A Dual-Band Resonance Sensor for Microwave Complex Permittivity Characterization of Polar Liquids

A highly sensitive, planar dual-band resonant sensor is proposed for the comprehensive characterization of the complex permittivity of polar liquids, which are often distinguished by high dielectric loss and strong frequency dispersion. The device integrates two distinct split-ring resonator (SRR) structures, each incorporating a set of interdigital electrodes (IDEs), onto a single microstrip transmission line. This architecture enables simultaneous, high-fidelity measurements at two discrete frequencies, providing a more robust characterization of a liquid's frequency-dependent dielectric properties than is possible with conventional single-band sensors. The IDEs are specifically engineered to concentrate the electric field within the sensing region, thereby maximizing interaction with the material under test and significantly enhancing sensitivity. The sensor, fabricated on a low-cost Rogers RO3006 substrate, was experimentally validated using a series of methanol-ethanol mixtures. Following a robust calibration procedure, the sensor demonstrated excellent performance, achieving sensitivities of 0.34% and 0.41% at its two unloaded operating frequencies of 4.94 and 6.22 GHz, respectively. The device provides accurate measurements over a broad permittivity range for the real part (epsilon')from 1 to 81, with experimental validation showing an error of less than 5% for values between 1 and 11. This study presents a low-cost, easily fabricated, and highly effective tool for the nuanced analysis of polar liquids, holding significant promise for applications in biochemical analysis, food quality control, and industrial process monitoring.