Generalization of Impedance Characterization Methods for Liquid Crystal-Embedded Tunable Transmission Lines and Applied Study into Guard Band Redundancy Evaluation.

In the trajectory of millimeter-wave (mmW) reconfigurable components advancement leveraging liquid crystals (LCs), a comprehensive array of planar and non-planar transmission lines, along with their diverse iterations, has emerged over the past two decades. This study introduces a unified framework for impedance characterization and benchmarking employing three distinct methodologies, i.e., Zpi (power-current), Zvi (voltage-current), and Zpv (power voltage). These frameworks are applied to three types of phase-variable transmission lines, namely, inverted microstrip line (IMSL), strip line (SL), and coaxial line (CL), each integrated with highly anisotropic nematic LCs as electronically tunable mediums. While these three topologies feature dual conductors, their fundamental disparity lies in the presence or absence of non-tunable dielectrics, thus influencing the interaction of mmW power with the transmission lines. Computational benchmarking at 60 GHz affirms the phenomenon of Zpi>Zvi>Zpv across the three topologies, albeit with varying degrees of deviation contingent upon geometric aspect ratios of the core line width to the LC layer thickness, as well as the permittivity ratio between tunable (LC) and non-tunable dielectrics (specifically pertinent to IMSL). Notably, aiming for 50 O at 60 GHz and with identical LC permittivity, the maximum deviation among the three impedances is 2.496 O for the IMSL, followed by 2.015 O for the strip line, and a minimal 0.254 O for the coaxial counterpart. Arguably, the deviations reduce with the enhancement of shielding towards a true TEM mode. Based on the impedance characterizations, a conservativeness assessment of TE11 cutoff guard band allowance for 300 GHz LC-filled coaxial phase shifters is conducted. Extending the implications of these findings, such a nuanced understanding of impedance characteristics and deviation patterns is instrumental in optimizing the design and performance of mmW reconfigurable components utilizing LC-based transmission lines towards enhanced adaptability and performance in diverse applications such as 5G/6G wireless communication networks, radar systems, and beyond. [ABSTRACT FROM AUTHOR]