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. Palmer Station, on Anvers Island, is Antarctica's only US station north of the Antarctic Circle. Radio frequency ( RF) is a Frequency or rate of Oscillation within the range of about 3 Hz to 300 GHz The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. 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. A band is a small section of the spectrum of Radio communication frequencies, in which channels are usually used or set aside for the same Radio frequency ( RF) is a Frequency or rate of Oscillation within the range of about 3 Hz to 300 GHz Radio navigation or radionavigation is the application of Radio frequencies to determining a position on the Earth. 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. An antenna is a Transducer designed to transmit or Receive electromagnetic waves In other words antennas convert electromagnetic waves into The Grimeton VLF transmitter is a VLF transmission facility near Grimeton close to Varberg in Sweden. Water is a common Chemical substance that is essential for the survival of all known forms of Life. VLF is used to communicate with submarines near the surface (for example using the transmitter DHO38), while ELF is used for deeply-submerged vessels. Communication with submarines when they are submerged is a difficult technological task which requires specific techniques and devices In Mathematics, specifically in Topology, a surface is a Two-dimensional Manifold. The VLF transmitter DHO38 is a VLF transmitter used by the German Navy near Rhauderfehn, Saterland, Germany. Extremely low frequency (ELF is the band of Radio frequencies from 3 to 30 Hz. VLF is also used for radio navigation beacons (alpha) and time signals (beta).

VLF is also used in electromagnetic geophysical surveys. Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of [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. Amplitude modulation ( AM) is a technique used in electronic communication most commonly for transmitting information via a Radio Carrier wave Single-sideband modulation ( SSB) is a refinement of Amplitude modulation that more efficiently uses electrical power and 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. Radio is the transmission of signals by Modulation of electromagnetic waves with frequencies below those of visible Light. In Physics, emission is the process by which the Energy of a Photon is released by another entity for example by an Atom whose Electrons A Whistler is a Very low frequency electromagnetic (radio wave generated by Lightning. ^[1]

In the USA, the time signal station WWVL began transmitting a 500 W signal on 20 kHz in August 1963. A time signal is a visible audible mechanical or electronic signal used as a reference to determine the time of day For the Virginia based broadcast radio station please see WWVB-FM. It used Frequency Shift Keying (FSK) to send data, shifting between 20 kHz and 26 kHz. Frequency-shift keying (FSK is a Frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a Carrier wave 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. The Grimeton VLF transmitter is a VLF transmission facility near Grimeton close to Varberg in Sweden. "Sverige" redirects here For other uses see Sweden (disambiguation and Sverige (disambiguation. The Alexanderson Day, named after the inventor Ernst Fredrik Werner Alexanderson, is the day of the open house (first Sunday of July each year at the VLF transmitter Grimeton

Details of VLF submarine communication methods

The VLF antenna of the II World War 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. The acre is a unit of Area in a number of different systems including the imperial and U Square Kilometre ( US spelling square kilometer) symbol km2, is a decimal multiple of the SI unit of 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. In Telecommunications, modulation is the process of varying a periodic Waveform, i Simple Morse code transmission mode where carrier on = mark and off = space. Morse code is a Character encoding for transmitting telegraphic information using standardized sequences of short and long elements to represent the letters numerals 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. Frequency-shift keying (FSK is a Frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a Carrier wave 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. In Digital modulation, Minimum-shift keying (MSK is a type of continuous phase Frequency-shift keying that was developed in the late 1960s 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. The Baudot code, invented by Émile Baudot, is a character set predating EBCDIC and ASCII, and the root predecessor to International Telegraph American Standard Code for Information Interchange ( 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. In Cryptography, the one-time pad (OTP is an Encryption Algorithm where the Plaintext is combined with a random key or "pad"

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. The spectrogram is the result of calculating the Frequency spectrum of Windowed frames of a compound signal. 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. Superpulse is the name for very strong radio signals which are transmitted by the VLF transmitter GBZ and by an unidentified station on 17 The strength of the signal received can vary with a Sudden Ionospheric Disturbance. A sudden ionospheric disturbance (SID is an abnormally high ionization/ plasma density in the D region of the Ionosphere caused by a Solar These cause the ionisation 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

See also

• Communication with submarines
• OMEGA Navigation System, 1971-1997

References

Further reading

• Romero, R. (2006). Radio Natura (in Italian). Albino, Italy: SANDIT S. r. l. .  
• Klawitter, G. ; Oexner, M. , Herold, K. (2000). Langwelle und Längstwelle (in German). Meckenheim: Siebel Verlag GmbH. ISBN 3-89632-043-2.  

External links

• Longwave club of America
• Radio waves below 22 kHz
• Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)".
• PC-based VLF-reception
• Gallery of VLF-signals
• NASA live streaming ELF -> VLF Receiver
• VLF radio art, 1
• VLF radio art, 2
• VLF radio art, 3
• World Wide Lightning Location Network
• Stanford University VLF group
• Larry's Very Low Frequency site
• Kiel Longwave Monitor, VLF/LF real time data
• Mark's Live Online VLF Receiver, UK