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.
- (Astronomy.SE) How do ASKAP's focal plane phased array feeds interact with the entire array phasing?
- (Astronomy.SE) What is the highest granularity focal-plane array on a dish radio telescope? Or is this the ONLY ONE?
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!
