Forward-Wave Nonuniform Microstrip Couplers

Wideband, entirely planar microstrip forward wave couplers are described. The technique uses a wiggly coupler structure in which the difference between odd- and even-mode phase velocities is controlled at any desired value by the amount of wiggle depth introduced between the conductors. In this way, forward coupling levels up to almost 0 dB can be achieved without the need for close conductor spacing. Exact closed-form expressions are given for the S-parameters of the coupler.