Abstract
This paper presents a sensitive microwave near-field sensor based on utilizing a three-dimensional capacitor within a planar split-ring resonator. The planar resonator is etched in the ground plane of a microstrip line, where the resonator's length is relatively smaller than the guided wavelength. The sensor was fabricated utilizing PCB technology and used to detect the presence of dielectric fluids. The numerical analysis shows that the three-dimensional capacitor significantly enhances the electric field in the sensing area (free space) leading to an increase in the sensitivity. The electric field enhancement was quantitatively investigated using numerical analysis. The analysis shows that a loaded-quality factor of 1556 can be achieved using a three-dimensional capacitor with a length of 29 mm, which is significantly higher than what can be achieved by using planar resonators working in the same range of frequency. In addition, the utilization of the high-dielectric substrate can enhance coupling to the resonator. The proposed sensor was experimentally tested on two fluids, chloroform and dichloromethane with the relative permittivity of 4.81 and 8.93, respectively. The proposed sensor shows higher sensitivity of 700% and 374.5% in detecting the presence of chloroform and dichloromethane, respectively. Furthermore, the proposed sensor was utilized to detect the changes in fluid levels.