High coupling achieved by unequal and nonuniform characteristic impedances within three-line couplers

Different from well known conventional designs of parallel-multi-line directional couplers, this paper presents a method to substantially increase the coupling of parallel-line coupler types. Emphasis is on three-line couplers. Enhanced coupling is achieved by using unequal characteristic impedances for the parallel-coupled lines and nonuniform characteristic impedances within a line. Moderate broadband design is obtained by optimization. The paper presents the basics and shows simulation and optimization results.