Contactless Microwave Sensor for Quality Assessment of Liquid Products

This article presents a contactless microwave sensor useful for monitoring the purity of liquid products. The device is a one-port reflective-mode microstrip structure terminated with a patch capacitor and a dumbbell defect-ground-structure (DB-DGS) resonator (the sensitive element), transversally etched in the ground plane. A small capillary, placed on top of the DB-DGS resonator, is used to drive the liquid under test (LUT) close to the DB-DGS resonator. The sensor operates at a single (operating) frequency and uses the phase of the reflection coefficient at such frequency as the output variable, highly dependent on the complex permittivity of the LUT. The device is first validated by means of solid samples, where the high sensitivity (1891° per unit of dielectric constant) is applied to detect small defects, and then by detecting the presence of water/detergent in commercial wine. In the latter application, sensor resolution is 0.25%, and the peak sensitivity is 1298°/%, very competitive values.