Abstract: A combined duplexer/mixer especially for use in an FMCW millimetric radar system has a waveguide section containing a fin line or suspended strip structure for responding to the linearly polarized radar return signal passing through the section in one direction while permitting passage of the orthogonally polarized, outgoing signal passing in the other direction. The fin line is coupled via double balanced mixer diodes to an IF output line to produce an IF signal by mixing the return signal with a portion of the outgoing signal tapped off by a field distorting probe, ridge section or like coupling means.