A Compact Frequency-Reconfigurable Rat-Race Coupler

A compact rat-race coupler with tunable frequency is presented. The frequency-reconfigurable operation is achieved by attaching the frequency-reconfigurable matching network (MN) to each port of the four-port network. The MN composed of a shunt tunable capacitor transforms the port equivalent impedance of the four-port network to the port impedance across the entire frequency-tuning range. Closed-form design equations are derived. For demonstration, a compact frequency-reconfigurable 3-dB rat-race coupler is designed and measured. The measured results agree well with simulated ones. The measured frequency tuning range is from 1.0 to 2.96 GHz, with the >20-dB return loss and >35 dB isolation. As compared with state of the arts, the proposed coupler exhibits wide frequency tuning range with smaller size and fewer tuning elements.