Diffraction of a Gaussian beam at an aperture

Measurements of the effect of a diffracting aperture on a Gaussian beam at 230 GHz ( lambda =1.3 mm) are reported, and are in good agreement with theoretically calculated beam shapes. This is of importance in the design of compact diplexers. It is found that apertures >or=2.4 times the 8.7 dB beam size (1/e field size, omega _o) should be used to ensure beams unmodified due to truncation.