Double-Layer Metamaterial Microwave Sensor for Olive Oil Quality

This paper presents a double-layer metamaterial microwave sensor for the selective detection of water adulteration in virgin olive oil. The proposed design employs an E-comb resonator configuration that enhances electric-field confinement and improves interaction between the resonator and the material under test compared with conventional single-layer resonant sensors. The sensor was first characterized through broadband full-wave electromagnetic simulations using CST and HFSS over the 1–15 GHz frequency range, followed by fabrication and experimental validation using vector network analyzer (VNA) measurements. Experimental results demonstrate a strong and selective response to polar contaminants, exhibiting a resonance frequency shift of approximately 0.5 GHz for only 5 % (v/v) water adulteration. In contrast, non-polar adulterants such as sunflower oil produce negligible resonance variation. The dielectric properties of the samples were extracted using the Cole–Cole relaxation model and showed good agreement with reported literature values, confirming the reliability of the proposed sensing mechanism. Furthermore, a quality factor-based index is introduced as a quantitative indicator for assessing oil purity. The proposed approach provides a non-destructive, real-time sensing technique with strong potential for automated quality control in the olive oil industry, using a compact and cost-effective microwave sensing platform.