The Brocken Transmitter ( German : Sender Brocken ) is a facility for FM - and TV -transmitters on the Brocken , the highest mountain in Northern Germany.
112-400: The facility includes two transmission towers. The old tower was built between 1936 and 1937. It is 53 metres high (including its antenna mast, which no longer exists, it had a height of 95 metres) and has an observation deck , which can be reached by elevator . This tower was intended to be used after 1939 for TV transmissions to central Germany, but due to the beginning of World War II , it
224-415: A burning glass . Parabolic reflectors are popular for use in creating optical illusions . These consist of two opposing parabolic mirrors, with an opening in the center of the top mirror. When an object is placed on the bottom mirror, the mirrors create a real image , which is a virtually identical copy of the original that appears in the opening. The quality of the image is dependent upon the precision of
336-430: A hemisphere ( 2 3 π R 2 D , {\textstyle ({\frac {2}{3}}\pi R^{2}D,} where D = R ) , {\textstyle D=R),} and a cone ( 1 3 π R 2 D ) . {\textstyle ({\frac {1}{3}}\pi R^{2}D).} π R 2 {\textstyle \pi R^{2}}
448-504: A sphere . Many nondirectional antennas, such as monopoles and dipoles , emit equal power in all horizontal directions, with the power dropping off at higher and lower angles; this is called an omnidirectional pattern and when plotted looks like a torus or donut. Parabolic reflector A parabolic (or paraboloid or paraboloidal ) reflector (or dish or mirror ) is a reflective surface used to collect or project energy such as light , sound , or radio waves . Its shape
560-535: A beam of light in flashlights , searchlights , stage spotlights , and car headlights . In radio , parabolic antennas are used to radiate a narrow beam of radio waves for point-to-point communications in satellite dishes and microwave relay stations, and to locate aircraft, ships, and vehicles in radar sets. In acoustics , parabolic microphones are used to record faraway sounds such as bird calls , in sports reporting, and to eavesdrop on private conversations in espionage and law enforcement. Strictly,
672-405: A boom; the boom is only for support and not involved electrically. Only one of the elements is electrically connected to the transmitter or receiver, while the remaining elements are passive. The Yagi produces a fairly large gain (depending on the number of passive elements) and is widely used as a directional antenna with an antenna rotor to control the direction of its beam. It suffers from having
784-405: A current of 1 Ampere will require 63 Volts, and the antenna will radiate 63 Watts (ignoring losses) of radio frequency power. Now consider the case when the antenna is fed a signal with a wavelength of 1.25 m; in this case the current induced by the signal would arrive at the antenna's feedpoint out-of-phase with the signal, causing the net current to drop while the voltage remains
896-463: A current will reflect when there are changes in the electrical properties of the material. In order to efficiently transfer the received signal into the transmission line, it is important that the transmission line has the same impedance as its connection point on the antenna, otherwise some of the signal will be reflected backwards into the body of the antenna; likewise part of the transmitter's signal power will be reflected back to transmitter, if there
1008-495: A fashion are known to be harmonically operated . Resonant antennas usually use a linear conductor (or element ), or pair of such elements, each of which is about a quarter of the wavelength in length (an odd multiple of quarter wavelengths will also be resonant). Antennas that are required to be small compared to the wavelength sacrifice efficiency and cannot be very directional. Since wavelengths are so small at higher frequencies ( UHF , microwaves ) trading off performance to obtain
1120-400: A feed-point impedance that matches that of a transmission line; a matching network between antenna terminals and the transmission line will improve power transfer to the antenna. A non-adjustable matching network will most likely place further limits the usable bandwidth of the antenna system. It may be desirable to use tubular elements, instead of thin wires, to make an antenna; these will allow
1232-436: A flux of 1 pW / m (10 Watts per square meter) and an antenna has an effective area of 12 m , then the antenna would deliver 12 pW of RF power to the receiver (30 microvolts RMS at 75 ohms). Since the receiving antenna is not equally sensitive to signals received from all directions, the effective area is a function of the direction to the source. Due to reciprocity (discussed above)
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#17327826939921344-403: A greater bandwidth. Or, several thin wires can be grouped in a cage to simulate a thicker element. This widens the bandwidth of the resonance. Amateur radio antennas that operate at several frequency bands which are widely separated from each other may connect elements resonant at those different frequencies in parallel. Most of the transmitter's power will flow into the resonant element while
1456-450: A long Beverage antenna can have significant directivity. For non directional portable use, a short vertical antenna or small loop antenna works well, with the main design challenge being that of impedance matching . With a vertical antenna a loading coil at the base of the antenna may be employed to cancel the reactive component of impedance ; small loop antennas are tuned with parallel capacitors for this purpose. An antenna lead-in
1568-416: A mirror that was parabolic would correct spherical aberration as well as the chromatic aberration seen in refracting telescopes . The design he came up with bears his name: the " Gregorian telescope "; but according to his own confession, Gregory had no practical skill and he could find no optician capable of actually constructing one. Isaac Newton knew about the properties of parabolic mirrors but chose
1680-603: A number of parallel dipole antennas with a certain spacing. Depending on the relative phase introduced by the network, the same combination of dipole antennas can operate as a "broadside array" (directional normal to a line connecting the elements) or as an "end-fire array" (directional along the line connecting the elements). Antenna arrays may employ any basic (omnidirectional or weakly directional) antenna type, such as dipole, loop or slot antennas. These elements are often identical. Log-periodic and frequency-independent antennas employ self-similarity in order to be operational over
1792-427: A proper resonant antenna at the trap frequency. At substantially higher or lower frequencies the trap allows the full length of the broken element to be employed, but with a resonant frequency shifted by the net reactance added by the trap. The bandwidth characteristics of a resonant antenna element can be characterized according to its Q where the resistance involved is the radiation resistance , which represents
1904-516: A pure resistance. Sometimes the resulting (lower) electrical resonant frequency of such a system (antenna plus matching network) is described using the concept of electrical length , so an antenna used at a lower frequency than its resonant frequency is called an electrically short antenna For example, at 30 MHz (10 m wavelength) a true resonant 1 / 4 wave monopole would be almost 2.5 meters long, and using an antenna only 1.5 meters tall would require
2016-504: A rather limited bandwidth, restricting its use to certain applications. Rather than using one driven antenna element along with passive radiators, one can build an array antenna in which multiple elements are all driven by the transmitter through a system of power splitters and transmission lines in relative phases so as to concentrate the RF power in a single direction. What's more, a phased array can be made "steerable", that is, by changing
2128-583: A reflector must be correct to within about 20 nm. For comparison, the diameter of a human hair is usually about 50,000 nm, so the required accuracy for a reflector to focus visible light is about 2500 times less than the diameter of a hair. For example, the flaw in the Hubble Space Telescope mirror (too flat by about 2,200 nm at its perimeter) caused severe spherical aberration until corrected with COSTAR . Microwaves, such as are used for satellite-TV signals, have wavelengths of
2240-443: A signal into the transmission line only when the source signal's frequency is close to that of the design frequency of the antenna, or one of the resonant multiples. This makes resonant antenna designs inherently narrow-band: Only useful for a small range of frequencies centered around the resonance(s). It is possible to use simple impedance matching techniques to allow the use of monopole or dipole antennas substantially shorter than
2352-440: A smaller physical size is usually not required. The quarter-wave elements imitate a series-resonant electrical element due to the standing wave present along the conductor. At the resonant frequency, the standing wave has a current peak and voltage node (minimum) at the feed. In electrical terms, this means that at that position, the element has minimum impedance magnitude , generating the maximum current for minimum voltage. This
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#17327826939922464-416: A spherical shape for his Newtonian telescope mirror to simplify construction. Lighthouses also commonly used parabolic mirrors to collimate a point of light from a lantern into a beam, before being replaced by more efficient Fresnel lenses in the 19th century. In 1888, Heinrich Hertz , a German physicist, constructed the world's first parabolic reflector antenna. The most common modern applications of
2576-498: A standard resistive impedance needed for its optimum operation. The feed point location(s) is selected, and antenna elements electrically similar to tuner components may be incorporated in the antenna structure itself, to improve the match . It is a fundamental property of antennas that most of the electrical characteristics of an antenna, such as those described in the next section (e.g. gain , radiation pattern , impedance , bandwidth , resonant frequency and polarization ), are
2688-477: A total 360 degree phase change, returning it to the original signal. The current in the element thus adds to the current being created from the source at that instant. This process creates a standing wave in the conductor, with the maximum current at the feed. The ordinary half-wave dipole is probably the most widely used antenna design. This consists of two 1 / 4 wavelength elements arranged end-to-end, and lying along essentially
2800-473: A wide range of bandwidths . The most familiar example is the log-periodic dipole array which can be seen as a number (typically 10 to 20) of connected dipole elements with progressive lengths in an endfire array making it rather directional; it finds use especially as a rooftop antenna for television reception. On the other hand, a Yagi–Uda antenna (or simply "Yagi"), with a somewhat similar appearance, has only one dipole element with an electrical connection;
2912-465: Is a monopole antenna, not balanced with respect to ground. The ground (or any large conductive surface) plays the role of the second conductor of a monopole. Since monopole antennas rely on a conductive surface, they may be mounted with a ground plane to approximate the effect of being mounted on the Earth's surface. More complex antennas increase the directivity of the antenna. Additional elements in
3024-434: Is a change in electrical impedance where the feedline joins the antenna. This leads to the concept of impedance matching , the design of the overall system of antenna and transmission line so the impedance is as close as possible, thereby reducing these losses. Impedance matching is accomplished by a circuit called an antenna tuner or impedance matching network between the transmitter and antenna. The impedance match between
3136-417: Is a component which due to its shape and position functions to selectively delay or advance portions of the electromagnetic wavefront passing through it. The refractor alters the spatial characteristics of the wave on one side relative to the other side. It can, for instance, bring the wave to a focus or alter the wave front in other ways, generally in order to maximize the directivity of the antenna system. This
3248-490: Is a consequence of the reciprocity theorem of electromagnetics. Therefore, in discussions of antenna properties no distinction is usually made between receiving and transmitting terminology, and the antenna can be viewed as either transmitting or receiving, whichever is more convenient. A necessary condition for the aforementioned reciprocity property is that the materials in the antenna and transmission medium are linear and reciprocal. Reciprocal (or bilateral ) means that
3360-462: Is a paraboloidal mirror which is rotated about axes that pass through its centre of mass, but this does not coincide with the focus, which is outside the dish. If the reflector were a rigid paraboloid, the focus would move as the dish turns. To avoid this, the reflector is flexible, and is bent as it rotates so as to keep the focus stationary. Ideally, the reflector would be exactly paraboloidal at all times. In practice, this cannot be achieved exactly, so
3472-410: Is adjusted according to the receiver tuning. On the other hand, log-periodic antennas are not resonant at any single frequency but can (in principle) be built to attain similar characteristics (including feedpoint impedance) over any frequency range. These are therefore commonly used (in the form of directional log-periodic dipole arrays ) as television antennas. Gain is a parameter which measures
Brocken Transmitter - Misplaced Pages Continue
3584-421: Is an electronic device that converts an alternating electric current into radio waves (transmitting), or radio waves into an electric current (receiving). It is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver . In transmission , a radio transmitter supplies an electric current to the antenna's terminals, and
3696-400: Is called an isotropic radiator ; however, these cannot exist in practice nor would they be particularly desired. For most terrestrial communications, rather, there is an advantage in reducing radiation toward the sky or ground in favor of horizontal direction(s). A dipole antenna oriented horizontally sends no energy in the direction of the conductor – this is called the antenna null – but
3808-407: Is connected to a transmission line . The conductor, or element , is aligned with the electrical field of the desired signal, normally meaning it is perpendicular to the line from the antenna to the source (or receiver in the case of a broadcast antenna). The radio signal's electrical component induces a voltage in the conductor. This causes an electrical current to begin flowing in the direction of
3920-438: Is equal to 1. Therefore, the effective area A eff in terms of the gain G in a given direction is given by: For an antenna with an efficiency of less than 100%, both the effective area and gain are reduced by that same amount. Therefore, the above relationship between gain and effective area still holds. These are thus two different ways of expressing the same quantity. A eff is especially convenient when computing
4032-462: Is greatest, and where the axis of symmetry intersects the paraboloid. However, if the reflector is used to focus incoming energy onto a receiver, the shadow of the receiver falls onto the vertex of the paraboloid, which is part of the reflector, so part of the reflector is wasted. This can be avoided by making the reflector from a segment of the paraboloid which is offset from the vertex and the axis of symmetry. The whole reflector receives energy, which
4144-465: Is its radiation pattern . The frequency range or bandwidth over which an antenna functions well can be very wide (as in a log-periodic antenna) or narrow (as in a small loop antenna); outside this range the antenna impedance becomes a poor match to the transmission line and transmitter (or receiver). Use of the antenna well away from its design frequency affects its radiation pattern , reducing its directive gain. Generally an antenna will not have
4256-457: Is offset from the axis of rotation. To make less accurate ones, suitable as satellite dishes, the shape is designed by a computer, then multiple dishes are stamped out of sheet metal. Off-axis-reflectors heading from medium latitudes to a geostationary TV satellite somewhere above the equator stand steeper than a coaxial reflector. The effect is, that the arm to hold the dish can be shorter and snow tends less to accumulate in (the lower part of)
4368-401: Is part of a circular paraboloid , that is, the surface generated by a parabola revolving around its axis. The parabolic reflector transforms an incoming plane wave travelling along the axis into a spherical wave converging toward the focus. Conversely, a spherical wave generated by a point source placed in the focus is reflected into a plane wave propagating as a collimated beam along
4480-434: Is redirected toward the desired direction, increasing the antenna's gain by a factor of at least 2. Likewise, a corner reflector can insure that all of the antenna's power is concentrated in only one quadrant of space (or less) with a consequent increase in gain. Practically speaking, the reflector need not be a solid metal sheet, but can consist of a curtain of rods aligned with the antenna's polarization; this greatly reduces
4592-421: Is rotated around axes that pass through the focus and around which it is balanced. If the dish is symmetrical and made of uniform material of constant thickness, and if F represents the focal length of the paraboloid, this "focus-balanced" condition occurs if the depth of the dish, measured along the axis of the paraboloid from the vertex to the plane of the rim of the dish, is 1.8478 times F . The radius of
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4704-418: Is the transmission line , or feed line , which connects the antenna to a transmitter or receiver. The " antenna feed " may refer to all components connecting the antenna to the transmitter or receiver, such as an impedance matching network in addition to the transmission line. In a so-called "aperture antenna", such as a horn or parabolic dish, the "feed" may also refer to a basic radiating antenna embedded in
4816-681: Is the aperture area of the dish, the area enclosed by the rim, which is proportional to the amount of sunlight the reflector dish can intercept. The area of the concave surface of the dish can be found using the area formula for a surface of revolution which gives A = π R 6 D 2 ( ( R 2 + 4 D 2 ) 3 / 2 − R 3 ) {\textstyle A={\frac {\pi R}{6D^{2}}}\left((R^{2}+4D^{2})^{3/2}-R^{3}\right)} . providing D ≠ 0 {\textstyle D\neq 0} . The fraction of light reflected by
4928-411: Is the focal length, D {\textstyle D} is the depth of the dish (measured along the axis of symmetry from the vertex to the plane of the rim), and R {\textstyle R} is the radius of the dish from the center. All units used for the radius, focal point and depth must be the same. If two of these three quantities are known, this equation can be used to calculate
5040-410: Is the ideal situation, because it produces the maximum output for the minimum input, producing the highest possible efficiency. Contrary to an ideal (lossless) series-resonant circuit, a finite resistance remains (corresponding to the relatively small voltage at the feed-point) due to the antenna's resistance to radiating , as well as any conventional electrical losses from producing heat. Recall that
5152-416: Is the radio equivalent of an optical lens . An antenna coupling network is a passive network (generally a combination of inductive and capacitive circuit elements) used for impedance matching in between the antenna and the transmitter or receiver. This may be used to minimize losses on the feed line, by reducing transmission line's standing wave ratio , and to present the transmitter or receiver with
5264-570: Is then focused onto the receiver. This is frequently done, for example, in satellite-TV receiving dishes, and also in some types of astronomical telescope ( e.g. , the Green Bank Telescope , the James Webb Space Telescope ). Accurate off-axis reflectors, for use in solar furnaces and other non-critical applications, can be made quite simply by using a rotating furnace , in which the container of molten glass
5376-440: Is unidirectional, designed for maximum response in the direction of the other station, whereas many other antennas are intended to accommodate stations in various directions but are not truly omnidirectional. Since antennas obey reciprocity the same radiation pattern applies to transmission as well as reception of radio waves. A hypothetical antenna that radiates equally in all directions (vertical as well as all horizontal angles)
5488-449: Is usable in most other directions. A number of such dipole elements can be combined into an antenna array such as the Yagi–Uda in order to favor a single horizontal direction, thus termed a beam antenna. The dipole antenna, which is the basis for most antenna designs, is a balanced component, with equal but opposite voltages and currents applied at its two terminals. The vertical antenna
5600-448: The 1 / 4 or 1 / 2 wave , respectively, at which they are resonant. As these antennas are made shorter (for a given frequency) their impedance becomes dominated by a series capacitive (negative) reactance; by adding an appropriate size " loading coil " – a series inductance with equal and opposite (positive) reactance – the antenna's capacitive reactance may be cancelled leaving only
5712-459: The Siege of Syracuse . This seems unlikely to be true, however, as the claim does not appear in sources before the 2nd century CE, and Diocles does not mention it in his book. Parabolic mirrors and reflectors were also studied extensively by the physicist Roger Bacon in the 13th century AD. James Gregory , in his 1663 book Optica Promota (1663), pointed out that a reflecting telescope with
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#17327826939925824-405: The lens antenna . The antenna's power gain (or simply "gain") also takes into account the antenna's efficiency, and is often the primary figure of merit. Antennas are characterized by a number of performance measures which a user would be concerned with in selecting or designing an antenna for a particular application. A plot of the directional characteristics in the space surrounding the antenna
5936-427: The natural logarithm of x , i.e. its logarithm to base " e ". The volume of the dish is given by 1 2 π R 2 D , {\textstyle {\frac {1}{2}}\pi R^{2}D,} where the symbols are defined as above. This can be compared with the formulae for the volumes of a cylinder ( π R 2 D ) , {\textstyle (\pi R^{2}D),}
6048-403: The resonance principle. This relies on the behaviour of moving electrons, which reflect off surfaces where the dielectric constant changes, in a fashion similar to the way light reflects when optical properties change. In these designs, the reflective surface is created by the end of a conductor, normally a thin metal wire or rod, which in the simplest case has a feed point at one end where it
6160-479: The Scheffler reflector is not suitable for purposes that require high accuracy. It is used in applications such as solar cooking , where sunlight has to be focused well enough to strike a cooking pot, but not to an exact point. A circular paraboloid is theoretically unlimited in size. Any practical reflector uses just a segment of it. Often, the segment includes the vertex of the paraboloid, where its curvature
6272-467: The addition of a loading coil. Then it may be said that the coil has lengthened the antenna to achieve an electrical length of 2.5 meters. However, the resulting resistive impedance achieved will be quite a bit lower than that of a true 1 / 4 wave (resonant) monopole, often requiring further impedance matching (a transformer) to the desired transmission line. For ever shorter antennas (requiring greater "electrical lengthening")
6384-402: The antenna consisting of a thin conductor. Antennas for use over much broader frequency ranges are achieved using further techniques. Adjustment of a matching network can, in principle, allow for any antenna to be matched at any frequency. Thus the small loop antenna built into most AM broadcast (medium wave) receivers has a very narrow bandwidth, but is tuned using a parallel capacitance which
6496-412: The antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception , an antenna intercepts some of the power of a radio wave in order to produce an electric current at its terminals, that is applied to a receiver to be amplified . Antennas are essential components of all radio equipment. An antenna is an array of conductors ( elements ), electrically connected to
6608-400: The antenna structure, which need not be directly connected to the receiver or transmitter, increase its directionality. Antenna "gain" describes the concentration of radiated power into a particular solid angle of space. "Gain" is perhaps an unfortunately chosen term, by comparison with amplifier "gain" which implies a net increase in power. In contrast, for antenna "gain", the power increased in
6720-474: The antenna to the power radiated by a half-wave dipole antenna I dipole {\displaystyle I_{\text{dipole}}} ; these units are called decibels-dipole (dBd) Since the gain of a half-wave dipole is 2.15 dBi and the logarithm of a product is additive, the gain in dBi is just 2.15 decibels greater than the gain in dBd High-gain antennas have the advantage of longer range and better signal quality, but must be aimed carefully at
6832-401: The axis (or if the emitting point source is not placed in the focus), parabolic reflectors suffer from an aberration called coma . This is primarily of interest in telescopes because most other applications do not require sharp resolution off the axis of the parabola. The precision to which a parabolic dish must be made in order to focus energy well depends on the wavelength of the energy. If
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#17327826939926944-417: The axis. Parabolic reflectors are used to collect energy from a distant source (for example sound waves or incoming star light). Since the principles of reflection are reversible, parabolic reflectors can also be used to collimate radiation from an isotropic source into a parallel beam . In optics , parabolic mirrors are used to gather light in reflecting telescopes and solar furnaces , and project
7056-427: The broadside direction. If higher gain is needed one cannot simply make the antenna larger. Due to the constraint on the effective area of a receiving antenna detailed below , one sees that for an already-efficient antenna design, the only way to increase gain (effective area) is by reducing the antenna's gain in another direction. If a half-wave dipole is not connected to an external circuit but rather shorted out at
7168-470: The degree of directivity of the antenna's radiation pattern . A high-gain antenna will radiate most of its power in a particular direction, while a low-gain antenna will radiate over a wide angle. The antenna gain , or power gain of an antenna is defined as the ratio of the intensity (power per unit surface area) I {\displaystyle I} radiated by the antenna in the direction of its maximum output, at an arbitrary distance, divided by
7280-557: The design operating frequency, f o , and antennas are normally designed to be this size. However, feeding that element with 3 f o (whose wavelength is 1 / 3 that of f o ) will also lead to a standing wave pattern. Thus, an antenna element is also resonant when its length is 3 / 4 of a wavelength. This is true for all odd multiples of 1 / 4 wavelength. This allows some flexibility of design in terms of antenna lengths and feed points. Antennas used in such
7392-463: The desired direction is at the expense of power reduced in undesired directions. Unlike amplifiers, antennas are electrically " passive " devices which conserve total power, and there is no increase in total power above that delivered from the power source (the transmitter), only improved distribution of that fixed total. A phased array consists of two or more simple antennas which are connected together through an electrical network. This often involves
7504-474: The dish is wrong by a quarter of a wavelength, then the reflected energy will be wrong by a half wavelength, which means that it will interfere destructively with energy that has been reflected properly from another part of the dish. To prevent this, the dish must be made correctly to within about 1 / 20 of a wavelength. The wavelength range of visible light is between about 400 and 700 nanometres (nm), so in order to focus all visible light well,
7616-467: The dish, from a light source in the focus, is given by 1 − arctan R D − F π {\textstyle 1-{\frac {\arctan {\frac {R}{D-F}}}{\pi }}} , where F , {\displaystyle F,} D , {\displaystyle D,} and R {\displaystyle R} are defined as above. The parabolic reflector functions due to
7728-480: The dish. The principle of parabolic reflectors has been known since classical antiquity , when the mathematician Diocles described them in his book On Burning Mirrors and proved that they focus a parallel beam to a point. Archimedes in the third century BCE studied paraboloids as part of his study of hydrostatic equilibrium , and it has been claimed that he used reflectors to set the Roman fleet alight during
7840-405: The electromagnetic field. Radio waves are electromagnetic waves which carry signals through the air (or through space) at the speed of light with almost no transmission loss . Antennas can be classified as omnidirectional , radiating energy approximately equally in all horizontal directions, or directional , where radio waves are concentrated in some direction(s). A so-called beam antenna
7952-427: The emission of energy from the resonant antenna to free space. The Q of a narrow band antenna can be as high as 15. On the other hand, the reactance at the same off-resonant frequency of one using thick elements is much less, consequently resulting in a Q as low as 5. These two antennas may perform equivalently at the resonant frequency, but the second antenna will perform over a bandwidth 3 times as wide as
8064-418: The entire system of reflecting elements (normally at the focus of the parabolic dish or at the throat of a horn) which could be considered the one active element in that antenna system. A microwave antenna may also be fed directly from a waveguide in place of a (conductive) transmission line . An antenna counterpoise , or ground plane , is a structure of conductive material which improves or substitutes for
8176-648: The equivalent: P = R 2 2 D {\textstyle P={\frac {R^{2}}{2D}}} ) and Q = P 2 + R 2 {\textstyle Q={\sqrt {P^{2}+R^{2}}}} , where F , D , and R are defined as above. The diameter of the dish, measured along the surface, is then given by: R Q P + P ln ( R + Q P ) {\textstyle {\frac {RQ}{P}}+P\ln \left({\frac {R+Q}{P}}\right)} , where ln ( x ) {\textstyle \ln(x)} means
8288-526: The feedline and antenna is measured by a parameter called the standing wave ratio (SWR) on the feedline. Consider a half-wave dipole designed to work with signals with wavelength 1 m, meaning the antenna would be approximately 50 cm from tip to tip. If the element has a length-to-diameter ratio of 1000, it will have an inherent impedance of about 63 ohms resistive. Using the appropriate transmission wire or balun, we match that resistance to ensure minimum signal reflection. Feeding that antenna with
8400-430: The feedpoint, then it becomes a resonant half-wave element which efficiently produces a standing wave in response to an impinging radio wave. Because there is no load to absorb that power, it retransmits all of that power, possibly with a phase shift which is critically dependent on the element's exact length. Thus such a conductor can be arranged in order to transmit a second copy of a transmitter's signal in order to affect
8512-445: The first half of the 1990s the transmitting aerial of the old tower was removed and replaced by a radome holding air traffic control radar equipment. The Brocken Transmitter is property of Deutsche Telekom . 51°48′02″N 10°36′51″E / 51.8005°N 10.6141°E / 51.8005; 10.6141 Antenna (radio) In radio engineering , an antenna ( American English ) or aerial ( British English )
8624-438: The focal point of parabolic reflectors for both transmitting and receiving. Starting in 1895, Guglielmo Marconi began development of antennas practical for long-distance, wireless telegraphy, for which he received the 1909 Nobel Prize in physics . The words antenna and aerial are used interchangeably. Occasionally the equivalent term "aerial" is used to specifically mean an elevated horizontal wire antenna. The origin of
8736-406: The focus to the dish can be transmitted outward in a beam that is parallel to the axis of the dish. In contrast with spherical reflectors , which suffer from a spherical aberration that becomes stronger as the ratio of the beam diameter to the focal distance becomes larger, parabolic reflectors can be made to accommodate beams of any width. However, if the incoming beam makes a non-zero angle with
8848-446: The gain of an antenna used for transmitting must be proportional to its effective area when used for receiving. Consider an antenna with no loss , that is, one whose electrical efficiency is 100%. It can be shown that its effective area averaged over all directions must be equal to λ /4π , the wavelength squared divided by 4π . Gain is defined such that the average gain over all directions for an antenna with 100% electrical efficiency
8960-410: The geometric properties of the paraboloidal shape: any incoming ray that is parallel to the axis of the dish will be reflected to a central point, or " focus ". (For a geometrical proof, click here .) Because many types of energy can be reflected in this way, parabolic reflectors can be used to collect and concentrate energy entering the reflector at a particular angle. Similarly, energy radiating from
9072-439: The geometrical divergence of the transmitted wave. For a given incoming flux, the power acquired by a receiving antenna is proportional to its effective area . This parameter compares the amount of power captured by a receiving antenna in comparison to the flux of an incoming wave (measured in terms of the signal's power density in watts per square metre). A half-wave dipole has an effective area of about 0.13 λ seen from
9184-474: The ground. It may be connected to or insulated from the natural ground. In a monopole antenna, this aids in the function of the natural ground, particularly where variations (or limitations) of the characteristics of the natural ground interfere with its proper function. Such a structure is normally connected to the return connection of an unbalanced transmission line such as the shield of a coaxial cable . An electromagnetic wave refractor in some aperture antennas
9296-413: The increase in signal power due to an amplifying device placed at the front-end of the system, such as a low-noise amplifier . The effective area or effective aperture of a receiving antenna expresses the portion of the power of a passing electromagnetic wave which the antenna delivers to its terminals, expressed in terms of an equivalent area. For instance, if a radio wave passing a given location has
9408-405: The intensity I iso {\displaystyle I_{\text{iso}}} radiated at the same distance by a hypothetical isotropic antenna which radiates equal power in all directions. This dimensionless ratio is usually expressed logarithmically in decibels , these units are called decibels-isotropic (dBi) A second unit used to measure gain is the ratio of the power radiated by
9520-580: The legs are three decks for directional radio transmission aerials. The new TV tower is not accessible to the public. While Germany was divided into East and West , the Brocken transmitter was used for TV and FM-transmissions, even though it lay in the restricted area of the east-west frontier (on the Eastern Side). Its location so close to the border enabled it to be received in parts of the West. In
9632-417: The loading coil, relative to the decreased radiation resistance, entail a reduced electrical efficiency , which can be of great concern for a transmitting antenna, but bandwidth is the major factor that sets the size of antennas at 1 MHz and lower frequencies. The radiant flux as a function of the distance from the transmitting antenna varies according to the inverse-square law , since that describes
9744-435: The log-periodic principle it obtains the unique property of maintaining its performance characteristics (gain and impedance) over a very large bandwidth. When a radio wave hits a large conducting sheet it is reflected (with the phase of the electric field reversed) just as a mirror reflects light. Placing such a reflector behind an otherwise non-directional antenna will insure that the power that would have gone in its direction
9856-553: The material has the same response to an electric current or magnetic field in one direction, as it has to the field or current in the opposite direction. Most materials used in antennas meet these conditions, but some microwave antennas use high-tech components such as isolators and circulators , made of nonreciprocal materials such as ferrite . These can be used to give the antenna a different behavior on receiving than it has on transmitting, which can be useful in applications like radar . The majority of antenna designs are based on
9968-451: The order of ten millimetres, so dishes to focus these waves can be wrong by half a millimetre or so and still perform well. It is sometimes useful if the centre of mass of a reflector dish coincides with its focus . This allows it to be easily turned so it can be aimed at a moving source of light, such as the Sun in the sky, while its focus, where the target is located, is stationary. The dish
10080-530: The origin and its axis of symmetry along the y-axis, so the parabola opens upward, its equation is 4 f y = x 2 {\textstyle 4fy=x^{2}} , where f {\textstyle f} is its focal length. (See " Parabola#In a cartesian coordinate system ".) Correspondingly, the dimensions of a symmetrical paraboloidal dish are related by the equation: 4 F D = R 2 {\textstyle 4FD=R^{2}} , where F {\textstyle F}
10192-653: The other parasitic elements interact with the electromagnetic field in order to realize a highly directional antenna but with a narrow bandwidth. Even greater directionality can be obtained using aperture antennas such as the parabolic reflector or horn antenna . Since high directivity in an antenna depends on it being large compared to the wavelength, highly directional antennas (thus with high antenna gain ) become more practical at higher frequencies ( UHF and above). At low frequencies (such as AM broadcast ), arrays of vertical towers are used to achieve directionality and they will occupy large areas of land. For reception,
10304-409: The other antenna. An example of a high-gain antenna is a parabolic dish such as a satellite television antenna. Low-gain antennas have shorter range, but the orientation of the antenna is relatively unimportant. An example of a low-gain antenna is the whip antenna found on portable radios and cordless phones . Antenna gain should not be confused with amplifier gain , a separate parameter measuring
10416-464: The other side connected to ground or an equivalent ground plane (or counterpoise ). Monopoles, which are one-half the size of a dipole, are common for long-wavelength radio signals where a dipole would be impractically large. Another common design is the folded dipole which consists of two (or more) half-wave dipoles placed side by side and connected at their ends but only one of which is driven. The standing wave forms with this desired pattern at
10528-401: The others present a high impedance. Another solution uses traps , parallel resonant circuits which are strategically placed in breaks created in long antenna elements. When used at the trap's particular resonant frequency the trap presents a very high impedance (parallel resonance) effectively truncating the element at the location of the trap; if positioned correctly, the truncated element makes
10640-503: The parabolic reflector are in satellite dishes , reflecting telescopes , radio telescopes , parabolic microphones , solar cookers , and many lighting devices such as spotlights , car headlights , PAR lamps and LED housings. The Olympic Flame is traditionally lit at Olympia, Greece , using a parabolic reflector concentrating sunlight , and is then transported to the venue of the Games. Parabolic mirrors are one of many shapes for
10752-465: The phases applied to each element the radiation pattern can be shifted without physically moving the antenna elements. Another common array antenna is the log-periodic dipole array which has an appearance similar to the Yagi (with a number of parallel elements along a boom) but is totally dissimilar in operation as all elements are connected electrically to the adjacent element with a phase reversal; using
10864-488: The power that would be received by an antenna of a specified gain, as illustrated by the above example. The radiation pattern of an antenna is a plot of the relative field strength of the radio waves emitted by the antenna at different angles in the far field. It is typically represented by a three-dimensional graph, or polar plots of the horizontal and vertical cross sections. The pattern of an ideal isotropic antenna , which radiates equally in all directions, would look like
10976-444: The radiation pattern (and feedpoint impedance) of the element electrically connected to the transmitter. Antenna elements used in this way are known as passive radiators . A Yagi–Uda array uses passive elements to greatly increase gain in one direction (at the expense of other directions). A number of parallel approximately half-wave elements (of very specific lengths) are situated parallel to each other, at specific positions, along
11088-421: The radiation resistance plummets (approximately according to the square of the antenna length), so that the mismatch due to a net reactance away from the electrical resonance worsens. Or one could as well say that the equivalent resonant circuit of the antenna system has a higher Q factor and thus a reduced bandwidth, which can even become inadequate for the transmitted signal's spectrum. Resistive losses due to
11200-480: The radio waves into a beam or other desired radiation pattern . Strong directivity and good efficiency when transmitting are hard to achieve with antennas with dimensions that are much smaller than a half wavelength . The first antennas were built in 1888 by German physicist Heinrich Hertz in his pioneering experiments to prove the existence of electromagnetic waves predicted by the 1867 electromagnetic theory of James Clerk Maxwell . Hertz placed dipole antennas at
11312-399: The receiver or transmitter. Antennas can be designed to transmit and receive radio waves in all horizontal directions equally ( omnidirectional antennas ), or preferentially in a particular direction ( directional , or high-gain, or "beam" antennas). An antenna may include components not connected to the transmitter, parabolic reflectors , horns , or parasitic elements , which serve to direct
11424-441: The reflector's weight and wind load . Specular reflection of radio waves is also employed in a parabolic reflector antenna, in which a curved reflecting surface effects focussing of an incoming wave toward a so-called feed antenna ; this results in an antenna system with an effective area comparable to the size of the reflector itself. Other concepts from geometrical optics are also employed in antenna technology, such as with
11536-495: The rim is 2.7187 F . The angular radius of the rim as seen from the focal point is 72.68 degrees. The focus-balanced configuration (see above) requires the depth of the reflector dish to be greater than its focal length, so the focus is within the dish. This can lead to the focus being difficult to access. An alternative approach is exemplified by the Scheffler reflector , named after its inventor, Wolfgang Scheffler . This
11648-458: The same axis (or collinear ), each feeding one side of a two-conductor transmission wire. The physical arrangement of the two elements places them 180 degrees out of phase, which means that at any given instant one of the elements is driving current into the transmission line while the other is pulling it out. The monopole antenna is essentially one half of the half-wave dipole, a single 1 / 4 wavelength element with
11760-425: The same whether the antenna is transmitting or receiving . For example, the "receiving pattern" (sensitivity to incoming signals as a function of direction) of an antenna when used for reception is identical to the radiation pattern of the antenna when it is driven and functions as a radiator, even though the current and voltage distributions on the antenna itself are different for receiving and sending. This
11872-432: The same. Electrically this appears to be a very high impedance. The antenna and transmission line no longer have the same impedance, and the signal will be reflected back into the antenna, reducing output. This could be addressed by changing the matching system between the antenna and transmission line, but that solution only works well at the new design frequency. The result is that the resonant antenna will efficiently feed
11984-469: The signal's instantaneous field. When the resulting current reaches the end of the conductor, it reflects, which is equivalent to a 180 degree change in phase. If the conductor is 1 / 4 of a wavelength long, current from the feed point will undergo 90 degree phase change by the time it reaches the end of the conductor, reflect through 180 degrees, and then another 90 degrees as it travels back. That means it has undergone
12096-420: The third. A more complex calculation is needed to find the diameter of the dish measured along its surface . This is sometimes called the "linear diameter", and equals the diameter of a flat, circular sheet of material, usually metal, which is the right size to be cut and bent to make the dish. Two intermediate results are useful in the calculation: P = 2 F {\textstyle P=2F} (or
12208-460: The three-dimensional shape of the reflector is called a paraboloid . A parabola is the two-dimensional figure. (The distinction is like that between a sphere and a circle.) However, in informal language, the word parabola and its associated adjective parabolic are often used in place of paraboloid and paraboloidal . If a parabola is positioned in Cartesian coordinates with its vertex at
12320-560: 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 father's estate near Bologna and soon began to experiment with long wire "aerials" suspended from a pole. In Italian a tent pole is known as l'antenna centrale , and the pole with the wire was simply called l'antenna . Until then wireless radiating transmitting and receiving elements were known simply as "terminals". Because of his prominence, Marconi's use of
12432-548: The word antenna spread among wireless researchers and enthusiasts, and later to the general public. Antenna may refer broadly to an entire assembly including support structure, enclosure (if any), etc., in addition to the actual RF current-carrying components. A receiving antenna may include not only the passive metal receiving elements, but also an integrated preamplifier or mixer , especially at and above microwave frequencies. Antennas are required by any radio receiver or transmitter to couple its electrical connection to
12544-497: Was transformed into a radar facility. Unlike most modern TV towers, the old tower looks like a block of flats with a square cross section. The arrangement of the windows in the observation deck is similar to those in the restaurant in the Radio tower Berlin . In 1973 a new TV tower was built on Brocken. This 123-metre, freestanding steel-tube tower stands on three legs, which hold shafts for cable and stairways for personnel access. Above
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