부산대학교 도서관

This article presents three low-cost microwave permittivity sensors for dielectric characterization of liquids. The sensors are based on a differential design concept to achieve robust performance in relation to fluctuations of the environmental parameters (humidity and temperature). The three circuits are implemented on substrate integrated waveguide (SIW) technology loaded with two complementary split-ring resonators (CSRRs). By incorporating three plated via-holes between them, the CSRRs can be effectively decoupled while maintaining a compact design. The sensors, operating in ${C}$ band, are manufactured by laser etching on a standard printed circuit board (PCB) process using an RO4003C substrate. They have two ports and do not require surface-mount soldering of discrete components. Sensors with different CSRR geometries (rectangular double ring, circular double ring, and circular triple ring) are analyzed by electromagnetic (EM) simulations and measurements. The measured results show good agreement in comparison to the simulations. Furthermore, the sensing performance is experimentally assessed by applying them to the dielectric characterization of deionized (DI) water and ethanol solutions with different concentrations, using capillary glass tubes as sample containers. The highest average measured frequency sensitivity achieved is 12.24 MHz per unit change in real permittivity with the sensor based on circular double ring CSRRs, and it is obtained with a very small liquid sample volume of about $1.5 \mu \text{L}$ .