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Which of the following uses a concave sub-reflector in a dual reflector antenna?

(a) Cassegrain

(b) Gregorian

(c) Both Gregorian &Cassegrain

(d) Neither Gregorian nor Cassegrain

The question was posed to me by my school teacher while I was bunking the class.

My doubt is from Feeding Systems topic in portion Reflector Antenna of Antennas

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Right option is (b) Gregorian

Easy explanation: Cassegrain and Gregorian are dual reflector antennas. Gregorian antenna has a concave sub-reflector which is elliptic. Cassegrain uses a hyperboloid reflector which is convex towards the feed. The sub-reflectors are used to reduce the spillover efficiency.

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