Abstract: A microwave harmonic absorption filter is disclosed which has an extended single mode pass-band ranging over one or more octaves as well as enhanced attenuation in the stop-band. This filter has a primary, rectangular waveguide having one or more ridges on the inner surfaces of its elongated wall members. Secondary waveguides are provided on the outer surfaces of these wall members and these secondary waveguides serve to diminish propagation of microwave energy in the stop-band. Apertures extend through the wall members of the primary, ridged, rectangular waveguide so that microwave energy within the range of frequencies of the stop-band can be transmitted from the primary waveguide into the secondary waveguides.