Rethinking Figure-of-Merits of Liquid Crystals Shielded Coplanar Waveguide Phase Shifters at 60 GHz

Jinfeng Li

doi:10.3390/j4030034

Abstract

The demand for reconfigurable millimetre-wave (mm-Wave) components based on highly anisotropic liquid crystals (LC) is higher than ever before for the UK and worldwide. In this work, 60 GHz investigation on a bespoke shielded coplanar waveguide (SCPW) phase shifter structure filled with 16 types of microwave-enabled nematic LCs respectively indicates that the patterns of the device’s figure-of-merit (FoM, defined as the ratio of maximum differential phase shift to maximum insertion loss) reshuffle from those of the characterised LC materials’ FoM (defined as the ratio of tunability to maximum dissipation factor). To be more specific, GT7-29001- and MDA-03-2838-based phase shifters exhibit the highest FoM for devices, outperforming phase shifters based on GT5-28004 and TUD-566 with the highest FoM for materials. Such a mismatch between the device’s FoM and LC’s FoM implies a nonlinearly perturbed wave-occupied volume ratio effect. Furthermore, the relationship between insertion loss and the effective delay line length is nonlinear, as evidenced by measurement results of two phase shifters (0–π and 0–2π, respectively). Such nonlinearities complicate the established FoM metrics and potentially lead to a renewed interest in the selection and material synthesis of LCs to optimise reconfigurable mmWave devices, and promote their technological exploitation in phased array systems targeting demanding applications such as inter-satellite links and satellite internet.

Highlights

Mobile broadband terminals [1,2,3] are becoming a prime need and rely increasingly on a continuously tunable liquid crystals (LC)-based [4,5,6,7] millimetre-wave beam steering flat-panel [8] antenna array, in lieu of rotating parabolic dishes [9]
The difference was attributed to the perturbed wave-occupied volume ratio of the tunable-to-non-tunable dielectrics confined in the shielded coplanar waveguide (SCPW) mode, with LCs of diverse permittivity values, whereas the PTFE substrate material and SCPW geometry remained unchanged
With a direct connection to wave-occupied-volume ratio (WoVR), geometrical factor (GF) represents the efficiency of the specific type of LC material fitting into the device geometry

Summary

Introduction

Mobile broadband terminals [1,2,3] are becoming a prime need and rely increasingly on a continuously tunable liquid crystals (LC)-based [4,5,6,7] millimetre-wave (mmWave) beam steering flat-panel [8] antenna array, in lieu of rotating parabolic dishes [9]. For the past two decades of development in the field of LC-based phase shifters (variable delay lines), FoM has received a considerable level of interest as a dominant framework for the interpretation of performance, which is instrumental when different research groups report their devices with diverse achievable phase-shifting ranges (at the same frequency) but intend to compare the maximum insertion loss in a fair manner. From the electronically tunable LC material to a reconfigurable device (e.g., a phase shifter) combined with LC, without loss of generality, it is tempting to ask whether the material’s characterised FoM could directly inform the LC-based phase shifting device’s FoM. Such a research topic has neither been raised nor been answered clearly in existing literature

Methods

Results

Conclusion