Measurement of Dielectric Constants of Liquid Crystals Using Double-Ridged Waveguide Cavity

Liquid crystals (LCs) are widely used in reconfigurable microwave devices for their voltage tunable dielectric constant and loss. For the design of materials and devices, accurate measurements of the complex permittivity of LCs are required. This paper presents a method of performing microwave characterizations of LCs using a double ridged waveguide cavity and the perturbation method. To realize the broadband test and make operating frequency points distribute uniformly, the multi-mode technique is introduced and 6 modes (TE_1,0,2n-1,n= 1, 2, 3, 4, 5, 6) are chosen as operating modes to work at 5-22 GHz by one cavity. Besides, the sample loading area in the double-ridged waveguide cavity is much smaller and the sample volume required is only about 0.02 ml. Several LCs are measured: a certain commercial mixture MM (Merck), and M1~M3, as developed LC compositions with large dielectric anisotropy and low microwave losses.