Discussion
Why is a Cassegrain arrangement unsuitable for a very small dish?
Answer: B
Blockage loss goes as the square of the blocked diameter fraction, so on a small dish the necessarily oversized subreflector removes an unacceptable share of the aperture. Prime focus or offset feeding is used instead below a diameter of perhaps twenty wavelengths. The feed sits at the vertex by design, tolerance scales with wavelength rather than with dish size, and gain is not the criterion. Knowing where a technique stops working is as useful as knowing how it works.
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