|
| |
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 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 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.
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.
| |
|
