- Very low frequency
Very low frequency or VLF refers to radio frequencies (RF) in the range of 3
kHzto 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).
VLF waves can penetrate
waterto 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
electromagneticgeophysical surveys. [http://www.geonics.com/html/vlfsystems.html]
Early in the history of radio engineering attempts were made to use radiotelephone using
amplitude modulationand single-sideband modulationwithin 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 Unionand may be used in some nations license-free. Many natural radioemissions, such as whistlers, can also be heard in this band. [cite book | author=Helliwell, R.A. | title=Whistlers and Related Ionospheric Phenomena | isbn=0-486-44572-0 | publisher=Dover Publications, Inc | date=2006 ]
In the USA, the
time signalstation WWVLbegan 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
Swedencan 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 great areas (many
acresor 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 [http://www.detica.com/indexed/CaseStudy_newcommsforroyalnavy.htm 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
* OOK / CWK: On-Off Keying / Continuous Wave Keying. Simple
Morse codetransmission 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.
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.
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
ITA2or 8-bit ASCII. Because these are military transmissions they are almost always encryptedfor 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 frequencysimultaneously 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 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
Communication with submarines
OMEGA Navigation System, 1971-1997
*cite book | author=Klawitter, G. | coauthors=Oexner, M., Herold, K. | title=Langwelle und Längstwelle | isbn=3-89632-043-2 | publisher=Siebel Verlag GmbH |location=Meckenheim | year=2000 | language=German
* [http://www.lwca.org Longwave club of America]
* [http://www.vlf.it Radio waves below 22 kHz]
* Tomislav Stimac, " [http://www.vlf.it/frequency/bands.html Definition of frequency bands (VLF, ELF... etc.)] ".
* [http://spaceweather.com/glossary/inspire.html NASA live streaming ELF -> VLF Receiver]
* [http://www.youtube.com/watch?v=-g6PcLEx6bs VLF radio art, 1]
* [http://www.youtube.com/watch?v=LQoYOXmUuE0 VLF radio art, 2]
* [http://www.youtube.com/watch?v=B13-09K-Ubc VLF radio art, 3]
* [http://webflash.ess.washington.edu/ World Wide Lightning Location Network]
* [http://www-star.stanford.edu/~vlf/ Stanford University VLF group]
* [http://www.vlfradio.com/ Larry's Very Low Frequency site]
* [http://home.arcor.de/df3lp/ Kiel Longwave Monitor, VLF/LF real time data]
* [http://www.markyd26uk.110mb.com/vlf.html Mark's Live Online VLF Receiver, UK]
* [http://noweenergie.org/index.php?p=artykuly&class=technika&s=vlf_teoria NoweEnergie.org and VLF, PL]
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