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
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".
