Tunable passive phase shifter for microwave applications using highly anisotropic liquid crystals

A tunable broadband inverted microstrip line phase shifter filled with Liquid Crystals (LCs) is investigated between 1.125 GHz and 35 GHz at room temperature. The effective dielectric anisotropy is tuned by a DC-voltage of up to 30 V. In addition to standard LCs like K15 (5CB), a novel highly anisotropic LC mixture is characterized by a resonator method at 8.5 GHz, showing a very high dielectric anisotropy Δn of 0.32 for the novel mixture compared to 0.13 for K15. These LCs are filled into two inverted microstrip line phase shifter devices with different polyimide films and heights. With a physical length of 50 mm, the insertion losses are about 4 dB for the novel mixture compared to 6 dB for K15 at 24 GHz. A differential phase shift of 360° can be achieved at 30 GHz with the novel mixture. The figure-of-merit of the phase shifter exceeds 110°/dB for the novel mixture compared to 21°/dB for K15 at 24 GHz. To our knowledge, this is the best value above 20 GHz at room temperature demonstrated for a tunable phase shifter based on nonlinear dielectrics up to now. This substantial progress opens up totally new low-cost LC applications beyond optics.