Phase Compensation Analysis of Rat-Race Coupler with Two Short Stubs

For the rat-race coupler, an effective method to improve the phase bandwidth is by adding quarter-wavelength short-circuited branch at center frequency. In this paper, two short stubs are placed in a rat-race coupler to achieve phase compensation. Through the odd-even mode method, the specific analysis process is presented, and the impedance value of the short stub to obtain optimal phase bandwidth is derived. Based on these analyses, a prototype operating at 3GHz are developed and simulated. Results express good impedance matching and transmission characteristics. By using open stubs, another demonstrator is exhibited with a compact size. Numerical predictions shows that phase difference bandwidth of the two samples are significantly improved, thus confirming the analysis of this principle.