What does the flange do over that rectangular cavity antenna?
The flange around a cavity-backed aperture antenna has a significant effect on the radiation pattern. If the flange dimensions are large compared to the wavelength, it can be approximated as going on to infinity, which makes mathematical analysis simpler. If the flange is around a wavelength or less, you need one of numerical simulation programs. Roughly speaking though, the infinite flange gives a narrower beam, and the narrower flange gives a broader beam.