A planar balun comprises a balun input, a first balun output, a second balun output,two primary coupled lines with a first cross-sectional area disposed on an inhomogeneous medium, and two secondary coupled lines with a second cross-sectional area, the second coupled lines disposed on an inhomogeneous medium. Each of the primary coupled lines are coupled to one of the secondary coupled lines with a predetermined uniform surface space therebetween. The first cross-sectional area and the second cross-sectional area are unequal, thereby producing pairs of asymmetric coupled lines.