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Longwave Radio frequencies are those below 500 KHz , which correspond to Wavelength s longer than 600 Meters . They have the property of following the curvature of the earth, making them ideal for continuous, continental Communication s. Unlike Shortwave radio, longwave signals do not Reflect or Refract using the Ionosphere , so there are fewer Phase -caused Fadeouts . Instead, the D-layer of the ionosphere and the surface of the earth serve as a Waveguide directing the signal. Because of limitations in the technology of the time, the earliest radio Transmitter s were all longwave transmitters. A device called a Radio Alternator or Alexanderson Alternator was commonly used to generate the radio frequency carrier wave. BROADCASTING In Europe , North Africa and Asia , longwave radio frequencies between 153 and 279 kHz are used for domestic and International Broadcasting . RADIO NAVIGATION In the Americas , frequencies between 200 and 430 kHz are used for Non-directional Beacon s (NDBs), and do not necessarily follow the same 9 kHz spacing that other areas do. STANDARD TIME SIGNALS In the frequency range 40-80 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 Clock s with an LF receiver for these signals, which penetrate buildings more effectively than those in higher frequency bands. In North America, 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 Submarine s 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 U.K. It is rumoured that they are to construe a sudden halt in transmission as an indicator that the U.K. is under attack, whereafter their sealed orders take effect. EXPERIMENTAL In most countries of the world, radio amateurs have a frequency allocation between 135.7 kHz and 137.8 kHz, mainly used for extremely slow speed telegraphy (QRSS). A single dot has about 5 seconds of duration and a single dash has about 15 seconds of duration. The signals are decoded by computers. According to the laws of physics the extremely slow speed gives an extremely narrow bandwidth of the signals. These signals can be decoded by a computer far below the noise level. Such weak signals cannot be heard by the human ear. Signals from the U.K. have been heard in Canada but the current world record for two way communication is between U.K. and Italy, some 2300km. In North America there is a special licence free allocation in the longwave range called LowFER . ANTENNAS As aerials are usually used: Mast Radiator s which are fed at the bottom and which are insulated against ground, mast antennas fed by the guy ropes (such masts are usually grounded), T-aerials, L-aerials and long wire aerials. In future the use of Crossed Field Antenna is to be expected, if claims for their efficiency are proven. T-aerials and L-aerials are depending on the requirements mounted on as well grounded as insulated masts or towers. The height of aerials differ from usage: for NDBs the height is just around 10 metres, while for more powerful navigation transmitters as DECCA masts with a height around 100 metres are used. T-aerials have a height between 50 and 200 metres, while mast aerials are usually taller than 150 metres. The height of mast aerials for LORAN-C is around 190 metres for such with radiation powers below 500 kW and around 400 metres for such with powers more than 1,000 kilowatts. The first type of LORAN-C aerials is insulated against ground. Longwave broadcasting stations use mast antennas with heights of more than 150 metres 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 Warszaw 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 centre. 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. LIST OF LONGWAVE BROADCASTING TRANSMITTERS List of the most important longwave broadcasting transmitters (Source: http://de.wikipedia.org/wiki/Langwelle ). SEE ALSO
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