Parabolic antenna
The Parabolic antenna is a high-gain, reflector antenna that
is used for radio, television and data communications on the
UHF and SHF
frequencies. The short wavelength of electromagnetic
(radio) waves at these frequencies allows reasonably sized
reflectors to exhibit a very desirable highly directional
response for both received and transmitted energy.
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 having the
reflective
property of parabolas in that a point light source at
the 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
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 waveguide.
Considering the parabolic antenna as a circular
aperture gives the following approximation for
the maximum gain:
- <math>G\approx \pi^2 D^2<math>
or
- <math>G\approx 9.87D^2<math>
where:
- <math>G \,\! <math> is power gain over isotropic
- <math>D \,\! <math> is reflector diameter in wavelengths
Practical considerations of antenna
effective area and sidelobe suppression reduce the actual
gain to about 0.35 to 0.55 of this 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
(decibels above the isotropic level).
With the advent of TVRO and DBS satellite
television, the parabolic antenna has become a ubiquitous feature of
urban, suburban, and even rural, landscapes. Extensive
terrestrial microwave links between cellphone base stations
and wireless WAN/LAN use 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.
