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Very low frequency

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This is an old revision of this page, as edited by EEPROM Eagle (talk | contribs) at 03:01, 3 July 2008 (If I'm not mistaken, using an aerial connected to a sound card should let you recieve frequencies below the Nyquist frequency, not just 24 kHz. Correct if wrong. Typo fixed too.). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

A VLF receiving antenna at Palmer Station, Antarctica, operated by Stanford University

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).

Applications

Part of the aerial of the Grimeton VLF transmitter

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. [1]

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. [1]

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

The VLF antenna of a World War II U-boat

High power land-based transmitters in countries that operate submarines send signals that can be received thousands of miles away. Transmitter sites typically cover great areas (many acres or square kilometers), 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:

  • OOK / CWK: On-Off Keying / Continuous Wave Keying. Simple Morse code transmission mode where carrier on = mark and off = space. This is the simplest possible form of radio transmission, but it is difficult for transmitters to transmit high power levels, and the signal can easily be swamped by atmospheric noise, so this is only really used for emergencies or basic testing.
  • FSK: Frequency-shift keying. The oldest and simplest form of digital radio data modulation. Frequency is increased by 25 Hz (for example) from the carrier to indicate a binary “1” and reduced by 25 Hz to indicate binary “0”. FSK is used at rates of 50 bit/s and 75 bit/s.
  • MSK: Minimum-shift keying. A more sophisticated modulation method that uses less bandwidth for a given data rate than FSK. This is the normal mode for submarine communications today, and can be used at data rates up to 300 bit/s.

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 the Nyquist frequency 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. The strength of the signal received can vary with a Sudden Ionospheric Disturbance. These cause the ionization level to drop in the atmosphere. The result of this is that the VLF signal will reflect down to Earth with greater strength.

List of VLF transmissions

Callsign Frequency Location of transmitter Remarks
- 11.905 kHz Russia (various locations) Alpha-Navigation
- 12.649 kHz Russia (various locations) Alpha-Navigation
- 14.881 kHz Russia (various locations)
- 15.625 kHz - Frequency for horizontal deflection of electron beam of TV sets (PAL)
- 15.734 kHz - Frequency for horizontal deflection of electron beam of TV sets (NTSC)
GBR 15.8 kHz Rugby, England (Regular transmissions ceased April 2003) Many publications listed its frequency as 16 kHz
JXN 16.4 kHz Helgeland (Norway)
SAQ 17.2 kHz Grimeton (Sweden) Only active at special occasions (Alexanderson Day)
- ca. 17.5 kHz ? Twenty second pulses
? 17.8 kHz ? Transmits occasionally Superpulses
RDL/UPD/UFQE/UPP/UPD8 18.1 kHz Russia (various locations)
HWU 18.3 kHz Le Blanc (France) Frequently inactive for longer periods
RKS 18.9 kHz Russia (various locations) Rarely active
GBZ 19.6 kHz Anthorn (Britain) Many operation modes, even Superpulses.
NWC 19.8 kHz Exmouth, Western Australia (AUS) Used for submarine communication, 1 megawatt.
ICV 20.27 kHz Tavolara (Italia)
RJH63, RJH66, RJH69, RJH77, RJH99 20.5 kHz Russia (various locations) Time signal transmitter Beta
ICV 20.76 kHz Tavolara (Italia)
HWU 20.9 kHz Le Blanc (France)
RDL 21.1 kHz Russia (various locations) rarely active
HWU 21.75 kHz Le Blanc (France)
GBZ 22.1 kHz Skelton (Britain)
- 22.2 kHz Ebino (Japan)
? 22.3 kHz Russia? Only active on 2nd of each month for a short period between 11:00 and 13:00 (respectively 10:00 and 12:00 in winter), if 2nd of each month is not a Sunday
RJH63, RJH66, RJH69, RJH77, RJH99 23 kHz Russia (various locations) Time signal transmitter Beta
DHO38 23.4 kHz near Rhauderfehn (Germany) submarine communication
NAA 24 kHz Cutler, Maine (USA) Used for submarine communication, at 2 megawatts. [2]

See also

References

  1. ^ Helliwell, R.A. (2006). Whistlers and Related Ionospheric Phenomena. Dover Publications, Inc. ISBN 0-486-44572-0.

Further reading

  • Romero, R. (2006). Radio Natura (in Italian). Albino, Italy: SANDIT S.r.l.
  • Klawitter, G. (2000). Langwelle und Längstwelle (in German). Meckenheim: Siebel Verlag GmbH. ISBN 3-89632-043-2. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
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