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
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
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
Super high frequency
Super high frequency is the ITU designation for radio frequencies in the range between 3 and 30 gigahertz. This band of frequencies is known as the centimetre band or centimetre wave as the wavelengths range from one to ten centimetres; these frequencies fall within the microwave band, so radio waves with these frequencies are called microwaves. The small wavelength of microwaves allows them to be directed in narrow beams by aperture antennas such as parabolic dishes and horn antennas, so they are used for point-to-point communication and data links and for radar; this frequency range is used for most radar transmitters, wireless LANs, satellite communication, microwave radio relay links, numerous short range terrestrial data links. They are used for heating in industrial microwave heating, medical diathermy, microwave hyperthermy to treat cancer, to cook food in microwave ovens. Frequencies in the SHF range are referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations.
Microwaves propagate by line of sight. Although in some cases they can penetrate building walls enough for useful reception, unobstructed rights of way cleared to the first Fresnel zone are required. Wavelengths are small enough at microwave frequencies that the antenna can be much larger than a wavelength, allowing directional antennas to be built which can produce narrow beams. Therefore, they are used in point-to-point terrestrial communications links, limited by the visual horizon to 30–40 miles; such high gain antennas allow frequency reuse by nearby transmitters. The wavelength of SHF waves allows strong reflections from metal objects the size of automobiles and ships, other vehicles. Thus, the narrow beamwidths possible with high gain antennas and the low atmospheric attenuation as compared with higher frequencies make SHF the main frequencies used in radar. Attenuation and scattering by moisture in the atmosphere increase with frequency, limiting the use of high SHF frequencies for long range applications.
Small amounts of microwave energy are randomly scattered by water vapor molecules in the troposphere. This is used in troposcatter communications systems, operating at a few GHz, to communicate beyond the horizon. A powerful microwave beam is aimed just above the horizon. Distances of 300 km can be achieved; these are used for military communication. The wavelengths of SHF waves are small enough that they can be focused into narrow beams by high gain antennas from a half meter to five meters in diameter. Directive antennas at SHF frequencies are aperture antennas, such as parabolic antennas, dielectric lens and horn antennas. Large parabolic antennas can produce narrow beams of a few degrees or less, must be aimed with the aid of a boresight. For omnidirectional applications like wireless devices and cellphones, small dipoles or monopoles are used; the patch antenna is another type integrated into the skin of aircraft. Another type of antenna practical at microwave frequencies is the phased array, consisting of many dipoles or patch antennas on a flat surface, each fed through a phase shifter, which allows the array's beam to be steered electronically.
The short wavelength requires great mechanical rigidity in large antennas, to ensure that the radio waves arrive at the feed point in phase. At microwave frequencies, the types of cable used to conduct lower frequency radio waves, such as coaxial cable, have high power losses. Therefore, to transport microwaves between the transmitter or receiver and the antenna with low losses, a special type of metal pipe called waveguide must be used; because of the high cost and maintenance requirements of long waveguide runs, in many microwave antennas the output stage of the transmitter or the RF front end of the receiver is located at the antenna. SHF frequencies occupy a "sweet spot" in the radio spectrum, being exploited by many new radio services, they are the lowest frequency band where radio waves can be directed in narrow beams by conveniently sized antennas so they do not interfere with nearby transmitters on the same frequency, allowing frequency reuse. On the other hand, they are the highest frequencies which can be used for long distance terrestrial communication.
The high frequency gives microwave communication links a large information-carrying capacity. In recent decades many new solid state sources of microwave energy have been developed, microwave integrated circuits for the first time allow significant signal processing to be done at these frequencies. Sources of EHF energy are much more limited and in an earlier state of development. Knife-edge effect Microwave burn Tomislav Stimac, "Definition of frequency bands". IK1QFK Home Page. Inés Vidal Castiñeira, "Celeria: Wireless Access To Cable Networks"
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
Low frequency or LF is the ITU designation for radio frequencies in the range of 30 kilohertz to 300 kHz. As its wavelengths range from ten kilometres to one kilometre it is known as the kilometre band or kilometre wave. LF radio waves exhibit low signal attenuation, making them suitable for long-distance communications. In Europe and areas of Northern Africa and Asia, part of the LF spectrum is used for AM broadcasting as the "longwave" band. In the western hemisphere, its main use is for aircraft beacon, navigation and weather systems. A number of time signal broadcasts are broadcast in this band; because of their long wavelength, low frequency radio waves can diffract over obstacles like mountain ranges and travel beyond the horizon, following the contour of the Earth. This mode of propagation, called ground wave, is the main mode in the LF band. Ground waves must be vertically polarized, so vertical monopole antennas are used for transmitting; the attenuation of signal strength with distance by absorption in the ground is lower than at higher frequencies.
Low frequency ground waves can be received up to 2,000 kilometres from the transmitting antenna. Low frequency waves can occasionally travel long distances by reflecting from the ionosphere, although this method, called skywave or "skip" propagation, is not as common as at higher frequencies. Reflection occurs at F layers. Skywave signals can be detected at distances exceeding 300 kilometres from the transmitting antenna. In Europe and Japan, many low-cost consumer devices have since the late 1980s contained radio clocks with an LF receiver for these signals. Since these frequencies propagate by ground wave only, the precision of time signals is not affected by varying propagation paths between the transmitter, the ionosphere, the receiver. In the United States, such devices became feasible for the mass market only after the output power of WWVB was increased in 1997 and 1999. Radio signals below 50 kHz are capable of penetrating ocean depths to 200 metres, the longer the wavelength, the deeper.
The British, Indian, Swedish, United States and other navies communicate with submarines on these frequencies. In addition, Royal Navy nuclear submarines carrying ballistic missiles are under standing orders to monitor the BBC Radio 4 transmission on 198 kHz in waters near the UK, it is rumoured that they are to construe a sudden halt in transmission of the morning news programme Today, as an indicator that the UK is under attack, whereafter their sealed orders take effect. In the US, the Ground Wave Emergency Network or GWEN operated between 150 and 175 kHz, until replaced by satellite communications systems in 1999. GWEN was a land based military radio communications system which could survive and continue to operate in the case of a nuclear attack; the 2007 World Radiocommunication Conference made this band a worldwide amateur radio allocation. An international 2.1 kHz allocation, the 2200 meter band, is available to amateur radio operators in several countries in Europe, New Zealand and French overseas dependencies.
The world record distance for a two-way contact is over 10,000 km from near Vladivostok to New Zealand. As well as conventional Morse code many operators use slow computer-controlled Morse code or specialized digital communications modes; the UK allocated a 2.8 kHz sliver of spectrum from 71.6 kHz to 74.4 kHz beginning in April 1996 to UK amateurs who applied for a Notice of Variation to use the band on a noninterference basis with a maximum output power of 1 Watt ERP. This was withdrawn on 30 June 2003 after a number of extensions in favor of the European-harmonized 136 kHz band. Slow Morse Code from G3AQC in the UK was received 3,275 miles away, across the Atlantic Ocean, by W1TAG in the US on 21-22 November 2001 on 72.401 kHz. In the United States, there is a exemption within FCC Part 15 regulations permitting unlicensed transmissions in the frequency range of 160 to 190 kHz. Longwave radio hobbyists refer to this as the' LowFER' band, experimenters, their transmitters are called'LowFERs'.
This frequency range between 160 kHz and 190 kHz is referred to as the 1750 Meter band. Requirements from 47CFR15.217 and 47CFR15.206 include: The total input power to the final radio frequency stage shall not exceed one watt. The total length of the transmission line and ground lead shall not exceed 15 meters. All emissions below 160 kHz or above 190 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier; as an alternative to these requirements, a field strength of 2400/F microvolts/meter may be used. In all cases, operation may not cause harmful interference to licensed services. Many experimenters in this band are amateur radio operators. A regular service transmitting RTTY marine meteorological information in SYNOP code on LF is the German Meteorological Service; the DWD operates station DDH47 on 147.3 kHz using standard ITA-2 alphabet with a transmission speed of 50 baud and FSK modulation with 85 Hz shift. In parts of the world where there is no longwave broadcasting service, Non-directional beacons used for aeronavigation operate on 190–300 kHz.
In Europe and Africa, the NDB allocation starts on 283.5 kHz. The LORAN-C radio navigation system operated on 100 kHz. In the past, the Decca Navigator System operated betw