Very low frequency
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very low frequency (VLF) |
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Cycles per second:3 kHz to 30 kHz Wavelength: 100 km to 10 km |
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. 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).
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[edit] Applications
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 deeply-submerged vessels.) This frequency range is used for the transmission of instructions to submerged submarines (for example with the transmitter DHO38), since radio waves can penetrate some tens 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 modulation 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 transmitter SAQ at Grimeton near Varberg in Sweden can be visited by the public at certain times, such as on Alexanderson Day. As a rule, very long wave transmitters work in the frequency range between 10 kHz and 30 kHz. There are also stations that 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 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. Many natural radio emissions, such as whistlers, can also be heard in this band. VLF is also used in Electromagnetic Geophysical Surveys
[edit] 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:
- 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.
[edit] 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.
[edit] List of VLF transmissions
Callsign | Frequency | Location of transmitter | Remarks | |
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- | 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 | Criggion (Britain) | Many operation modes, even Superpulses. Closed 2003. | |
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 | Anthorn (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, it is the most powerful station in the world [1] |
[edit] See also
- OMEGA Navigation System, 1971-1997
[edit] External links
- Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).
- PC-based VLF-reception
- Gallery of VLF-signals
- NASA live streaming ELF -> VLF Receiver
- http://webflash.ess.washington.edu/
- Stanford University VLF group
Radio spectrum | ||||||||||
ELF | SLF | ULF | VLF | LF | MF | HF | VHF | UHF | SHF | EHF |
3 Hz | 30 Hz | 300 Hz | 3 kHz | 30 kHz | 300 kHz | 3 MHz | 30 MHz | 300 MHz | 3 GHz | 30 GHz |
30 Hz | 300 Hz | 3 kHz | 30 kHz | 300 kHz | 3 MHz | 30 MHz | 300 MHz | 3 GHz | 30 GHz | 300 GHz |
The Electromagnetic Spectrum | |
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(Sorted by wavelength, short to long)
Gamma ray • X-ray • Ultraviolet • Visible spectrum • Infrared • Terahertz radiation • Microwave • Radio waves |
|
Visible (optical) spectrum | Violet • Blue • Green • Yellow • Orange • Red |
Microwave spectrum | |
Radio spectrum | EHF • SHF • UHF • VHF • HF • MF • LF • VLF • ULF • SLF • ELF |
Wavelength designations |