Very low frequency

Last updated
Very low frequency
Frequency range
3–30  kHz
Wavelength range
100–10  km
A VLF receiving antenna at Palmer Station, Antarctica, operated by Stanford University. VLFatPalmer.JPG
A VLF receiving antenna at Palmer Station, Antarctica, operated by Stanford University.

Very low frequency or VLF is the ITU designation [1] 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 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.

Contents

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. Ground waves are less important beyond several hundred to a thousand miles, and the main mode of long distance propagation is an Earth-ionosphere waveguide mechanism. [2] 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, [3] 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 1,000 km, [2] with little of the "fading" experienced at higher frequencies, [3] 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 to 20,000 km have been realized. [2] However, atmospheric noise (sferics) is high in the band, [3] including such phenomena as "whistlers", caused by lightning.

VLF waves can penetrate seawater to a depth of at least 10 to 40 meters (30 to 130 feet), 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. [4]

Antennas

Cutler VLF antenna array.png
"Trideco" antenna tower array at the US Navy's Naval Radio Station Cutler in Cutler, Maine, USA. The central mast is the radiating element, while the star-shaped horizontal wire array is the capacitive top load. About 1.2 miles in diameter, it communicates with submerged submarines at 24 kHz at a power of 1.8 megawatts, the most powerful radio station in the world.
VLF umbrella antenna - Anthorn Radio Station UK - central mast.jpg
Central mast of a similar "trideco" antenna of the NATO VLF transmitter at Anthorn radio station, UK, showing 6 insulator strings attaching the toploads to the 6 vertical radiator wires
Jim Creek VLF antenna.png
Another type of large VLF antenna: the "valley-span" antenna, consisting of one or more long horizontal topload cables spanning a valley, fed in the center by vertical radiator cables. This example is at the US Navy Jim Creek station near Seattle, which transmits on 24.8 kHz at a power of 1.2 MW.
Tsushima Omega Tower 1977 2.jpg
Umbrella antenna of the Omega navigation system beacon on Tsushima Island, Japan, which transmitted at 10 - 14 kHz. 389 meters high, it was dismantled in 1977.

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 kilometres (8,200 feet) high. So practical transmitting antennas are electrically short, a small fraction of the length at which they would be self-resonant. [5] [6] 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, [2] with the rest of the power dissipated in the antenna/ground system resistances. Very high power transmitters (~1 megawatt) are required for long distance communication, so the efficiency of the antenna is an important factor.

High power transmitting antennas for VLF frequencies are very large wire antennas, up to several kilometers long. [7] [8] 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 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. [9] 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 withstand, before the onset of air breakdown, corona, and arcing from the antenna.

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.

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 (AM or FM radiotelephony). [10] 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 Q factor of the antenna. [11] The huge capacitively-loaded antenna and loading coil form a high Q tuned circuit, which stores oscillating electrical energy. The Q of large VLF antennas is typically over 200; this means the antenna stores far more energy (200 times as much) than is supplied or radiated in each cycle of the transmitter current. The energy is stored alternately as electrostatic energy in the topload and ground system, and magnetic energy in the 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:

Dynamic antenna tuning

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. [12] [13] 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.

Applications

Flattop antenna towers of the Grimeton VLF transmitter, Varberg, Sweden Grimetonmasterna.jpg
Flattop antenna towers of the Grimeton VLF transmitter, Varberg, Sweden

The frequency range below 8.3 kHz is not allocated by the International Telecommunication Union and may be used in some nations license-free.

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, the US Jim Creek Naval Radio Station in Oso, Washington state, which transmits on 24.8 kHz with a power of 1.2 MW; and 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.

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 to 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.

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.

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. [14]

VLF can also penetrate soil and rock for some distance, so these frequencies are also used for through-the-earth mine communications systems. Geophysicists use VLF-electromagnetic receivers to measure conductivity in the near surface of the Earth. [15]

Communication with submarines

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 MW. 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 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. 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

Radio amateurs in some countries have been granted permission (or have assumed permission) to operate at frequencies below 8.3 kHz. [16]

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. [17] Bandwidths of a few tens of µHz[ citation needed ] 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 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.

PC based reception

Spectrogram of an 18.1 kHz VLF signal, picked up using a small loop antenna and a sound card. The vertical stripes are distant lightning strikes. VLF 18.1 kHz spectrogram.svg
Spectrogram of an 18.1 kHz VLF signal, picked up using a small loop antenna and a sound card. 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). [18] [19] 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

CallsignFrequencyLocation of transmitterRemarks
-11.905 kHzRussia (various locations) Alpha-Navigation
-12.649 kHzRussia (various locations) Alpha-Navigation
-14.881 kHzRussia (various locations) Alpha-Navigation
HWU 15.1 kHzRosnay, France400 kW.
-15.625 kHz-Frequency for horizontal deflection of electron beam in CRT televisions (576i)
-15.734 kHz-Frequency for horizontal deflection of electron beam in CRT televisions (480i)
JXN 16.4 kHz Gildeskål (Norway)
SAQ 17.2 kHz Grimeton (Sweden)Only active at special occasions (Alexanderson Day)
-17.5 kHz (ca.)?Twenty second pulses
NAA 17.8 kHzVLF station (NAA) at Cutler, Maine
RDL/UPD/UFQE/UPP/UPD818.1 kHzRussia (various locations including Matotchkinchar, Russia)
HWU 18.3 kHz Le Blanc (France)Frequently inactive for longer periods
RKS18.9 kHzRussia (various locations)Rarely active
GQD 19.6 kHz Anthorn (Britain)Many operation modes.
NWC 19.8 kHz Exmouth, Western Australia (AUS)Used for submarine communication, 1 Megawatt. [20]
ICV20.27 kHz Tavolara (Italy)
RJH63, RJH66, RJH69, RJH77, RJH9920.5 kHzRussia (various locations) Time signal transmitter Beta
ICV20.76 kHzTavolara (Italy)
HWU 20.9 kHzSaint-Assise, France [http://www.mdpi.com/2076-3263/1/1/3/pdf The Central Italy Electromagnetic Network and the 2009 L'Aquila Earthquake: Observed Electric Activity, Geosciences, Cristiano Fidani, Dec 2011]
RDL21.1 kHzRussia (various locations)rarely active
NPM 21.4 kHzHawaii (USA)
HWU 21.75 kHzRosnay, France
GZQ 22.1 kHz Skelton (Britain)
JJI 22.2 kHz Ebino (Japan)
?22.3 kHzRussia?Only active on 2nd of each month for a short period between 11:00 and 13:00 (respectively 10:00 and 12:00 in winter), if 2nd of each month is not a Sunday
RJH63, RJH66, RJH69, RJH77, RJH9923 kHzRussia (various locations)Time signal transmitter Beta
DHO38 23.4 kHznear Rhauderfehn (Germany)submarine communication
NAA 24 kHzCutler, Maine (USA)Used for submarine communication, at 2 megawatts
NLK 24.6 kHz Seattle, Washington (USA)192 kW.
NLF24.8 kHz Arlington, Washington (USA)Used for submarine communication.
NML25.2 kHz LaMoure, North Dakota (USA)
PNSH 14–25.2? kHz Karachi coast, Sindh (Pakistan)

See also

Related Research Articles

Frequency-shift keying frequency modulation scheme

Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. The technology is used for communication systems such as telemetry, weather balloon radiosondes, caller ID, garage door openers, and low frequency radio transmission in the VLF and ELF bands. The simplest FSK is binary FSK (BFSK). BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called the mark frequency and the "0" is called the space frequency.

Shortwave radio radio frequencies in the range of 1.6-30 megahertz (ITU region 1) or 1.7-30 megahertz (ITU region 2)

Shortwave radio is radio transmission using shortwave radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3–30 MHz ; above the medium frequency band (MF), to the bottom of the VHF band.

Transmitter Electronic device that emits radio waves

In electronics and telecommunications a transmitter or radio transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves.

Radio wave Type of electromagnetic radiation

Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as high as 300 gigahertz (GHz) to as low as 30 hertz (Hz). At 300 GHz, the corresponding wavelength is 1 mm, and at 30 Hz is 10,000 km. The wavelength of a radio wave can be anywhere from shorter than a grain of rice to longer than the radius of the Earth. Like all other electromagnetic waves, radio waves travel at the speed of light in vacuum. They are generated by electric charges undergoing acceleration, such as time varying electric currents. Naturally occurring radio waves are emitted by lightning and astronomical objects.

Medium wave Part of the medium frequency radio band

Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. The spectrum provides about 120 channels with limited sound quality. During daytime, only local stations can be received. Propagation in the night allows strong signals within a range of about 2000 km. This can cause massive interference because on most channels, about 20 to 50 transmitters operate simultaneously worldwide. In addition to that, amplitude modulation (AM) is prone to interference by all sorts of electronic devices, especially power supplies and computers. Strong transmitters cover larger areas than on the FM broadcast band but require more energy. Digital modes are possible but have not reached the momentum yet.

Low frequency (LF) is the ITU designation for radio frequencies (RF) in the range of 30–300 kHz. Since its wavelengths range from 10–1 km, respectively, it is also known as the kilometre band or kilometre wave.

Medium frequency The range 300-3000 kHz of the electromagnetic spectrum

Medium frequency (MF) is the ITU designation for radio frequencies (RF) in the range of 300 kilohertz (kHz) to 3 megahertz (MHz). Part of this band is the medium wave (MW) AM broadcast band. The MF band is also known as the hectometer band as the wavelengths range from ten to one hectometer. Frequencies immediately below MF are denoted low frequency (LF), while the first band of higher frequencies is known as high frequency (HF). MF is mostly used for AM radio broadcasting, navigational radio beacons, maritime ship-to-shore communication, and transoceanic air traffic control.

Longwave radio broadcast band

In radio, longwave, long wave or long-wave, and commonly abbreviated LW, refers to parts of the radio spectrum with wavelengths longer than what was originally called the medium-wave broadcasting band. The term is historic, dating from the early 20th century, when the radio spectrum was considered to consist of longwave (LW), medium-wave (MW), and short-wave (SW) radio bands. Most modern radio systems and devices use wavelengths which would then have been considered 'ultra-short'.

High frequency The range 3-30 MHz of the electromagnetic spectrum

High frequency (HF) is the ITU designation for the range of radio frequency electromagnetic waves between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten decameters. Frequencies immediately below HF are denoted medium frequency (MF), while the next band of higher frequencies is known as the very high frequency (VHF) band. The HF band is a major part of the shortwave band of frequencies, so communication at these frequencies is often called shortwave radio. Because radio waves in this band can be reflected back to Earth by the ionosphere layer in the atmosphere – a method known as "skip" or "skywave" propagation – these frequencies are suitable for long-distance communication across intercontinental distances and for mountainous terrains which prevent line-of-sight communications. The band is used by international shortwave broadcasting stations (2.31–25.82 MHz), aviation communication, government time stations, weather stations, amateur radio and citizens band services, among other uses.

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DCF77 German longwave time signal and standard-frequency radio station

DCF77 is a German longwave time signal and standard-frequency radio station. It started service as a standard-frequency station on 1 January 1959. In June 1973 date and time information was added. Its primary and backup transmitter are located at 50°0′56″N9°00′39″E in Mainflingen, about 25 km south-east of Frankfurt am Main, Germany. The transmitter generates a nominal power of 50 kW, of which about 30 to 35 kW can be radiated via a T-antenna.

Extremely low frequency The range 3-30 Hz of the electromagnetic spectrum

Extremely low frequency (ELF) is the ITU designation for electromagnetic radiation with frequencies from 3 to 30 Hz, and corresponding wavelengths of 100,000 to 10,000 kilometers, respectively. In atmospheric science, an alternative definition is usually given, from 3 Hz to 3 kHz. In the related magnetosphere science, the lower frequency electromagnetic oscillations are considered to lie in the ULF range, which is thus also defined differently from the ITU radio bands.

Communication with submarines is a field within military communications that presents technical challenges and requires specialized technology. Because radio waves do not travel well through good electrical conductors like salt water, submerged submarines are cut off from radio communication with their command authorities at ordinary radio frequencies. Submarines can surface and raise an antenna above the sea level, then use ordinary radio transmissions, however this makes them vulnerable to detection by anti-submarine warfare forces. Early submarines during World War II mostly traveled on the surface because of their limited underwater speed and endurance; they dived mainly to evade immediate threats. During the Cold War, however, nuclear-powered submarines were developed that could stay submerged for months. In the event of a nuclear war, submerged ballistic missile submarines have to be ordered quickly to launch their missiles. Transmitting messages to these submarines is an active area of research. Very low frequency (VLF) radio waves can penetrate seawater a few hundred feet, and many navies use powerful shore VLF transmitters for submarine communications. A few nations have built transmitters which use extremely low frequency (ELF) radio waves, which can penetrate seawater to reach submarines at operating depths, but these require huge antennas. Other techniques that have been used include sonar and blue lasers.

Grimeton Radio Station working life museum in Varberg Municipality, Sweden

Grimeton Radio Station in southern Sweden, close to Varberg in Halland, is an early longwave transatlantic wireless telegraphy station built in 1922-1924, that has been preserved as a historical site. From the 1920s through the 1940s it was used to transmit telegram traffic by Morse code to North America and other countries, and during World War 2 was Sweden's only telecommunication link with the rest of the world. It is the only remaining example of an early pre-electronic radio transmitter technology called an Alexanderson alternator. It was added to the UNESCO World Heritage List in 2004, with the statement: "Grimeton Radio Station, Varberg is an outstanding monument representing the process of development of communication technology in the period following the First World War." The radio station is also an anchor site for the European Route of Industrial Heritage. The transmitter is still in operational condition, and each year on a day called Alexanderson Day is started up and transmits brief Morse code test transmissions, which can be received all over Europe.

The 80 meter or 3.5 MHz band is a band of radio frequencies allocated for amateur radio use, from 3.5 to 4.0 MHz in IARU Region 2, and generally 3.5 to 3.8 or 3.9 MHz in Regions 1 and 3 respectively. The upper portion of the band, which is usually used for phone (voice), is sometimes referred to as 75 meters. In Europe, 75m is a shortwave broadcast band, with a number of national radio services operating between 3.9 & 4.0 MHz.

Non-line-of-sight (NLOS) and near-line-of-sight are radio transmissions across a path that is partially obstructed, usually by a physical object in the innermost Fresnel zone.

Ground dipole

In radio communication, a ground dipole, also referred to as an earth dipole antenna, transmission line antenna, and in technical literature as a horizontal electric dipole (HED), is a huge, specialized type of radio antenna that radiates extremely low frequency (ELF) electromagnetic waves. It is the only type of transmitting antenna that can radiate practical amounts of power in the frequency range of 3 Hz to 3 kHz, commonly called ELF waves A ground dipole consists of two ground electrodes buried in the earth, separated by tens to hundreds of kilometers, linked by overhead transmission lines to a power plant transmitter located between them. Alternating current electricity flows in a giant loop between the electrodes through the ground, radiating ELF waves, so the ground is part of the antenna. To be most effective, ground dipoles must be located over certain types of underground rock formations. The idea was proposed by U.S. Dept. of Defense physicist Nicholas Christofilos in 1959.

Radio Technology of using radio waves to carry information

Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connected to an antenna which radiates the waves, and received by a radio receiver connected to another antenna. Radio is very widely used in modern technology, in radio communication, radar, radio navigation, remote control, remote sensing and other applications.

Aguada transmission station is a tall guyed radio mast erected by the US Navy. It is used as a facility of the US Navy for transmitting orders to submerged submarines near Aguada, Puerto Rico at 18°23′55″N67°10′38″W by using radio waves in the very low frequency range.

References

  1. "Rec. ITU-R V.431-7, Nomenclature of the frequency and wavelength bands used in telecommunications" (PDF). ITU. Archived from the original (PDF) on 31 October 2013. Retrieved 20 February 2013.
  2. 1 2 3 4 Hunsucker, R. D.; John Keith Hargreaves (2002). The high-latitude ionosphere and its effects on radio propagation. Cambridge University Press. p. 419. ISBN   978-0-521-33083-1.
  3. 1 2 3 Ghosh, S. N. (2002). Electromagnetic theory and wave propagation. CRC Press. p. 89. ISBN   978-0-8493-2430-7.
  4. Marina Koren (May 18, 2017). "Humans Accidentally Created a Protective Bubble Around Earth". The Atlantic . Retrieved May 20, 2017.
  5. Seybold, John S. (2005). Introduction to RF Propagation. John Wiley and Sons. pp. 55–58. ISBN   978-0471743682.
  6. Johnson, Richard (1993) Antenna Engineering Handbook, 3rd Ed., p. 24.5-24.6
  7. Manual NAVELEX 0101-113: Naval Shore Electronics Criteria - VLF, LF, and MF communications systems (PDF). Washington, D.C.: Naval Electronics Systems Command, US Navy. August 1972. pp. 3.9–3.21.
  8. Johnson, Richard C. (1993). Antenna Engineering Handbook, 3rd Ed (PDF). McGraw-Hill. pp. 24.8–24.12. ISBN   007032381X.
  9. Watt, Arthur D. (1967). VLF Radio Engineering. Pergamon Press. pp. 129–162.
  10. Holtet, Ed., J.A. (1974). ELF-VLF Radio Wave Propagation: Proceedings of the NATO Advanced Study Institute held at Spåtind, Norway, April 17–27, 1974. Springer Science and Business Media. pp. 372–373. ISBN   9789401022651.
  11. Manual NAVELEX 0101-113: Naval Shore Electronics Criteria - VLF, LF, and MF communications systems, 1972, U.S. Navy, section 3.1.1 p.3.2-3.4]
  12. Johnson, Richard (1993)Antenna Engineering Handbook, 3rd Ed., p. 24.7
  13. Manual NAVELEX 0101-113: Naval Shore Electronics Criteria - VLF, LF, and MF communications systems, 1972, U.S. Navy, p. 3.36]
  14. "AWDANet".
  15. "Geonics Limited - VLF Receivers" . Retrieved 13 June 2014.
  16. "Sub 9kHz spectrum in the Amateur Service" . Retrieved 13 May 2017.
  17. "Some recent milestones with amateur radio experiments at VLF" . Retrieved 13 May 2017.
  18. Renato Romero, IK1QFK (2008). Radio Nature. Radio Society of Great Britain. p. 77. ISBN   9781-9050-8637-5.
  19. Mardina Abdullah; et al. (2013). "Development of UKM-SID teaching module for space science education (6th International Forum on Engineering Education 2012 (IFEE 2012))". Procedia - Social and Behavioral Sciences. 102: 80–85. doi: 10.1016/j.sbspro.2013.10.716 .
  20. Naval base link to jet plunge - The Sydney Morning Herald, 14 November 2008, retrieved on 14 November 2008.

Further reading