Analysis and design of double-ring crossover junction with arbitrary diagonal port impedances

A planar four-port double-ring crossover junction with arbitrary diagonal port impedances is designed and realized. The circuit consists of two concentric rings connected with four transmission line sections. It allows the two pairs of diagonal ports of the junction to have different characteristic impedances. Based on the transmission-line theory, design equations are formulated and design curves are provided. Two microstrip circuits are realized to validate the idea. One operates at 2.45 GHz and the other at 5.2 GHz. Both circuits have excellent performance such as good return loss and isolation. The measured total power loss is less than 4.8%. The measured results agree very well with simulation counterparts.