thumb|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. Therefore, this design is used for antennas with bulky or complicated feeds, Other dish illumination patterns can also be synthesized, such as patterns with high taper at the dish edge for ultra-low spillover sidelobes, and patterns with a central "hole" to reduce feed shadowing.

  • Another reason for using the Cassegrain design is to increase the focal length of the antenna, to reduce sidelobes, among other advantages. Parabolic reflectors used in dish antennas have a large curvature and short focal length; the focal point is located 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. In a front-fed antenna, 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 is sensitive to small deviations from the focal point: the angular width that it can effectively focus is small. 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 the good off-axis focusing characteristics of a large focal ratio, and because the convex secondary reflector of the Cassegrain antenna increases it significantly, these antennas typically use a Cassegrain design.
  • The longer focal length also improves crosspolarization discrimination of off-axis feeds, The Voyager 1 spacecraft launched in 1977 is, , 24.6 billion kilometers from Earth, the furthest manmade object in space, and its 3.7 meter S and X-band Cassegrain antenna (picture below) is still able to communicate with ground stations.

See also

  • Cassegrain reflector
  • Nasmyth telescope

References

  • Cassegrain subreflector design article

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