Cassegrain antenna

Types of parabolic antenna

In telecommunications and radar, a Cassegrain antenna is a parabolic antenna in which the feed antenna 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. The Cassegrain design is widely used in parabolic antennas, particularly in large antennas such as those in satellite ground stations, radio telescopes, and communication satellites.

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" or "prime focus", in which the feed antenna itself is mounted suspended in front of the dish at the focus, pointed back toward the dish. The Cassegrain design has several advantages over front feed that can justify its increased complexity:

A beam waveguide antenna, a type of Cassegrain design, showing the complicated signal path.

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 be longer for a given wavelength.

Beam waveguide antenna

A beam waveguide antenna is a type of complicated Cassegrain antenna with a long radio wave path to allow the feed electronics to be located at ground level. It is used in very large steerable radio telescopes and satellite ground antennas, where the feed electronics is too complicated and bulky, or requires too much maintenance and alterations, to locate on the dish; for example those using cryogenically-cooled amplifiers. The beam of incoming radio waves from the secondary reflector is reflected by additional mirrors in a long twisting path through the axes of the altazimuth mount, so the antenna can be steered without interrupting the beam, and then down through the antenna tower to a feed building at ground level.

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.[6]

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.
Cassegrain antenna on the Voyager spacecraft

See also

References

  1. 1 2 3 Chatterjee, Rajeswari (2006). Antenna theory and practice, 2nd Ed. New Age International. p. 188. ISBN 81-224-0881-8.
  2. 1 2 3 4 Welch, W.J. (1976). "Types of Astronomical Antennas". Methods of experimental physics: Astrophysics. Radio telescopes, Volume 12. US: Academic Press. pp. 13–14. ISBN 0-12-475952-1. Retrieved 2012-01-14.
  3. Galindo, V. (1964). "Design of dual-reflector antennas with arbitrary phase and amplitude distributions". Antennas and Propagation, IEEE Transactions on. IEEE. 12 (4): 403–408. doi:10.1109/TAP.1964.1138236.
  4. Willams, WF (1983). "RF Design and Predicted Performance for a Future 34-Meter Shaped Dual-Reflector Antenna System Using the Common Aperture XS Feedhorn" (PDF). Telecommunications and Data Acquisition Progress Report. 73: 74–84.
  5. Cheng, Jingquan (2009). The principles of astronomical telescope design. New York: Springer. pp. 359–360. ISBN 0-387-88790-3.
  6. Makino, Shigero (2006). "Historical review of reflector antenna systems developed for satellite communication by MELCO" (PDF). ISAP2006-International Symposium on Antennas and Propagation. Mitsubishi Electric Corp. Retrieved 2011-12-24. on ISAP website

External links

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