Parabolic antenna
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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 frequencies. 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:
- [G\approx (\pi^2 D^2)/\lambda^2\,]
- [G\approx (9.87D^2)/\lambda^2\,]
- [G \,\! ] is power gain over isotropic
- [D \,\! ] is reflector diameter in same units as wavelength
- [\lambda \,\! ] is wavelength
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 an 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 weight less, are easier to construct and have nearly ideal characteristics if the holes or gaps are kept under 1/10 of the wavelength.
More exotic types include the off-set parabolic antenna and Cassegrain type. In the off-set type only one half of the parabola is needed, the feed element is still located at the focal point but is aimed to illumate only the half reflector. This removes the feed element and support from disturbing the main beam and allows for easier maintenance of the feed. The Cassegrain type uses a secondary reflector allowing for the feed element to be located behind 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.
Feeding parabolic antennas.
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. 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
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