Information about Parabolic Antenna
The parabolic antenna is a high-gain reflector antenna used for radio, television and data communications, and also for radiolocation (RADAR), on the UHF and SHF parts of the electromagnetic spectrum. The relatively short wavelength of electromagnetic (radio) energy at these frequencies allows reasonably sized reflectors to exhibit the very desirable highly directional response for both receiving and transmitting.
A typical parabolic antenna consists of a parabolic reflector illuminated by a small feed antenna.
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. This paraboloid possesses a distinct focal point by virtue of having the reflective property of parabolas in that a point light source at this focus produces a parallel light beam aligned with the axis of revolution.
The feed antenna is placed at the reflector focus. This antenna is typically a low-gain type such as a half-wave dipole or a small waveguide horn. In more complex designs, such as the Cassegrain antenna, a sub-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) transmitting or receiving equipment by means of a coaxial cable transmission line or hollow waveguide.
Considering the parabolic antenna as a circular aperture gives the following approximation for the maximum gain:
or
where:
Practical considerations of antenna effective area and sidelobe suppression reduce the actual gain obtained to between 35 and 55 percent of this theoretical value. For theoretical considerations of mutual interference (at frequencies between 2 and c. 30 GHz - typically in the 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.
Applying the formula to just one of the 25-meter-diameter VLA antennas shown in the illustration for 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 (decibels above the isotropic level).
With the advent of TVRO and DBS satellite television, the parabolic antenna became a ubiquitous feature of urban, suburban, and even rural, landscapes. Extensive terrestrial microwave links, such as those between cellphone base stations, and wireless WAN/LAN applications have also proliferated this antenna type. Earlier applications included ground-based and airborne radar and radio astronomy. The largest "dish" antenna in the world is the radio telescope at Arecibo, PR, but, for beam-steering reasons, it is actually a spherical, rather than parabolic, reflector.
More exotic types include the off-set parabolic antenna, Gregorian and Cassegrain types. In the off-set, the feed element is still located at the focal point, which because of the angles utilized, is usually located below the reflector so that the feed element and support do not interfere with the the main beam. This also allows for easier maintenance of the feed, but is usually only found in smaller antennas.
The Gregorian and Cassegrain types, sometimes generically referred to as "dual optics" antennas, utilize a secondary reflector, or "sub-reflector", allowing for better control over the colimnated beam as well as allowing the antenna feed system to be more compact. These antennas are usually much larger where prime focus and off-set construction are not as practical. The feed element is usually located in a "feed horn" which protrudes out from the main reflector. This setup is used when the feed element is bulky or heavy such as when it contains a pre-amplifier or even the actual receiver or transmitter. Parabolic antenna theory closely follows optics theory. So a Gregorian antenna can be identified by the fact that it uses a concave sub-reflector, while a Cassegrain antenna uses a convex sub-reflector.
The actual 'antenna' in a parabolic antenna, that is, the device that interfaces the transmission line or waveguide containing the radio-frequency energy to free space, is the feed element. The reflector surface is entirely passive. This feed element should usually be at the center of the reflector at the focal point of that dish. The focal point is the point where all reflected waves will be concentrated. The focal length
(distance of focal point from the center of the reflector) is calculated with the following equation:
where:
The radiation from the feed element induces a current flow in the conductive reflector surface which, in turn, re-radiates in the desired direction, perpendicular to the directrix plane of the paraboloid. The feed element can be any one of a multitude of antenna types. Whichever type is used, it must exhibit a directivity that efficiently illuminates the reflector and must have the correct polarization for the application -- the polarization of the feed determining the polarization of the entire antenna system. The simplest feed is a half-wave dipole which is commonly used at lower frequencies, sometimes in conjunction with a closely coupled parasitic reflector or "splash plate". At higher frequencies a horn-type becomes more feasible and efficient. To adapt the horn to a coaxial antenna cable, a length of waveguide is used to effect the transition.
)
Parabolic antennas at the Very Large Array Radio Telescope in New Mexico, USA
A typical parabolic antenna consists of a parabolic reflector illuminated by a small feed antenna.
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. This paraboloid possesses a distinct focal point by virtue of having the reflective property of parabolas in that a point light source at this focus produces a parallel light beam aligned with the axis of revolution.
The feed antenna is placed at the reflector focus. This antenna is typically a low-gain type such as a half-wave dipole or a small waveguide horn. In more complex designs, such as the Cassegrain antenna, a sub-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) transmitting or receiving equipment by means of a coaxial cable transmission line or hollow waveguide.
)
Main types of parabolic antennas
Considering the parabolic antenna as a circular aperture gives the following approximation for the maximum gain:
or
where:
is power gain over isotropic
is reflector diameter in same units as wavelength
is wavelength
Practical considerations of antenna effective area and sidelobe suppression reduce the actual gain obtained to between 35 and 55 percent of this theoretical value. For theoretical considerations of mutual interference (at frequencies between 2 and c. 30 GHz - typically in the 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.
Applying the formula to just one of the 25-meter-diameter VLA antennas shown in the illustration for 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 (decibels above the isotropic level).
With the advent of TVRO and DBS satellite television, the parabolic antenna became a ubiquitous feature of urban, suburban, and even rural, landscapes. Extensive terrestrial microwave links, such as those between cellphone base stations, and wireless WAN/LAN applications have also proliferated this antenna type. Earlier applications included ground-based and airborne radar and radio astronomy. The largest "dish" antenna in the world is the radio telescope at Arecibo, PR, but, for beam-steering reasons, it is actually a spherical, rather than parabolic, reflector.
Structure
The reflector dish can be solid, mesh or wire in construction and it can be either fully circular or somewhat rectangular depending on the radiation pattern of the feeding element. Solid antennas have more ideal characteristics but are troublesome because of weight and high wind load. Mesh and wire types weigh less, are easier to construct and have nearly ideal characteristics if the holes or gaps are kept under 1/10 of the wavelength.)
Wire-type parabolic antenna (Wi-Fi / WLAN antenna at 2,4Ghz). Oriented to provide horizontal polarization: the reflector wires and the feed element are both horizontal. This antenna has a greater extent in the vertical plane and hence, a narrower beamwidth in that plane. The feed element has a wider beam in the vertical direction than the horizontal and hence matches the reflector by illuminating it fully.
More exotic types include the off-set parabolic antenna, Gregorian and Cassegrain types. In the off-set, the feed element is still located at the focal point, which because of the angles utilized, is usually located below the reflector so that the feed element and support do not interfere with the the main beam. This also allows for easier maintenance of the feed, but is usually only found in smaller antennas.
The Gregorian and Cassegrain types, sometimes generically referred to as "dual optics" antennas, utilize a secondary reflector, or "sub-reflector", allowing for better control over the colimnated beam as well as allowing the antenna feed system to be more compact. These antennas are usually much larger where prime focus and off-set construction are not as practical. The feed element is usually located in a "feed horn" which protrudes out from the main reflector. This setup is used when the feed element is bulky or heavy such as when it contains a pre-amplifier or even the actual receiver or transmitter. Parabolic antenna theory closely follows optics theory. So a Gregorian antenna can be identified by the fact that it uses a concave sub-reflector, while a Cassegrain antenna uses a convex sub-reflector.
Feeding parabolic antennas.
)
Antenna feeders
The actual 'antenna' in a parabolic antenna, that is, the device that interfaces the transmission line or waveguide containing the radio-frequency energy to free space, is the feed element. The reflector surface is entirely passive. This feed element should usually be at the center of the reflector at the focal point of that dish. The focal point is the point where all reflected waves will be concentrated. The focal length
(distance of focal point from the center of the reflector) is calculated with the following equation:
where:
is the focal length of the reflector
- is reflector diameter in same units as wavelength
is depth of the reflector
The radiation from the feed element induces a current flow in the conductive reflector surface which, in turn, re-radiates in the desired direction, perpendicular to the directrix plane of the paraboloid. The feed element can be any one of a multitude of antenna types. Whichever type is used, it must exhibit a directivity that efficiently illuminates the reflector and must have the correct polarization for the application -- the polarization of the feed determining the polarization of the entire antenna system. The simplest feed is a half-wave dipole which is commonly used at lower frequencies, sometimes in conjunction with a closely coupled parasitic reflector or "splash plate". At higher frequencies a horn-type becomes more feasible and efficient. To adapt the horn to a coaxial antenna cable, a length of waveguide is used to effect the transition.
See also
- Satellite dish
- Simulsat (a quasi parabolic antenna which is spherical in one plane and parabolic in another)
- Corner reflector
External links
- Antenna types: Parabolic Dish Antenna from Educypedia
Radar is a system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain.
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Cycles per second: 300 MHz to 3 GHz
Wavelength: 1 m to 100 mm Ultra high frequency (UHF) designates a range (band) of electromagnetic waves whose frequency is between 300 MHz and 3 GHz, which is 300 MHz to 3,000 MHz.
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Wavelength: 1 m to 100 mm Ultra high frequency (UHF) designates a range (band) of electromagnetic waves whose frequency is between 300 MHz and 3 GHz, which is 300 MHz to 3,000 MHz.
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In satellite dish and antenna design parlance, a feedhorn (or feed horn) is a horn antenna used to convey radio waves between the transceiver (transmitter and/or receiver) and the reflector.
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paraboloid is a quadric, a type of surface in three dimensions, described by the equation:
or
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- (elliptical paraboloid, opens upward),
or
- (hyperbolic paraboloid, opens up on x-axis and down on y-axis).
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A focal point may mean:
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- Focus (optics), the point at which initially collimated rays of light meet after passing through a convex lens, or reflecting off of a concave mirror.
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The low-gain antenna (LGA) is an antenna with a broad radiowave beam width. This very wide beam allows for a more reliable signal that is best used in mountainous regions, where the signal will propagate reasonably well regardless of terrain.
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dipole antenna, developed by Heinrich Rudolph Hertz around 1886, is an antenna with a center-fed driven element for transmitting or receiving radio frequency energy. These antennas are the simplest practical antennas from a theoretical point of view.
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Electromagnetic waveguides
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horn has the following meanings:
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- In radio transmission, an open-ended waveguide, of increasing cross-sectional area, which radiates directly in a desired direction or feeds a reflector that forms a desired beam.
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A transmission line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic waves or acoustic waves, as well as electric power transmission.
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A waveguide is a structure which guides waves, such as electromagnetic waves, light, or sound waves. There are different types of waveguide for each type of wave.
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Electromagnetic waveguides
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As a receiver, antenna aperture can be visualised as the area of a circle constructed broadside to incoming radiation where all radiation passing within the circle is delivered by the antenna to a matched load.
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side lobes are the lobes of the radiation pattern that are not the main beam. The power density in the side lobes is generally much less than that in the main beam. It is generally desirable to minimize the sidelobe level (SLL), which is measured in decibels relative to the peak of
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Fixed Service Satellite (or FSS), is the official classification (used chiefly in North America) for geostationary communications satellites used for broadcast feeds for television and radio stations and networks, as well as for telephony and data communications.
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The ITU Radiocommunication Sector (ITU-R) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU) and is responsible for radio communication.
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Very Large Array
Organization: National Radio Astronomy Observatory [1]
Location: Socorro, New Mexico, USA
Coordinates:
Altitude: 2124 m (6970 ft)
Wavelength: radio
Diameter: 27 x 25m
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Organization: National Radio Astronomy Observatory [1]
Location: Socorro, New Mexico, USA
Coordinates:
Altitude: 2124 m (6970 ft)
Wavelength: radio
Diameter: 27 x 25m
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Radio astronomy is a subfield of astronomy that studies celestial objects in the radio frequency portion of the electromagnetic spectrum. Radio astronomy techniques are similar to optical techniques but radio telescopes have to be much larger due to the longer wavelengths being
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The decibel (dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power) relative to a specified or implied reference level.
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An isotropic radiator is a theoretical point source of waves which exhibits the same magnitude or properties when measured in all directions. It has no preferred direction of radiation. It radiates uniformly in all directions over a sphere centred on the source.
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The acronym DBS may stand for the following, depending on context:
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- Aston Martin DBS or Aston Martin DBS V12, an automobile model
- The dB's, a power pop band of the '70s and '80s
- d.b.s.
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The of this article or section may be compromised by "weasel words".
You can help Wikipedia by removing weasel words. Satellite television is television delivered by way of communications satellites, as compared to conventional terrestrial television and cable television.
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You can help Wikipedia by removing weasel words. Satellite television is television delivered by way of communications satellites, as compared to conventional terrestrial television and cable television.
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The term base station can be used in the context of land surveying, wireless computer networking, and wireless communications.
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Land surveying
In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to..... Click the link for more information.
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Arecibo, Puerto Rico
Flag Seal
Nickname: "La Villa del Capitán Correa", "Muy Leal", "Ciudad del CetÃ", "Diamante del Norte" (the north diamond), "La Ribera del Arecibo" (the shore of the Arecibo)
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Flag Seal
Nickname: "La Villa del Capitán Correa", "Muy Leal", "Ciudad del CetÃ", "Diamante del Norte" (the north diamond), "La Ribera del Arecibo" (the shore of the Arecibo)
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Location: Holmdel, New Jersey United States
Coordinates: _ ]
Built/Founded: 1964
Architect: Crawford,A.B.
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Coordinates: _ ]
Built/Founded: 1964
Architect: Crawford,A.B.
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satellite dish is a type of parabolic antenna designed with the specific purpose of transmitting signals to and/or receiving from satellites. A satellite dish is a particular type of microwave antenna.
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Herod_Archelaus
