180° Differential Phase Shifting of 60 GHz Signals Using Liquid Crystal-filled Strip-Line Designs with Enhanced FoM Performance and Compact Footprint

Existing liquid crystals (LC) filled phase shifters are predominately structured in inverted microstrip lines at microwave frequencies, and coplanar waveguides (and their variants such as enclosed coplanar waveguides) or waveguides at millimetre-wave frequencies. Designs continue to evolve by fusing electromagnetic knowledge with state-of-the-art computer-aided design. This work shows a renaissance of interest in strip-lines by presenting a strip-line-structured liquid crystal phase shifter design reported for 3 mm-effective-wavelength agile reconfigurable applications (60 GHz), featuring a true-strip-line mode by having a fully symmetric substrates arrangement. Two designs (based on impedance-matching references on the two extreme values of the LC permittivity) are proposed and comparatively evaluated in this work. The phase shift to insertion loss ratio (FoM, i.e., figure-of-merit) surpasses 120°/dB, with the maximum insertion loss reporting 1.76 dB, under the device line length of 1.047 cm and LC thickness of 100 µm. These performance indicators and geometry footprints show a comprehensive improvement over the coaxial counterpart. FoM increases by 18°/dB at 60 GHz.