Very low frequency
Low frequency or VLF is the ITU designation for radio frequencies in the range of 3 to 30 kilohertz, corresponding to wavelengths from 100 to 10 kilometers, respectively. The band is 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 impractical in this band, 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 for secure military communication. Since VLF waves can penetrate at least 40 meters into saltwater, they are used for military communication with submarines; because of their large wavelengths, VLF radio waves can diffract around large obstacles and so are not blocked by mountain ranges or the horizon, can propagate as ground waves following the curvature of the Earth. 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 60 to 90 km 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 zigzag path around the Earth, reflected alternately by the Earth and the ionosphere, in TM mode. VLF waves have low path attenuation, 2-3 dB per 1000 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, spend most of their journey in the ionosphere, so they are much more affected by ionization gradients and turbulence. Therefore, VLF transmissions are stable and reliable, are used for long distance communication. Propagation distances of 5000 to 20000 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 10 to 40 meters, 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. A major practical drawback to this band is that because of the length of the waves, full size resonant 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 high. So practical transmitting antennas are electrically short, a small fraction of a wavelength long. Due to their low radiation resistance they are inefficient, radiating only 10% to 50% of the transmitter power at most, with the rest of the power dissipated in the antenna/ground system resistances.
High power transmitters are required for long distance communication, so the efficiency of the antenna is an important factor. High power transmitting antennas for VLF frequencies are large wire antennas, up to a mile across, they consist of a series of steel radio masts, linked at the top with a network of cables shaped like an umbrella or clotheslines. Either the towers themselves or vertical wires serve as monopole radiators, 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 antennas. For low power transmitters, inverted-L and T antennas are used. A large loading coil is required at the antenna feed point to cancel the capacitive reactance of the antenna to make it resonant. To minimize power dissipated in the ground, these antennas require low resistance ground systems; because of soil resistance and dielectric losses in the ground, the buried cable ground systems used by higher frequency transmitters tend to have unacceptably high losses, counterpoise systems are used, consisting of radial networks of copper cables supported several feet above the ground under the antenna, extending out radially from the mast or vertical element.
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 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 have bandwidths of only a few tens of hertz, when transmitting frequency shift keying, 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 of the antenna results in high voltages at the ends of the horizontal topload wires where the nodes of the standing wave pattern occur, good insulation is required; the practical limit to the power of large VLF transmitters is determined by onset of air breakdown and arcing from the antenna. The re
Very high frequency
High frequency is the ITU designation for the range of radio frequency electromagnetic waves from 30 to 300 megahertz, with corresponding wavelengths of ten meters to one meter. Frequencies below VHF are denoted high frequency, the next higher frequencies are known as ultra high frequency. Common uses for radio waves in the VHF band are FM radio broadcasting, television broadcasting, two way land mobile radio systems, long range data communication up to several tens of kilometers with radio modems, amateur radio, marine communications. Air traffic control communications and air navigation systems work at distances of 100 kilometres or more to aircraft at cruising altitude. In the Americas and many other parts of the world, VHF Band I was used for the transmission of analog television; as part of the worldwide transition to digital terrestrial television most countries require broadcasters to air television in the VHF range using digital rather than analog format. Radio waves in the VHF band propagate by line-of-sight and ground-bounce paths.
They do not follow the contour of the Earth as ground waves and so are blocked by hills and mountains, although because they are weakly refracted by the atmosphere they can travel somewhat beyond the visual horizon out to about 160 km. They can penetrate building walls and be received indoors, although in urban areas reflections from buildings cause multipath propagation, which can interfere with television reception. Atmospheric radio noise and interference from electrical equipment is less of a problem in the band than at lower frequencies; the VHF band is the first band at which efficient transmitting antennas are small enough that they can be mounted on vehicles and portable devices, so the band is used for two-way land mobile radio systems, such as walkie-talkies, two way radio communication with aircraft and ships. When conditions are right, VHF waves can travel long distances by tropospheric ducting due to refraction by temperature gradients in the atmosphere. For analog TV, VHF transmission range is a function of transmitter power, receiver sensitivity, distance to the horizon, since VHF signals propagate under normal conditions as a near line-of-sight phenomenon.
The distance to the radio horizon is extended over the geometric line of sight to the horizon, as radio waves are weakly bent back toward the Earth by the atmosphere. An approximation to calculate the line-of-sight horizon distance is: distance in nautical miles = 1.23 × A f where A f is the height of the antenna in feet distance in kilometers = 12.746 × A m where A m is the height of the antenna in meters. These approximations are only valid for antennas at heights that are small compared to the radius of the Earth, they may not be accurate in mountainous areas, since the landscape may not be transparent enough for radio waves. In engineered communications systems, more complex calculations are required to assess the probable coverage area of a proposed transmitter station; the accuracy of these calculations for digital TV signals is being debated. VHF is the first band at which wavelengths are small enough that efficient transmitting antennas are short enough to mount on vehicles and handheld devices, a quarter wave whip antenna at VHF frequencies is 25 cm to 2.5 meter long.
So the VHF and UHF wavelengths are used for two-way radios in vehicles and handheld transceivers and walkie-talkies. Portable radios use whips or rubber ducky antennas, while base stations use larger fiberglass whips or collinear arrays of vertical dipoles. For directional antennas, the Yagi antenna is the most used as a high gain or "beam" antenna. For television reception, the Yagi is used, as well as the log-periodic antenna due to its wider bandwidth. Helical and turnstile antennas are used for satellite communication since they employ circular polarization. For higher gain, multiple Yagis or helicals can be mounted together to make array antennas. Vertical collinear arrays of dipoles can be used to make high gain omnidirectional antennas, in which more of the antenna's power is radiated in horizontal directions. Television and FM broadcasting stations use collinear arrays of specialized dipole antennas such as batwing antennas. Certain subparts of the VHF band have the same use around the world.
Some national uses are detailed below. 50–54 MHz: Amateur Radio 6-meter band. 108–118 MHz: Air navigation beacons VOR and Instrument Landing System localizer. 118–137 MHz: Airband for air traffic control, AM, 121.5 MHz is emergency frequency 144–148 MHz: Amateur Radio 2-meter band. The VHF TV band in Australia was allocated channels 1 to 10-with channels 2, 7 and 9 assigned for the initial services in Sydney and Melbourne, the same channels were assigned in Brisbane and Perth. Other capital cities and regional areas used a combination of these and other frequencies as available; the initial commercial services in Hobart and Darwin were allocated channels 6 and 8 rather than 7 or 9. By the early 1960s it became apparent that the 10 VHF channels were insufficient to support the growth of television services; this was rectified by the addition of th
Ultra low frequency
Ultra low frequency is the ITU designation for the frequency range of electromagnetic waves between 300 hertz and 3 kilohertz. In magnetosphere science and seismology, alternative definitions are given, including ranges from 1 mHz to 100 Hz, 1 mHz to 1 Hz, 10 mHz to 10 Hz. Frequencies above 3 Hz in atmospheric science are assigned to the ELF range. Many types of waves in the ULF frequency band can be observed in the magnetosphere and on the ground; these waves represent important physical processes in the near-Earth plasma environment. The speed of the ULF waves is associated with the Alfvén velocity that depends on the ambient magnetic field and plasma mass density; this band is used for communications in mines. Some monitoring stations have reported that earthquakes are sometimes preceded by a spike in ULF activity. A remarkable example of this occurred before the 1989 Loma Prieta earthquake in California, although a subsequent study indicates that this was little more than a sensor malfunction.
On December 9, 2010, geoscientists announced that the DEMETER satellite observed a dramatic increase in ULF radio waves over Haiti in the month before the magnitude 7.0 Mw 2010 earthquake. Researchers are attempting to learn more about this correlation to find out whether this method can be used as part of an early warning system for earthquakes. ULF has been used by the military for secure communications through the ground. NATO AGARD publications from the 1960s detailed many such systems, although it is possible that the published papers left a lot unsaid about what was developed secretly for defense purposes. Communications through the ground using conduction fields is known as "Earth-Mode" communications and was first used in World War I. Radio amateurs and electronics hobbyists have used this mode for limited range communications using audio power amplifiers connected to spaced electrode pairs hammered into the soil. At the receiving end, the signal is detected as a weak electric current between a further pair of electrodes.
Using weak signal reception methods with PC-based DSP filtering with narrow bandwidths, it is possible to receive signals at a range of a few kilometers with a transmitting power of 10-100 W and electrode spacing of around 10–50 m. Earth's field NMR Through the earth mine communications Voice frequency Tomislav Stimac, "Definition of frequency bands". IK1QFK Home Page. NASA live streaming ELF -> VLF Receiver Amateur Radio Below 10 kHz "G3XBM's page on Earth Mode Communication" Review of Earth Mode Communications "1966 abstract about Earth Mode Comms by Ames and Orange" Radio communications within the Earth's crust "Abstract of article by Burrows written in 1963"
Extremely high frequency
High frequency is the International Telecommunication Union designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz. It lies between the super high frequency band, the far infrared band, the lower part of, referred to as the terahertz gap. Radio waves in this band have wavelengths from ten to one millimetre, so it is called the millimetre band and radiation in this band is called millimetre waves, sometimes abbreviated MMW or mmW. Millimetre-length electromagnetic waves were first investigated in the 1890s by Indian scientist Jagadish Chandra Bose. Compared to lower bands, radio waves in this band have high atmospheric attenuation: they are absorbed by the gases in the atmosphere. Therefore, they have a short range and can only be used for terrestrial communication over about a kilometer. Absorption by humidity in the atmosphere is significant except in desert environments, attenuation by rain is a serious problem over short distances; however the short propagation range allows smaller frequency reuse distances than lower frequencies.
The short wavelength allows modest size antennas to have a small beam width, further increasing frequency reuse potential. Millimeter waves propagate by line-of-sight paths, they are not reflected by the ionosphere nor do they travel along the Earth as ground waves as lower frequency radio waves do. At typical power densities they are blocked by building walls and suffer significant attenuation passing through foliage. Absorption by atmospheric gases is a significant factor throughout the band and increases with frequency. However, it is maximum at a few specific absorption lines those of oxygen at 60 GHz and water vapor at 24 GHz and 184 GHz. At frequencies in the "windows" between these absorption peaks, millimeter waves have much less atmospheric attenuation and greater range, so many applications use these frequencies. Millimeter wavelengths are the same order of size as raindrops, so precipitation causes additional attenuation due to scattering as well as absorption; the high free space loss and atmospheric absorption limits useful propagation to a few kilometers.
Thus, they are useful for densely packed communications networks such as personal area networks that improve spectrum utilization through frequency reuse. Millimeter waves show "optical" propagation characteristics and can be reflected and focused by small metal surfaces and dielectric lenses around 5 to 30 cm diameter; because their wavelengths are much smaller than the equipment that manipulates them, the techniques of geometric optics can be used. Diffraction is less than at lower frequencies. At millimeter wavelengths, surfaces appear rougher so diffuse reflection increases. Multipath propagation reflection from indoor walls and surfaces, causes serious fading. Doppler shift of frequency can be significant at pedestrian speeds. In portable devices, shadowing due to the human body is a problem. Since the waves penetrate clothing and their small wavelength allows them to reflect from small metal objects they are used in millimeter wave scanners for airport security scanning; this band is used in radio astronomy and remote sensing.
Ground-based radio astronomy is limited to high altitude sites such as Kitt Peak and Atacama Large Millimeter Array due to atmospheric absorption issues. Satellite-based remote sensing near 60 GHz can determine temperature in the upper atmosphere by measuring radiation emitted from oxygen molecules, a function of temperature and pressure; the ITU non-exclusive passive frequency allocation at 57–59.3 GHz is used for atmospheric monitoring in meteorological and climate sensing applications and is important for these purposes due to the properties of oxygen absorption and emission in Earth's atmosphere. Operational U. S. satellite sensors such as the Advanced Microwave Sounding Unit on one NASA satellite and four NOAA satellites and the special sensor microwave/imager on Department of Defense satellite F-16 make use of this frequency range. In the United States, the band 36.0 – 40.0 GHz is used for licensed high-speed microwave data links, the 60 GHz band can be used for unlicensed short range data links with data throughputs up to 2.5 Gbit/s.
It is used in flat terrain. The 71–76, 81–86 and 92–95 GHz bands are used for point-to-point high-bandwidth communication links; these higher frequencies do not suffer from oxygen absorption, but require a transmitting license in the US from the Federal Communications Commission. There are plans for 10 Gbit/s links using these frequencies as well. In the case of the 92–95 GHz band, a small 100 MHz range has been reserved for space-borne radios, limiting this reserved range to a transmission rate of under a few gigabits per second; the band is undeveloped and available for use in a broad range of new products and services, including high-speed, point-to-point wireless local area networks and broadband Internet access. WirelessHD is another recent technology. Directional, "pencil-beam" signal characteristics permit different systems to operate close to one another without causing interference. Potential applications include radar systems with high resolution; the Wi-Fi standard IEEE 802.11ad operates in the 60 GHz spectrum to achieve data transfer rates as high as 7 Gbit/s.
Uses of the millimeter wave bands include point-to-point communications, intersatellite links, point-to-multipoint communications. There are tentative plans to use millimeter waves in future 5G mobile phones. In addition, use of millimeter wave
The X band is the designation for a band of frequencies in the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of the X band is rather indefinitely set at 7.0 to 11.2 GHz. In radar engineering, the frequency range is specified by the IEEE at 8.0 to 12.0 GHz. The X band is used for radar, satellite communication, wireless computer networks. X band is used in radar applications including continuous-wave, single-polarization, dual-polarization, synthetic aperture radar, phased arrays. X band radar frequency sub-bands are used in civil and government institutions for weather monitoring, air traffic control, maritime vessel traffic control, defense tracking, vehicle speed detection for law enforcement. X band is used in modern radars; the shorter wavelengths of the X band allow for higher resolution imagery from high-resolution imaging radars for target identification and discrimination. In Ireland, Saudi Arabia and Canada, the X band 10.15 to 10.7 segment is used for terrestrial broadband.
Alvarion, CBNL, CableFree and Ogier make systems for this. The Ogier system is a full duplex Transverter used for DOCSIS over microwave; the home / Business CPE has a single coaxial cable with a power adapter connecting to an ordinary cable modem. The local oscillator is 9750 MHz, the same as for Ku band satellite TV LNB. Two way applications such as broadband use a 350 MHz TX offset. Portions of the X band are assigned by the International Telecommunications Union for deep space telecommunications; the primary user of this allocation is the American NASA Deep Space Network. DSN facilities are in Goldstone, near Canberra and near Madrid, Spain; these three stations, located 120 degrees apart in longitude, provide continual communications from the Earth to any point in the Solar System independent of Earth rotation. DSN stations are capable of using the older and lower S band deep-space radio communications allocations, some higher frequencies on a more-or-less experimental basis, such as in the K band.
Notable deep space probe programs that have employed X band communications include the Viking Mars landers. The new European double Mars Mission ExoMars will use X band communication, on the instrument LaRa, to study the internal structure of Mars, to make precise measurements of the rotation and orientation of Mars by monitoring two-way Doppler frequency shifts between the surface platform and Earth, it will detect variations in angular momentum due to the redistribution of masses, such as the migration of ice from the polar caps to the atmosphere. An important use of the X band communications came with the two Viking program landers; when the planet Mars was passing near or behind the Sun, as seen from the Earth, a Viking lander would transmit two simultaneous continuous-wave carriers, one in the S band and one in the X band in the direction of the Earth, where they were picked up by DSN ground stations. By making simultaneous measurements at the two different frequencies, the resulting data enabled theoretical physicists to verify the mathematical predictions of Albert Einstein's General Theory of Relativity.
These results are some of the best confirmations of the General Theory of Relativity. The International Telecommunications Union, the international body which allocates radio frequencies for civilian use, is not authorised to allocate frequency bands for military radio communication; this is the case pertaining to X band military communications satellites. However, in order to meet military radio spectrum requirements, e.g. for fixed-satellite service and mobile-satellite service, the NATO nations negotiated the so-called NATO Joint Civil/Military Frequency Agreement. The Radio Regulations of the International Telecommunication Union allow amateur radio operations in the frequency range 10.000 to 10.500 GHz, amateur satellite operations are allowed in the range 10.450 to 10.500 GHz. This is known as the 3-centimeter band by amateurs and the X-band by AMSAT. Motion detectors use 10.525 GHz. 10.4 GHz is proposed for traffic light crossing detectors. Comreg in Ireland has allocated 10.450 GHz for Traffic Sensors as SRD.
Many electron paramagnetic resonance spectrometers operate near 9.8 GHz. Particle accelerators may be powered by X-band RF sources; the frequencies are standardized at 11.9942 GHz or 11.424 GHz, the second harmonic of C-band and fourth harmonic of S-band. The European X-band frequency is used for the Compact Linear Collider. Cassegrain reflector Directional antenna XTAR Sea-based X band Radar New Horizons telecommunications Voyager program#Spacecraft design Earth observation satellites transmission frequencies TerraSAR-X: a German Earth observation satellite http://www.ntia.doc.gov/osmhome/allochrt.pdf http://www.g3pho.free-online.co.uk/microwaves/wideband.htm
The metre or meter is the base unit of length in the International System of Units. The SI unit symbol is m; the metre is defined as the length of the path travelled by light in vacuum in 1/299 792 458 of a second. The metre was defined in 1793 as one ten-millionth of the distance from the equator to the North Pole – as a result the Earth's circumference is 40,000 km today. In 1799, it was redefined in terms of a prototype metre bar. In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. In 1983, the current definition was adopted; the imperial inch is defined as 0.0254 metres. One metre is about 3 3⁄8 inches longer than a yard, i.e. about 39 3⁄8 inches. Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations except the United States and the Philippines, which use meter. Other Germanic languages, such as German and the Scandinavian languages spell the word meter. Measuring devices are spelled "-meter" in all variants of English.
The suffix "-meter" has the same Greek origin as the unit of length. The etymological roots of metre can be traced to the Greek verb μετρέω and noun μέτρον, which were used for physical measurement, for poetic metre and by extension for moderation or avoiding extremism; this range of uses is found in Latin, French and other languages. The motto ΜΕΤΡΩ ΧΡΩ in the seal of the International Bureau of Weights and Measures, a saying of the Greek statesman and philosopher Pittacus of Mytilene and may be translated as "Use measure!", thus calls for both measurement and moderation. In 1668 the English cleric and philosopher John Wilkins proposed in an essay a decimal-based unit of length, the universal measure or standard based on a pendulum with a two-second period; the use of the seconds pendulum to define length had been suggested to the Royal Society in 1660 by Christopher Wren. Christiaan Huygens had observed that length to be 39.26 English inches. No official action was taken regarding these suggestions.
In 1670 Gabriel Mouton, Bishop of Lyon suggested a universal length standard with decimal multiples and divisions, to be based on a one-minute angle of the Earth's meridian arc or on a pendulum with a two-second period. In 1675, the Italian scientist Tito Livio Burattini, in his work Misura Universale, used the phrase metro cattolico, derived from the Greek μέτρον καθολικόν, to denote the standard unit of length derived from a pendulum; as a result of the French Revolution, the French Academy of Sciences charged a commission with determining a single scale for all measures. On 7 October 1790 that commission advised the adoption of a decimal system, on 19 March 1791 advised the adoption of the term mètre, a basic unit of length, which they defined as equal to one ten-millionth of the distance between the North Pole and the Equator. In 1793, the French National Convention adopted the proposal. In 1791, the French Academy of Sciences selected the meridional definition over the pendular definition because the force of gravity varies over the surface of the Earth, which affects the period of a pendulum.
To establish a universally accepted foundation for the definition of the metre, more accurate measurements of this meridian were needed. The French Academy of Sciences commissioned an expedition led by Jean Baptiste Joseph Delambre and Pierre Méchain, lasting from 1792 to 1799, which attempted to measure the distance between a belfry in Dunkerque and Montjuïc castle in Barcelona to estimate the length of the meridian arc through Dunkerque; this portion of the meridian, assumed to be the same length as the Paris meridian, was to serve as the basis for the length of the half meridian connecting the North Pole with the Equator. The problem with this approach is that the exact shape of the Earth is not a simple mathematical shape, such as a sphere or oblate spheroid, at the level of precision required for defining a standard of length; the irregular and particular shape of the Earth smoothed to sea level is represented by a mathematical model called a geoid, which means "Earth-shaped". Despite these issues, in 1793 France adopted this definition of the metre as its official unit of length based on provisional results from this expedition.
However, it was determined that the first prototype metre bar was short by about 200 micrometres because of miscalculation of the flattening of the Earth, making the prototype about 0.02% shorter than the original proposed definition of the metre. Regardless, this length became the French standard and was progressively adopted by other countries in Europe; the expedition was fictionalised in Le mètre du Monde. Ken Alder wrote factually about the expedition in The Measure of All Things: the seven year odyssey and hidden error that transformed the world. In 1867 at the second general conference of the International Association of Geodesy held in Berlin, the question of an international standard unit of length was discussed in order to combine the measurements made in different countries to determine the size and shape of the Earth; the conference recommended the adoption of the metre and the creation of an internatio