Design of a narrow dual-band BPF with an independently-tunable passband

Using polygonal resonators and bended lines, a new type of dual-band bandpass filter (BPF) is presented generating a fixed passband centred at 2.70 (f01) and an independently-electronically tunable passband (f02) which can be tuned between 3.65 and 6.51 GHz (56% tuning range). The measured results indicate that the return losses of both passbands are >21 dB throughout the tuning range. Furthermore, the use of bended lines leads to extremely small size of $0.136 \times 0.098\lambda _{\rm g}^2 $0.136×0.098λg2, where $\lambda _{\rm g}$λg is the guided wavelength at the lowest working frequency (f01). The proposed BPF also benefits from an ultra-wide upper stopband from 7.03 up to 14.41 GHz (based on 21 dB points), suppressing unwanted signal by more than 21 dB considered as a positive point. The filter prototype is investigated in four design stages with the LC analysis to support the design process theoretically and fabrication results to validate the design practically.