Microwave tunable components, such as filters and phase shifters, have attracted considerable attention for use in mobile communications and satellite broadband applications. Nematic liquid crystals have already been used to realize microwave phase shifters. On the other hand, ferroelectric liquid crystal possesses spontaneous polarization and has fast switching speed. In this study, we discuss the microwave variable phase shifter of microstrip and coplanar waveguide structures using ferroelectric liquid crystal material.

A microstrip line is deposited on a dielectric substrate, and another brass substrate is to enable it to be used as a ground plane. Both surfaces of the glass and brass substrates were coated with polyimide because the initial molecular orientation is parallel to the substrate. The dc voltage was applied to the microstrip line using a function generator and a power amplifier for applying the voltage to the liquid crystal layer.

In the ferroelectric liquid crystal, a helical structure forms in the absence of applied voltage and the helical structure unwinds under application of the voltage higher than a certain voltage. The dielectric permittivities perpendicular to the substrate are different in the wound and unwound states. Therefore, the phase delay of the microstrip line changes under applying the dc voltage to the liquid crystal layer. Figure 1 shows the applied voltage dependence of the phase shift in the case of an offset voltage of 20 V. The phase shift increased with the voltage and was saturated above 100 V. This result indicates that the phase shift can be controlled by the application of appropriate voltage. The phase shift response with and without the application of stepwise voltage was measured. The response times depend on the voltage and were less than 30 ms under application of sufficient voltage. In particular, the decay time of this device is much less than that using nematic liquid crystal. The phase shifter of a coplanar waveguide structure using ferroelectric liquid crystal is also discussed in this presentation.