A
parabolic antenna is a high-gain reflector antenna used for radio, television and data communications, and also for radiolocation (
radarRadar is an object detection 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. The term RADAR was coined in 1941 as an acronym for RAdio Detection And...
), on the
UHFUltra high frequency designates a range of electromagnetic waves with frequencies between 300 MHz and 3 GHz . Also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
and
SHFSuper high frequency refers to radio frequencies in the range of 3 GHz and 30 GHz. Also known as the centimeter band or centimeter wave as the wavelengths range from ten to one centimeters.-Description:...
parts of the electromagnetic spectrum. The relatively short wavelength of electromagnetic radiation at these frequencies allows reasonably sized reflectors to exhibit the desired highly directional response for both receiving and transmitting.
With the advent of TVRO and
DBSDirect broadcast satellite is a term used to refer to satellite television broadcasts intended for home reception, also referred to more broadly as direct-to-home signals...
satellite televisionSatellite television is television delivered by the means of communications satellite and received by a satellite dish and set-top box. In many areas of the world it provides a wide range of channels and services, often to areas that are not serviced by terrestrial or cable providers.-History:The...
, the parabolic antenna became a ubiquitous feature of urban, suburban, and even rural landscapes.
A
parabolic antenna is a high-gain reflector antenna used for radio, television and data communications, and also for radiolocation (
radarRadar is an object detection 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. The term RADAR was coined in 1941 as an acronym for RAdio Detection And...
), on the
UHFUltra high frequency designates a range of electromagnetic waves with frequencies between 300 MHz and 3 GHz . Also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
and
SHFSuper high frequency refers to radio frequencies in the range of 3 GHz and 30 GHz. Also known as the centimeter band or centimeter wave as the wavelengths range from ten to one centimeters.-Description:...
parts of the electromagnetic spectrum. The relatively short wavelength of electromagnetic radiation at these frequencies allows reasonably sized reflectors to exhibit the desired highly directional response for both receiving and transmitting.
With the advent of TVRO and
DBSDirect broadcast satellite is a term used to refer to satellite television broadcasts intended for home reception, also referred to more broadly as direct-to-home signals...
satellite televisionSatellite television is television delivered by the means of communications satellite and received by a satellite dish and set-top box. In many areas of the world it provides a wide range of channels and services, often to areas that are not serviced by terrestrial or cable providers.-History:The...
, the parabolic antenna became a ubiquitous feature of urban, suburban, and even rural landscapes. Extensive terrestrial microwave links, such as those between cellphone
base stationThe term base station can be used in the context of land surveying, wireless computer networking, and wireless communications.-Land surveying:...
s, and wireless WAN/LAN applications have also proliferated this antenna type. Earlier applications included ground-based and airborne
radarRadar is an object detection 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. The term RADAR was coined in 1941 as an acronym for RAdio Detection And...
and
radio astronomyRadio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, but subsequent advances have identified a number of different sources of radio emission...
.
However a term
dish antenna is often used for a parabolic antenna instead, it connote a spheric antenna as well, which has a portion of spherical surface as the reflector shape.
History
Heinrich Hertz constructed the world's first parabolic reflector antenna in 1888. The antenna had an aperture 1.2 meters wide, a focal distance of 0.12 meters, and was used at an operating frequency of about 450 MHz. The reflector was made of zinc sheet metal supported by a wooden frame, and had a spark-gap excited dipole along the focal line. With two such antennas, one used for transmitting and the other for receiving, Hertz successfully demonstrated the existence of the electromagnetic waves which had been predicted by
James Clerk MaxwellJames Clerk Maxwell was a Scottish theoretical physicist and mathematician. His most significant achievement was the development of the classical electromagnetic theory, synthesizing all previous unrelated observations, experiments and equations of electricity, magnetism and even optics into a...
some 22 years earlier.
Design
A typical parabolic antenna consists of a
parabolic reflectorA parabolic reflector is a reflective device used to collect or project energy such as light, sound, or radio waves. Its shape is that of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis...
with a small
feed antennaIn satellite dish and antenna design parlance, a feedhorn is a horn antenna used to convey radio waves between the transceiver and the reflector....
at its focus.
The reflector is a metallic surface formed into a
paraboloidIn mathematics, a paraboloid is a quadric surface of special kind. There are two kinds of paraboloids: elliptic and hyperbolic. The elliptic paraboloid is shaped like an oval cup and can have a maximum or minimum point....
of revolution and (usually) truncated in a circular rim that forms the diameter of the antenna. This paraboloid possesses a distinct
focal pointA focal point may mean:* Focus , the point at which initially collimated rays of light meet after passing through a convex lens, or reflecting from a concave mirror....
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 at the reflector's focus is typically a
low-gainThe low-gain antenna 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. The mountains become the equivalent of rocks in a stream,...
type such as a
half-wave dipoleA dipole antenna, created by Heinrich Rudolph Hertz around 1886, is an antenna that can be made by a simple wire, with a center-fed driven element for transmitting or receiving radio frequency energy...
or a small
waveguideA waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguide for each type of wave...
hornIn telecommunications, the term horn has the following meanings:* 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....
. In more complex designs, such as the
Cassegrain antennaIn telecommunications and radar, a Cassegrain antenna is an antenna in which the feed radiator is mounted at or near the surface of a concave main reflector and is aimed at a convex secondary reflector slightly inside the focus of the main reflector....
, 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 cableCoaxial cable, or coax, is an electrical cable with an inner conductor surrounded by a tubular insulating layer typically of a flexible material with a high dielectric constant, all of which are surrounded by a conductive layer , and finally covered with a thin insulating layer on the outside...
transmission lineA 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....
or hollow
waveguideA waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguide for each type of wave...
.
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
wavelengthIn physics, the wavelength of a sinusoidal wave is the spatial period of the wave – the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
Practical considerations of
antenna effective areaIn telecommunications, antenna effective area or effective aperture is the functionally equivalent area from which an antenna directed toward the source of the received signal gathers or absorbs the energy of an incident electromagnetic wave....
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 ServiceFixed Service Satellite , is the official classification for geostationary communications satellites used for broadcast feeds for television and radio stations and networks, as well as for telephony and data communications.FSS satellites have also been used for Direct-To-Home satellite TV...
) where specific antenna performance has not been defined, a
reference antenna based on Recommendation
ITU-RThe ITU Radiocommunication Sector is one of the three sectors of the International Telecommunication Union and is responsible for radio communication....
S.465 is used to calculate the interference, which will include the likely sidelobes for off-axis effects.
Applying the formula to the 25-meter-diameter antennas used by the
VLAThe Very Large Array is a radio astronomy observatory located on the Plains of San Augustin, between the towns of Magdalena and Datil, some fifty miles west of Socorro, New Mexico, USA. U.S. Route 60 passes through the complex, which is adjacent to the Boy Scout Double H High Adventure Base. The...
and
VLBAThe Very Long Baseline Array is a system of ten radio telescopes controlled remotely from the Array Operations Center in Socorro, New Mexico by the National Radio Astronomy Observatory. The array works together as the world's largest dedicated, full-time astronomical instrument using the...
radio telescopes at a wavelength of 21 cm (1.42 GHz, a common
radio astronomyRadio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, but subsequent advances have identified a number of different sources of radio emission...
frequency) yields an approximate maximum gain of 140,000 times or about 50 dBi (
decibelThe decibel is a logarithmic unit of measurement that expresses the magnitude of a physical quantity relative to a specified or implied reference level. Since it expresses a ratio of two quantities with the same unit, it is a dimensionless unit...
s above the isotropic level).
As of 2009, the largest "dish" antenna in the world is the
Arecibo ObservatoryThe Arecibo Observatory is a radio telescope located close to the city of Arecibo in Puerto Rico. It is operated by Cornell University under cooperative agreement with the National Science Foundation. The observatory works as the National Astronomy and Ionosphere Center although both names are...
's
radio telescopeA radio telescope is a form of directional radio antenna used in radio astronomy. The same types of antennas are also used in tracking and collecting data from satellites and space probes...
, which has a diameter of 1,000 ft. (305m) at
AreciboArecibo may refer to:*Arecibo, Puerto Rico, a municipality located by the Atlantic Ocean*Arecibo Observatory, a very sensitive radio telescope located approximately south-southwest from the city of Arecibo...
, Puerto Rico.
For beam-steering reasons, the Arecibo telescope is actually a spherical, rather than parabolic reflector.
Generally a spherical reflector does not have a focal point as a
point, however, the Arecibo radio telescope is a three-reflector variety of
Gregorian telescopeThe Gregorian telescope is a type of reflecting telescope designed by Scottish mathematician and astronomer, James Gregory in the 17th century and first built in 1673 by Robert Hooke...
, and uses its main, secondary and tertiary reflectors to focus the radio waves to a single point.
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.
Common 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 main beam. This also allows for easier maintenance of the feed, but is usually only found in smaller antennas. The maximum size of offset feed antennas found is typically 3.8 to 4.5 meters, with all larger reflector dishes using a Gregorian or Cassegrain design.
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
collimatedCollimated light is light whose rays are nearly parallel, and therefore will spread slowly as it propagates. The word is related to "colinear" and implies light that does not disperse with distance , or that will disperse minimally...
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, with the Rx and Tx equipment located immediately behind the center of the primary 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.
See also
- Radio telescope
A radio telescope is a form of directional radio antenna used in radio astronomy. The same types of antennas are also used in tracking and collecting data from satellites and space probes...
- Satellite dish
A dish is a type of parabolic antenna designed to receive microwaves from communications satellites, which transmit data transmissions or broadcasts, such as satellite television.-Principle of operation:...
- Simulsat
Simulsat is a quasi-parabolic antenna product line designed and manufactured by Antenna Technology Communications. Whereas satellite dishes with one low-noise block converter are able to receive a satellite television broadcast from one communications satellite at a time, Simulsat antennas are...
(a quasi parabolic antenna which is spherical in one plane and parabolic in another)
- Corner reflector
A corner reflector is a retroreflector consisting of three mutually perpendicular, intersecting flat surfaces, which reflects electromagnetic waves back towards the source. The three intersecting surfaces often have square shapes. This is also known as a corner cube.-Usage:Corner reflectors are...
External links