A Compact Rat-Race Coupler With Widely Tunable Frequency- and Power-Dividing Ratio

This letter presents a novel compact rat-race coupler that allows for simultaneous tuning of its operating frequency and power-dividing ratio. By substituting the tunable transmission-line section of the varactor-based rat-race coupler with two novel types of tunable units, it enables a wide tuning range for both frequency and power division ratio. To demonstrate the effectiveness of the design, a compact rat-race coupler was designed, processed, and measured. In the frequency-tunable mode, the measured 3-dB operating frequency can be adjusted from 1.2 to 3.1 GHz, while maintaining return loss and isolation of ${>}15$ and ${>}35$ dB, respectively. Under the power-dividing ratio-tunable mode, the measured power-dividing ratio of the coupler can be adjusted within ranges from $-$ 5 to 17 dB, $-$ 11.4 to 8 dB, and $-$ 12.1 to 4 dB at 1.2, 2.2, and 3.0 GHz, respectively. Compared to the existing couplers of the same type, the proposed coupler has the widest frequency tuning range.