Systematic design of reconfigurable quadrature directional couplers

This paper presents an approach for the systematic design of reconfigurable quadrature directional couplers. From even-odd-analysis of four-ports with two-fold symmetry four eigenreflections are derived. The scattering-parameters are linear combinations of them. The eigenreflections can be positioned deliberately with tuning reactances to create directional couplers which are reconfigurable in coupling ratio, operating frequency or the arrangement of the output ports. Two exemplary couplers, a LC-bridge and a modified branchline-coupler, are presented and measurement results are shown. Directional couplers with adjustable coupling ratio can also be created with cascaded couplers and additional phase shifters or discontinuities. A compact directional coupler using this principle is presented together with measurement results.