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Very low frequency or VLF is the ITU designation for Radio frequency (RF) in the range of 3–30 kHz, corresponding to from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave as the wavelengths range from one to ten (an obsolete metric unit equal to 10 kilometers). Due to its limited bandwidth, Audio signal (voice) transmission is highly impractical in this band, and therefore only low-data-rate coded signals are used. The VLF band is used for a few radio navigation services, government Time signal (broadcasting time signals to set ) and secure military communication. Since VLF waves can penetrate at least into saltwater, they are used for military communication with .
VLF waves have very low path attenuation, 2–3 dB per 1,000 km, with little of the "fading" experienced at higher frequencies. This is because VLF waves are reflected from the bottom of the ionosphere, while higher frequency shortwave signals are returned to Earth from higher layers in the ionosphere, the F1 and F2 layers, by a refraction process, and spend most of their journey in the ionosphere, so they are much more affected by ionization gradients and turbulence. Therefore, VLF transmissions are very stable and reliable, and are used for long-distance communication. Propagation distances of 5,000–20,000 km have been realized. However, atmospheric noise ("sferics") is high in the band, including such phenomena as "whistlers", caused by lightning.
Due to the low radiation resistance, to minimize power dissipated in the ground these antennas require extremely low resistance ground (Earthing) systems, consisting of radial networks of buried copper wires under the antenna. To minimize in the soil, the ground conductors are buried shallowly, only a few inches in the ground, and the ground surface near the antenna is sometimes protected by copper ground screens. Counterpoise systems have also been used, consisting of radial networks of copper cables supported several feet above the ground under the antenna.
A large loading coil is required at the antenna feed point to cancel the capacitive reactance of the antenna to make it resonant. At VLF the design of this coil is challenging; it must have low resistance at the operating RF frequency, Q factor, must handle very high currents, and must withstand the extremely high voltage on the antenna. These are usually huge air core coils 2–4 meters high wound on a nonconductive frame, with RF resistance reduced by using thick litz wire several centimeters in diameter, consisting of thousands of insulated strands of fine wire braided together.
The high capacitance and inductance and low resistance of the antenna-loading coil combination makes it act electrically like a Q factor tuned circuit. VLF antennas have very narrow bandwidth and to change the transmitting frequency requires a variable inductor (variometer) to tune the antenna. The large VLF antennas used for high-power transmitters usually have bandwidths of only 50–100 hertz. The high results in very high voltages (up to 250 kV) on the antenna and very good insulation is required. Large VLF antennas usually operate in 'voltage limited' mode: the maximum power of the transmitter is limited by the voltage the antenna can accept without air breakdown, corona discharge, and arcing from the antenna.
In high power VLF transmitters, to increase the allowable data rate, a special form of FSK called minimum-shift keying (MSK) is used. This is required due to the high of the antenna. The huge capacitively-loaded antenna and loading coil form a high tuned circuit, which stores oscillating electrical energy. The of large VLF antennas is typically over 200; this means the antenna stores far more energy (200 times as much) than is supplied or radiated in any single cycle of the transmitter current. The energy is stored alternately as electrostatic energy in the topload and ground system, and magnetic energy in the vertical wires and loading coil. VLF antennas typically operate "voltage-limited", with the voltage on the antenna close to the limit that the insulation will stand, so they will not tolerate any abrupt change in the voltage or current from the transmitter without arcing or other insulation problems. As described below, MSK is able to modulate the transmitted wave at higher data rates without causing voltage spikes on the antenna.
The three types of modulation that have been used in VLF transmitters are:
In the 1920s the discovery of the skywave (skip) radio propagation method allowed lower power transmitters operating at high frequency to communicate at similar distances by reflecting their radio waves off a layer of ionization atoms in the ionosphere, and long-distance radio communication stations switched to the shortwave frequencies. The Grimeton VLF transmitter at Grimeton near Varberg in Sweden, one of the few remaining transmitters from that era that has been preserved as a historical monument, can be visited by the public at certain times, such as on Alexanderson Day.
The worldwide Omega system used frequencies from 10 to 14 kHz, as did Russia's Alpha.
VLF was also used for standard time and frequency broadcasts. In the US, 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.
Geophysicists use VLF-electromagnetic receivers to measure conductivity in the near surface of the Earth.
VLF signals can be measured as a Geophysics electromagnetic survey that relies on transmitted currents inducing secondary responses in conductive geologic units. A VLF anomaly represents a change in the attitude of the electromagnetic vector overlying conductive materials in the subsurface.
Examples of naval VLF transmitters are
Since 2004 the US Navy has stopped using ELF transmissions, with the statement that improvements in VLF communication has made them unnecessary, so it may have developed technology to allow submarines to receive VLF transmissions while at operating depth.
High power land-based and aircraft 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 2,000 kW. Submarines receive signals from land based and aircraft transmitters using some form of towed antenna that floats just under the surface of the water – for example a Buoyant Cable Array Antenna (BCAA).
Modern receivers use sophisticated digital signal processing 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. Strategic nuclear bombers of the United States Air Force receive VLF signals as part of hardened nuclear resilient operations.
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, enemies cannot decode the encrypted messages; military communications usually use unbreakable one-time pad since the amount of text is so small.
Operations tend to congregate around the frequencies 8.27 kHz, 6.47 kHz, 5.17 kHz, and 2.97 kHz. Transmissions typically last from one hour up to several days and both receiver and transmitter must have their frequency locked to a stable reference such as a GPS disciplined oscillator or a rubidium standard in order to support such long duration coherent detection and decoding.
The transmitter generally consists of an audio amplifier of a few hundred watts, an impedance matching transformer, a loading coil and a large wire antenna. Receivers employ an electric field probe or magnetic loop antenna, a sensitive audio preamplifier, isolating transformers, and a PC sound card to digitise the signal. Extensive digital signal processing is required to retrieve the weak signals from beneath interference from power line harmonics and VLF radio atmospherics. Useful received signal strengths are as low as volts/meter (electric field) and tesla (magnetic field), with typically between 1 and 100 bits per hour.
Because CRT monitors are strong sources of noise in the VLF range, it is recommended to record the spectrograms with any PC CRT monitors turned off. These spectrograms show many signals, which may include VLF transmitters and the horizontal electron beam deflection of TV sets. The strength of the signal received can vary with a sudden ionospheric disturbance. These cause the ionization level to increase in the ionosphere producing a rapid change to the amplitude and phase of the received VLF signal.
Applications
Early wireless telegraphy
Historically, this band was used for long distance transoceanic radio communication during the wireless telegraphy era between about 1905 and 1925. Nations built networks of high-power LF and VLF radiotelegraphy stations that transmitted text information by Morse code, to communicate with other countries, their colonies, and naval fleets. Early 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 the available bandwidth was insufficient to contain the .
Navigation beacons and time signals
Due to its long propagation distances and stable phase characteristics, during the 20th century the VLF band was used for long range hyperbolic radio navigation systems which allowed ships and aircraft to determine their geographical position by comparing the phase of radio waves received from fixed VLF navigation beacon transmitters.
Geophysical and atmospheric measurement
Naturally occurring signals in the VLF band are used by for long range lightning location and for research into atmospheric phenomena such as the aurora. Measurements of whistlers are employed to infer the physical properties of the magnetosphere.
Mine communication systems
VLF can also penetrate soil and rock for some distance, so these frequencies are also used for through-the-earth mine communications systems.
Military communications
Powerful VLF transmitters are used by the military to communicate with their forces worldwide. The advantage of VLF frequencies is their long range, high reliability, and the prediction that in a nuclear war VLF communications will be less disrupted by nuclear explosions than higher frequencies. Since it can penetrate seawater VLF is used by the military to communicate with submarines near the surface, while ELF frequencies are used for deeply submerged subs.
Amateur use
The frequency range below 8.3 kHz is not allocated by the International Telecommunication Union and in some nations may be used license-free.
Radio amateurs in some countries have been granted permission (or have assumed permission) to operate at frequencies below 8.3 kHz.
Amateur equipment
Radiated power from amateur stations is very small, ranging from 1 μW to 100 μW for fixed base station antennas, and up to 10 mW from kite or balloon antennas. Despite the low power, stable propagation with low attenuation in the earth-ionosphere cavity enable very narrow bandwidths to be used to reach distances up to several thousand kilometers. The modes used are QRSS, MFSK, and coherent BPSK.
PC based reception
VLF signals are often monitored by using simple homemade VLF based on personal computers (PCs). 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 meters 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 .
List of VLF transmissions
For a more detailed list, see List of VLF transmitters
See also
Further reading
(2025). 9783896320438, Siebel Verlag GmbH. ISBN 9783896320438
External links
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