Broadband complex permittivity measurements of nematic liquid crystals based on cavity perturbation method

Abstract

Liquid crystals (LCs) have already been widely used in reconfigurable microwave devices as an excellent phase change material for their electric tunable dielectric properties. An accurate value of complex permittivity on a given LCs is the most important parameter on designing microwave devices. In this paper, a novel method is proposed to realise a broadband permittivity measurement. On the basis of the theory of perturbation, it consisted of a double-ridge waveguide cavity and the given vector network analyser. In order to have a broadband measuring range and uniformly distribute operating frequency points, multi-mode double-ridged waveguide cavity is introduced to resonate at 11 operating modes (TE 1,0,2n−1,n = 1,2, …, 11). Meanwhile, to suppress the undesired-modes effectively, several non-radiating slots are applied to keep the operating modes easily recognisable, so as to complete the broadband complex permittivity measurement in 5–40 GHz using only one cavity. Besides, another feature of this method is that the total-required sample is only about 0.02 mL because the sample loading area in the double-ridged waveguide cavity is perfectly designed very small. Therefore, several NLCs are measured, including a certain commercial mixture MM (Merck), and M1~ M7, as developed LC mixtures with large dielectric anisotropy and low microwave losses.