Liquid Crystal Based SIR Bandpass Filter for Millimeter Wave Application

A liquid crystal wideband bandpass filter based on inner coupled ring SIR structure (stepped impedance resonator) for millimeter wave application is presented. The filter adopts a periodically loaded in-coupled ring-shaped SIR structure, and the insertion loss introduced by the filter with the coupling structure is reduced by cascading a different number of resonant elements. The filter has high frequency selectivity and out-off-band suppression. By encapsulating the liquid crystal material inside the multilayer substrate and using the dielectric anisotropy of the liquid crystal material to realize the tuning characteristic of the circuit, the design of a reconfigurable bandpass of the millimeter wave center frequency is realized. The 3dB bandwidth of the 6-order millimeter wave SIR filter cascaded with three resonant elements is 10.4GHz, and the return loss is less than −16.5dB and the insertion loss is greater than −1.2dB. By changing the dielectric constant of the liquid crystal material (from 2.5 to 3.1), the frequency of the filter center varies from 28.2GHz to 29.7GHz.