Abstract: The present invention is a passband-type waveguide filter in which a periodic distortion is employed which has a sinusoidal, or alternately configured, distortion, which changes phase at a point along the periodic distortion. The passband filter comprises essentially any waveguide, designed as a waveguide, with a periodic distortion added to at least one of the sides of the waveguide. The waveguide can be a rectangular cross section metallic microwave type, cylindrical, elliptical, or microstrip. The waveguide may be optical, a SAW type waveguide or any waveguide of the prior art. In a preferred embodiment a waveguide structure supports propagation of a wave along the waveguide, the waveguide structure containing a boundary and the waveguide structure containing a filter portion having a portion of said boundary with a sinusoidal distortion following:f(z)=1+A sin (k.sub.w z) for 0<=z<=L/2f(z)=1-A sin (k.sub.