1.
Telecommunication
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Telecommunication is the transmission of signs, signals, messages, writings, images and sounds or intelligence of any nature by wire, radio, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology and it is transmitted either electrically over physical media, such as cables, or via electromagnetic radiation. Such transmission paths are divided into communication channels which afford the advantages of multiplexing. The term is used in its plural form, telecommunications. Early means of communicating over a distance included visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, other examples of pre-modern long-distance communication included audio messages such as coded drumbeats, lung-blown horns, and loud whistles. Zworykin, John Logie Baird and Philo Farnsworth, the word telecommunication is a compound of the Greek prefix tele, meaning distant, far off, or afar, and the Latin communicare, meaning to share. Its modern use is adapted from the French, because its use was recorded in 1904 by the French engineer. Communication was first used as an English word in the late 14th century, in the Middle Ages, chains of beacons were commonly used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could pass a single bit of information. One notable instance of their use was during the Spanish Armada, in 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system between Lille and Paris. However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres, as a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880. Homing pigeons have occasionally used throughout history by different cultures. Pigeon post is thought to have Persians roots and was used by the Romans to aid their military, frontinus said that Julius Caesar used pigeons as messengers in his conquest of Gaul. The Greeks also conveyed the names of the victors at the Olympic Games to various cities using homing pigeons, in the early 19th century, the Dutch government used the system in Java and Sumatra. And in 1849, Paul Julius Reuter started a service to fly stock prices between Aachen and Brussels, a service that operated for a year until the gap in the telegraph link was closed. Sir Charles Wheatstone and Sir William Fothergill Cooke invented the telegraph in 1837. Also, the first commercial electrical telegraph is purported to have constructed by Wheatstone and Cooke. Both inventors viewed their device as an improvement to the electromagnetic telegraph not as a new device, samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837
2.
Antenna array
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An antenna array is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves. The individual antenna elements are connected to a receiver or transmitter by feedlines that feed the power to the elements in a specific phase relationship. An antenna array can achieve higher gain, that is a beam of radio waves. In general, the larger the number of antenna elements used, the higher the gain. Some antenna arrays are composed of thousands of individual antennas, the term antenna array most commonly means a driven array consisting of multiple identical driven elements all connected to the receiver or transmitter, often half-wave dipoles fed in phase. A parasitic array consists of a driven element connected to the feedline. It is usually another name for a Yagi-Uda antenna, the beam of radio waves can be steered electronically to point instantly in any direction over a wide angle. However the term phased array is used to mean an ordinary array antenna. To create a high gain antenna, which radio waves in a narrow beam. A second technique is to use multiple antennas which are fed from the transmitter or receiver. In directions in which the waves from the individual antennas arrive in phase, in directions in which the individual waves arrive out of phase, with the peak of one wave coinciding with the valley of another, the waves cancel reducing the power radiated in that direction. Arrays in which the elements are fed in phase are broadside arrays. Using the technique called aperture synthesis such an array can have the resolution of an antenna with a equal to the distance between the antennas. In the technique called Very Long Baseline Interferometry dishes on separate continents have been linked, Array antennas can be divided into two classes based on how the component antennas axis is related to the direction of radiation. A broadside array is a one or two dimensional array in which the direction of radiation is perpendicular to the plane of the antennas, an endfire array is a linear array in which the direction of radiation is along the line of antennas. Driven arrays - These are arrays in which the component antennas are all driven - connected to the transmitter or receiver. Collinear array - a broadside array consisting of multiple identical dipole antennas oriented vertically in a line and this is a high gain omnidirectional antenna, often used in the VHF band as base station antennas for land mobile services. Superturnstile or Batwing array - specialized vertical antenna used for television broadcasting consisting of multiple crossed-dipole antennas mounted collinearly on a mast, high gain omnidirectional radiation pattern with wide bandwidth
3.
Dipole antenna
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In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna. A dipole antenna consists of two identical conductive elements such as metal wires or rods, which are usually bilaterally symmetrical. The driving current from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna, each side of the feedline to the transmitter or receiver is connected to one of the conductors. This contrasts with an antenna, which consists of a single rod or conductor with one side of the feedline connected to it. A common example of a dipole is the rabbit ears television antenna found on broadcast television sets and this is the simplest type of antenna from a theoretical point of view. Dipoles are resonant antennas, meaning that the serve as resonators, with standing waves of radio current flowing back. So the length of the elements is determined by the wavelength of the radio waves used. The most common form is the dipole, in which each of the two rod elements is approximately 1/4 wavelength long, so the whole antenna is a half-wavelength long. The radiation pattern of a dipole is omnidirectional, it radiates equal power in all azimuthal directions perpendicular to the axis of the antenna. For a half-wave dipole the radiation is maximum,2.15 dBi perpendicular to the axis, falling monotonically with elevation angle to zero on the axis. Several different variations of the dipole are also used, such as the dipole, short dipole, cage dipole, bow-tie. The dipole was the earliest type of antenna, it was invented by German physicist Heinrich Hertz around 1886 in his investigations of radio waves. The feedpoint impedance of an antenna is very sensitive to its electrical length. Therefore, a dipole will generally only perform optimally over a narrow bandwidth. The real and imaginary components of impedance, as a function of electrical length, are shown in the accompanying graph. The detailed calculation of these numbers are described below, note that the value of the reactance is highly dependent on the diameter of the conductors, this plot is for conductors with a diameter of.001 wavelengths. Dipoles that are smaller than the wavelength of the signal are called short dipoles. These have a low radiation resistance making them inefficient antennas
4.
Antenna (radio)
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In radio and electronics, an antenna, or aerial, is an electrical device which converts electric power into radio waves, and vice versa. It is usually used with a transmitter or radio receiver. In reception, an antenna intercepts some of the power of a wave in order to produce a tiny voltage at its terminals. Antennas are essential components of all equipment that uses radio, typically an antenna consists of an arrangement of metallic conductors, electrically connected to the receiver or transmitter. These time-varying fields radiate away from the antenna into space as a transverse electromagnetic field wave. Antennas can be designed to transmit and receive radio waves in all directions equally. The first antennas were built in 1888 by German physicist Heinrich Hertz in his experiments to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. Hertz placed dipole antennas at the point of parabolic reflectors for both transmitting and receiving. He published his work in Annalen der Physik und Chemie, the words antenna and aerial are used interchangeably. Occasionally the term aerial is used to mean a wire antenna, however, note the important international technical journal, the IEEE Transactions on Antennas and Propagation. In the United Kingdom and other areas where British English is used, the origin of the word antenna relative to wireless apparatus is attributed to Italian radio pioneer Guglielmo Marconi. In the summer of 1895, Marconi began testing his wireless system outdoors on his fathers estate near Bologna, Marconi discovered that by raising the aerial wire above the ground and connecting the other side of his transmitter to ground, the transmission range was increased. Soon he was able to transmit signals over a hill, a distance of approximately 2.4 kilometres, in Italian a tent pole is known as lantenna centrale, and the pole with the wire was simply called lantenna. Until then wireless radiating transmitting and receiving elements were simply as aerials or terminals. Because of his prominence, Marconis use of the word spread among wireless researchers. In common usage, the antenna may refer broadly to an entire assembly including support structure, enclosure. Especially at microwave frequencies, an antenna may include not only the actual electrical antenna. An antenna, in converting radio waves to electrical signals or vice versa, is a form of transducer, Antennas are required by any radio receiver or transmitter to couple its electrical connection to the electromagnetic field
5.
Collinearity
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In geometry, collinearity of a set of points is the property of their lying on a single line. A set of points with this property is said to be collinear, in greater generality, the term has been used for aligned objects, that is, things being in a line or in a row. In any geometry, the set of points on a line are said to be collinear, in Euclidean geometry this relation is intuitively visualized by points lying in a row on a straight line. However, in most geometries a line is typically an object type. For instance, in geometry, where lines are represented in the standard model by great circles of a sphere. Such points do not lie on a line in the Euclidean sense. A mapping of a geometry to itself which sends lines to lines is called a collineation, the linear maps of vector spaces, viewed as geometric maps, map lines to lines, that is, they map collinear point sets to collinear point sets and so, are collineations. In projective geometry these linear mappings are called homographies and are just one type of collineation, the de Longchamps point also has other collinearities. Any vertex, the tangency of the side with an excircle. Any vertex, the tangency of the side with the incircle. From any point on the circumcircle of a triangle, the nearest points on each of the three extended sides of the triangle are collinear in the Simson line of the point on the circumcircle, the lines connecting the feet of the altitudes intersect the opposite sides at collinear points. A triangles incenter, the midpoint of an altitude, and the point of contact of the side with the excircle relative to that side are collinear. The incenter, the centroid, and the Spieker circles center are collinear, the circumcenter, the Brocard midpoint, and the Lemoine point of a triangle are collinear. Two perpendicular lines intersecting at the orthocenter of a triangle intersect each of the triangles extended sides. The midpoints on the three sides of these points of intersection are collinear in the Droz–Farny line. In a convex quadrilateral ABCD whose opposite sides intersect at E and F, the midpoints of AC, BD, and EF are collinear, if the quadrilateral is a tangential quadrilateral, then its incenter also lies on this line. In a convex quadrilateral, the quasiorthocenter H, the area centroid G, and the quasicircumcenter O are collinear in this order, other collinearities of a tangential quadrilateral are given in Tangential quadrilateral#Collinear points. In a cyclic quadrilateral, the circumcenter, the centroid
6.
Omnidirectional antenna
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This radiation pattern is often described as doughnut shaped. Note that this is different from an antenna, which radiates equal power in all directions and has a spherical radiation pattern. Higher-gain omnidirectional antennas can also be built, higher gain in this case means that the antenna radiates less energy at higher and lower elevation angles and more in the horizontal directions. High-gain omnidirectional antennas are generally realized using collinear dipole arrays and these consist of multiple half-wave dipoles mounted collinearly, fed in phase. The coaxial collinear antenna uses transposed coaxial sections to produce in-phase half-wavelength radiators, a Franklin Array uses short U-shaped half-wavelength sections whose radiation cancels in the far-field to bring each half-wavelength dipole section into equal phase. Another type is the Omnidirectional Microstrip Antenna, Omnidirectional radiation patterns are produced by the simplest practical antennas, monopole and dipole antennas, consisting of one or two straight rod conductors on a common axis. Antenna gain is defined as antenna efficiency multiplied by antenna directivity which is expressed mathematically as, G = e D. A useful relationship between omnidirectional radiation pattern directivity in decibels and half-power beamwidth based on the assumption of a sin b θ / b θ pattern shape is, D =10 log 10 d B
7.
Radiation pattern
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In the field of antenna design the term radiation pattern refers to the directional dependence of the strength of the radio waves from the antenna or other source. Particularly in the fields of optics, lasers, and integrated optics. This refers to the dependence of the electromagnetic field in the near-field. The near-field pattern is most commonly defined over a plane placed in front of the source, the far-field radiation pattern can also be calculated from the antenna shape by computer programs such as NEC. Other software, like HFSS can also compute the near field, very often, only the relative amplitude is plotted, normalized either to the amplitude on the antenna boresight, or to the total radiated power. The plotted quantity may be shown on a scale, or in dB. The plot is typically represented as a graph, or as separate graphs in the vertical plane. This is often known as a polar diagram and it is a fundamental property of antennas that the receiving pattern of an antenna when used for receiving is identical to the far-field radiation pattern of the antenna when used for transmitting. This is a consequence of the reciprocity theorem of electro-magnetics and is proved below, therefore, in discussions of radiation patterns the antenna can be viewed as either transmitting or receiving, whichever is more convenient. The simplest antennas, monopole and dipole antennas, consist of one or two metal rods along a common axis. This illustrates the principle that if the shape of an antenna is symmetrical. In most antennas, the radiation from the different parts of the antenna interferes at some angles, therefore, the radiation plot of most antennas shows a pattern of maxima called lobes at various angles, separated by nulls at which the radiation goes to zero. The larger the antenna is compared to a wavelength, the more lobes there will be. In a directive antenna in which the objective is to direct the waves in one particular direction, the lobe in that direction is larger than the others. The axis of maximum radiation, passing through the center of the lobe, is called the beam axis or boresight axis. In some antennas, such as antennas, there may exist more than one major lobe. A minor lobe is any lobe except a major lobe, the other lobes, representing unwanted radiation in other directions, are called side lobes. The side lobe in the direction from the main lobe is called the back lobe
8.
Azimuth
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An azimuth is an angular measurement in a spherical coordinate system. An example is the position of a star in the sky, the star is the point of interest, the reference plane is the horizon or the surface of the sea, and the reference vector points north. The azimuth is the angle between the vector and the perpendicular projection of the star down onto the horizon. Azimuth is usually measured in degrees, the concept is used in navigation, astronomy, engineering, mapping, mining and artillery. In land navigation, azimuth is usually denoted alpha, α, azimuth has also been more generally defined as a horizontal angle measured clockwise from any fixed reference plane or easily established base direction line. Today, the plane for an azimuth is typically true north, measured as a 0° azimuth. Moving clockwise on a 360 degree circle, east has azimuth 90°, south 180°, there are exceptions, some navigation systems use south as the reference vector. Any direction can be the vector, as long as it is clearly defined. Quite commonly, azimuths or compass bearings are stated in a system in which either north or south can be the zero, the reference direction, stated first, is always north or south, and the turning direction, stated last, is east or west. The directions are chosen so that the angle, stated between them, is positive, between zero and 90 degrees. If the bearing happens to be exactly in the direction of one of the cardinal points and this is the reason why the X and Y axis in the above formula are swapped. If the azimuth becomes negative, one can always add 360°, the formula in radians would be slightly easier, α = atan2 Caveat, Most computer libraries reverse the order of the atan2 parameters. We are standing at latitude φ1, longitude zero, we want to find the azimuth from our viewpoint to Point 2 at latitude φ2, longitude L. The difference is usually small, if Point 2 is not more than 100 km away. Various websites will calculate geodetic azimuth, e. g. GeoScience Australia site, formulas for calculating geodetic azimuth are linked in the distance article. Normal-section azimuth is simpler to calculate, Bomford says Cunninghams formula is exact for any distance, replace φ2 with declination and longitude difference with hour angle, and change the sign. There is a variety of azimuthal map projections. They all have the property that directions from a point are preserved
9.
Polarization (waves)
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Polarization is a parameter applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave the direction of the oscillation is transverse to the direction of motion of the wave, a simple example of a polarized transverse wave is vibrations traveling along a taut string for example in a musical instrument like a guitar string. Depending on how the string is plucked, the vibrations can be in a direction, horizontal direction. Transverse waves that exhibit polarization include electromagnetic waves such as light and radio waves, gravitational waves, in some types of transverse waves the wave displacement is limited to a single direction, so these also dont exhibit polarization. For example, in waves in liquids the wave displacement of the particles is always in a vertical plane. In linear polarization the fields oscillate in a single direction, in circular or elliptical polarization the fields rotate at a constant rate in a plane as the wave travels. Polarized light can be produced by passing unpolarized light through a polarizing filter, some of these are used to make polarizing filters. Light is also partially polarized when it reflects from a surface, according to quantum mechanics, electromagnetic waves consist of particles called photons. When viewed in this way, the polarization of a wave is determined by a quantum mechanical property of photons called their spin. A photon has one of two spins, it can either spin in a right hand sense or a left hand sense about its direction of travel. Circularly polarized electromagnetic waves are composed of photons with one type of spin. Linearly polarized waves consist of numbers of right and left hand spinning photons. Polarization is an important parameter in areas of science dealing with transverse waves, such as optics, seismology, radio, especially impacted are technologies such as lasers, wireless and optical fiber telecommunications, and radar. And incoherent states can be modeled stochastically as a combination of such uncorrelated waves with some distribution of frequencies, phases. Considering a monochromatic wave of optical frequency f, let us take the direction of propagation as the z axis. Being a transverse wave the E and H fields must then contain components only in the x and y directions whereas Ez=Hz=0. Using complex notation, we understand the instantaneous electric and magnetic fields to be given by the real parts of the complex quantities occurring in the following equations. Here ex, ey, hx, and hy are complex numbers, in the second more compact form, as these equations are customarily expressed, these factors are described using the wavenumber k =2 π n / λ and angular frequency ω =2 π f
10.
Decibel
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The decibel is a logarithmic unit used to express the ratio of two values of a physical quantity. One of these values is often a reference value, in which case the decibel is used to express the level of the other value relative to this reference. When used in way, the decibel symbol is often qualified with a suffix that indicates the reference quantity that has been used or some other property of the quantity being measured. For example, dBm indicates a power of one milliwatt. There are two different scales used when expressing a ratio in decibels depending on the nature of the quantities, when expressing power quantities, the number of decibels is ten times the logarithm to base 10 of the ratio of two power quantities. That is, a change in power by a factor of 10 corresponds to a 10 dB change in level, when expressing field quantities, a change in amplitude by a factor of 10 corresponds to a 20 dB change in level. The difference in scales relates to the square law of fields in three-dimensional linear space. The decibel scales differ so that comparisons can be made between related power and field quantities when they are expressed in decibels. The definition of the decibel is based on the measurement of power in telephony of the early 20th century in the Bell System in the United States. One decibel is one tenth of one bel, named in honor of Alexander Graham Bell, however, today, the decibel is used for a wide variety of measurements in science and engineering, most prominently in acoustics, electronics, and control theory. In electronics, the gains of amplifiers, attenuation of signals, the decibel originates from methods used to quantify signal loss in telegraph and telephone circuits. The unit for loss was originally Miles of Standard Cable, the standard telephone cable implied was a cable having uniformly distributed resistance of 88 ohms per loop mile and uniformly distributed shunt capacitance of 0.054 microfarad per mile. 1 TU was defined such that the number of TUs was ten times the logarithm of the ratio of measured power to a reference power level. The definition was conveniently chosen such that 1 TU approximated 1 MSC, in 1928, the Bell system renamed the TU into the decibel, being one tenth of a newly defined unit for the base-10 logarithm of the power ratio. It was named the bel, in honor of the telecommunications pioneer Alexander Graham Bell, the bel is seldom used, as the decibel was the proposed working unit. However, the decibel is recognized by international bodies such as the International Electrotechnical Commission. The term field quantity is deprecated by ISO 80000-1, which favors root-power, in spite of their widespread use, suffixes are not recognized by the IEC or ISO. The ISO Standard 80000-3,2006 defines the following quantities, the decibel is one-tenth of a bel,1 dB =0.1 B
11.
Two-way radio
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A two-way radio is a radio that can do both transmit and receive a signal, unlike a broadcast receiver which only receives content. A two-way radio allows the operator to have a conversation with other similar radios operating on the radio frequency. Two-way radios are available in mobile, stationary base and hand-held portable configurations, hand-held radios are often called walkie-talkies, handie-talkies, or just hand-helds. Two-way radio systems operate in a half-duplex mode, that is, the operator can talk, or he can listen. A push-to-talk or Press To Transmit button activates the transmitter, when it is released the receiver is active, a mobile phone or cellular telephone is an example of a two-way radio that can transmit and receive at the same time, i. e. in full-duplex mode. Full-duplex is generally achieved by the use of two different frequencies or by frequency-sharing methods to carry the two directions of the conversation simultaneously, methods for mitigating the self interference caused by simultaneous same-frequency transmission and reception include using two antennas, or dynamic solid-state filters. Installation of receivers and transmitters at the fixed location allowed exchange of messages wirelessly. As early as 1907, two-way telegraphy traffic across the Atlantic Ocean was commercially available, by 1912 commercial and military ships carried both transmitters and receivers, allowing two-way communication in close to real-time with a ship that was out of sight of land. The first truly mobile two-way radio was developed in Australia in 1923 by Senior Constable Frederick William Downie of the Victorian Police. The Victoria Police were the first in the world to use communication in cars. The first sets took up the back seat of the Lancia patrol cars. As radio equipment became more powerful, compact, and easier to use, during World War II walkie-talkie hand-held radio transceivers were extensively used by air and ground troops, both by the Allies and the Axis. Early two-way schemes allowed only one station to transmit at a time while others listened, the first radio systems could not transmit voice. This required training of operators in use of Morse code, on a ship, the radio operating officers typically had no other duties than handling radio messages. When voice transmission possible, dedicated operators were no longer required. Two-way radio systems can be classified in several ways depending on their attributes, conventional radios operate on fixed RF channels. In the case of radios with multiple channels, they operate on one channel at a time, the proper channel is selected by a user. The user operates a channel selector on the control panel to pick the appropriate channel
12.
Radio broadcasting
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Radio broadcasting is a unidirectional wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a radio format. Audio broadcasting also can be done via radio, local wire television networks, satellite radio. The signal types can be either analog audio or digital audio, the earliest radio stations were simply radiotelegraphy systems and did not carry audio. For audio broadcasts to be possible, electronic detection and amplification devices had to be incorporated, the thermionic valve was invented in 1904 by the English physicist John Ambrose Fleming. He developed a device he called an oscillation valve, the heated filament, or cathode, was capable of thermionic emission of electrons that would flow to the plate when it was at a higher voltage. Electrons, however, could not pass in the direction because the plate was not heated. Later known as the Fleming valve, it could be used as a rectifier of alternating current and this greatly improved the crystal set which rectified the radio signal using an early solid-state diode based on a crystal and a so-called cats whisker. However, what was required was an amplifier. The triode was patented on March 4,1906, by the Austrian Robert von Lieben independent from that, on October 25,1906 and it wasnt put to practical use until 1912 when its amplifying ability became recognized by researchers. By about 1920, valve technology had matured to the point where radio broadcasting was quickly becoming viable, however, an early audio transmission that could be termed a broadcast may have occurred on Christmas Eve in 1906 by Reginald Fessenden, although this is disputed. Charles Herrold started broadcasting in California in 1909 and was carrying audio by the next year, in The Hague, the Netherlands, PCGG started broadcasting on November 6,1919, making it, arguably the first commercial broadcasting station. In 1916, Frank Conrad, an engineer employed at the Westinghouse Electric Corporation, began broadcasting from his Wilkinsburg. Later, the station was moved to the top of the Westinghouse factory building in East Pittsburgh, Westinghouse relaunched the station as KDKA on November 2,1920, as the first commercially licensed radio station in America. The commercial broadcasting designation came from the type of broadcast license, the first licensed broadcast in the United States came from KDKA itself, the results of the Harding/Cox Presidential Election. In 1920, wireless broadcasts for entertainment began in the UK from the Marconi Research Centre 2MT at Writtle near Chelmsford, England. A famous broadcast from Marconis New Street Works factory in Chelmsford was made by the famous soprano Dame Nellie Melba on 15 June 1920 and she was the first artist of international renown to participate in direct radio broadcasts. The 2MT station began to broadcast regular entertainment in 1922, the BBC was amalgamated in 1922 and received a Royal Charter in 1926, making it the first national broadcaster in the world, followed by Czech Radio and other European broadcasters in 1923
13.
General Services Administration
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The General Services Administration is an independent agency of the United States government, established in 1949 to help manage and support the basic functioning of federal agencies. GSA employs about 12,000 federal workers and has an operating budget of roughly $26.3 billion. GSA oversees $66 billion of procurement annually and it contributes to the management of about $500 billion in U. S. federal property, divided chiefly among 8,300 owned and leased buildings and a 210,000 vehicle motor pool. GSAs business lines include the Federal Acquisition Service, the Public Buildings Service, the GSA is member of the Procurement G6, an informal group leading the use of framework agreements and e-procurement instruments in public procurement. In 1947 President Harry Truman asked former President Herbert Hoover to lead what became known as the Hoover Commission to make recommendations to reorganize the operations of the federal government, one of the recommendations of the commission was the establishment of an Office of the General Services. This proposed office would combine the responsibilities of the following organizations, General Jess Larson, Administrator of the War Assets Administration, was named GSAs first Administrator. The first job awaiting Administrator Larson and the newly formed GSA was a renovation of the White House. The structure had fallen into such a state of disrepair by 1949 that one inspector of the time said the structure was standing “purely from habit. Everything, except the four walls without a roof, was stripped down. GSA completed the renovation in 1952, GSA headquarters, located at Eighteenth and F Streets, NW, was U. S. General Services Administration Building listed on the National Register of Historic Places in 1986 as Interior Department Offices, in 1960, GSA created the Federal Telecommunications System, a government-wide intercity telephone system. In 1970, the Nixon administration created the Consumer Product Information Coordinating Center, the Federal Buildings Fund was initiated in 1974, which allowed the GSA to issue rent bills to federal agencies. In 1972, the GSA established the Automated Data and Telecommunications Service, in 1973, the GSA created the Office of Federal Management Policy. GSA’s Office of Acquisition Policy centralized procurement policy in 1978, in 1984, GSA introduced the federal government to the use of charge cards, known as the GMA SmartPay system. The National Archives and Records Administration was spun off into an independent agency in 1985, the same year, the GSA began to provide governmentwide policy oversight and guidance for federal real property management as a result of an Executive Order signed by President Ronald Reagan. In 2003, the Federal Protective Service was moved to the Department of Homeland Security, in 2005, GSA reorganized to merge the Federal Supply Service and Federal Technology Service business lines into the Federal Acquisition Service. On April 3,2009, President Barack Obama nominated Martha N. Johnson to serve as the GSA Administrator, after a 9-month delay, the United States Senate confirmed her nomination on February 4,2010. In July 1991, GSA contractors began the excavation of what is now the Ted Weiss Federal Building in New York City, when initial excavation disturbed burials, destroying skeletons and artifacts, GSA sent archaeologists to excavate—but hid their findings from the public
14.
Isotropy
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Isotropy is uniformity in all orientations, it is derived from the Greek isos and tropos. Precise definitions depend on the subject area, exceptions, or inequalities, are frequently indicated by the prefix an, hence anisotropy. Anisotropy is also used to describe situations where properties vary systematically, Isotropic radiation has the same intensity regardless of the direction of measurement, and an isotropic field exerts the same action regardless of how the test particle is oriented. Within mathematics, isotropy has a few different meanings, Isotropic manifolds A manifold is isotropic if the geometry on the manifold is the same regardless of direction, a manifold can be homogeneous without being isotropic, but if it is inhomogeneous, it is necessarily anisotropic. Isotropic quadratic form A quadratic form q is said to be if there is a non-zero vector v such that q =0. In complex geometry, a line through the origin in the direction of a vector is an isotropic line. Isotropic coordinates Isotropic coordinates are coordinates on a chart for Lorentzian manifolds. Isotropy group An isotropy group is the group of isomorphisms from any object to itself in a groupoid, Isotropic position A probability distribution over a vector space is in isotropic position if its covariance matrix is the identity. This follows from invariance of the Hamiltonian, which in turn is guaranteed for a spherically symmetric potential. Kinetic theory of gases is also an example of isotropy and it is assumed that the molecules move in random directions and as a consequence, there is an equal probability of a molecule moving in any direction. Thus when there are molecules in the gas, with high probability there will be very similar numbers moving in one direction as any other hence demonstrating approximate isotropy. Fluid dynamics Fluid flow is isotropic if there is no directional preference, an example of anisotropy is in flows with a background density as gravity works in only one direction. The apparent surface separating two differing isotropic fluids would be referred to as an isotrope, thermal expansion A solid is said to be isotropic if the expansion of solid is equal in all directions when thermal energy is provided to the solid. Electromagnetics An isotropic medium is one such that the permittivity, ε, and permeability, μ, of the medium are uniform in all directions of the medium, optics Optical isotropy means having the same optical properties in all directions. The individual reflectance or transmittance of the domains is averaged if the macroscopic reflectance or transmittance is to be calculated, cosmology The Big Bang theory of the evolution of the observable universe assumes that space is isotropic. It also assumes that space is homogeneous and these two assumptions together are known as the Cosmological Principle. As of 2006, the observations suggest that, on scales much larger than galaxies, galaxy clusters are Great features. Here homogeneous means that the universe is the same everywhere and isotropic implies that there is no preferred direction, in the study of mechanical properties of materials, isotropic means having identical values of a property in all directions
15.
Isotropic radiator
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An isotropic radiator is a theoretical point source of electromagnetic or sound waves which radiates the same intensity of radiation in all directions. It has no preferred direction of radiation and it radiates uniformly in all directions over a sphere centred on the source. Isotropic radiators are used as radiators with which other sources are compared. A coherent isotropic radiator of electromagnetic waves is theoretically impossible, an isotropic sound radiator is possible because sound is a longitudinal wave. Whether a radiator is isotropic is independent of whether it obeys Lamberts law, in physics, an isotropic radiator is a point radiation or sound source. At a distance, the sun is a radiator of electromagnetic radiation. The Big Bang is another example of an isotropic radiator - the Cosmic Microwave Background, in antenna theory, an isotropic antenna is a hypothetical antenna radiating the same intensity of radio waves in all directions. It thus is said to have a directivity of 0 dBi in all directions, in reality, a coherent isotropic radiator of linear polarization can be shown to be impossible. Its radiation field could not be consistent with the Helmholtz wave equation in all directions simultaneously, consider a large sphere surrounding the hypothetical point source, so that at that radius the wave over a reasonable area is essentially planar. The electric field of a wave in free space is always perpendicular to the direction of propagation of the wave. So the electric field would have to be tangent to the surface of the sphere everywhere, incoherent isotropic radiators are possible and do not violate Maxwells equations. Acoustic isotropic radiators are possible because sound waves in a gas or liquid are longitudinal waves, even though an isotropic antenna cannot exist in practice, it is used as a base of comparison to calculate the directivity of actual antennas. This is called isotropic gain, G = I I iso Thus the gain of any perfectly efficient antenna averaged over all directions is unity, the parameter used to define accuracy in the measurements is called isotropic deviation. Imagine two cavities in thermal equilibrium, a lossless antenna in one cavity is connected to a matched impedance inside the second cavity. Being isotropic, Ae is constant in direction, thus A e k T λ2 ∫4 π d Ω = k T. In optics, a radiator is a point source of light. The sun approximates an isotropic radiator of light, certain munitions such as flares and chaff have isotropic radiator properties. An isotropic radiator is a perfect speaker exhibiting equal sound volume in all directions
16.
Batwing antenna
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A batwing or super turnstile antenna is a type of broadcasting antenna used at VHF and UHF frequencies, named for its distinctive shape which resembles a bat wing or bow tie. Stacked arrays of batwing antennas are used as television broadcasting antennas due to their omnidirectional characteristics, batwing antennas generate a horizontally polarized signal. The advantage of the design for television broadcasting is that it has a wide bandwidth. It was the first widely used television broadcasting antenna, batwing antennas are a specialized type of crossed dipole antenna, a variant of the turnstile antenna. Two pairs of identical vertical batwing-shaped elements are mounted at right angles around a common mast, element “wings” on opposite sides are fed as a dipole. To generate a pattern, the two dipoles are fed 90° out of phase. The antenna radiates horizontally polarized radiation in the horizontal plane, each group of four elements at a single level is referred to as a bay. The radiation pattern is close to omnidirectional but has four lobes in the directions of the four elements. This generates an omnidirectional radiation pattern with increased horizontal gain, suitable for terrestrial broadcasting, the batwing shape is used because of its wide bandwidth of approximately 20% of operating frequency at a VSWR of 1.1,1. Lee Antenna Handbook Vol III, Antenna Applications, ISBN V100442015941 / ISDN V13 978-0442015947 Markley, Don
17.
Biconical antenna
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In radio systems, a biconical antenna is a broad-bandwidth antenna made of two roughly conical conductive objects, nearly touching at their points. Biconical antennas are broadband dipole antennas, typically exhibiting a bandwidth of three octaves or more, a common subtype is the bowtie antenna, essentially a two-dimensional version of the biconial design which is often used for short-range UHF television reception. These are also referred to as butterfly antennas. The biconical antenna has a bandwidth because it is an example of a travelling wave structure. For an infinite antenna, the impedance at the point of connection is a function of the cone angle only and is independent of the frequency. Practical antennas have finite length and a resonant frequency. A simple conical monopole antenna is an approximation of the solid biconical antenna and has increased bandwidth. Biconical antennas are used in electromagnetic interference testing either for immunity testing. While the bicon is very broadband, it exhibits poor efficiency at low frequencies, log periodic dipole arrays, Yagi-Uda antennas, and reverberation chambers have shown to achieve much higher field strengths for the power input than a simple biconical antenna in an anechoic chamber
18.
Choke ring antenna
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A choke ring antenna is a particular form of omnidirectional antenna for use at high frequencies. It consists of a number of concentric cylinders around a central antenna. Due to its construction, it is often enclosed in a protective cover or radome when placed outside. Choke ring antennas are notable for their ability to reject multipath signals fading from a source, since the path that a signal takes from a transmitter to receiver can be used to measure the distance between the two, this makes it highly suited for GPS and radar applications. In a GPS ground based receiver, a choke ring antenna can provide millimeter precision measurements for surveying, the choke ring design originated at the Jet Propulsion Laboratory. Trimble Navigation - one manufacturer of this product, patch antenna - another antenna for GPS applications. Magellan Professional Products - another manufacturer of this product, http, //www. javad. com/jns/technology/Choke%20Ring%20Theory. html - Basic choke Ring theory of operation
19.
Crossed field antenna
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A crossed field antenna, or CFA, is a controversial type of radio antenna for long and mediumwave broadcasting, patented by F. M. Kabbary and M. C. Hately in 1986, which was claimed to have the same efficiency as a conventional antenna, although the antenna was installed in a few broadcasting stations in the 1990s, performance has not borne out the claims of the inventors. As with other low frequency antennas, the crossed field antenna is installed above a ground plane which may be the Earth, the antennas operation is described in its inventors literature. In general, peer-reviewed journals have not accepted papers on CFAs, the data presented in figure 31 of the Trainotti report shows measured field values up to 15 dB lower than from a theoretical 100% efficient monopole. There are a handful of CFAs operating in Egypt, at powers ranging from 1 kW to 100 kW and these have been operational for over ten years and were developed by the engineering sector of the ERTU the Egyptian state broadcaster for their own use. Many CFA projects in other countries failed including those in Australia, Brazil, China, Germany, Italy, Isle of Man, the last commercial CFA was installed in 2003. Results of the test are unavailable
20.
Discone antenna
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A discone antenna is a version of a biconical antenna in which one of the cones is replaced by a disc. It is usually mounted vertically, with the disc at the top, omnidirectional, vertically polarized and with gain similar to a dipole, it is exceptionally wideband, offering a frequency range ratio of up to approximately 10,1. The radiation pattern in the plane is quite narrow, making its sensitivity highest in the direction of the horizon. The discone antenna has a frequency range of at least 10 to 1. When employed as an antenna, it is often less efficient than an antenna designed for a more limited frequency range. SWR is typically 1.5,1 or less over several octaves of frequency, a discone antenna consists of three main parts, the disc, the cone, and the insulator. The disc, The disc should have a diameter of 0.7 times a quarter wavelength of the antennas lowest frequency. The antennas feed point is at the center of the disc and it is usually fed with 50-ohm coaxial cable, with the center conductor connected to the disc, and the outer conductor to the cone. The cone, The length of the cone should be a quarter wavelength of the antennas lowest operating frequency, the cone angle is generally from 25 to 40 degrees. The insulator, The disc and cone must be separated by an insulator, a discone may be made from solid metal sheet, which is practical for small indoor UHF antennas, such as for Wi-Fi. At lower frequencies a sufficient number of wires or rods in a spoke configuration is often used to approximate a solid surface. This simplifies construction and reduces wind loading, the spokes may be made of stiff wire, brazing rods or even coat hanger wire. The optimal number of rods comprising the disc and cone is often quoted as being from 8 to 16, the discones wideband coverage makes it attractive in commercial, military, amateur radio and radio scanner applications. The discones inherently wideband nature permits it to broadcast undesirable spurious emissions from faulty or improperly filtered transmitters, a vertical whip may be affixed to the center of the disc in order to extend the low frequency response, but this may compromise efficiency at higher frequencies. In this configuration, at lower frequencies the discone may more resemble a ground plane antenna or a coaxial dipole
21.
Folded unipole antenna
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The folded unipole antenna was first devised for broadcast use by John H. Mullaney, an American radio broadcast pioneer, and consulting engineer. A variation of an antenna, it consists of a vertical metal rod or mast mounted over a conductive surface called a ground plane. The mast is surrounded by a skirt of vertical wires electrically attached to the top of the mast, the wires are connected by a metal ring at the bottom and the feed line is connected between the bottom of the wires and ground. A typical AM broadcast antenna is a monopole antenna above a ground system. The ground system normally comprises 120 buried copper or phosphor bronze radial wires at least one-quarter wavelength long, all the ground system components are bonded together, usually by brazing or using coin silver solder to minimize corrosion. If required, insulated guy wires are used, radio frequency power is fed across the base insulator between the ground system and the tower itself. P = I2 R Electrically short monopole antennas have low resistance, longer antennas may have impedances that are more advantageous unless the electrical height exceeds about 5/8 of a wavelength. In any case, a network at the base of the tower matches the antenna to its transmission line. If the tower is short, it will have its capacitive reactance tuned out by this matching network. This network and tower combination often results in a narrow bandwidth, Electrically short antenna systems have relatively small apertures and high losses. Some claims have made that ground system losses are reduced for the folded unipole antenna. However the experimental work of Ronald Rackley, et al. have shown this to be incorrect, such claims do not consider that for the same conditions, the total displacement current in the ground system is nearly the same for a folded unipole as for a conventional series-fed unipole. Therefore ground system losses for these two configurations are very similar. Using a folded unipole will increase the bandwidth of electrically short antennas because its feed-point comprises less reactance that needs to be tuned out, one usually constructs the folded-unipole from an existing monopole antenna by shorting out the base insulator, thus connecting the tower directly to the ground system. The resulting skirt connects to the top and remains insulated throughout its entire length. The wires of the skirt connect at the feed-point near the bottom, below is another folded unipole antenna constructed from an existing series fed monopole. This antenna has three folds comprising the skirt. A possible improvement over the basic folded-unipole antenna is the “self resonant” unipole antenna and this reactance cancels the reactive component of the antenna input impedance causing the input impedance to appear resistive at the feed point
22.
Whip antenna
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A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the receiver or transmitter. The antenna is designed to be flexible so that it does not easily break, whip antennas for portable radios are often made of a series of interlocking telescoping metal tubes, so they can be retracted when not in use. Longer ones, made for mounting on vehicles and structures, are made of a fiberglass rod around a wire core. The most common type is the whip, which is approximately one-quarter of a wavelength long. Whips are the most common type of antenna, and are used in the HF, VHF and UHF radio bands. Larger versions mounted on roofs and radio masts are used as base station antennas for police, fire, ambulance, taxi, whip antennas 1/4 wavelength long or less have a single main lobe, with field strength maximum in horizontal directions, falling monotonically to zero on the axis. This is because they transmit equally well in all horizontal directions, whip antennas are normally designed as resonant antennas, the rod acts as a resonator for radio waves, with standing waves of voltage and current reflected back and forth from its ends. Therefore, the length of the rod is determined by the wavelength of the radio waves used. The most common length is approximately one-quarter of the wavelength, called a quarter-wave whip. If mounted above a ground plane, a quarter-wave whip has a gain twice that of a half wave dipole, or 5.19 dBi. However without a plane the gain is reduced and the radiation resistance increased. Whips mounted on vehicles use the skin of the vehicle as a ground plane. In hand-held devices usually no explicit ground plane is provided, therefore, the radio itself, and possibly the users hand, serves as a rudimentary ground plane. Since these are no larger than the size of the antenna itself and this type of antenna is called a ground plane antenna. To reduce the length of an antenna to make it less cumbersome. This allows the antenna to be much shorter than the normal length of a quarter-wavelength. The coil is added at the base of the whip or occasionally in the middle, in the most widely used form, the rubber ducky antenna, the loading coil is integrated with the antenna itself by making the whip out of a narrow helix of springy wire
23.
Halo antenna
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A halo antenna, or halo, is a horizontally polarized, omnidirectional 1/2 wavelength dipole antenna. It is shaped like a loop with a break on the side of the loop directly opposite the feedpoint. The double loop system was the method described in the original patent. The effective difference is that the loop is balanced, while a gamma match is unbalanced. The halo antenna is distinct from the loop antenna, which is similar, but quite a bit smaller, and the full wave loop. When constructed correctly, the antenna will present a good match to 50-ohm coaxial cable with a low SWR, the antenna provides a low-angle omnidirectional pattern. The radiating element of the halo is grounded, which tends to reduce static buildup, on the higher VHF bands and above, the physical diameter of a halo is small enough to be effectively used as a mobile antenna. Halos may be stacked, narrowing the vertical pattern. Halos may be made from a pipe with an embedded small wire conducting loop. The interior loop connects to the feedline, and that provides a means of tuning, due to their narrow pattern, halo antennas perform poorly for NVIS and skip propagation. Halos have very little vertical polarization components, and will perform poorly for vertically polarized signals, US2324462, High Frequency Antenna System, issued July 14,1943 www. kr1st. com/2mhalo Construct a halo antenna for two meters. Fedler. com/radio/2m_loop Another two meter vhf loop design
24.
Helical antenna
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A helical antenna is an antenna consisting of a conducting wire wound in the form of a helix. In most cases, helical antennas are mounted over a ground plane, the feed line is connected between the bottom of the helix and the ground plane. Helical antennas can operate in one of two principal modes — normal mode or axial mode, in the normal mode or broadside helical antenna, the dimensions of the helix are small compared with the wavelength. The radiation is linearly polarised parallel to the helix axis and these are used for compact antennas for portable and mobile two-way radios, and for UHF television broadcasting antennas. In the axial mode or end-fire helical antenna, the dimensions of the helix are comparable to a wavelength, the antenna functions as a directional antenna radiating a beam off the ends of the helix, along the antennas axis. It radiates circularly polarised radio waves and these are used for satellite communication. If the circumference of the helix is significantly less than a wavelength and its pitch is less than a quarter wavelength. The antenna acts similar to an antenna, with an omnidirectional radiation pattern. However, because of the inductance added by the helical shape, as with other electrically short antennas, the gain, and thus the communication range, of the helix will be less than that of a full sized antenna. Their compact size makes helicals useful as antennas for mobile and portable communications equipment on the HF, VHF, and UHF bands. An effect of using a conductor rather than a straight one is that the matching impedance is changed from the nominal 50 ohms to between 25 and 35 ohms base impedance. Many examples of type have been used extensively for 27 MHz CB radio with a wide variety of designs originating in the US. Multi-frequency versions with plug-in taps have become the mainstay for multi-band Single-sideband modulation HF communications, most examples were wound with copper wire using a fiberglass rod as a former. This flexible radiator is then covered with heat-shrink tubing which provides a resilient, the most common use for broadside helixes is in the rubber ducky antenna found on most portable VHF and UHF radios. Specialized normal-mode helical antennas are used for FM radio and television broadcasting on the VHF and UHF bands, when the helix circumference is near the wavelength of operation, the antenna operates in axial mode. This is a nonresonant traveling wave mode, in which instead of standing waves, instead of radiating linearly polarized waves normal to the antennas axis, it radiates a beam of radio waves with circular polarisation along the axis, off the ends of the antenna. The main lobes of the pattern are along the axis of the helix. Since in an antenna only radiation in one direction is wanted
25.
J-pole antenna
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The J-pole antenna, more properly known as the J antenna, was first invented by Hans Beggerow in 1909 for use in Zeppelin airships. Trailed behind the airship, it consisted of a single element and this concept evolved to the J configuration by 1936 attaining the name J Antenna by 1943. The J-pole antenna is an end-fed omnidirectional half-wave antenna that is matched to the feedline by a quarter wave transmission line stub of Lecher system form. Matching to the feed-line is achieved by sliding the connection of the feedline back, being a half-wave antenna, it provides a small gain over a quarter-wave ground-plane antenna. Primarily a dipole, the J-pole antenna exhibits a mostly circular pattern in the H plane with an average free-space gain near 2.2 dBi, at right angles to the J-stub, the gain is closer to the overall average, about 2.2 dBi. The slight increase over a dipoles 2.15 dBi gain represents the contribution to the pattern made by the current imbalance on the matching section. The pattern in the E plane reveals a slight elevation of the pattern in the direction of the J element while the pattern opposite the J element is mostly broadside. The net effect of the caused by quarter-wave stub is an H-plane approximate gain from 1.5 to 2.6 dBi. The quarter wave transmission line stub has an external electromagnetic field with strength. The J-Pole is very sensitive to support structures and will achieve best performance with no electrical bonding between antenna conductors and the mounting structure. Typical construction materials include metal tubing, ladder line, or twin-lead, the J-pole antenna and its variations may be fed with balanced line. A coax feed line may be used if it includes a means to suppress feed-line RF currents, a folded-balun, sleeve balun or common-mode choke will suppress feed-line RF currents. The feed-point of the J-pole is somewhere between the closed low-impedance bottom and open top of the J stub. Between these two extremes a match to any impedance between the low to high points is available. The J-pole design functions well when fed with a balanced feed, later research confirms the tendency of the mast or grounding wire to draw current from the antenna potentially spoiling the antenna pattern. A common approach extends the conductor below the bottom of the J-pole resulting in additional and this modifies the far field antenna pattern typically, but not always, raising the primary lobes above the horizon reducing antenna effectiveness for terrestrial service. J-pole antennas with electrical connection to their supports often fare no better, a mast decoupling stub reduces mast currents. A variation of the J-pole is the Slim Jim antenna, also known as 2BCX Slim Jim, the Slim Jim is one of many ways to form a J-Pole
26.
Mast radiator
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A mast radiator is a radio mast or tower in which the entire structure functions as an antenna. This design, developed around 1930, is used for transmitting antennas operating at low frequencies, in the VLF, LF and MF ranges. The metal mast is connected to the transmitter. Its base is mounted on a nonconductive support to insulate it from the ground. A mast radiator is a form of monopole antenna, most mast radiators are built as guyed masts insulated from the ground at the base. Steel lattice masts of triangular cross-section are the most common type, square lattice masts and tubular masts are also sometimes used. To ensure that the tower is a conductor, the towers structural sections are electrically bonded at the joints by short copper jumpers which are soldered to each side. The actual transmitter is located in a separate building, which supplies RF power to the helix building via a transmission line. Free-standing towers are used as radiating structures. These towers can have a triangular or a cross section. One of the best-known radiating towers is the Blosenbergturm in Beromünster, fiberglass masts are sometimes used for small constructions. The ideal height of a mast radiator depends on frequency, demographics for the location. When this is not possible, masts with a loading coil, the height of the mast determines the radiation properties. A focus of radiated power towards the ground is much desired on frequencies below 3 megahertz, masts longer than five eighths of the wavelength are normally not used, because they show bad vertical radiation patterns, so masts for mediumwave transmitters do not normally exceed 300 metres. For longwave transmitters, however, the construction of masts with heights of half-wave wavelength is not economically viable. The only longwave radio mast with a height of the length of the radiated wavelength was the Warszawa Radio Mast at Konstantynów. At the time of its collapse in 1991 it was the tallest manmade structure in the world, at 646.38 metres tall, for a wavelength of 1292.76 metres. For frequencies below longwave, masts are electrically enlarged by loading coils or capacity hats on the top, sectional, the structure is divided into usually two sections with insulators between, usually center-fed
27.
Monopole antenna
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A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the end of the monopole and the ground plane. One side of the feedline is attached to the lower end of the monopole, and the other side is attached to the ground plane. This contrasts with an antenna which consists of two identical rod conductors, with the signal from the transmitter applied between the two halves of the antenna. The monopole is a resonant antenna, the rod functions as a resonator for radio waves, oscillating with standing waves of voltage. Therefore, the length of the antenna is determined by the wavelength of the waves it is used with. The most common form is the quarter-wave monopole, in which the antenna is approximately ¼ of the wavelength of the radio waves, the monopole antenna was invented in 1895 by radio pioneer Guglielmo Marconi, for this reason it is sometimes called the Marconi antenna. Common types of antenna are the whip, rubber ducky, helical, random wire, umbrella, inverted-L and T-antenna, inverted-F, mast radiator. The monopole antenna was invented in 1895 and patented 1896 by radio pioneer Guglielmo Marconi during his historic first experiments in radio communication and he began by using dipole antennas invented by Heinrich Hertz consisting of two identical horizontal wires ending in metal plates. For this reason the monopole is also called a Marconi antenna, like a dipole antenna, a monopole has an omnidirectional radiation pattern. It radiates vertically polarized radio waves, a monopole can be visualized as being formed by replacing the bottom half of a vertical dipole antenna with a conducting plane at right-angles to the remaining half. The general effect of small ground planes, as well as imperfectly conducting earth grounds, is to tilt the direction of maximum radiation up to higher elevation angles. This design is used for the mast radiator antennas employed in radio broadcasting at low frequencies, as well as low frequency antennas such as the T-antenna. At VHF and UHF frequencies the size of the ground plane needed is smaller, the most common antenna used in mobile phones is the inverted-F antenna, which is a variant of the inverted-L monopole. Bending over the antenna saves space and keeps the it within the bounds of the mobiles case, to improve the match the antenna is not fed from the end, rather some intermediate point, and the end is grounded instead. The quarter-wave whip and rubber ducky antennas used with handheld radios such as walkie-talkies and these dont use a ground plane, and the ground side of the transmitter is just connected to the ground connection on its circuit board. The hand and body of the person holding them may function as a ground plane. When used for broadcasting, the radio frequency power from the broadcasting transmitter is fed across the base insulator between the tower and a ground system
28.
Random wire antenna
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The wire may be straight or it may be strung back and forth between trees or walls just to get enough wire into the air, this type of antenna sometimes is called a zig-zag antenna. Such antennas are not as effective as antennas whose length is adjusted to resonate at the wavelength to be used. Random wire antennas are a type of antenna and the other side of the receiver or transmitter antenna terminal must be connected to an earth ground. Often random wire antennas are also referred to as long-wire antenna, long-wire antennas require a length greater than a quarter-wavelength or half of the radio waves, whereas random wire antennas have no such constraint. The radiation pattern of a random wire antenna is unpredictable and depends on its electrical length. The radiation will drop off to zero on the axis, however it may have several lobes at angles to the antenna axis, under about 0. 6λ a wire antenna will have a single lobe with a maximum at right angles to the axis. Above this the lobe will split into two lobes with their maximum directed at equal angles to the wire, and a null between them. This results in four azimuth angles at which the gain is maximum, as the length of wire in wavelengths increases, the number of lobes increases and the maxima become increasingly sharp. A folded or zig-zag antenna will exhibit a pattern that is more complicated. Long wire antennas are reported to be effective for reception than multielement antennas such as Yagi or quad antennas with the same length of wire. A random wire antenna usually consists of a wire with one end connected to the radio. Ideally, it is a wire strung as high as possible between trees or buildings, the ends insulated from supports with strain insulators. Typically it is made from number 12 or 14 AWG copperclad wire, folding the wire into a zigzag pattern to fit in a limited space such as an apartment or attic will reduce effectiveness and make theoretical analysis extremely difficult. If used for transmitting, a wire antenna usually will also require an antenna tuner. One side of the output of the tuner is connected directly to the antenna, without a transmission line, a quarter-wavelength sized wire works best, and unless fed through a unun, a half-wavelength will exceed the matching ability of most tuners. Even without a good earth, the antenna will also radiate, the ground for a random wire antenna may be chosen by experimentation. RF feedback, or ‘backlash current’, can be an issue, RF feedback can be minimized by selecting a wire length that causes the low feed-point impedance at a current loop to occur at the transmitter. Alternately, a remote tuner can be fed with feedline, and the tuner located on the antenna
29.
Rubber ducky antenna
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The rubber ducky antenna is an electrically short monopole antenna that functions somewhat like a base-loaded whip antenna. It consists of a wire in the shape of a narrow helix. Rubber ducky antenna is a form of normal-mode helical antenna, two rumors link the naming of the antenna with the Kennedy family. In the early 1960s the rubber became the antenna of choice for personal walkie-talkie transceivers used by police and security services. Secret Service, which guards the President of the United States, according to one rumor, the young Caroline Kennedy, daughter of President John F. Kennedy, named the flexible device when she pointed at one on an agents transceiver and said, rubber ducky. On the other hand, Dr. Thomas A. Clark, an alternative name is based on the short stub format, the stubby antenna. Before the rubber ducky, antennas on portable radios usually consisted of quarter-wave whip antennas, in the VHF range where they were used, these antennas were 2 or 3 feet long, making them cumbersome. They were often made of telescoping tubes that could be retracted when not in use, to make the antenna more compact, electrically short antennas, shorter than one-quarter wavelength, began to be used. Electrically short antennas have considerable capacitive reactance, so to make them resonant at the frequency a inductor is added in series with the antenna. Antennas which have these inductors built into their bases are called base-loaded whips, the rubber ducky is an electrically short quarter-wave antenna in which the inductor, instead of being in the base, is built into the antenna itself. The antenna is made of a helix of wire like a spring. The springy wire is flexible, making it prone to damage than a stiff antenna. The spring antenna is further enclosed in a plastic or rubber-like covering to prevent it catching on things. The technical name for type of antenna is a normal-mode helix. Rubber ducky antennas are typically 4% to 15% of a wavelength long and this is because the inductance is distributed throughout the antenna and so allows somewhat greater current in the antenna. Rubber ducky antennas have lower gain than a full size quarter-wavelength antenna and they are typically used in short-range two way radios where maximum range is not a requirement. Their design is a compromise between antenna gain and small size and they are difficult to characterize electrically because the current distribution along the element is not sinusoidal as is the case with a thin linear antenna. In common with other inductively loaded short monopoles, the rubber ducky has a high Q factor and this means that as the frequency departs from the antennas designed center frequency, its SWR increases and thus its efficiency falls off quickly
30.
Turnstile antenna
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The name reflects the notion the antenna looks like a turnstile when mounted horizontally. The antenna can be used in two possible modes, in normal mode the antenna radiates horizontally polarized radio waves perpendicular to its axis. In axial mode the antenna radiates circularly polarized radiation along its axis, specialized normal mode turnstile antennas called superturnstile or batwing antennas are used as television broadcasting antennas. The turnstile antenna was invented by George Brown in 1935 and described in scholarship in 1936, the patent history reveals the popularity of the turnstile antenna over the years. The antenna can be used in two different modes, normal mode and axial mode, in directions perpendicular to its axis the antenna radiates linearly polarized radio waves. The radiation pattern, a superposition of the two patterns, is close to omnidirectional but actually cloverleaf shaped, with four small maxima off the ends of the elements. The pattern departs from omnidirectional by only ±5 percent, the radiation in these horizontal directions is often increased by vertically stacking multiple turnstile antennas fed in phase. These stacked normal mode turnstile antennas are used at VHF and UHF frequencies for FM, since the first turnstiles invented by Brown operated in this mode, the normal mode turnstile is occasionally called the George Brown turnstile antenna. Off the ends of the axis, perpendicular to the plane of the elements. The radiation off one end is righthand-circularly-polarized and the end is lefthand-circularly-polarized. Which end produces which polarization is determined by the phase of the feed connections, the waves in that direction are reflected back 180° and the reflection reverses the polarization sense, so the reflected waves reinforce the forward radiation. For example, if the waves radiated forward are right-circularly-polarized. The flat reflector reverses the sense so the reflected waves are right-circularly-polarized. By locating the reflector λ/4 behind the elements the direct and reflected waves are in phase, addition of the reflector increases the axial radiation by a factor of 2. Another common way to increase the axial mode radiation is to replace each dipole with a Yagi array, axial mode turnstile antennas are often used for satellite and missile antennas, since circular polarization is used in satellite communication. This is because with circularly polarized waves the relative orientation of the antenna elements does not affect the gain, the fundamental requirement for the turnstile to function is ensuring each dipoles currents are of equal magnitude and in phase quadrature. This is done with feed-line techniques or by adding reactance in series with the dipoles, each phase is applied to one of the dipoles. By modifying the length and shape of the dipoles, the combined terminal impedance presented to a single feed-point can achieve pure resistance and this method of changing the physical dimensions of the antenna element to yield quadrature currents is known as turnstile feeding
31.
T2FD antenna
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The Tilted Terminated Folded Dipole or Balanced Termination, Folded Dipole - also known as W3HH antenna - is a general-purpose shortwave antenna developed in the late 1940s by the United States Navy. It performs reasonably well over a frequency range, without marked dead spots in terms of either frequency, direction. The history of the TTFD antenna divides conveniently into three different phases, It was first developed for use as a general purpose antenna on Naval ships in the 1940s. Recently, with the advent of new frequency bands which are not even-integer multiples of existing bands’ frequencies. One of the developers of the original Navy antenna, Captain Gil L. Countryman, was also a radio enthusiast. He introduced the design to other amateurs at the beginning of the 1950s, also contributing to its falling popularity was the growing fad for low-impedance antenna feeds. The T2FD is useful for hidden indoor systems, or where several optimised frequency-specific antennas cannot be accommodated, for example, an indoor antenna only 24 feet long will allow operation on all amateur HF bands above 14 MHz on transmit, and down to 7 MHz on receive. A typical T2FD is built as follows, out of two conductors, Span near a half-wavelength of the lowest required frequency. Distance between upper and lower conductors equal to 1/100 of the wavelength and this distance is maintained by a number of insulating dowels. At least two dowels at the ends are tied to non-conducting ropes, which in turn are tied to supports, the upper and lower ends of the conductors are connected at the ends, by wire sections that follow the end dowels. Fed in the middle of the conductor, with an impedance in the order of 300 Ω, balanced. This provides an acceptable match to commonly available 75 Ω coaxial cable. Terminated in the middle of the conductor with a 400–480 Ω non-inductive resistor. The resistor absorbs a portion of the RF power as the operating frequency nears the lower limit of the design range. The resistor can be built of 3 pairs of 10 serial 1600 Ω,1 W resistors, the commercially available B&W AC3-30 antenna varies from the above to cover 3 to 30 MHz with a 90 foot length with an 18-inch spacing of the wires. The balun is a 16,1 ratio, thereby transforming the 50 ohm coax to an 800 Ω feed at the antenna, the resistor load is also 800 Ohms, non-inductive. This allows the antenna impedance to swing from 400 to 1600 Ω over the frequency range intended, an antenna such as the one described above is usable for both local and medium-long distance communication across a frequency range of about 1,6. For example, an antenna for the portion of shortwave will be roughly 33 m long
32.
T-antenna
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A T-antenna, T-aerial, flat-top antenna, or top-hat antenna is a vertically polarized simple wire radio antenna used in the VLF, LF, MF and shortwave bands. T-antennas are widely used as transmitting antennas for radio stations, long wave. They are also used as receiving antennas for shortwave listening, the antenna consists of one or more horizontal wires suspended between two supporting radio masts or buildings and insulated from them at the ends. A vertical wire is connected to the center of the wires and hangs down close to the ground. Combined, the two form a T shape, hence the name. The transmitter power is applied, or the receiver is connected, the T-antenna functions as a monopole antenna with capacitive top-loading, other antennas in this category include the inverted-L, umbrella, delta, and triatic antennas. It was invented during the first decades of radio, in the wireless telegraphy era, instead it serves to add capacitance to the top of the antenna. The top load wire can increase radiated power by 2 to 4 times for a given base current, however, the antenna is still typically not as efficient as a full-height λ/4 vertical monopole, and has a higher Q and thus a narrower bandwidth. 1λ or less. Since the radiation resistance and efficiency increases with height, the antenna should be suspended as high as possible, the antenna has a base impedance with a capacitive component that must be tuned out with an added inductor. To increase the capacitance, several parallel horizontal wires are often used, connected together at the center where the vertical wire attaches. The capacitance does not increase proportionally with the number of wires, however, since the vertical wire is the actual radiating element, the antenna radiates vertically polarized radio waves in an omnidirectional radiation pattern, with equal power in all azimuthal directions. The axis of the horizontal wire makes little difference, the power is maximum in a horizontal direction or at a shallow elevation angle, decreasing to zero at the zenith. The effect of ground conductivity is generally to tilt the pattern up. Since it is shorter than λ/4 the T antenna has a capacitive reactance. To operate over a frequency range the loading coil often must be adjustable. The high Q also causes a voltage on the antenna. The insulators at the ends must be designed to withstand these voltages, in high power transmitters the output power is often limited by the onset of corona discharge on the wires. At low frequencies the resistance is very low, often less than an ohm
33.
Umbrella antenna
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The outer end of each wire is connected by an insulator to a supporting rope or cable anchored to the ground. The radial wires can also function as guy wires to support the mast, the radial wires make the antenna look like a giant umbrella without the cloth, accounting for the name. Either the central mast itself, insulated from the ground, is connected to the transmitter and serves as the radiating element, or vertical wires parallel to the mast. The umbrella wires serve as the plate of a capacitor, with the serving as the other plate. Umbrella antennas with heights of 15 to 460 metres are in service, the tallest umbrella antennas are actually used by Lualualei VLF transmitter, INS Kattabomman and the CHAYKA-transmitters at Inta and Dudinka. Umbrella antennas radiate vertically polarised ground waves in a radiation pattern. Because they are compared to a wavelength of the radio waves, they have low radiation resistance and are usually inefficient. Umbrella antennas can be built as single mast antennas or as multiple mast antennas, umbrella antennas are in common use for commercial medium-wave AM broadcasting as well as for LF and VLF transmitters for broadcasting and navigational aids. They were used at most OMEGA Navigation System transmitters, operating around 10 kHz, umbrella antennas were invented during the wireless telegraphy era, about 1900 to 1920, and used with spark-gap transmitters on longwave bands to transmit information by Morse code. Small umbrella antennas were used with portable transmitters by military signal corps during World War I. It is also possible to build an umbrella antenna with a grounded tower, such an antenna requires a separate feeder to the umbrella wires, which are also insulated against the mast and the ground
34.
Directional antenna
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A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing for increased performance and reduced interference from unwanted sources. Directional antennas provide increased performance over dipole antennas – or omnidirectional antennas in general – when greater concentration of radiation in a direction is desired. A high-gain antenna is an antenna with a focused, narrow radiowave beam width. This narrow beam allows more precise targeting of the radio signals. Most commonly referred to during space missions, these antennas are also in use all over Earth, most successfully in flat, open areas where no mountains lie to disrupt radiowaves. Low gain antennas are used in spacecraft as a backup to the high-gain antenna. The most common types are the Yagi antenna, the antenna, and the corner reflector antenna. Cellular repeaters often make use of directional antennas to give a far greater signal than can be obtained on a standard cell phone. Satellite Television receivers usually use parabolic antennas, for long and medium wavelength frequencies, tower arrays are used in most cases as directional antennas. When transmitting, a high-gain antenna allows more of the power to be sent in the direction of the receiver. When receiving, a high gain antenna captures more of the signal, as a consequence of their directivity, directional antennas also send less signal from directions other than the main beam. This property may be used to reduce interference, there are many ways to make a high-gain antenna - the most common are parabolic antennas, helical antennas, yagi antennas, and phased arrays of smaller antennas of any kind. Horn antennas can also be constructed with high gain, but are commonly seen. Still other configurations are possible - the Arecibo Observatory uses a combination of a line feed with a spherical reflector. Antenna gain is often quoted with respect to an antenna that radiates equally in all directions. This gain, when measured in decibels, is called dBi, conservation of energy dictates that high gain antennas must have narrow beams. For example, if a high gain antenna makes a 1 watt transmitter look like a 100 watt transmitter, in turn this implies that high-gain antennas must be physically large, since according to the diffraction limit, the narrower the beam desired, the larger the antenna must be. Antenna gain can also be measured in dBd, which is gain in Decibels compared to the maximum intensity direction of a half wave dipole
35.
Adcock antenna
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The Adcock antenna is an antenna array consisting of four equidistant vertical elements which can be used to transmit or receive directional radio waves. The Adcock array was invented and patented by British engineer Frank Adcock in 1919 as British Patent No,130,490, and has been used for a variety of applications, both civilian and military, ever since. Although originally conceived for receiving Low Frequency waves, it has also used for transmitting. In the early 1930s, the Adcock antenna became a key feature of the newly created radio navigation system for aviation, the Low Frequency radio range network, which consisted of hundreds of Adcock antenna arrays, defined the airways used by aircraft for instrument flying. The LFR remained as the main aerial navigation technology until it was replaced by the VOR system in the 1950s and 1960s, prior to Adcocks invention, engineers had been using loop antennas to achieve directional sensitivity. This created the equivalent of square loops, but without their horizontal members, the same principles remain valid today, and the Adcock antenna array and its variants are still used for radio direction finding. In the late 1920s, the Adcock antenna was adopted for navigation, in what became known as the Low Frequency radio range. Hundreds of transmitting stations, each consisting of four or five Adcock antenna towers, were constructed around the U. S. the result was a network of electronic airways, which allowed pilots to navigate at night and in poor visibility, under virtually all weather conditions. The LFR remained as the main aerial navigation system in the U. S. and other countries until the 1950s, by the 1980s all LFR stations were decommissioned. Direction finding Radio direction finder Huff-Duff
36.
Near vertical incidence skywave
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Near vertical incidence skywave, or NVIS, is a skywave radio-wave propagation path that provides usable signals in the range between groundwave and conventional skywave distances—usually 30–400 miles. It is used for military and paramilitary communications, broadcasting, especially in the tropics, the radio waves travel near-vertically upwards into the ionosphere, where they are refracted back down and can be received within a circular region up to 650 km from the transmitter. If the frequency is too high, refraction fails to occur and if it is too low, there is no fundamental difference between NVIS and conventional skywave propagation, the practical distinction arises solely from different desirable radiation patterns of the antennas. The most reliable frequencies for NVIS communications are between 1.8 MHz and 8 MHz, above 8 MHz, the probability of success begins to decrease, dropping to near zero at 30 MHz. Usable frequencies are dictated by local conditions, which have a strong systematic dependence on geographical location. Common bands used in radio at mid-latitudes are 3.5 MHz at night and 7 MHz during daylight. Optimum NVIS frequencies tend to be higher towards the tropics and lower towards the arctic regions and they are also higher during high sunspot activity years. Another interesting aspect of NVIS communication is, that direction finding of the sender is more difficult than for ground-wave communication, an NVIS antenna configuration is a horizontally polarized radiating element that is from 1/20th wavelength to 1/4 wavelength above the ground. That proximity to the forces the majority of the radiation to go straight up. Overall efficiency of the antenna can be increased by placing a ground wire slightly longer than the parallel to. One source says that a ground wire can provide antenna gain in the 3–6 dB range. Another source indicates 2 dB for a wire and nearly 4 dB for multiple ground wires. Ground wires are more necessary when using lower dipoles over poor soils as without them considerable energy goes into heating the ground, significant increases in communication will obviously be realized when both the transmitting station and the receiving station use NVIS configuration for their antennas. In particular for low profile operations NVIS antennas are a good option, for broadcasting, typical antennas consist of a dipole about 1/4 wavelength above ground, or arrays of such dipoles. Up to 16 dipoles can be used, allowing strong signals with relatively low power by concentrating the signal in a smaller area, limiting the coverage may be dictated by licensing, language or political considerations. Arrays of dipoles can be used to slew the pattern, so that the transmitter need not be in the center of the coverage footprint, broadcast NVIS antennas usually use an extensive ground screen to increase gain and stabilize the pattern and feed impedance with changing ground moisture. A military NVIS antenna is the AS-2259 Antenna, which consists of two V-shaped dipoles, the four dipole wires also serve as guy rope for the antenna mast, an alternative configuration consists of a transmitting loop antenna which is configured for maximum signal transmission upwards. Refraction Analysis of height vs gain QSL. net NVIS Article Make A Quick, Easy, Cheap, NVIS Antenna for Roadside Operating Field-deployed NVIS An example of mobile NVIS NVIS tutorial
37.
AWX antenna
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An AWX antenna is an antenna which means that it can be used for All Wave radio frequency and is shaped like X. It has two V shaped elements, two types to place these two elements exist for this antenna. First, one element is placed left side spreading out for left, second, one element is placed upper side spreading out for upward and the other element is placed lower side spreading out for downward. RF energy is fed at two vertices of each element and it can be used in very wide frequency range whose gain is higher than a dipole antenna. To obtain enough gain, the antenna must be set at enough high place off the ground to avoid the effect of the ground, the antenna is used by radio stations which need to communicate in wide frequency range such as amateur radios, military radios and maritime mobile services
38.
Beverage antenna
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The Beverage antenna or wave antenna is a long-wire receiving antenna mainly used in the low frequency and medium frequency radio bands, invented by Harold H. Beverage in 1921. It is used by radio, shortwave listening, and longwave radio DXers. However the antenna must be built so the points at the location of the transmitter. Some Beverage antennas use a design that allows reception in two directions from a single Beverage antenna. The advantages of the Beverage are excellent directivity, and wider bandwidth than resonant antennas and its disadvantages are its physical size, requiring considerable land area, and inability to rotate to change the direction of reception. Installations often use multiple antennas to provide wide azimuth coverage, Harold H. Beverage experimented with receiving antennas similar to the Beverage antenna in 1919 at the Otter Cliffs Radio Station. He discovered in 1920 that an otherwise nearly bidirectional long-wire antenna becomes unidirectional by placing it close to the lossy earth, the antenna was patented in 1921 and named for its inventor Beverage. The Beverage antenna consists of a horizontal wire one-half to several long, suspended close to the ground, usually 10 to 20 feet high. At the other end it is connected to the receiver with a transmission line, the lack of resonance gives it a wider bandwidth than resonant antennas. It receives vertically polarized waves, but unlike other vertically polarized antennas it is suspended close to the ground. The Beverage antenna relies on wave tilt for its operation, the electric field is not perpendicular to the ground but at an angle, producing an electric field component parallel to the Earths surface. The RF currents traveling along the wire add in phase and amplitude throughout the length of the wire, the antenna wire and the ground under it together can be thought of as a leaky transmission line which absorbs energy from the radio waves. The velocity of the current waves in the antenna is less than the speed of light due to the ground, the velocity of the wavefront along the wire is also less than the speed of light due to its angle. At a certain angle θmax the two velocities are equal, at this angle the gain of the antenna is maximum, so the radiation pattern has a main lobe at this angle. The angle of the lobe is θ max = arccos . The antenna is particularly suited to radio waves reflected from the ionosphere, called skywave, or skip propagation. The radio waves arrive at the Earths surface with shallow angles of approximately 5 to 45 degrees. While Beverage antennas have excellent directivity, because they are close to lossy Earth, they do not produce absolute gain and this is rarely a problem, because the antenna is used at frequencies where there are high levels of atmospheric radio noise
39.
Cantenna
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A cantenna is a homemade directional waveguide antenna, made out of an open-ended metal can. Cantennas are typically used to increase the range of Wi-Fi networks, the cylinder portion of the can may consist of metal-coated paperboard. Although some designs are based on a Pringles potato chips can, however, a cantenna can be made from various cans or tubes of an appropriate diameter. Some designs include a pole mount to elevate the cantenna, at 2.4 GHz, losses can occur if the cable from the cantenna to the Wi-Fi circuitry is too long. A more efficient cantenna can be made by minimising this length or connecting the cantenna directly to the Wi-Fi circuitry, cantennas are typically used for extending a wireless local area network. The tiny design makes them ideal for applications such as wardriving. Cantennas can be used to increase cell phone range, improve reception, a cantenna can be used as a satellite dish feed horn. The 5.5 GHz cantenna dimensions are almost perfect in that make a perfect fit for the standard TV satellite dish. The resulting setup is a low-cost high-quality high-gain antenna, such setups are widely used in wireless community networks for long-distance Wi-Fi links. Cantennas may be used with other RF devices such as security cameras
40.
Cassegrain antenna
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The beam of radio waves from the feed illuminates the secondary reflector, which reflects it back to the main reflector dish, which reflects it forward again to form the desired beam. The Cassegrain design is used in parabolic antennas, particularly in large antennas such as those in satellite ground stations, radio telescopes. The primary reflector is a paraboloid, while the shape of the secondary reflector is a hyperboloid. Usually the secondary reflector and the antenna are located on the central axis of the dish. Therefore, this design is used for antennas with bulky or complicated feeds, such as satellite communication ground antennas, radio telescopes, in receiving antennas this reduces reception of ground noise, resulting in a lower antenna noise temperature. Dual reflector shaping, The presence of a reflecting surface in the signal path allows additional opportunities for tailoring the radiation pattern for maximum performance. For example, the gain of ordinary parabolic antennas is reduced because the radiation of the feed antenna falls off toward the outer parts of the dish, however, this results in a secondary that is no longer precisely hyperbolic, so the constant phase property is lost. This phase error, however, can be compensated for by slightly tweaking the shape of the primary mirror, the result is a higher gain, or gain/spillover ratio, at the cost of surfaces that are trickier to fabricate and test. The focal ratio of typical parabolic antennas is 0.25 -0.8, in a front-fed antenna, a flatter parabolic dish with a long focal length would require an impractically elaborate support structure to hold the feed rigid with respect to the dish. However, the drawback of this small focal ratio is that the antenna has a field of view. Modern parabolic antennas in radio telescopes and communications satellites often use arrays of feedhorns clustered around the focal point and these require good off-axis focusing characteristics. The convex secondary reflector of the Cassegrain increases the focal length, the angular width the secondary reflector subtends at the feed horn is typically 10° - 15°, as opposed to 120° - 180° the main reflector subtends in a front-fed dish. Therefore, the horn must be longer for a given wavelength. A beam waveguide antenna is a type of complicated Cassegrain antenna with a radio wave path to allow the feed electronics to be located at ground level. The Cassegrain antenna design was adapted from the Cassegrain telescope, a type of reflecting telescope developed around 1672, the first Cassegrain antenna was invented and built in Japan in 1963 by NTT, KDDI and Mitsubishi Electric. The 20 meter I-1 antenna operated at 6.4,4.2, and 1.7 GHz, Cassegrain reflector Nasmyth telescope Cassegrain subreflector design article
41.
Corner reflector antenna
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A corner reflector antenna is a type of directional antenna used at VHF and UHF frequencies. It was invented by John D. Kraus in 1938 and it consists of a dipole driven element mounted in front of two flat rectangular reflecting screens joined at an angle, usually 90°. Corner reflectors have moderate gain of 10-15 dB, high ratio of 20-30 dB. They are widely used for UHF television receiving antennas, point-to-point communication links and data links for wireless WANs, and amateur radio antennas on the 144,420 and they radiate linearly polarized radio waves and can be mounted for either horizontal or vertical polarization. The corner reflector antenna should not be confused with a corner reflector, the flat reflecting surfaces can be metal sheets, but are more often made of wire screen or rod elements parallel to the driven element, to reduce weight and wind loads on the antenna. The angle θ between the sides is most commonly 90°, the gain increases as the angle narrows, but the increase below 90° is minimal, and requires longer reflector screens be used. However, angles down to 45° have been used, the radiation resistance of the dipole increases with this spacing, so the spacing can be adjusted to match the driven element to the feed line. Bowtie driven elements are used for wide bandwidth applications like television antennas. The antenna can be regarded as a form with an intermediate between a plane reflective array antenna and a parabolic antenna. It is particularly suitable in applications where a directional antenna around one to 1 1/2 wavelengths in size is needed. A parabolic dish this size has no advantage in gain over the corner reflector, so its simplicity of design, the single driven element can be replaced by a Yagi array. UHF Yagi television antennas often use a corner reflector. Monopole versions for use at lower frequencies have been built by placing vertical reflecting screens behind a vertical monopole antenna, the square-corner reflector beam antenna for ultra high frequencies. Technical article about the antenna by the inventor
42.
Curtain array
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Curtain arrays are a class of large multielement directional wire radio transmitting antennas, used in the shortwave radio bands. These are suspended by support wires strung between pairs of steel towers, up to 300 feet high. They are used for long-distance skywave transmission, they transmit a beam of waves at a shallow angle into the sky. Curtain antennas are used by international short wave radio stations to broadcast to large areas at transcontinental distances. Because of their powerful directional characteristics, curtain arrays are used in government propaganda radio stations to beam propaganda broadcasts over national borders into other nations. For example, curtain arrays were used by Radio Free Europe, curtain arrays were originally developed during the 1920s and 1930s when there was a lot of experimentation with long distance shortwave broadcasting. The underlying concept was to achieve improvements in gain and/or directionality over the dipole antenna. Franklin invented the first curtain array aerial system in 1924, known as the Franklin or English system, other early curtain arrays included the Bruce array patented by Edmond Bruce in 1927, and the Sterba curtain, patented by Ernest J. Sterba in 1929. The Bruce array produces a signal, Sterba arrays produce a horizontally-polarised signal. The Sterba array was used by Bell Labs and others during the 1930s and 1940s, the Sterba curtain is however a narrowband design and is only steerable by mechanical means. However, as far back as the mid-1930s, Radio Netherlands was using a rotatable HRS antenna for global coverage, since the 1950s the HRS design has become more or less the standard for long distance high power shortwave broadcasting. Curtain arrays were used in some of the first radar systems, the driven elements are usually half-wave dipoles, fed in phase, mounted in a plane 1/4 wavelength in front of the reflector plane. The reflector wires are oriented parallel to the dipoles, the dipoles may be vertical, radiating in vertical polarization, but are most often horizontal, because horizontally polarized waves are less absorbed by earth reflections. The lowest row of dipoles are mounted more than 1/2 wavelength above the ground and this allows most of the radiation to be concentrated in a narrow main lobe aimed a few degrees above the horizon, which is ideal for skywave transmission. A curtain array may have a gain of 20 dB greater than a dipole antenna. In order to allow the beam to be steered, sometimes the entire array is suspended by cantilever arms from a large tower which can be rotated. Alternatively, some versions are constructed as phased arrays in which the beam can be steered electronically. Adding a constant phase shift between adjacent horizontal dipoles allows the direction of the beam to be rotated in azimuth by a limited angle, V indicates the dipoles are oriented vertically, so the antenna radiates vertically polarized radio waves
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".
