Cassegrain antenna
Encyclopedia
In telecommunication
s and radar
, a Cassegrain antenna is a parabolic antenna
in which the feed radiator
is mounted at or behind the surface of the concave main parabolic reflector
dish and is aimed at a smaller convex secondary reflector suspended in front of the primary reflector. The beam of radio waves from the feed illuminates the secondary reflector, which reflects it back to the main reflector dish, which reflects it forward again to form the desired beam.
This design is an alternative to the most common parabolic antenna design, called "front feed", in which the feed antenna
itself is mounted suspended in front of the dish at the focus. One advantage of the Cassegrain design is that the feed antennas and associated waveguides and "front end
" electronics can be located on or behind the dish, rather than suspended in front where they block part of the outgoing beam. Therefore this design is used for antennas with bulky or complicated feeds, such as satellite communication ground antennas, radio telescope
s, and the antennas on some communication satellites. Another reason for using the Cassegrain design is that modifying the shape of the secondary reflector offers additional possibilities for shaping the beam pattern of the antenna over what is possible with a simple parabolic antenna.
, while the shape of the convex secondary reflector is a hyperboloid. The geometrical condition for radiating a collimated, plane wave beam is that the feed antenna is located at the far focus
of the hyperboloid, while the focus of the primary reflector coincides with the near focus of the hyperboloid. Usually the secondary reflector and the feed antenna are located on the central axis of the dish. However in offset Cassegrain configurations, the primary dish reflector is asymmetric, and its focus, and the secondary reflector, are located to one side of the dish, so that the secondary reflector does not partially obstruct the beam.
developed around 1672 and attributed to French priest Laurent Cassegrain
. The first Cassegrain antenna was invented and built in Japan by NTT
, KDDI
and Mitsubishi Electric
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s and radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
, a Cassegrain antenna is a parabolic antenna
Parabolic antenna
A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish...
in which the feed radiator
Antenna feed
In telecommunications and electronics, an antenna feed refers to the components of an antenna which feed the radio waves to the rest of the antenna structure, or in receiving antennas collect the incoming radio waves, convert them to electric currents and transmit them to the receiver...
is mounted at or behind the surface of the concave main parabolic reflector
Parabolic reflector
A 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...
dish and is aimed at a smaller convex secondary reflector suspended in front of the primary reflector. The beam of radio waves from the feed illuminates the secondary reflector, which reflects it back to the main reflector dish, which reflects it forward again to form the desired beam.
This design is an alternative to the most common parabolic antenna design, called "front feed", in which the feed antenna
Antenna feed
In telecommunications and electronics, an antenna feed refers to the components of an antenna which feed the radio waves to the rest of the antenna structure, or in receiving antennas collect the incoming radio waves, convert them to electric currents and transmit them to the receiver...
itself is mounted suspended in front of the dish at the focus. One advantage of the Cassegrain design is that the feed antennas and associated waveguides and "front end
RF front end
In a radio receiver circuit, the RF front end is a generic term for all the circuitry between the antenna and the first intermediate frequency stage. It consists of all the components in the receiver that process the signal at the original incoming radio frequency , before it is converted to a...
" electronics can be located on or behind the dish, rather than suspended in front where they block part of the outgoing beam. Therefore this design is used for antennas with bulky or complicated feeds, such as satellite communication ground antennas, radio telescope
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...
s, and the antennas on some communication satellites. Another reason for using the Cassegrain design is that modifying the shape of the secondary reflector offers additional possibilities for shaping the beam pattern of the antenna over what is possible with a simple parabolic antenna.
Geometry
The primary reflector is a paraboloidParaboloid
In 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....
, while the shape of the convex secondary reflector is a hyperboloid. The geometrical condition for radiating a collimated, plane wave beam is that the feed antenna is located at the far focus
Focus (geometry)
In geometry, the foci are a pair of special points with reference to which any of a variety of curves is constructed. For example, foci can be used in defining conic sections, the four types of which are the circle, ellipse, parabola, and hyperbola...
of the hyperboloid, while the focus of the primary reflector coincides with the near focus of the hyperboloid. Usually the secondary reflector and the feed antenna are located on the central axis of the dish. However in offset Cassegrain configurations, the primary dish reflector is asymmetric, and its focus, and the secondary reflector, are located to one side of the dish, so that the secondary reflector does not partially obstruct the beam.
History
The Cassegrain antenna design was adapted from the Cassegrain telescope, a type of reflecting telescopeReflecting telescope
A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...
developed around 1672 and attributed to French priest Laurent Cassegrain
Laurent Cassegrain
Laurent Cassegrain was a Catholic priest who is notable as the probable inventor of the Cassegrain reflector, a folded two mirror reflecting telescope design.-Biography:...
. The first Cassegrain antenna was invented and built in Japan by NTT
Nippon Telegraph and Telephone
, commonly known as NTT, is a Japanese telecommunications company headquartered in Tokyo, Japan. Ranked the 31st in Fortune Global 500, NTT is the largest telecommunications company in Asia, and the second-largest in the world in terms of revenue....
, KDDI
KDDI
is a Japanese telecommunications operator formed in October 2000 through the merger of DDI Corp., KDD Corp., and IDO Corp. It has its headquarters in the Garden Air Tower in Iidabashi, Chiyoda, Tokyo....
and Mitsubishi Electric
Mitsubishi Electric
is a multinational electronics and information technology company headquartered in Tokyo, Japan. It is one of the core companies of the Mitsubishi Group....