Very low frequency
Very low frequency or VLF refers to radio frequencies (RF) in the range of 3 to 30 kHz. Since there is not much bandwidth in this band of the radio spectrum, only the very simplest signals are used, such as for radionavigation. Because VLF waves can penetrate water only to a depth of roughly 10 to 40 metres (30 to 130 feet), depending on the frequency and the salinity of the water, they are used to communicate with submarines near the surface. (ELF is used for fully submerged vessels.) This frequency range is used nowadays for the transmission of instructions to submerged submarines (for example with the transmitter DHO38), since radio waves can penetrate some dozen of meters in sea water in this frequency band. They are also used for radio navigation (alpha) and for the transmission of time signals (beta). Early in the history of radio engineering within the band starting from 20 kHz attempts were made to use radiotelephone using amplitude and single-sideband modulation, but the result was unsatisfactory, because of the small available bandwidth.
The frequency range under 30 kHz also is used for time signals and radio navigation beacons. The very long wave transmitters, SAQ in Grimeton and Varberg in Sweden can be visited by public at certain times, such as on Alexanderson Day. As a rule very long wave transmitters work in the frequency range between 10kHz and 30kHz. There are also stations, which work in the frequency range under 10 kHz. This frequency range is subject to no control on the part of the international communications organization (International Telecommunication Union) and may be used in some states by license-free. In the USA, the time signal station WWVL began transmitting a 500 W signal on 20 kHz in August 1963. It used Frequency Shift Keying (FSK) to send data, shifting between 20 kHz and 26 kHz. The WWVL service was discontinued in July 1972.
PC-based VLF reception A very convenient way using all advantages of modern computer technology is PC based VLF reception. Therefore you connect an aerial for VLF in form of a coil of insulated wire at the entrance of the soundcard of the PC and put it a few metres away from it. Then you download a FFT-software (e.g.'SpecLab' from http://www.qsl.net/dl4yhf/spectra1.html ).
This software allows in combination with your soundcard a reception of all frequencies below 24 kilohertz simultaneously in form of spectrogrammes.
Because PC monitors are strong sources of noise in the VLF range, it is recommanded to record the spectrograms time steered on hard disk with the PC monitor turned off.
These spectrograms show many interesting signals: the signals of VLF transmitters, the signal of the frequency of the horizontal electron beam deflection of TV sets and sometimes superpulses and twenty second pulses.
(Source: Längstwelle in German-language Wikipedia) Many natural radio emissions, such as whistlerss, can be also heard in this band.
List of VLF transmitters
The well known VLF transmitter GBR Rugby on 16 kHz was shut down on April 12003.
|
|
