Abstract
Non-destructive evaluation of permittivity and thickness is a critical requirement in industries such as aerospace, automotive, construction, and medical engineering. This study presents a simple yet highly efficient method for characterizing the permittivity and thickness of solid materials using symmetrical Split Square Resonators (SSRs) incorporating a central via. The proposed configuration enhances electric and magnetic field confinement within the sensing area, thereby improving measurement sensitivity and spatial resolution. The sensor consists of a Substrate Integrated Waveguide (SIW) structure hosting two centrally via-loaded SSRs positioned at locations of maximum electromagnetic field intensity to achieve high sensitivity. The design operates through a combined band-pass and band-stop response: a transmission pole (TP) at 1.62 GHz and a transmission zero (TZ) at 2.95 GHz. The TZ frequency exhibits an approximately linear relationship with the permittivity and thickness of the material under test (MUT). As the MUT permittivity and thickness increase, the TZ resonance shifts from 2.95 GHz to 2.1 GHz and from 2.95 GHz to 2.48 GHz, respectively, while the TP frequency remains nearly constant. This differential behavior reduces environmental interference and enhances measurement accuracy. The sensor is fabricated using PCB technology on an RO-4003c substrate. Experimental results demonstrate high sensitivity of 3.27% for permittivity and 30.63% for thickness measurements, showing competitive performance compared to similar reported works. The overall dimensions of the sensor are 64 mm × 60 mm × 0.508 mm, corresponding to 0.0119 of the guided wavelengths.