Very low frequency
Very low frequency or VLF is the ITU designation for radio frequencies 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. Due to its limited bandwidth, audio 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 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 mode.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 is high in the band, including such phenomena as "whistlers", caused by lightning.
- VLF waves can penetrate seawater to a depth of at least, depending on the frequency employed and the salinity of the water, so they are used to communicate with submarines.
- VLF waves at certain frequencies have been found to cause electron precipitation.
- VLF waves used to communicate with submarines have created an artificial bubble around the Earth that can protect it from solar flares and coronal mass ejections; this occurred through interaction with high-energy radiation particles.
Antennas
VLF transmitting antennas
High power VLF transmitting stations use capacitively-toploaded monopole antennas. These are very large wire antennas, up to several kilometers long. 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 antennas. 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 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.
The high capacitance and inductance and low resistance of the antenna-loading coil combination makes it act electrically like a high tuned circuit. VLF antennas have very narrow bandwidth and to change the transmitting frequency requires a variable inductor 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 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.
Dynamic antenna tuning
The bandwidth of large capacitively loaded VLF antennas is so narrow 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, 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. 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.VLF receiving antennas
The requirements for receiving antennas are less stringent, because of the high level of natural atmospheric noise in the band. At VLF frequencies 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. Ferrite loop antennas are usually used for reception.Modulation
Because of the small bandwidth of the band, and the extremely narrow bandwidth of the antennas used, it is impractical to transmit audio signals. 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 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 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 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:
;Continuous Wave, Interrupted Continuous Wave, or On-Off Keying: Morse code radiotelegraphy transmission with unmodulated carrier. The carrier is turned on and off, with carrier on representing the Morse code "dots" and "dashes" and carrier off representing spaces. The simplest and earliest form of radio data transmission, this was used from the beginning of the 20th century to the 1960s in commercial and military VLF stations. Because of the high antenna the carrier cannot be switched abruptly on and off but requires a long time constant, many cycles, to build up the oscillating energy in the antenna when the carrier turns on, and many cycles to dissipate the stored energy when the carrier turns off. This limits the data rate that can be transmitted to 15–20 words/minute. CW is now only used in small hand-keyed transmitters, and for testing large transmitters.
;Frequency-shift keying : FSK is the second oldest and second simplest form of digital radio data modulation, after CW. For FSK, the carrier shifted between two frequencies, one representing the binary digit '1' and the other representing binary '0'. For example, a frequency of 9070 Hz might be used to indicate a '1' and the frequency 9020 Hz, 50 Hz lower, to indicate a '0'. The two frequencies are generated by a continuously running frequency synthesizer. The transmitter is periodically switched between these frequencies to represent 8 bit ASCII codes for the characters of the message. A problem at VLF is that when the frequency is switched the two sine waves usually have different phases, which creates a sudden phase-shift transient which can cause arcing on the antenna. To avoid arcing, FSK can only be used at slow rates of 50–75 bit/s.
;Minimum-shift keying : A continuous phase version of FSK designed specifically for small bandwidths, this was adopted by naval VLF stations in the 1970s to increase the data rate and is now the standard mode used in military VLF transmitters. If the two frequencies representing '1' and '0' are 50 Hz apart, the standard frequency shift used in military VLF stations, their phases coincide every 20 ms. In MSK the frequency of the transmitter is switched only when the two sine waves have the same phase, at the point both sine waves cross zero in the same direction. This creates a smooth continuous transition between the waves, avoiding transients which can cause stress and arcing on the antenna. MSK can be used at data rates up to 300 bit/s, or about 35 ASCII characters per second, approximately 450 words per minute.