Tunable bandpass filter with low insertion loss based on electrically controlled liquid crystal

This paper presents a low loss electrically tunable bandpass filter based on liquid crystal. By utilizing structure of transmission line and direct-connected feeding lines without coupling gaps which eliminate mismatch and radiation losses, then the proposed filter gets a low insertion loss. And the tunability of the filter is realized by utilizing electrically controlled liquid crystal (LC) material as the substrate. In order to verify the design concept, a liquid crystal tunable prototype filter is designed, fabricated and measured. Measured results agree well with the simulated results. With no bias voltage applied, the filter operates at center frequency of 11.5 GHz with fractional bandwidth of 11.3%. And with voltage tuning from 0 V to 10 V, the filter gets a tuning range of 500 MHz and the insertion losses are 2-3.1 dB, respectively.