Abstract: A stripline to waveguide transition device efficiently and simply converts he TEM stripline mode to the TE.sub.10 rectangular waveguide mode. A stripline is inserted longitudinally, end-to-end, into a waveguide. A conductive strip extends from the conductive strip of the stripline into the waveguide passing between two metallic plates, which separate the waveguide from the stripline terminal end and establish a TE.sub.10 boundary condition. The conductive strip is orthogonally coupled at its distal end to a broad interior wall of the waveguide to provide a transition area supporting both TEM and TE.sub.10 modes.