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The simplest dish antennas have a concave reflector and their feed horns at the primary focus which is "up in the air".

That has several inconveniences, it's hard to go up there to do any kind of band or amplifier switching or add a liquid helium refrigerator for front-end cooling, much less repairs.

So bringing a secondary focus back down closer to the ground and often inside a large enclosure behind the dish has several advantages.

But as discussed in this answer to Schmidt corrector plate on a Newtonian = Schmidt Cassegrain? and all of the questions and answers linked therein, the main selling point of a Cassegrain telescope is its correction of off-axis aberrations.

Except for some fancy radio astronomy imaging applications, most dish antennas are primarily on-axis power collectors for weak signals, especially in deep-space applications.

Question: Why are modern satellite ground station and deep space large communications dishes with secondary mirrors almost always Cassegrain? Besides providing the folded path, are there ever situations where the off-axis aberration correction from the hyperbolic shape of the secondary provides additional benefits in performance, design or operation?

There are of course some notable exceptions!

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  • $\begingroup$ just trying to clarify, are you asking why Cassegrain over a system with only a primary reflector or why Cassegrain over say Gregorian? (sorry, but StackExchange is eating my ATs again...) $\endgroup$ Jan 15 at 0:20
  • $\begingroup$ @Grapefruit I don’t remember nor can I reconstruct my exact thinking now. I do point out that Cassegrains have off-axis corrections and wonder if those are ever taken advantage of, and I think that “besides folded path” means I’m not asking about secondary versus primary only. Does that help? Do you have an interesting answer that you think the wording currently excludes? I can edit if it helps. $\endgroup$
    – uhoh
    Jan 15 at 13:48

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Short Answer: The main benefits of the Cassegrain design for an antenna are lower loss, higher aperture efficiency, and more convenient location of the feed horn / low noise amplifier.

Advantages of Cassegrain Design

Additional References

Parabolic Reflector Antenna Feed Systems
LET’S TALK ABOUT REFLECTOR ANTENNAS
A COMPARISON BETWEEN PRIME FOCUS AND CASSEGRAIN ANTENNAS

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