An over-mode metal rectangular waveguide is widely used in the generation, propagation, coupling, and transition of microwaves. When applied as the beam-wave interaction circuit of some high power microwave devices, a rectangular waveguide is expected to operate at a single electromagnetic mode. To do that, unwanted modes resulted from spurious oscillations should be suppressed. In this paper, a method of selective suppression of electromagnetic modes in rectangular waveguides by loading distributed losses in some special position of waveguide inner wall is presented. By using the method, the unwanted modes can be attenuated much larger relative to the operating mode. The presented method can be used to improve the stability of rectangular waveguide beam-wave interaction circuit.
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