Abstract: An apparatus that utilizes a solid, dielectric-filled waveguide filter in a microstrip circuit is dislosed. The microstrip circuit couples directly into and out from the waveguide filter through slot openings in waveguide filter walls without penetrating the interior of the waveguide. Adjustment of the coupling is accomplished through varying the length of the slot. Resonant frequency tuning of the filter is accomplished using additional slots, or by removing metallic plating from metallized cavities which extend into the waveguide core from a central region of a waveguide broad wall.