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definition : VLF

Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3 to 30 kilohertz (kHz), corresponding to wavelengths from 100 to 10 kilometers, respectively. The band is also known as the myriameter band or myriameter wave as the wavelengths range from one to ten myriameters (an obsolete metric unit equal to 10 kilometers). Due to its limited bandwidth, audio (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 radio stations (broadcasting time signals to set radio clocks) and for secure military communication. Since VLF waves can penetrate at least 40 meters (120 ft) into saltwater, they are used for military communication with submarines.

==Propagation characteristics== Because of their large wavelengths, VLF radio waves can diffract around large obstacles and so are not blocked by mountain ranges or the horizon, and can propagate as ground waves following the curvature of the Earth. The main mode of long distance propagation is an Earth-ionosphere waveguide mechanism. The Earth is surrounded by a conductive layer of electrons and ions in the upper atmosphere at the bottom of the ionosphere called the D layer at 60 to 90 km (37 to 56 miles) altitude, which reflects VLF radio waves. The conductive ionosphere and the conductive Earth form a horizontal "duct" a few VLF wavelengths high, which acts as a waveguide confining the waves so they don't escape into space. The waves travel in a zigzag path around the Earth, reflected alternately by the Earth and the ionosphere, in TM (transverse magnetic) mode.

VLF waves have very low path attenuation, 2-3 dB per 1000 km,


A major practical drawback to this band is that because of the length of the waves, full size resonant antennas (half wave dipole or quarter wave monopole antennas) cannot be built because of their physical height. Vertical antennas must be used because VLF waves propagate in vertical polarization, but a quarter-wave vertical antenna at 30 kHz would be 2.5 kilometers high. So practical transmitting antennas are electrically short, a small fraction of a wavelength long. Due to their low radiation resistance they are inefficient, radiating only 10% to 50% of the transmitter power at most,

Geophysicists use VLF-electromagnetic receivers to measure conductivity in the near surface of the Earth.

==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 acres or 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 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.

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- or about 35 8-bit ASCII characters per second (or the equivalence of a sentence every two seconds) – a total of 450 words per minute.

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 because they most likely use one-time pads since the amount of text is so small.

==PC-based VLF reception==

VLF signals are often monitored by radio amateurs using simple homemade VLF radio receivers 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 spectrogrammes. 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.

==Amateur radio== Radio amateurs in some countries have been granted permission (or have assumed permission) to operate at frequencies below 8.3 kHz.

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 km. The modes used are QRSS, MFSK, and coherent BPSK.

Operations tend to congregate around the frequencies 8.27 kHz, 6.47 kHz, 5.17 kHz and 2.97 kHz. Bandwidths of a few tens of uHz are typical and both receiver and transmitter must have their frequency locked to a stable reference such as a GPS disciplined oscillator or a rubidium standard.

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 signaling rates typically between 1 and 100 bits per hour.

==List of VLF transmissions== For a more detailed list, see List of VLF-transmitters

==See also== *Communication with submarines *OMEGA Navigation System, 1971–1997 *Radio atmospheric


==Further reading== * * *[ Friese "Very low wave reception with ferrite antennas 5-50 kHz]

==External links== * [ Longwave club of America] * [ Radio waves below 22 kHz] * [ VLF Discussion Group] * Tomislav Stimac, "[ Definition of frequency bands (VLF, ELF... etc.)]". * PC-based VLF-reception * Gallery of VLF-signals * [ NASA live streaming ELF -> VLF Receiver] NOTE: As of 05/03/2014, the "Listen live" links are down, but the site has some previously recorded examples to listen to. * [ VLF radio art, 1] * [ VLF radio art, 2] * [ VLF radio art, 3] * [ World Wide Lightning Location Network] * [ Stanford University VLF group] * [ University of Louisville VLF Monitor] * [ Larry's Very Low Frequency site] * [ Mark's Live Online VLF Receiver, UK] * [ IW0BZD VLF TUBE receiver] * [ Internet based VLF listening guide with server list] * [ List of VLF-transmitters]

Category:Radio spectrum Category:Radio electronics

Texte soumis à la licence CC-BY-SA. Source : Article de Wikipédia

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