Very low frequency

High power transmitting antennas for VLF frequencies are very large wire antennas, up to several kilometers long.[7]^{(pp 3.9–3.21)}[6]^{(pp 24.8–24.12)} They consist of a series of steel radio masts, linked at the top with a network of cables, often shaped like an umbrella or clotheslines. Either the towers themselves or vertical wires serve as monopole radiators, and the horizontal cables form a capacitive top-load to increase the current in the vertical wires, increasing the radiated power and efficiency of the antenna. High power stations use variations on the umbrella antenna such as the "delta" and "trideco" antennas, or multiwire flattop (triatic) antennas.[8] 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 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 Q, and must withstand the high voltage on the antenna end. The RF resistance is usually reduced by using litz wire.

The high capacitance and inductance and low resistance of the antenna-loading coil combination makes it act electrically like a high Q 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, and when transmitting frequency shift keying (FSK), the usual mode, the resonant frequency of the antenna must sometimes be dynamically shifted with the modulation, between the two FSK frequencies. The high Q results in very high voltages (up to 200 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, and arcing from the antenna.

The bandwidth of large capacitively loaded VLF antennas is so narrow (50–100 Hz) that even the small frequency shifts of FSK and MSK modulation may exceed it, throwing the antenna out of resonance, causing the antenna to reflect some power back down the feedline. The traditional solution is to use a "bandwidth resistor" in the antenna which reduces the Q, increasing the bandwidth; however this also reduces the power output. A recent alternative used in some military VLF transmitters is a circuit which dynamically shifts the antenna's resonant frequency between the two output frequencies with the modulation.[6]^(p 24.7)[7]^(p 3.36) This is accomplished with a saturable reactor in series with the antenna loading coil. This is a ferromagnetic core inductor with a second control winding through which a DC current flows, which controls the inductance by magnetizing the core, changing its permeability. The keying datastream is applied to the control winding. So when the frequency of the transmitter is shifted between the '1' and '0' frequencies, the saturable reactor changes the inductance in the antenna resonant circuit to shift the antenna resonant frequency to follow the transmitter's frequency.

The requirements for receiving antennas are less stringent, because of the high level of natural atmospheric noise in the band. Atmospheric radio noise is far above the receiver noise introduced by the receiver circuit and determines the receiver signal to noise ratio. So small inefficient receiving antennas can be used, and the low voltage signal from the antenna can simply be amplified by the receiver without introducing significant noise. Loop antennas are usually used for reception.

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 sidebands.

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 ionized 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.

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.

The worldwide Omega system used frequencies from 10–14 kHz, as did Russia's Alpha.

VLF was also used for standard time and frequency broadcasts. 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.

Naturally occurring signals in the VLF band are used by geophysicists 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.[10]

Geophysicists use VLF-electromagnetic receivers to measure conductivity in the near surface of the Earth.[11]

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.

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.

Due to the narrow bandwidth of the band, audio (voice) transmission cannot be used, and text transmission is limited to a slow data rate of around 300 bits per second, or about 35 eight-bit ASCII characters per second. 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.

VLF can also penetrate soil and rock for some distance, so these frequencies are also used for through-the-earth mine communications systems.

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 3×10⁻⁸ volts/meter (electric field) and 1×10⁻¹⁶ tesla (magnetic field), with signaling rates typically between 1 and 100 bits per hour.

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).[14][15] 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.