I am trying to figure out Ku-band antenna consisting a prime focus parabolic reflector sub-reflector-->circular outer corrugated waveguide omt. It is backfire feed so that feed assembly ( OMT) is located behind of reflector. I am having a hard time to understand what is function of outer corrugation of waveguide. The corrugation looks like rings.
Does anyone has any clue of the function of outer corrugation of waveguide ?