A Versatile Reflective-Mode Phase-Variation Microwave Sensor Based on a Dumbbell Defect-Ground-Structure (DB-DGS) Resonator for Solid and Liquid Characterization

The paper presents a single-frequency planar microwave sensor designed for characterizing solid and liquid samples. The sensor operates in reflective-mode one-port configuration utilizing a microstrip line with a series inductive strip terminated with a transversely oriented dumbbell defect-ground-structure (DB-DGS) resonator (etched in the ground plane), the sensing element. The output variable is the phase of the reflection coefficient at a specific (operating) frequency. The device is modelled by a lumped circuit consisting of a parallel resonator (accounting for the DB-DGS resonator) series-connected to an inductor and a parasitic capacitor. This configuration enables the generation of a steep phase slope in the reflection coefficient at the operating frequency, which is essential for achieving high sensitivity. The sensor can be applied to the detection of small variations in the permittivity of the environment surrounding the sensing element in the vicinity of a reference (REF) value, where such REF value can be selected at wish by conveniently tuning the operating frequency. To demonstrate the versatility of the proposed device, three different scenarios are considered: (i) dielectric constant of solid samples measurement, (ii) ethanol concentration in deionized (DI) water determination, and (iii) detection of water/detergent in wine. The maximum sensitivity achieved for the dielectric characterization of solids is -382.44° per unit of dielectric constant. For liquid sensing, the sensor exhibits sensitivities of 9°/% for ethanol concentration in water and 0.88°/% for detecting water/detergent mixtures in wine. Despite the similar electromagnetic characteristics of water/detergent and wine, the proposed sensor can resolve a volume fraction (percentage of water/detergent in wine) as little as 2.5%, demonstrating high sensitivity. For the mixture of ethanol and DI water, the sensor is capable of detecting a concentration of 1% of ethanol.