There are 3 flange types - the plain, the “O” ring and the choke

Plain flange - UBR

It has a flat face – it is also sometimes called a plain or a cover flange.

When mated to another plain or ‘O’ ring flange it gives the best match performance. It can be used with either an ‘O’ ring flange, a choke flange or another plain flange

”O” ring flange PBR

It has a flat face with a groove cut in it to take an ‘O’ ring for environmental or pressure sealing. It is used together with a plain flange.

It is permissible, but not recommended, to use with another PBR flange and “O” ring.

Choke flange - CBR

It has 2 grooves cut into it. The outer groove is for an ‘O’ ring (the same as the PBR flange). The inner groove is the choke and the region between the waveguide aperture and the inner groove is sub flush. The distance from the waveguide to the bottom of the choke groove is ¼ wavelength so the reflection presents a short circuit at the waveguide.

It must be used together with a plain flange, and is used in very high power systems.

It essential that the flange is aligned precisely to minimise the return loss degradation.

If the flange is correctly aligned and used at the frequency the choke is designed for (typically the band centre) then it offers the best leakage performance (but the worst match performance - it is possible to achieve similar leakage performance using rf shims without the degradation in match.)