A Frequency-Compensated Rat-Race Coupler With Wide Bandwidth and Tunable Power Dividing Ratio

This paper presents, for the first time, the theoretical and experimental study of broadband rat-race coupler design with a tunable power dividing ratio. Wide fractional bandwidth (around 40%) is accomplished by the adoption of frequency compensating networks with specific derivatives of even- and odd-mode admittances. General formulation is analytically carried out to lay down the design principles of these networks. The proposed design also features wide tuning range (almost 20 dB), low insertion loss, high port isolation (over 25 dB), good return loss, simple structure, compact size, zero dc power consumption, and single voltage operation with a minimal number of tuning diodes. For demonstration, both simulated and measured results of a 1-GHz tunable rat-race coupler implemented on microstrip are shown.