A novel rat-race coupler with widely tunable frequency

This paper presents a novel design method of rat-race couplers with tunable frequency. The port equivalent impedances of a reciprocity four-port network being matched and isolated are derived by employing the even-odd mode analyses. A frequency-reconfigurable matching network (MN) transforming the port equivalent impedances to normalized unit resistance is first designed, and then the frequency-reconfigurable couplers are constructed by attaching the MN to each port of the four-port network. Based on the proposed method, two frequency-tunable rat-race couplers with power-dividing ratios of 1:1 and 2:1 are designed and measured. The measurements agree well with simulations. The measured center frequencies of the couplers with power-dividing ratios of 1:1 and 2:1 are tuned from 840 to 2480 and 840 to 2410 MHz, respectively, with a return loss of better than 15 dB and isolation of better than 20 dB.