Misplaced Pages

#388611

71-647: ELF 3 Hz/100 Mm 30 Hz/10 Mm SLF 30 Hz/10 Mm 300 Hz/1 Mm ULF 300 Hz/1 Mm 3 kHz/100 km VLF 3 kHz/100 km 30 kHz/10 km LF 30 kHz/10 km 300 kHz/1 km MF 300 kHz/1 km 3 MHz/100 m HF 3 MHz/100 m 30 MHz/10 m VHF 30 MHz/10 m 300 MHz/1 m UHF 300 MHz/1 m 3 GHz/100 mm SHF 3 GHz/100 mm 30 GHz/10 mm EHF 30 GHz/10 mm 300 GHz/1 mm THF 300 GHz/1 mm 3 THz/0.1 mm This article about wireless technology

142-785: 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  volts/meter (electric field) and 1 × 10  tesla (magnetic field), with signaling rates typically between 1 and 100 bits per hour. VLF signals are often monitored by radio amateurs using simple homemade VLF radio receivers based on personal computers (PCs). An aerial in

213-598: A few nations have built naval ELF transmitters to communicate with their submarines while submerged. It was reported in 2018 that China had constructed the world's largest ELF facility roughly the size of New York City in order to communicate with its submarine forces without requiring them to surface. The United States Navy in 1982 built the first ELF submarine communications facility, two coupled ELF transmitters at Clam Lake, Wisconsin , and Republic, Michigan . They were shut down in 2004. The Russian Navy operates an ELF transmitter called ZEVS (Zeus) at Murmansk on

284-480: 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 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

355-487: A huge cavity resonator , enhancing ELF radiation at its resonant frequencies . These are called Schumann resonances after German physicist Winfried Otto Schumann , who predicted them in 1952, and were detected in the 1950s. Modeling the Earth–ionosphere cavity with perfectly conducting walls, Schumann calculated the resonances should occur at frequencies of The actual frequencies differ slightly from this due to

426-778: A pooled analysis of epidemiological evidence supported the hypothesis that exposure to power-frequency magnetic fields is related to childhood leukemia. No other study has found any evidence to support the hypothesis that ELF exposure is a contributing factor to leukemia in children. A 2014 study estimated the cases of childhood leukemia attributable to exposure to ELF magnetic fields in the European Union (EU27), assuming that correlations seen in epidemiological studies were causal. It reported that around 50–60 cases of childhood leukemia might be attributable to ELF magnetic fields annually, corresponding to between ~1.5% and ~2.0% of all incident cases of childhood leukemia occurring in

497-468: A possible association between long-term occupational exposure to ELF and Alzheimer's disease . There have been some concerns over the possible ecological impact of ELF signals. In 1984 a federal judge halted construction, requiring more environmental and health studies. This judgment was overruled by a federal appeals court on the basis that the US Navy claimed to have spent over $ 25 million studying

568-402: A quarter-wave vertical antenna at 30 kHz (10 km wavelength) 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. 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, with

639-553: A second. When the refractive index of the medium is greater than one, ELF waves propagate slower than the speed of light in vacuum. As used in military applications, the wavelength is 299,792 km (186,282 mi) per second divided by 50–85 Hz, which equals around 3,500–6,000 km (2,200–3,700 mi) long. This is comparable to the Earth 's diameter of around 12,742 km (7,918 mi). Because of this huge size requirement, to transmit internationally using ELF frequencies,

710-404: 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

781-460: A shallow depth where it could receive some other form of communication. One of the difficulties posed when broadcasting in the ELF frequency range is antenna size, because the length of the antenna must be at least a substantial fraction of the length of the waves. For example, a 3  Hz signal has a wavelength equal to the distance electromagnetic waves travel through a given medium in one third of

SECTION 10

#1732780911389

852-478: A task group of scientific experts to assess any risks to health that might exist from "exposure to ELF electric and magnetic fields in the frequency range >0 to 100,000 Hz (100 kHz) in regards to childhood leukemia". The long-term, low-level exposure is evaluated as average exposure to residential power-frequency magnetic field above 0.3–0.4  μT , and it is estimated that only between 1% and 4% of children live in such conditions. Subsequently, in 2010,

923-497: Is a stub . You can help Misplaced Pages by expanding it . Extremely low frequency Extremely low frequency ( ELF ) is the ITU designation for electromagnetic radiation ( radio waves ) 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

994-452: Is high in the band, including such phenomena as " whistlers ", caused by lightning . A major practical drawback to 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. Vertical antennas must be used because VLF waves propagate in vertical polarization, but

1065-411: 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

1136-672: 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. 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

1207-494: Is so low that they can travel completely around the Earth several times before decaying to negligible amplitude, and thus waves radiated from a source in opposite directions circumnavigating the Earth on a great circle path interfere with each other. At certain frequencies these oppositely directed waves are in phase and add (reinforce), causing standing waves . In other words, the closed spherical Earth–ionosphere cavity acts as

1278-504: 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

1349-495: 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 (131 ft) into saltwater, they are used for military communication with submarines . Because of their long wavelengths, VLF radio waves can diffract around large obstacles and so are not blocked by mountain ranges, and can propagate as ground waves following

1420-454: 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 of the antenna. The huge capacitively-loaded antenna and loading coil form

1491-446: 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

SECTION 20

#1732780911389

1562-414: The Earth's electromagnetic field . Increasing the playback speed increases the pitch , bringing the tone into the audio frequency range. Since the 2000s, very low frequencies have been used successfully at sea for oil geophysical prospecting. Naturally occurring ELF waves are present on Earth, resonating in the region between ionosphere and surface seen in lightning strikes that make electrons in

1633-602: The Kola Peninsula . The Indian Navy has an ELF communication facility at the INS Kattabomman naval base to communicate with its Arihant-class and Akula-class submarines . Because of its electrical conductivity , seawater shields submarines from most higher-frequency radio waves, making radio communication with submerged submarines at ordinary frequencies impossible. Signals in the ELF frequency range, however, can penetrate much deeper. Two factors limit

1704-666: 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

1775-414: The electric field vertical and the magnetic field horizontal. ELF waves have extremely low attenuation of 1–2 dB per 1,000 km (620 mi), giving a single transmitter the potential to communicate worldwide. ELF waves can also travel considerable distances through "lossy" media like earth and seawater, which would absorb or reflect higher-frequency radio waves. The attenuation of ELF waves

1846-536: The ionosphere called the D layer at 60–90 km (37–56 miles) 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, with little of

1917-426: 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. Because of the small bandwidth of the band, and the extremely narrow bandwidth of

1988-596: 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

2059-446: The umbrella antenna such as the "delta" and " trideco " antennas, or multiwire flattop (triatic) 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 (Earthing) systems, consisting of radial networks of buried copper wires under the antenna. To minimize dielectric losses in

2130-660: 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 (" sferics ")

2201-501: The Crab nebula radiates powers of this order at 30 Hz. Radiation of this frequency is below the plasma frequency of the interstellar medium , thus this medium is opaque to it, and it cannot be observed from Earth. In electromagnetic therapy and electromagnetic radiation and health research, electromagnetic spectrum frequencies between 0 and 100 hertz are considered extremely low-frequency fields. A common source of exposure of

I band (NATO) - Misplaced Pages Continue

2272-428: The ELF spectrum and are called Schumann resonance . ELF waves have also been tentatively identified on Saturn's moon Titan . Titan's surface is thought to be a poor reflector of ELF waves, so the waves may instead be reflecting from the liquid–ice boundary of a subsurface ocean of water and ammonia, the existence of which is predicted by some theoretical models. Titan's ionosphere is also more complex than Earth's, with

2343-488: The EU27 each year. At present, however, ICNIRP and IEEE consider the scientific evidence related to possible health effects from long-term, low-level exposure to ELF fields insufficient to justify lowering these quantitative exposure limits. In summary, when all of the studies are evaluated together, the evidence suggesting that EMFs may contribute to an increased risk of cancer is non-existent. Epidemiological studies suggest

2414-903: The Earth itself forms a significant part of the antenna, and extremely long leads into the ground are necessary. Various means, such as electrical lengthening , are used to construct practical radio stations with smaller sizes. The United States maintained two sites: in the Chequamegon-Nicolet National Forest , Wisconsin , and in the Escanaba River State Forest , Michigan (originally named Project Sanguine , then downsized and renamed Project ELF prior to construction), until they were dismantled, beginning in late September 2004. Both sites used long power lines , so-called ground dipoles , as leads. These leads were in multiple strands ranging from 22.5 to 45 kilometres (14.0 to 28.0 mi) long. Because of

2485-496: 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. VLF can also penetrate soil and rock for some distance, so these frequencies are also used for through-the-earth mine communications systems. Powerful VLF transmitters are used by

2556-459: The Schumann resonances can be used to monitor global thunderstorm activity. Interest in Schumann resonances was renewed in 1993 when E. R. Williams showed a correlation between the resonance frequency and tropical air temperatures, suggesting that the resonance could be used to monitor global warming . Since ELF radio waves can penetrate seawater deeply, to the operating depths of submarines,

2627-438: 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

2698-404: 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

2769-559: 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 , 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

2840-420: 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. The high Q results in very high voltages (up to 250 kV) on

2911-615: The antennas used, it is impractical to transmit audio signals ( AM or FM radiotelephony ). 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

I band (NATO) - Misplaced Pages Continue

2982-471: The atmosphere oscillate. Although signals generated from lightning discharges were predominantly VLF, it was found that an observable ELF component (slow tail) followed the VLF component in almost all cases. Also, the fundamental mode of the Earth–ionosphere cavity has the wavelength equal to the circumference of the Earth, which gives a resonance frequency of 7.8 Hz. This frequency, and higher resonance modes of 14, 20, 26, and 32 Hz, appear as peaks in

3053-466: 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

3124-400: The body, which, at very high field strengths, cause nerve and muscle stimulation and changes in nerve cell excitability in the central nervous system. ELF at human-perceivable kV/m levels was said to create an annoying tingling sensation in the areas of the body in contact with clothing, particularly the arms, due to the induction of a surface charge by the ELF. Of the volunteers, 7% described

3195-539: The concept of "extremely low frequency (ELF) electric and magnetic fields (EMF)". The WHO also stated that at frequencies between 0 and 300 Hz, "the wavelengths in air are very long (6,000 km (3,700 mi) at 50 Hz and 5,000 km (3,100 mi) at 60 Hz), and, in practical situations, the electric and magnetic fields act independently of one another and are measured separately". Due to their extremely long wavelength, ELF waves can diffract around large obstacles, are not blocked by mountain ranges or

3266-432: The conduction properties of the ionosphere. The fundamental Schumann resonance is at approximately 7.83 Hz, the frequency at which the wavelength equals the circumference of the Earth, and higher harmonics occur at 14.1, 20.3, 26.4, and 32.4 Hz, etc. Lightning strikes excite these resonances, causing the Earth–ionosphere cavity to "ring" like a bell, resulting in a peak in the noise spectrum at these frequencies, so

3337-453: The conductive Earth's surface and the conductive D layer acts as a parallel-plate waveguide which confines ELF waves, allowing them to propagate long distances without escaping into space. In contrast to VLF waves, the height of the layer is much less than one wavelength at ELF frequencies, so the only mode that can propagate at ELF frequencies is the TEM mode in vertical polarization , with

3408-407: 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

3479-543: The difficulty of building antennas that can radiate such long waves, ELF have been used in only a very few human-made communication systems. ELF waves can penetrate seawater , which makes them useful in communication with submarines , and a few nations have built military ELF transmitters to transmit signals to their submerged submarines, consisting of huge grounded wire antennas ( ground dipoles ) 15–60 km (9–37 mi) long driven by transmitters producing megawatts of power. The United States, Russia, India, and China are

3550-1171: The effects of the electromagnetic fields, with results indicating that they were similar to the effect produced by standard power-distribution lines. However, during the time that ELF was in use, some Wisconsin politicians, such as Democratic Senators Herb Kohl , Russ Feingold and Congressman Dave Obey , continued to call for its closure. Extremely low frequency (ELF) electromagnetic fields (EMFs), typically ranging from 0.3 Hz to 300 Hz, have various ecological impacts on both flora and fauna. ELF 3 Hz/100 Mm 30 Hz/10 Mm SLF 30 Hz/10 Mm 300 Hz/1 Mm ULF 300 Hz/1 Mm 3 kHz/100 km VLF 3 kHz/100 km 30 kHz/10 km LF 30 kHz/10 km 300 kHz/1 km MF 300 kHz/1 km 3 MHz/100 m HF 3 MHz/100 m 30 MHz/10 m VHF 30 MHz/10 m 300 MHz/1 m UHF 300 MHz/1 m 3 GHz/100 mm SHF 3 GHz/100 mm 30 GHz/10 mm EHF 30 GHz/10 mm 300 GHz/1 mm THF 300 GHz/1 mm 3 THz/0.1 mm Very low frequency Very low frequency or VLF

3621-506: 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

SECTION 50

#1732780911389

3692-406: 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. Radiated power from amateur stations

3763-463: The horizon, and can travel around the curvature of the Earth . ELF and VLF waves propagate long distances by an Earth–ionosphere waveguide mechanism. The Earth is surrounded by a layer of charged particles ( ions and electrons ) in the atmosphere at an altitude of about 60 km (37 mi) at the bottom of the ionosphere , called the D ;layer , which reflects ELF waves. The space between

3834-457: The inefficiency of this method, considerable amounts of electrical power were required to operate the system. Transmitters in the 22 Hz range are also used in pipeline maintenance, or pigging . The signal is generated as an alternating magnetic field, and the transmitter is mounted to, or to part of, the "pig", the cleaning device inserted into the pipe. The pig is pushed through a mostly metal pipeline. The ELF signal can be detected through

3905-510: The main ionosphere at an altitude of 1,200 km (750 mi) but with an additional layer of charged particles at 63 km (39 mi). This splits Titan's atmosphere into two separate resonating chambers. The source of natural ELF waves on Titan is unclear, as there does not appear to be extensive lightning activity. Huge ELF radiation power outputs of 100,000 times the Sun's output in visible light may be radiated by magnetars . The pulsar in

3976-512: The metal, allowing its location to be detected by receivers located outside of the pipe. It is used to check whether a pig has passed a certain location or to locate a stuck pig. Some radio hobbyists record ELF signals using antennas ranging in size from 18-inch active antennas up to several thousand feet in length taking advantage of fences, highway guard rails, and even decommissioned railroad tracks. They then replay them at higher speeds to more easily observe natural low-frequency fluctuations in

4047-485: 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 Since 2004

4118-582: The only countries known to have constructed these ELF communication facilities. The U.S. facilities were used between 1985 and 2004 but are now decommissioned. ELF is a subradio frequency . Some medical peer reviewed journal articles refer to ELF in the context of "extremely low frequency (ELF) magnetic fields (MF)" with frequencies of 50 Hz and 50–80 Hz. United States Government agencies, such as NASA, describe ELF as non-ionizing radiation with frequencies between 0 and 300 Hz. The World Health Organization (WHO) have used ELF to refer to

4189-588: 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 US, 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

4260-428: The public to ELF fields is 50 Hz / 60 Hz electric and magnetic fields from high-voltage electric power transmission lines and secondary distribution lines, such as those supplying electricity to residential neighborhoods. Since the late 1970s, various conspiracy theories have risen around exposure to ELF electric and magnetic fields (EMF). External ELF magnetic fields induce electric fields and currents in

4331-455: The related magnetosphere science, the lower-frequency electromagnetic oscillations (pulsations occurring below ~3 Hz) are considered to lie in the ULF range, which is thus also defined differently from the ITU radio bands . ELF radio waves are generated by lightning and natural disturbances in Earth's magnetic field , so they are a subject of research by atmospheric scientists. Because of

SECTION 60

#1732780911389

4402-399: 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 VLF transmitting stations use capacitively-toploaded monopole antennas . These are very large wire antennas, up to several kilometers long. They consist of

4473-428: 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

4544-413: The spark discharges as painful when the subject was well-insulated and touched a grounded object within a 5 kV/m field, whereas 50% described a similar spark discharge as painful in a 10 kV/m field. There is high uncertainty regarding correlations between long-term, low-level exposure to ELF fields and a number of health effects, including leukemia in children. In October 2005, WHO convened

4615-720: 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

4686-409: 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. At VLF frequencies atmospheric radio noise is far above

4757-414: 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

4828-404: The usefulness of ELF communications channels: the low data transmission rate of a few characters per minute and, to a lesser extent, the one-way nature due to the impracticality of installing an antenna of the required size on a submarine (the antenna needs to be of an exceptional size in order to achieve successful communication). Generally, ELF signals have been used to order a submarine to rise to

4899-451: 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: Historically, this band

4970-464: 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 . Geophysicists use VLF- electromagnetic receivers to measure conductivity in the near surface of

5041-438: 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

#388611