Cassegrain Antenna - Advantages

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, pointed back toward the dish. The Cassegrain design has several advantages over front feed that can justify its increased complexity:

• 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 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. 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 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, 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 be longer for a given wavelength.