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| [[Image:Erdfunkstelle Raisting 2.jpg|250px|thumb|A parabolic [[satellite communications]] antenna at Erdfunkstelle Raisting, the biggest facility for satellite communication in the world, in [[Raisting]], [[Bavaria]], [[Germany]]. It has a [[Cassegrain antenna|Cassegrain]] type feed.]]
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| A '''parabolic antenna''' is an [[antenna (radio)|antenna]] that uses a [[parabolic reflector]], a curved surface with the cross-sectional shape of a [[parabola]], to direct the radio waves. The most common form is shaped like a [[dishware|dish]] and is popularly called a '''dish antenna''' or '''parabolic dish'''. The main advantage of a parabolic antenna is that it has high [[directivity]]. It functions similarly to a [[searchlight]] or [[flashlight]] reflector to direct the radio waves in a narrow beam, or receive radio waves from one particular direction only. Parabolic antennas have some of the highest [[antenna gain|gain]]s, that is, they can produce the narrowest [[beamwidth]]s, of any antenna type.<ref name="ARRL1">{{cite book
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| | last = Straw
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| | first = R. Dean, Ed.
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| | authorlink =
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| | coauthors =
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| | title = The ARRL Antenna Book, 19th Ed.
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| | publisher = American Radio Relay League
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| | year = 2000
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| | location = USA
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| | pages = 19.15
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| | url =
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| | doi =
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| | isbn = 0-87259-817-9}}</ref><ref name="Stutzman">{{cite book
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| | last = Stutzman
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| | first = Warren L.
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| | coauthors = Gary A. Thiele
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| | title = Antenna Theory and Design, 3rd Ed.
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| | publisher = John Wiley & Sons
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| | date = 2012
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| | location = US
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| | pages = 391-392
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| | url = http://books.google.com/books?id=xhZRA1K57wIC&pg=PA838&lpg=PA838&dq=%22partial+gain%22+antenna
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| | doi =
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| | id =
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| | isbn = 0470576642}}</ref> In order to achieve narrow beamwidths, the parabolic reflector must be much larger than the [[wavelength]] of the radio waves used,<ref name="Stutzman" /> so parabolic antennas are used in the high frequency part of the [[radio spectrum]], at [[ultrahigh frequency|UHF]] and [[microwave]] ([[super high frequency|SHF]]) frequencies, at which the wavelengths are small enough that conveniently-sized reflectors can be used.
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| Parabolic antennas are used as [[high-gain antenna]]s for [[point-to-point (telecommunications)|point-to-point communications]], in applications such as [[microwave transmission|microwave relay]] links that carry telephone and television signals between nearby cities, [[wireless network|wireless WAN/LAN]] links for data communications, [[satellite communications]] and spacecraft communication antennas. They are also used in [[radio telescope]]s.
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| The other large use of parabolic antennas is for [[radar]] antennas, in which there is a need to transmit a narrow beam of [[radio waves]] to locate objects like ships, [[airplane]]s, and [[guided missile]]s.<ref name="Stutzman" /> With the advent of [[direct broadcast satellite|home satellite television]] receivers, parabolic antennas have become a common feature of the landscapes of modern countries.<ref name="Stutzman" />
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| [[Image:Parabola with focus and arbitrary line.svg|thumb|upright=1.3|Parabolic antennas are based on the geometrical property of the paraboloid that the paths ''FP<sub>1</sub>Q<sub>1</sub>, FP<sub>2</sub>Q<sub>2</sub>, FP<sub>3</sub>Q<sub>3</sub>'' are all the same length. So a spherical wavefront emitted by a feed antenna at the dish's focus ''F'' will be reflected into an outgoing plane wave ''L'' travelling parallel to the dish's axis ''VF''.]]
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| ==Design==
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| The operating principle of a parabolic antenna is that a point source of radio waves at the [[Focus (optics)|focal point]] in front of a paraboloidal reflector of [[Electrical conduction|conductive]] material will be reflected into a [[Collimated light|collimated]] [[plane wave]] beam along the axis of the reflector. Conversely, an incoming plane wave parallel to the axis will be focused to a point at the focal point. | |
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| A typical parabolic antenna consists of a metal [[parabolic reflector]] with a small [[feed horn|feed antenna]] suspended in front of the reflector at its focus,<ref name="Stutzman" /> pointed back toward the reflector. The reflector is a metallic surface formed into a [[paraboloid]] of revolution and usually truncated in a circular rim that forms the diameter of the antenna.<ref name="Stutzman" /> In a transmitting antenna, [[radio frequency]] [[electric current|current]] from a [[transmitter]] is supplied through a [[transmission line]] cable to the [[Antenna feed|feed antenna]], which converts it into radio waves. The radio waves are emitted back toward the dish by the feed antenna and reflect off the dish into a parallel beam. In a receiving antenna the incoming radio waves bounce off the dish and are focused to a point at the feed antenna, which converts them to electric currents which travel through a [[transmission line]] to the [[radio receiver]].
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| ===Parabolic reflector===
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| [[Image:Screen dish antenna.jpg|thumb|Wire grid-type parabolic antenna used for [[Multichannel Multipoint Distribution Service|MMDS]] data link at a [[frequency]] of 2.5-2.7 GHz. It is fed by a vertical [[dipole antenna|dipole]] under the small aluminum reflector on the boom. It radiates [[vertical polarization|vertically polarized]] microwaves. ]]
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| The reflector can be of sheet metal, metal screen, or wire grill construction, and it can be either a circular "dish" or various other shapes to create different beam shapes. A metal screen reflects radio waves as well as a solid metal surface as long as the holes are smaller than one-tenth of a [[wavelength]], so screen reflectors are often used to reduce weight and wind loads on the dish. To achieve the maximum [[antenna gain|gain]], it is necessary that the shape of the dish be accurate within a small fraction of a wavelength, to ensure the waves from different parts of the antenna arrive at the focus [[in phase]]. Large dishes often require a supporting [[truss]] structure behind them to provide the required stiffness.
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| A reflector made of a grill of parallel wires or bars oriented in one direction acts as a ''[[polarizing filter]]'' as well as a reflector. It only reflects [[linear polarization|linearly polarized]] radio waves, with the [[electric field]] parallel to the grill elements. This type is often used in [[radar]] antennas. Combined with a linearly polarized [[feed horn]], it helps filter out noise in the receiver and reduces false returns.
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| ===Feed antenna===
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| The feed antenna at the reflector's focus is typically a [[low-gain antenna|low-gain]] type such as a [[dipole antenna|half-wave dipole]] or more often a small [[horn antenna]] called a [[feed horn]]. In more complex designs, such as the [[Cassegrain antenna|Cassegrain]] and Gregorian, a secondary reflector is used to direct the energy into the parabolic reflector from a feed antenna located away from the primary focal point. The feed antenna is connected to the associated radio-frequency (RF) [[transmitter|transmitting]] or [[radio receiver|receiving]] equipment by means of a [[coaxial cable]] [[transmission line]] or [[waveguide]].
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| An advantage of parabolic antennas is that most of the structure of the antenna (all of it except the feed antenna) is [[Resonance|nonresonant]], so it can function over a wide range of [[Frequency|frequencies]], that is a wide [[bandwidth (signal processing)|bandwidth]]. All that is necessary to change the frequency of operation is to replace the feed antenna with one that works at the new frequency. Some parabolic antennas transmit or receive at multiple frequencies by having several feed antennas mounted at the focal point, close together.
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| {{multiple image
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| |header = '''Dish parabolic antennas'''
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| |align = center
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| |image1 = Parabolic antennas on a telecommunications tower on Willans Hill.jpg
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| |width1 = 168
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| |caption1 = Shrouded microwave relay dishes on a communications tower in Australia.
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| |image2 = SuperDISH121.jpg
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| |width2 = 96
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| |caption2 = A satellite television dish, an example of an offset fed dish.
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| |image3 = Antenna 03.JPG
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| |width3 = 84
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| |caption3 = Cassegrain satellite communication antenna in Sweden.
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| |image4 = ATA-gregorian.jpg
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| |width4 = 160
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| |caption4 = Offset Gregorian antenna used in the [[Allen Telescope Array]], a [[radio telescope]] at the University of California at Berkeley, USA.
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| |image5 =
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| |caption5 =
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| }}
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| {{multiple image
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| |align = center
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| |header = '''Shaped-beam parabolic antennas'''
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| |image1 = Mps-16-1.jpg
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| |width1 = 108
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| |caption1 = Vertical "orange peel" antenna for military height finder radar, Germany.
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| |image2 = Bundesarchiv Bild 102-12453, Nauen, Richtungsweiser für Funkwellen.jpg
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| |width2 = 101
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| |caption2 = Early cylindrical parabolic antenna, 1931, Nauen, Germany.
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| |image3 = Deister-radar.jpg
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| |width3 = 105
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| |caption3 = Air traffic control radar antenna, near Hannover, Germany.
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| |image4 = ASR-9 Radar Antenna.jpg
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| |width4 = 180
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| |caption4 = ASR-9 Airport surveillance radar antenna.
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| |image5 = Antenna_radar_L-band_TAR_Finland.JPG
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| |width5 = 105
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| |caption5 = "Orange peel" antenna for air search radar, Finland.
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| }}
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| ==Types==
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| [[Image:Parabolic antenna types2.svg|thumb||Main types of parabolic antenna feeds.]]
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| Parabolic antennas are distinguished by their shapes:
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| * '''''Paraboloidal''''' or '''''dish''''' – The reflector is shaped like a [[paraboloid]] truncated in a circular rim. This is the most common type. It radiates a narrow pencil-shaped beam along the axis of the dish.
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| ** '''''Shrouded dish''''' – Sometimes a cylindrical metal shield is attached to the rim of the dish.<ref name="Lehpamer">{{cite book
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| | last = Lehpamer
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| | first = Harvey
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| | authorlink =
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| | coauthors =
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| | title = Microwave transmission networks: Planning, Design, and Deployment
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| | publisher = McGraw Hill Professional
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| | year = 2010
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| | location = USA
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| | pages = 268–272
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| | url = http://books.google.com/?id=-kiH5WZy88UC&pg=PA263&dq=%22beamwidth%22+%22parabolic+antenna%22#v=onepage&q=%22beamwidth%22%20%22parabolic%20antenna%22&f=false
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| | doi =
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| | isbn = 0-07-170122-2}}</ref> The shroud shields the antenna from radiation from angles outside the main beam axis, reducing the [[sidelobe]]s. It is sometimes used to prevent interference in terrestrial microwave links, where several antennas using the same frequency are located close together. The shroud is coated inside with microwave absorbent material. Shrouds can reduce back lobe radiation by 10 dB.<ref name="Lehpamer" />
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| * '''''Cylindrical''''' – The reflector is curved in only one direction and flat in the other. The radio waves come to a focus not at a point but along a line. The feed is sometimes a [[dipole antenna]] located along the focal line. Cylindrical parabolic antennas radiate a fan-shaped beam, narrow in the curved dimension, and wide in the uncurved dimension. The curved ends of the reflector are sometimes capped by flat plates, to prevent radiation out the ends, and this is called a ''pillbox'' antenna.
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| * '''''Shaped-beam antennas''''' – Modern reflector antennas can be designed to produce a beam or beams of a particular shape, rather than just the narrow "pencil" or "fan" beams of the simple dish and cylindrical antennas above.<ref>A. David Olver (1994) ''[http://books.google.com/books?id=soJiuUwevRIC&pg=PA59&dq=%22shaped-beam+antenna% Microwave Horns and Feeds]'', p. 61-62</ref> Two techniques are used, often in combination, to control the shape of the beam:
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| ** '''''Shaped reflectors''''' – The parabolic reflector can be given a noncircular shape, and/or different curvatures in the horizontal and vertical directions, to alter the shape of the beam. This is often used in radar antennas. As a general principle, the wider the antenna is in a given transverse direction, the narrower the radiation pattern will be in that direction.
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| *** '''''"Orange peel" antenna''''' – Used in search radars, this is a long narrow antenna shaped like the letter "C". It radiates a narrow vertical fan shaped beam.
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| ** '''''Arrays of feeds''''' – In order to produce an arbitrary shaped beam, instead of one feed horn, an array of feed horns clustered around the focal point can be used. Array-fed antennas are often used on communication satellites, particularly [[direct broadcast satellite]]s, to create a downlink radiation pattern to cover a particular continent or coverage area. They are often used with secondary reflector antennas such as the Cassegrain.
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| Parabolic antennas are also classified by the type of ''[[Antenna feed|feed]]'', that is, how the radio waves are supplied to the antenna:<ref name="Lehpamer" />
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| * '''''Axial''''' or '''''front feed''''' – This is the most common type of feed, with the [[Antenna feed|feed antenna]] located in front of the dish at the focus, on the beam axis, pointed back toward the dish. A disadvantage of this type is that the feed and its supports block some of the beam, which limits the aperture efficiency to only 55–60%.<ref name="Lehpamer" />
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| * '''''Off-axis''''' or '''''[[Offset dish antenna|offset feed]]''''' – The reflector is an asymmetrical segment of a paraboloid, so the focus, and the feed antenna, are located to one side of the dish. The purpose of this design is to move the feed structure out of the beam path, so it does not block the beam. It is widely used in home [[satellite television]] dishes, which are small enough that the feed structure would otherwise block a significant percentage of the signal. Offset feed is also used in multiple reflector designs such as the Cassegrain and Gregorian, below.
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| * '''''Cassegrain''''' – In a [[Cassegrain antenna]], the feed is located on or behind the dish, and radiates forward, illuminating a convex [[hyperboloid]]al secondary reflector at the focus of the dish. The radio waves from the feed reflect back off the secondary reflector to the dish, which forms the outgoing beam. An advantage of this configuration is that the feed, with its waveguides and "[[RF front end|front end]]" electronics does not have to be suspended in front of the dish, so it is used for antennas with complicated or bulky feeds, such as large [[satellite communication]] antennas and [[radio telescope]]s. Aperture efficiency is on the order of 65–70%<ref name="Lehpamer" />
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| * '''''Gregorian''''' – Similar to the Cassegrain design except that the secondary reflector is concave, ([[ellipsoid]]al) in shape. Aperture efficiency over 70% can be achieved.<ref name="Lehpamer" />
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| ==Feed pattern==
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| [[Image:Efecto spillover 4.png|thumb|Effect of the feed antenna [[radiation pattern]] ''(small pumpkin-shaped surface)'' on spillover. ''Left:'' With a low gain feed antenna, significant parts of its radiation fall outside the dish. ''Right:'' With a higher gain feed, almost all its radiation is emitted within the angle of the dish.]]
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| The [[radiation pattern]] of the [[Antenna feed|feed antenna]] has to be tailored to the shape of the dish, because it has a strong influence on the ''aperture efficiency'', which determines the antenna gain (see [[#Gain|Gain]] section below). Radiation from the feed that falls outside the edge of the dish is called "''spillover''" and is wasted, reducing the gain and increasing the [[sidelobe|backlobes]], possibly causing [[Electromagnetic interference|interference]] or (in receiving antennas) increasing susceptibility to ground noise. However, maximum gain is only achieved when the dish is uniformly "illuminated" with a constant field strength to its edges. So the ideal radiation pattern of a feed antenna would be a constant field strength throughout the solid angle of the dish, dropping abruptly to zero at the edges. However, practical feed antennas have radiation patterns that drop off gradually at the edges, so the feed antenna is a compromise between acceptably low spillover and adequate illumination. For most front feed horns, optimum illumination is achieved when the power radiated by the feed horn is 10 [[Decibel|dB]] less at the dish edge than its maximum value at the center of the dish.<ref name="ARRL">{{cite book
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| | last = Straw
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| | first = R. Dean, Ed.
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| | authorlink =
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| | coauthors =
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| | title = The ARRL Antenna Book, 19th Ed.
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| | publisher = American Radio Relay League
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| | year = 2000
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| | location = USA
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| | pages = 18.14
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| | url =
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| | doi =
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| | isbn = 0-87259-817-9}}</ref>
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| ===Dual reflector shaping===
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| When the highest performance is required, a technique called "dual reflector shaping" may be used in Cassegrain antennas. This involves changing the shape of the sub-reflector to direct more signal power to outer areas of the dish, to map the known pattern of the feed into a uniform illumination of the primary, to maximize the gain. However, this results in a secondary that is no longer precisely hyperbolic (though it is still very close), 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.<ref>{{cite journal
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| |title=Design of dual-reflector antennas with arbitrary phase and amplitude distributions
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| |author=Galindo, V.
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| |journal=Antennas and Propagation, IEEE Transactions on
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| |volume=12
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| |issue=4
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| |pages=403–408
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| |year=1964
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| |publisher=IEEE
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| |url=http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1138236&tag=1
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| |doi=10.1109/TAP.1964.1138236 }}</ref><ref>{{cite journal
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| |title=RF Design and Predicted Performance for a Future 34-Meter Shaped Dual-Reflector Antenna System Using the Common Aperture XS Feedhorn
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| |author=Willams, WF
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| |journal=Telecommunications and Data Acquisition Progress Report
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| |volume=73
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| |pages=74–84
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| |year=1983
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| |url=http://tmo.jpl.nasa.gov/progress_report/42-73/73H.PDF }}</ref> Other dish illumination patterns can also be synthesized, such as patterns with high taper at the dish edge for ultra-low spillover [[sidelobe]]s, and patterns with a central "hole" to reduce feed shadowing.
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| ==History==
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| [[Image:Hertz spark gap transmitter and parabolic antenna.png|thumb|The first parabolic antenna, built by Heinrich Hertz in 1888.]]
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| The idea of using parabolic reflectors for radio antennas was taken from [[optics]], where the power of a parabolic mirror to focus light into a beam has been known since [[classical antiquity]]. The designs of some specific types of parabolic antenna, such as the [[Cassegrain antenna|Cassegrain]] and [[Gregorian telescope|Gregorian]], come from similarly named analogous types of [[reflecting telescope]], which were invented by [[astronomer]]s during the 15th century.<ref name="Olver">{{cite book
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| | last = Olver
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| | first = A. David
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| | authorlink =
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| | coauthors =
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| | title = Microwave horns and feeds
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| | publisher = IET
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| | year = 1994
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| | location = USA
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| | pages = 3
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| | url = http://books.google.com/books?id=soJiuUwevRIC&pg=PA9&dq=how+horns+work+impedance&hl=en&ei=-V7DTpyxEILJiQKS_InyCw&sa=X&oi=book_result&ct=result&resnum=2&sqi=2&ved=0CDIQ6AEwAQ#v=onepage&q=how%20horns%20work%20impedance&f=false
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| | doi =
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| | id =
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| | isbn = 0-7803-1115-9}}</ref><ref name="Stutzman" />
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| German physicist [[Heinrich Hertz]] constructed the world's first parabolic reflector antenna in 1888.<ref name="Stutzman" /> The antenna was a cylindrical parabolic reflector made of zinc sheet metal supported by a wooden frame, and had a spark-gap excited dipole along the focal line. Its aperture was 2 meters high by 1.2 meters wide, with a [[focal length]] of 0.12 meters, and was used at an operating frequency of about 450 MHz. With two such antennas, one used for transmitting and the other for receiving, Hertz demonstrated the existence of [[radio wave]]s which had been predicted by [[James Clerk Maxwell]] some 22 years earlier.<ref>{{cite web |url=http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900009937_1990009937.pdf |title=Large Space Antenna Concepts for ESGP |accessdate=2009-07-31 |last=Love |first=Allan W. |format=PDF |publisher=Rockwell International }}</ref> However, the early development of radio was limited to lower frequencies at which parabolic antennas were unsuitable, and they were not widely used until after World War 2, when microwave frequencies began to be exploited.
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| Italian radio pioneer [[Guglielmo Marconi]] used a parabolic reflector during the 1930s in investigations of UHF transmission from his boat in the Mediterranean.<ref name="Olver" /> In 1931 a 1.7 GHz [[microwave relay]] telephone link across the [[English Channel]] using 10 ft. (3 meter) diameter dishes was demonstrated.<ref name="Olver" /> The first large parabolic antenna, a 9 m dish, was built in 1937 by pioneering radio astronomer [[Grote Reber]] in his backyard,<ref name="Stutzman" /> and the sky survey he did with it was one of the events that founded the field of [[radio astronomy]].<ref name="Olver" />
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| The development of [[radar]] during [[World War II]] provided a great impetus to parabolic antenna research, and saw the evolution of shaped-beam antennas, in which the curve of the reflector is different in the vertical and horizontal directions, tailored to produce a beam with a particular shape.<ref name="Olver" /> During the 1960s dish antennas became widely used in terrestrial [[Microwave transmission|microwave relay]] communication networks, which carried telephone calls and television programs across continents.<ref name="Olver" /> The first parabolic antenna used for satellite communications was constructed in 1962 at [[Goonhilly Satellite Earth Station|Goonhilly]] in [[Cornwall]], England to communicate with the [[Telstar]] satellite. The Cassegrain antenna was developed in Japan in 1963 by [[Nippon Telegraph and Telephone|NTT]], [[KDDI]] and [[Mitsubishi Electric]].<ref name="Makino">{{cite conference
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| | first = Shigero
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| | last = Makino
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| | authorlink =
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| | coauthors =
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| | title = Historical review of reflector antenna systems developed for satellite communication by MELCO
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| | booktitle = ISAP2006-International Symposium on Antennas and Propagation
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| | pages =
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| | publisher = Mitsubishi Electric Corp.
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| | date = 2006
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| | location =
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| | url = http://ap-s.ei.tuat.ac.jp/isapx/2006/pdf/1D2a-1.pdf
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| | doi =
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| | id =
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| | accessdate = 2011-12-24}} on ISAP website</ref> The advent in the 1970s of computer design tools such as [[Numerical Electromagnetics Code|NEC]] capable of calculating the radiation pattern of parabolic antennas has led to the development of sophisticated asymmetric, multireflector and multifeed designs in recent years.
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| ==Gain==
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| {{main|Antenna gain}}
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| [[File:Arecibo Observatory Aerial View.jpg|thumb|upright=1.5|The largest "dish" antenna in the world, the [[radio telescope]] at [[Arecibo Observatory]], Puerto Rico, 1000 feet (305 meters) in diameter. It has a gain of about 10 million, or 70 dBi, at 2.38 GHz.<ref>{{cite book
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| | last = Drentea
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| | first = Cornell
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| | authorlink =
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| | coauthors =
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| | title = Modern Communications Receiver Design and Technology
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| | publisher = Artech House
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| | year = 2010
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| | location = USA
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| | pages = 369
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| | url = http://books.google.com/?id=9juUwbKP-58C&pg=PA369&dq=Arecibo+observatory+%22radio+telescope%22+gain#v=onepage&q=Arecibo%20observatory%20%22radio%20telescope%22%20gain&f=false
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| | doi =
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| | isbn = 1-59693-309-7}}</ref> The dish is constructed into a valley in the landscape, so it is not steerable. To steer the antenna to point to different regions in the sky, the feed antenna suspended by cables over the dish is moved. The dish actually has a [[spherical reflector|spherical]] rather than a parabolic shape, which reduces the [[Aberration in optical systems|aberration]]s caused by moving the feed point, but also means that the received energy comes to a focus along a line rather than a single point. This spherical aberration can be corrected by a secondary reflector of the proper shape. ]]
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| The directive qualities of an antenna are measured by a dimensionless parameter called its [[Antenna gain|gain]], which is the ratio of the power received by the antenna from a source along its beam axis to the power received by a hypothetical [[Isotropic radiator|isotropic antenna]]. The gain of a parabolic antenna is:<ref name="Anderson">{{cite book
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| | last = Anderson
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| | first = Harry R.
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| | authorlink =
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| | coauthors =
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| | title = Fixed broadband wireless system design
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| | publisher = John Wiley & Sons
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| | year = 2003
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| | location = USA
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| | pages = 206–207
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| | url = http://books.google.com/?id=r-o3SmNsvD8C&pg=PA205&dq=parabolic+antenna+design#v=onepage&q=parabolic%20antenna%20design&f=false
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| | doi =
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| | isbn = 0-470-84438-8}}</ref>
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| :<math>G = \frac{4 \pi A}{\lambda^2}e_A = \frac{\pi^2d^2}{\lambda^2}e_A</math>
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| where:
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| :<math>A</math> is the area of the antenna aperture, that is, the mouth of the parabolic reflector
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| :<math>d</math> is the diameter of the parabolic reflector, if it is circular
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| :<math>\lambda</math> is the wavelength of the radio waves.
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| :<math>e_A</math> is a dimensionless parameter between 0 and 1 called the ''[[Antenna aperture|aperture efficiency]]''. The aperture efficiency of typical parabolic antennas is 0.55 to 0.70.
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| It can be seen that, as with any ''aperture antenna'', the larger the aperture is, compared to the [[wavelength]], the higher the gain. The gain increases with the square of the ratio of aperture width to wavelength, so large parabolic antennas, such as those used for spacecraft communication and [[radio telescope]]s, can have extremely high gain. Applying the above formula to the 25-meter-diameter antennas often used in [[radio telescope]] arrays and satellite ground antennas at a wavelength of 21 cm (1.42 GHz, a common [[radio astronomy]] frequency), yields an approximate maximum gain of 140,000 times or about 50 dBi ([[decibel]]s above the [[isotropic antenna|isotropic]] level).
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| Aperture efficiency ''e''<sub>A</sub> is a catchall variable which accounts for various losses that reduce the gain of the antenna from the maximum that could be achieved with the given aperture. The major factors reducing the aperture efficiency in parabolic antennas are:.<ref name="Pattan">{{cite book
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| | last = Pattan
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| | first = Bruno
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| | authorlink =
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| | coauthors =
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| | title = Satellite systems: principles and technologies
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| | publisher = Springer
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| | year = 1993
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| | location = USA
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| | pages = 267
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| | url = http://books.google.com/?id=0GJWEro9ea4C&pg=PA267&dq=aperture+efficiency#v=onepage&q=horn%20antenna&f=false
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| | doi =
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| | isbn = 0-442-01357-4}}</ref>
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| *''Feed spillover'' - Some of the radiation from the [[Antenna feed|feed antenna]] falls outside the edge of the dish and so doesn't contribute to the main beam.
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| *''Feed illumination taper'' - The maximum gain for any aperture antenna is only achieved when the intensity of the radiated beam is constant across the entire aperture area. However the radiation pattern from the feed antenna usually tapers off toward the outer part of the dish, so the outer parts of the dish are "illuminated" with a lower intensity of radiation. Even if the feed provided constant illumination across the angle subtended by the dish, the outer parts of the dish are farther away from the feed antenna than the inner parts, so the intensity would drop off with distance from the center. So the intensity of the beam radiated by a parabolic antenna is maximum at the center of the dish and falls off with distance from the axis, reducing the efficiency.
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| *''Aperture blockage'' - In front-fed parabolic dishes where the feed antenna is located in front of the dish in the beam path (and in Cassegrain and Gregorian designs as well), the feed structure and its supports block some of the beam. In small dishes such as home satellite dishes, where the size of the feed structure is comparable with the size of the dish, this can seriously reduce the antenna gain. To prevent this problem these types of antennas often use an ''offset'' feed, where the feed antenna is located to one side, outside the beam area. The aperture efficiency for these types of antennas can reach 0.7 to 0.8.
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| *''Shape errors'' - random surface errors in the shape of the reflector reduce efficiency. The loss is approximated by [[Ruze's Equation]].
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| For theoretical considerations of mutual interference (at frequencies between 2 and c. 30 GHz - typically in the [[Fixed Service Satellite|Fixed Satellite Service]]) where specific antenna performance has not been defined, a ''reference antenna'' based on Recommendation [[ITU-R]] S.465 is used to calculate the interference, which will include the likely sidelobes for off-axis effects.
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| ==Radiation pattern==
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| [[Image:Uplink3.png|thumb|[[Radiation pattern]] of a German parabolic antenna. The main lobe ''(top)'' is only a few degrees wide. The sidelobes are all at least 20 dB below (1/100 the power density of) the main lobe, and most are 30 dB below. ]]
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| In parabolic antennas, virtually all the power radiated is concentrated in a narrow [[main lobe]] along the antenna's axis. The residual power is radiated in [[sidelobe]]s, usually much smaller, in other directions. Because in parabolic antennas the reflector aperture is much larger than the wavelength, due to diffraction there are usually many narrow sidelobes, so the sidelobe pattern is complex. There is also usually a [[sidelobe|backlobe]], in the opposite direction to the main lobe, due to the spillover radiation from the feed antenna that misses the reflector.
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| ===Beamwidth===
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| The angular width of the beam radiated by high-gain antennas is measured by the ''[[Beam width|half-power beam width]]'' (HPBW), which is the angular separation between the points on the antenna [[radiation pattern]] at which the power drops to one-half (-3 dB) its maximum value. For parabolic antennas, the HPBW ''θ'' is given by:<ref name="ARRL" /><ref name="Minoli">{{cite book
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| | last = Minoli
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| | first = Daniel
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| | authorlink =
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| | coauthors =
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| | title = Satellite Systems Engineering in an IPv6 Environment
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| | publisher = CRC Press
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| | year = 2009
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| | location = USA
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| | pages = 78
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| | url = http://books.google.com/?id=4yJi1UQDPp8C&pg=PA80&dq=%22beamwidth%22+%22parabolic+antenna%22#v=onepage&q=%22beamwidth%22%20%22parabolic%20antenna%22&f=false
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| | doi =
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| | isbn = 1-4200-7868-2}}</ref>
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| :<math>\theta = k\lambda / d \,</math>
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| where ''k'' is a factor which varies slightly depending on the shape of the reflector and the feed illumination pattern. For a "typical" parabolic antenna ''k'' = 70 when ''θ'' is in degrees.<ref name="Minoli" />
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| For a typical 2 meter [[satellite dish]] operating on [[C band]] (4 GHz), this formula gives a beamwidth of about 2.6°. For the Arecibo antenna at 2.4 GHz the beamwidth is 0.028°. It can be seen that parabolic antennas can produce very narrow beams, and aiming them can be a problem. Some parabolic dishes are equipped with a [[Antenna boresight|boresight]] so they can be aimed accurately at the other antenna.
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| It can be seen there is an inverse relation between gain and beam width. By combining the beamwidth equation with the gain equation, the relation is:<ref name="Minoli" />
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| :<math>G = \left ( \frac{\pi k}{\theta} \right )^2 \ e_A </math>
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| ==See also==
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| {{commons category|Parabolic antennas}}
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| *[[Parabolic reflector]]
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| *[[Radio telescope]]
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| *[[Satellite dish]]
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| *[[Cassegrain antenna]]
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| *[[Offset dish antenna]]
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| *[[Simulsat]] (a quasi parabolic antenna which is [[spherical reflector|spherical]] in one plane and parabolic in another)
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| *[[Feed horn]]
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| ==References==
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| {{Reflist}}
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| ==External links==
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| * [http://urbanwireless.info/antennas/dish-with-biquad-feed WiFi: Parabolic Dish with BiQuad feeder]
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| * [http://www.sat-direction.com/ Online Satellite Finder Based on Google Maps]
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| * [http://www.freeantennas.com/projects/template2/index.html Antenna types: Parabolic Antenna for WiFi]
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| * Online pointing utility using google maps, and each satellite channel list: http://www.dishpointer.com/
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| * [http://www.youtube.com/watch?v=785kRIZ7aeI Animation of Propagation from a Parabolic Dish Antenna] from YouTube
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| * [http://www.radio-electronics.com/info/antennas/parabolic/parabolic_reflector.php Parabolic reflector antenna tutorial] Theory and practice
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| {{Antenna Types}}
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| {{Telecommunications}}
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| [[Category:Radio frequency antenna types]]
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| [[Category:Satellite broadcasting]]
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