Variable propagation constant directional coupler

Abstract

Variable propagation constant directional coupler is proposed and demonstrated. Compared with its conventional counterpart, the proposed coupler uses variable waveguide propagation constants as an extra degree of design freedom to achieve the required signal addition and cancellation. The operation principle is mathematically formulated and a step-by-step design methodology is presented. Two double-layer two-hole substrate integrated waveguide couplers: namely, one forward directional coupler and one backward directional coupler, are fabricated to validate the proposed concept. A coupling of −20 dB is achieved in both couplers and the maximum isolation of −47 and −52 dB are obtained by the forward and backward structures, respectively.