the actual mechanism is one of refraction), although this method, called skywave or "skip" propagation, is not as common as at higher frequencies. Reflection occurs at the ionospheric E layer or F layers. Skywave signals can be detected at distances exceeding 300 km from the transmitting antenna.
Standard time signals
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)
- BPC in Shangqiu, China (68.5 kHz)
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.
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  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.
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
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, 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. 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. Very slow Morse Code from G3AQC in the UK was received 3,275 miles (5,271 km) away, across the Atlantic Ocean, by W1TAG in the US on November 21–22, 2001 on 72.401 kHz.
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 of any mode that falls inside the 30 kHz bandwidth is permitted, except where interference would occur to 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
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A regular service transmitting RTTY marine meteorological information in SYNOP code 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.
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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.
The commercial "DATATRAK" radio navigation system operates on a number of frequencies, varying by country, between 120 and 148 kHz.
The longwave radio broadcasting service operates on frequencies between 148.5 and 283.5 kHz in Europe and parts of Asia.
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.
Since the ground waves used in this band require vertical polarization, vertical antennas are used for transmission, usually mast radiators, either insulated from the ground and fed at the bottom, or occasionally fed through guy-wires. T-antennas and inverted L-antennas are used when antenna height is an issue. Nearly all LF antennas are electrically 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 need loading coils of high inductance to bring them into resonance. Many antenna types, such as the umbrella 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. The capacitance improves the efficiency of the antenna without increasing its height 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. Amateur radio operators have achieved good LF reception using active antennas with a short whip.
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.
- "Rec. ITU-R V.431-7, Nomenclature of the frequency and wavelength bands used in telecommunications". ITU. Retrieved 20 February 2013.
- Alan Melia, G3NYK. "Understanding LF Propagation". Radcom (Bedford, UK: Radio Society of Great Britain) 85 (9): 32.
- "Very Low Frequency (VLF) - United States Nuclear Forces". 1998. Retrieved 2008-01-09.
- "The Human Button". 2008-12-02. BBC. BBC Radio 4. http://www.bbc.co.uk/programmes/b00fq2sy.
- CEPT/ERC Recommendation 62-01 E (Mainz 1997): Use of the band 135.7-137.8 kHz by the Amateur Service.
- "QSO ZL/UA0 on 136 kHz". The World of LF.
- "UK Spectrum Strategy 2002". Ofcom.
- "G3AQC'S SIGNAL SPANS THE ATLANTIC ON 73 KHZ!". The ARRL Letter. ARRL. November 30, 2001. Retrieved 12 January 2014. "Low-frequency experimenter Lawrence "Laurie" Mayhead, G3AQC, has added another LF accomplishment to his list--transatlantic reception of his 73 kHz signal. [...] Mayhead reports that on the night of November 21-22, his signal on 72.401 kHz was received in the US. "I managed to transmit a full call sign to John Andrews, W1TAG, in Holden, Massachusetts," he said. Mayhead was using dual-frequency CW--or DFCW--featuring elements that are two minutes long, and Andrews detected his signal using ARGO DSP software."
- "DWD Sendeplan". Retrieved 2008-01-08.[dead link]
- Alan Gale, G4TMV (2011). "World DGPS database for DXers" (PDF). 4.6. Retrieved 2008-01-14.
- Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)".
- IK1QFK Home Page.
- Klawitter, G.; Oexner, M.; Herold, K. (2000). Langwelle und Längstwelle (in German). Meckenheim: Siebel Verlag GmbH. ISBN 3-89632-043-2.
- Marten, M. (2007). Spezial-Frequenzliste 2007/08 (in German). 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.