Very low frequency
Due to its limited bandwidth, audio (voice) transmission is highly impractical in this band, and therefore only low-data-rate coded signals are used.
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.
VLF waves have very low path attenuation, 2–3 dB per 1,000 km,[3] with little of the "fading" experienced at higher frequencies.
[9]: 3.9–3.21 [8]: 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.
In high power VLF transmitters, to increase the allowable data rate, a special form of FSK called minimum-shift keying (MSK) is used.
[9]: 3.2–3.4, §3.1.1 The huge capacitively-loaded antenna and loading coil form a high Q tuned circuit, which stores oscillating electrical energy.
The three types of modulation that have been used in VLF transmitters are: 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.
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.
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.
[12] 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.
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.
High power land-based and aircraft transmitters in countries that operate submarines send signals that can be received thousands of miles away.
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.
[14] 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.
[15][16] 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.
These spectrograms show many signals, which may include VLF transmitters and the horizontal electron beam deflection of TV sets.
These cause the ionization level to increase in the ionosphere producing a rapid change to the amplitude and phase of the received VLF signal.
