Novel Hybrid Electric/Magnetic Bias Concept for Tunable Liquid Crystal Based Filter

This paper presents a novel hybrid bias concept for liquid crystal (LC) filter by using simultaneously electric and magnetic bias fields. It enables continuous tuning with a simplified electrode design, decreased number of electrodes and reduced bias voltage. Furthermore, the tuning efficiency is increased, since the homogeneous bias fields enable a full exploitation of the LC's anisotropy. To demonstrate this concept, a novel reconfigurable gap waveguide two-pole bandpass filter with tunable center frequency and coupling elements is designed at $30 \,\mathrm{GHz}$. Liquid crystal technology is applied as tunable material, whereby tunability of the center frequency and coupling elements is obtained by controlling the LC's effective permittivity. The presented filter's center frequency can be tuned from $28.88 \,\mathrm{GHz}\,\mathrm{to}\, 29.88 \,\mathrm{GHz}$ with a maximum bias voltage of 100 V, with a return loss of 20 dB and low insertion loss of only 1.65 to 1.95 dB.