A reconfigurable narrow-band bandpass filter using electrically-coupled open-loop resonators based on liquid crystals

This paper introduces an electrically reconfigurable narrowband bandpass filter that exploits liquid-crystal technology. The filter uses two electrically-coupled open-loop resonators to produce two transmission zeros, resulting in a more compact structure than a single open-loop resonator. The spacing between resonators contributes to easy manipulation of the fractional bandwidth (FBW) and return loss. Theoretical calculations of the electrical length using multilayer microstrip line equations are presented. Experimental results demonstrate that the center frequency can be tuned from 9.70 GHz to 10.86 GHz with a maximum bias voltage of 30 V, achieving a tuning range of 11.34%. With applied bias, the maximum FBW reaches 8.10% and the maximum return loss attains 15.97 dB in each biased state.