VERY LOW FREQUENCY

Part of the aerial of Transmitter Grimeton

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 fully submerged vessels.) This frequency range is used presently for the transmission of instructions to submerged submarines (for example with the transmitter DHO38), since radio waves can penetrate some dozen 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 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 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, which 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 be also heard in this band.

Details of VLF submarine communication methods

High powered land-based transmitters in the countries that operate submaries send out 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, thereby 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 hard 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 e.g. 25 Hz 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. More sophisticated modulation method that uses less bandwidth for a given data rate than FSK. The normal mode for submarine communications today. 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. Given that these are military transmissions, they are nearly always heavily encrypted, so although it is relatively easy to pick up the transmissions and convert them to a string of characters, it is not possible for civilians to read the messages.

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. You connect an aerial in the form of a coil of insulated wire to the input of the soundcard of the PC (via a jack plug) and put it a few metres away from it. Then you download some Fast Fourier transform (FFT) software (e.g.SpecLab). This software in combination with your 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: the signals of VLF transmitters, the signal of the frequency of the horizontal electron beam deflection of TV sets and sometimes superpulses and twenty second pulses.

• Using Spectrum Lab for the reception of natural radio - A broadband VLF application.
• Using a PC with soundcard as a VLF receiver - How to use your PC as a receiver for narrow-band signals in the VLF radio spectrum.

List of VLF transmitters

The well known VLF transmitter GBR Rugby on 16 kHz was shut down on April 1, 2003.

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