Tunable Microwave Bandpass Filters With Complementary Split Ring Resonator and Liquid Crystal Materials

In this paper, a novel planar waveguide filter based on liquid crystal materials is devised, and a half-mode corrugated substrate integrated waveguide (HMCSIW) tunable bandpass filter with complementary split ring resonator (CSRR) structure is proposed. The HMCSIW uses an array of quarter-wave open-circuit stubs to replace conventional substrate integrated waveguide (SIW). While retaining the original cut-off frequency of corrugated substrate integrated waveguide (CSIW), we reduce the circuit size by splitting the central axis to achieve the miniaturization. By utilizing band-stop features of the CSRR in microstrip structure and combining with the electronically controlled dielectric constant of liquid crystal materials with high modulation efficiency of inverted microstrip line structure, a CSRR-loaded HMCSIW bandpass filter with liquid crystal materials is implemented. The experimental results demonstrate that the center frequency of the new filter can be adjusted in a range of 0.65 GHz, the pass-band insertion loss is less than 3.4 dB, the return loss is less than -10 dB, and the structure size is 32 mm×76 mm, which verify that the designed tunable band-pass filter achieves good frequency selection performance.