Low frequency

Low Frequency 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 broadcast service. In the western hemisphere, its main use is for aircraft beacon, navigation (LORAN), information, and weather systems. Time signal stations MSF, DCF77, JJY and WWVB are found in this band. Also known as the kilometer band or kilometer wave as the wavelengths range from ten to one kilometers.

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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, USA (60 kHz)
• HBG in Prangins, Switzerland (75 kHz)
• DCF77 in Mainflingen near Frankfurt am Main, Germany (77.5 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.

Military

Radio signals below 50 kHz are capable of penetrating ocean depths to approximately 200 meters, the longer the wavelength, the deeper. The British, German, Indian, Russian, Swedish, United States and probably more 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.

For more details on this topic, see Communication with submarines.

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, 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.^[1] As well as conventional Morse code many operators use very slow computer controlled Morse code 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.^[2] A 1-watt transmission of very slow Morse Code between G3AQC (in the UK) and W1TAG (in the USA) spanned the Atlantic Ocean for 3275 miles 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, and an 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.

Antennas

Antennas 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-aerials, L-aerials and long wire aerials.

The height of aerials differ by usage. For non-directional beacons (NDBs) the height is just around 10 meters, while for more powerful navigation transmitters such as DECCA, masts with a height around 100 meters are used. T-aerials have a height between 50 and 200 meters, while mast aerials are usually taller than 150 meters.

The height of mast aerials 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 aerial is insulated from ground.

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 aerials on grounded masts.

Nearly all longwave aerials are not as high as one quarter of the radiated wavelength. The only longwave transmission aerial realized with a height corresponding to a half radiated wavelength was Warsaw Radio Mast.

For broadcasting stations often directional aerials are required. They consist of multiple masts, which often have the same height. Some longwave aerials consist of multiple mast antennas arranged in a circle with or without a mast antenna in the center. Such aerials focus the transmitted power toward ground and gave a large zone of fade-free reception. This aerial type 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 aerial of this kind was used by transmitter Orlunda in Sweden.

Longwave 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.

References

See also

• Longwave
• Time Signal
• WGU-20

External articles

• Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).