High-Sensitivity Sensor Loaded With Coupled Complementary Spiral Resonators for Simultaneous Permittivity and Thickness Measurement of Solid Dielectric Sheet

High-precision measurement of permittivity and thickness enables advanced quality control of microwave substrates. In this article, a subwavelength planar microwave sensor is proposed, which is composed of a single-microstrip transmission line with a pair of coupled circular complementary spiral resonators (CSRs). Thus, two robust band-stop notches in the transmission response are developed, which can be utilized for simultaneous thickness and permittivity measurements of a dielectric sheet. The mutual capacitance and inductance, which quantify the coupling effect between the two resonators, are added to the equivalent lumped circuit model. They enhance the frequency shifts, thus improving sensitivity. The results of full-wave simulations and measurements on five different types of samples verify the concept's effectiveness.