Abstract: A filter includes an electromagnetic wave supporting structure having a cavity defined by an encircling sidewall, and microwave feeds coupled to the cavity for inputting and for outputting electromagnetic power to and from the wave supporting structure. At least a portion of an interior surface of the sidewall has a succession of corrugations. Successive ones of the corrugations are spaced apart by a distance less than approximately 0.2 wavelength of the electromagnetic wave, and each of the corrugations has a height greater than the spacing distance but less than approximately 0.5 wavelength of the electromagnetic wave. The geometry of the corrugated sidewall reduces interaction between the electromagnetic wave and the sidewall to inhibit dissipation of power of the electromagnetic wave within the sidewall.