In telecommunications 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.
Another advantage, important in satellite ground antennas, is that because the feed antenna is directed forward, rather than backward toward the dish as in a front-fed antenna, the spillover sidelobes caused by portions of the beam that miss the secondary reflector are directed upwards toward the sky rather than downwards towards the warm earth.[2] In receiving antennas this reduces reception of ground noise, resulting in a lower antenna noise temperature.
Another reason for using the Cassegrain design is to increase the focal length of the antenna, to improve the field of view[2][3] Parabolic reflectors used in dish antennas have a large curvature and short focal length, to locate the focal point near the mouth of the dish, to reduce the length of the supports required to hold the feed structure or secondary reflector. The focal ratio (f-number, the ratio of the focal length to the dish diameter) of typical parabolic antennas is 0.25 - 0.8, compared to 3 - 8 for parabolic mirrors used in optical systems such as telescopes. A "flatter" parabolic dish with a long focal length would require an impractically elaborate support structure to hold the feed rigid with respect to the dish. However, the drawback of this small focal ratio is that the antenna has a small field of view, the angular width that it can effectively focus. Modern parabolic antennas in radio telescopes and communications satellites often use arrays of feedhorns clustered around the focal point, to create a particular beam pattern. These require good off-axis focusing characteristics. The convex secondary reflector of the Cassegrain increases the focal length, and thus the field of view, so these antennas usually use a Cassegrain design.
The longer focal length also improves crosspolarization discrimination of off-axis feeds,[2] important in satellite antennas that use the two orthogonal polarization modes to transmit separate channels of information.
A disadvantage of the Cassegrain is that the feed horn(s) must have a narrower beamwidth (higher gain) to focus its radiation on the smaller secondary reflector, instead of the wider primary reflector as in front-fed dishes. The angular width the secondary reflector subtends at the feed horn is typically 10° - 15°, as opposed to 120° - 180° the main reflector subtends in a front-fed dish. Therefore the feed horn must have a larger aperture for a given wavelength.
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Geometry
The primary reflector is a paraboloid, 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.[1] 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.Advantages
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.[1][2] Therefore this design is used for antennas with bulky or complicated feeds,[1] such as satellite communication ground antennas, radio telescopes, and the antennas on some communication satellites.Another advantage, important in satellite ground antennas, is that because the feed antenna is directed forward, rather than backward toward the dish as in a front-fed antenna, the spillover sidelobes caused by portions of the beam that miss the secondary reflector are directed upwards toward the sky rather than downwards towards the warm earth.[2] In receiving antennas this reduces reception of ground noise, resulting in a lower antenna noise temperature.
Another reason for using the Cassegrain design is to increase the focal length of the antenna, to improve the field of view[2][3] Parabolic reflectors used in dish antennas have a large curvature and short focal length, to locate the focal point near the mouth of the dish, to reduce the length of the supports required to hold the feed structure or secondary reflector. The focal ratio (f-number, the ratio of the focal length to the dish diameter) of typical parabolic antennas is 0.25 - 0.8, compared to 3 - 8 for parabolic mirrors used in optical systems such as telescopes. A "flatter" parabolic dish with a long focal length would require an impractically elaborate support structure to hold the feed rigid with respect to the dish. However, the drawback of this small focal ratio is that the antenna has a small field of view, the angular width that it can effectively focus. Modern parabolic antennas in radio telescopes and communications satellites often use arrays of feedhorns clustered around the focal point, to create a particular beam pattern. These require good off-axis focusing characteristics. The convex secondary reflector of the Cassegrain increases the focal length, and thus the field of view, so these antennas usually use a Cassegrain design.
The longer focal length also improves crosspolarization discrimination of off-axis feeds,[2] important in satellite antennas that use the two orthogonal polarization modes to transmit separate channels of information.
A disadvantage of the Cassegrain is that the feed horn(s) must have a narrower beamwidth (higher gain) to focus its radiation on the smaller secondary reflector, instead of the wider primary reflector as in front-fed dishes. The angular width the secondary reflector subtends at the feed horn is typically 10° - 15°, as opposed to 120° - 180° the main reflector subtends in a front-fed dish. Therefore the feed horn must have a larger aperture for a given wavelength.
History
The Cassegrain antenna design was adapted from the Cassegrain telescope, a type of reflecting telescope developed around 1672 and attributed to French priest Laurent Cassegrain. The first Cassegrain antenna was invented and built in Japan in 1963 by NTT, KDDI and Mitsubishi Electric. The 20 meter I-1 antenna operated at 6.4, 4.2, and 1.7 GHz, and was used in October 1963 in the first trans-Pacific satellite television relay experiments.[4]
Cassegrain satellite
communication antenna in Sweden. The convex secondary reflector can be
seen suspended above the dish, and the feed horn is visible projecting from the center of the dish.
Closeup of the convex secondary reflector in a large satellite communications antenna in Pleumeur-Bodou, France
Cassegrain spacecraft communication antenna in Canberra, Australia, part of NASA's Deep Space Network. The advantage of the Cassegrain design is that the heavy complicated feed structure (bottom) doesn't have to be suspended over the dish.
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