Electromagnetic Impacts of Interface Discontinuity in Geometry and Material between Tunable and Non-Tunable PCBs for 79 GHz Liquid Crystal Phase Shifters in Joint Communication and Sensing

Beyond on-wafer measurements that focus exclusively on transmission line characterization, the realization of reconfigurable millimeter-wave (mmW) phase shifters requires the integration of connectors, which introduces the need for electromagnetic (EM) quantification of the interfaces between these connectors and the printed circuit board (PCB) - including both the tunable core line and the non-tunable transition sections. This paper investigates the EM effects of interface discontinuities in hybrid tunable/non-tunable PCB structures for liquid crystal (LC)-based phase shifters operating at 79 GHz for joint communication and sensing (JC&S). Two legacy topologies are examined: Design 1 (symmetric CPW with LC-filled gaps) and Design 2 (partially shielded CPW with integrated top electrodes). The non-tunable PCB sections are deliberately maintained as straight lines - rather than optimized shapes - to isolate material and geometric discontinuity impacts. Full-wave simulations using CST and HFSS are conducted across 1-79 GHz, quantifying S-parameters, surface current density, differential phase shifts, and figure-of-merit (FoM). At 79 GHz, Design 1 achieves a maximum insertion loss of 7.62654 dB at 0 V bias and a maximum phase shift of 408.74755° at 10 V bias, yielding an FoM of 53.59°/dB. Design 2 exhibits 9.64885 dB insertion loss and 347.79376° phase shift, resulting in an FoM of 38.19°/dB. Results demonstrate that interface discontinuities induce higher-order mode propagation and impedance mismatches, significantly affecting performance. The study further assesses the quarter-wavelength transformation approach in multi-interface configurations, revealing deviations from standard 50 Ω impedance strategy. These findings illustrate the importance of rigorous interface characterization for LC-based mmW phase shifters in emerging JC&S applications.