In orthogonally polarized H（11）° Multi-mode circular waveguide systems

filtration of several lower high-order modes such as E（01）° E（11）° has been treated favorably. However

no report on further research in the filtration of higher-order modes such as H（21）° has so far been revealed. In this paper

operational principle and design scheme of an H（21）° mode filter are dealt with

approximate formula for determining mode coupling quantities according to Bethe’s small Holes diffraction theorems is derived

and methods for compu-ting mode attenuations for cascaded multicavity filter and reflection characteristics of dominant mode are proposed. Experimental results have checked well with the theoretical analysis and design formulas; the transmission characteristics of the dominant and higher-order modes are perfectly satisfactory.