Nonuniform transmission line codirectional couplers for hybrid MIMIC and superconductive applications

A new design approach for thin-film codirectional quadrature couplers and their applications is described. An in-depth analysis and semi-empirical design curves are presented for these couplers. Forward-wave coupling is achieved by making use of the difference between even- and odd-mode phase velocities. Modified nonuniform codirectional couplers with a dummy channel for continuously decreasing or increasing taper and employing wiggly, serpentined and smooth coupled edges have been designed and tested. It is found that a wiggly coupler can achieve a 50% length reduction compared to a smooth-edge coupler. A further 60% length reduction compared to a wiggly coupler is achieved by a serpentine coupler. Coupler performance for wiggly and serpentined configurations is computed by choosing a realizable phase velocity function for a given coupler length. Either constant 90° or -90° phase shift is possible with these couplers giving significant design flexibility in some applications. The results for a K_u-band Σ-Δ magic-T circuit employing a 0 dB wiggly coupler and a -3 dB smooth-edge coupler are also presented in the paper