Parallel ring-line rat-race circuit with very loose coupling

This article describes rat-race circuits (RRCs) in which a pair of ring-lines have parallel lines of +/- 180-degree difference for electrical length to realize the RRC with very loose coupling such as -10 dB or less. The proposed RRCs with parallel ring-lines realize a coupling factor of -10 dB and -20 dB by using moderate characteristic impedance lines with a view point of fabrication tolerance for general PCB technologies. We confirm the validity of this circuit construction and design method using a commercial electromagnetic simulator and decide the circuit pattern for fabrication at a center frequency of 2.45 GHz. The fabricated RRCs with parallel ring-lines realize the desired coupling factors and in-/out-of-phase outputs, and the measurement results of scattering parameters for -10 dB and -20 dB RRCs are in good agreement with the simulation results.