As I mentioned, I used the long stove pipes made of galvanized steel to guide the external zenith noise into underground radiometers. The polarization was determined by standard-gain horns underground. I tested the radiation patterns of both systems by moving an absorber block across the pipe openings above ground, and determined that the main lobes had the same shape as the horns alone. No more work was needed for this application.
In large parabolic antennas (mostly Cassegrain) the primary feed from a high-power transmitter and to a low-noise receiver is required to be low-loss at two separate frequency bands, either 4/6 GHz, or, 12/14 GHz. To my knowledge, overmoded waveguides (beam-guides) are typically 25-40 cm in diameter, and planar or convex 45-degree mirrors are used at bends. Antenna can rotate in azimuth and elevation while circular polarization is used. By keeping the circular shape, no special provisions are needed to keep "pure" modes.