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Very low frequency
Very low frequency or VLF refers to radio frequencies (RF) in the range of 3 kHz 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 radio navigation. Also known as the myriameter band or myriameter wave as the wavelengths range from ten to one myriameters (an obsolete metric unit equal to 10 kilometers).
Additional recommended knowledge
VLF waves can penetrate water to a depth of roughly 10 to 40 metres (30 to 130 feet), depending on the frequency employed and the salinity of the water. VLF is used to communicate with submarines near the surface (for example using the transmitter DHO38), while ELF is used for deeply-submerged vessels. VLF is also used for radio navigation beacons (alpha) and time signals (beta).
VLF is also used in electromagnetic geophysical surveys. 
Early in the history of radio engineering attempts were made to use radiotelephone using amplitude modulation and single-sideband modulation within the band starting from 20 kHz, but the result was unsatisfactory because of the small available bandwidth.
The frequency range below 9 kHz is not allocated by the International Telecommunication Union and may be used in some nations license-free. Many natural radio emissions, such as whistlers, can also be heard in this band.
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.
The very long wave transmitter SAQ at Grimeton near Varberg in Sweden can be visited by the public at certain times, such as on Alexanderson Day.
Details of VLF submarine communication methods
High power land-based transmitters in countries that operate submarines send signals that can be received thousands of miles away. Transmitter sites typically cover many acres, with transmitted power anywhere from 20 kW to 2 MW. Submarines receive the signal using some form of towed antenna which floats just under the surface of the water - for example a BCAA (Buoyant Cable Array Antenna). Modern receivers, such as those produced by Detica, use sophisticated digital signal processing (DSP) techniques to remove the effects of atmospheric noise (largely caused by lightning strikes around the world) and adjacent channel signals, extending the useful reception range.
Because of the low bandwidth available it is not possible to transmit audio signals, therefore all messaging is done with alphanumeric data at very low bit rates. Three types of modulation are used:
Two alternative character sets may be used: 5-bit ITA2 or 8-bit ASCII. Because these are military transmissions they are almost always encrypted for security reasons. Although it is relatively easy to receive the transmissions and convert them into a string of characters, civilians cannot decode any encrypted messages because they most likely use one time pads since the amount of text is so small.
PC-based VLF reception
PC based VLF reception is a simple method whereby anyone can pick up VLF signals using the advantages of modern computer technology. An aerial in the form of a coil of insulated wire is connected to the input of the soundcard of the PC (via a jack plug) and placed a few metres away from it. Fast Fourier transform (FFT) software in combination with a sound card allows reception of all frequencies below 24 kilohertz simultaneously in the form of spectrogrammes. Because PC monitors are strong sources of noise in the VLF range, it is recommended to record the spectrograms on hard disk with the PC monitor turned off. These spectrograms show many interesting signals, which may include VLF transmitters, the horizontal electron beam deflection of TV sets and sometimes superpulses and twenty second pulses.
List of VLF transmissions
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Very_low_frequency". A list of authors is available in Wikipedia.|