A Novel and Compact Eight-Port Forward-Wave Directional Coupler With Arbitrary Coupling Level Design Using Four-Mode Control Technology

We propose a novel, eight-port, forward-wave directional coupler with periodically patterned ground structures. The input impedances of the four modes for the proposed coupler can be matched, and the phase difference between the even-even and the other three modes is also strongly enhanced. Equivalent models have been constructed to predict the propagation properties based on the Bloch-Floquet theorem. A design procedure is also proposed completely based on the equivalent models. The proposed couplers, with both equal and unequal power divisions, are fabricated on a Duroid substrate. Then, there is good agreement between theoretical simulations and measurements.