| • Science | • People | • Locations | • Timeline |
A typical parabolic antenna consists of a 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.
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 ( decibelThe decibel is a "dimensionless unit" (like percent) that is a measure of ratios on a logarithmic scale. Usually, it is ten times the base-10 logarithm of the ratio. It's not an SI unit, although the International Committee for Weights and Measures (BIPM)s above the isotropic level).
With the advent of TVRO and DBSDBS may stand for: Direct broadcast satellite Deep brain stimulation Development Bank of Singapore Diocesan Boys' School, Hong Kong Den Beste Sykkel a norwegian bike manufacturer TLAs. satellite television, the parabolic antenna has become 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 radarThis article is about the device. For the fictional character in M A S H see Corporal Walter (Radar) O'Reilly. antenna (approximately 40m (130ft) in diameter) rotates on a track to observe activities near the horizon. Radar is an acronym for ra dio d etec and radio astronomyRadio astronomy is the study of celestial phenomena through measurement of the characteristics of radio waves emitted by physical processes occurring in space. Radio waves are much longer than light waves. In order to receive good signals, 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. Radio frequency antenna types
See also: satellite dishA satellite dish is a type of parabolic reflector 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. Satellite dishes come in varying sizes an
Satellites