On the use of corrugations in mode filters for oversized rectangular waveguides

The filtering of unwanted modes that can propagate in oversized rectangular waveguides is addressed, focusing on longitudinal corrugations partially filled with an absorber. Lengthwise slots, in the middle of top and bottom waveguide walls, can indeed extract the power of some modes from the main waveguide with negligible insertion losses for the working dominant mode. Once some power is coupled through the slot into the junction, it travels toward the absorbing material, where it is damped. This behavior is studied comparing the performance of a single continuous slot, where various modes can propagate, with the one achieved by several shorter single-mode apertures. The excitation and absorption of modes in the corrugations is estimated by means of analytical expressions under a small coupling approximation. Mono-modal corrugations achieve better performances with respect to the overmoded junction.