A Compact Tunable Filtering Rat-Race Coupler

This paper presents, for the first time, a compact frequency-tunable filtering rat-race coupler. The design employs four high-Q tunable Substrate Integrated Waveguide (SIW) cavity resonators with two opposite polarity inter-coupling structures. The coupler is synthesized using the coupling matrix method. The design process as well as experimental results are presented in the paper. The total size of the proposed rat-race coupler is (0.38λg × 0.38λg [λg at 1.3 GHz]) which represents 42% reduction in the structure footprint compared to a conventional one. The coupler can be tuned starting from 1.3–2.21 GHz (70% tuning range). Moreover, it has an equal power dividing ratio with a measured insertion loss ranging from 4.2–3.7 dB. Over the full range, the measured 3-dB fractional bandwidth ranges from 5.5–6.46%, and the isolation is better than 30 dB.