Abstract: A coplanar microwave balun is disclosed in which a signal propagating along a coplanar waveguide in an unbalanced mode is launched onto another coplanar transmission line in the balanced mode. One of the ground planes of the latter transmission line itself is separated into a pair of regions which constitute the ground planes of the coplanar waveguide. These pair of regions are electrically connected, e.g., by ribbon bonds that bridge across the transmission line strip of the coplanar waveguide. The end of that transmission line strip bridges across the coplanar transmission line and acts to launch the signal thereto. The inventive balun advantageously is employed in a three port device having isolated inputs from which separate signals may be multiplexed onto a common line, and may be used in other multiplexer, mixer and modulator assemblies.