Very low frequency

236px|thumb|A VLF receiving antenna at [[Palmer Station, Antarctica, operated by Stanford University ]]

Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30 kHz, corresponding to wavelengths 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 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 secure military communication. Since VLF waves can penetrate at least into saltwater, they are used for military communication with submarines.

Propagation characteristics

Because of their long wavelengths, VLF radio waves can diffract around large obstacles and so are not blocked by mountain ranges, and they can propagate as ground waves following the curvature of the Earth and so are not limited by the horizon. Ground waves are absorbed by the resistance of the Earth and are less important beyond several hundred to a thousand kilometres/miles, and 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 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 zig-zag path around the Earth, reflected alternately by the Earth and the ionosphere, in transverse magnetic (TM) mode.

VLF waves have very low path attenuation, 2–3 dB per 1,000 km,

Antennas

A major practical drawback of the VLF 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. Due to their low radiation resistance (often less than one ohm) they are inefficient, radiating only 10% to 50% of the transmitter power at most, For low-power transmitters, inverted-L and T antennas are used.

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 dielectric losses 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, high , 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. A typical AM radio signal with a bandwidth of 10 kHz would occupy one third of the VLF band. More significantly, it would be difficult to transmit any distance because it would require an antenna with 100 times the bandwidth of current VLF antennas, which due to the Chu-Harrington limit would be enormous in size. Therefore, only text data can be transmitted, at low bit rates. In military networks frequency-shift keying (FSK) modulation is used to transmit radioteletype data using 5 bit ITA2 or 8 bit ASCII character codes. A small frequency shift of 30–50 hertz is used due to the small bandwidth of 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.

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

VLF signals can be measured as a geophysical 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.

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.

Examples of naval VLF transmitters are

Britain's Skelton Transmitting Station in Skelton, Cumbria
Germany's DHO38 in Rhauderfehn, which transmits on 23.4 kHz with a power of 800 kW
U.S. Jim Creek Naval Radio Station in Oso, Washington state, which transmits on 24.8 kHz with a power of 1.2 MW
U.S. Cutler Naval Radio Station at Cutler, Maine which transmits on 24 kHz with 1.8 MW.

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 ciphers since the amount of text is so small.

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.

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.

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.

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.

PC based reception

thumb|Timing diagram of a frequency-shift keyed 18.1 kHz VLF signal, picked up using a small [[loop antenna and a sound card. The Morse code says "..33376.."; the vertical stripes are distant lightning strikes.]]

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.

List of VLF transmissions

For a more detailed list, see List of VLF transmitters

{| class="wikitable sortable"

! Callsign

! Frequency

! Location of transmitter

! Remarks