High-Performance Tunable Narrowband SIW Cavity-Based Quadrature Hybrid Coupler

This letter presents, for the first time, a tunable narrowband substrate integrated waveguide (SIW) cavity-based quadrature hybrid coupler. Four high-quality factor (Q) SIW evanescent-mode cavity resonators are employed in the design and tuned using piezoelectric actuators. This allows the proposed coupler to have wide spurious-free range (SFR), low-power consumption, and high-power handling capability compared to other tunable planar couplers. The coupler can be tuned starting from 1.32 to 2.22 GHz with a measured insertion loss range from 1.29 to 0.7 dB. Over the full range, the measured 3-dB fractional bandwidth ranges from 5% to 18% while the measured +/- 5 degrees phase error (angle S-21 - angle S-31) absolute bandwidth ranges from 40 to 82 MHz with an amplitude imbalance range between 1 and 0.3 dB. The measured reflection and isolation are better than 12 and 17 dB, respectively. Moreover, the coupler has a measured SFR of 5.1-3 f(o) (lowest-highest frequency).