High Q-Factor Compact Permittivity Sensor Based on Coupled SRR-ELC Metamaterial Element and Metasurfaces Shield

This article describes the development of a microwave sensor with a high Q-factor for measuring permittivity. The compact size of the sensor is deemed essential, leading to the utilization of a coupled line with two coupled split-ring resonators (SRRs) as the main section of this sensor. The coupling between the SRR and two transmission line resonators creates a trap for the electrical field, forming a hot spot and resulting in a high Q-factor sensor. To further improve the Q-factor, the electric inductive capacitive (ELC) element in the ground layer is utilized. However, losses due to the dielectric and surface wave effects are common problems faced in obtaining optimal results. To address these challenges, electromagnetic band gap (EBG) unit-cells are used as metasurfaces to enclose the main sensor element. This set-up establishes an electromagnetic shield, minimizing losses and improving the overall performance of the sensor. The sensor is specifically tuned to operate at 3.36 GHz. The study employs simulation using the finite element method (FEM) and compares the results with experimental data. The material under test (MUT) in this study is FR-4 substrates with photonic band gap (PBG) properties, having a permittivity range from 2 to 4.2. The obtained outcomes are then compared with the effective permittivity derived from the Maxwell–Garnett equation. The measured average sensitivity ( ${S}_{\text {avg}}$ ) of the developed sensor is determined to be 4.68%. Despite its high sensitivity, the sensor maintains a compact size, with total dimensions of $0.4\lambda _{{0}} \times 0.4\lambda _{{0}}$ , where $\lambda _{{0}}$ represents the free-space wavelength at the operating frequency.