Three-Point Calibration Method for Conductive Liquid Characterization Using High-Dynamic-Range Microwave Sensor

Accurately quantifying ionic compound levels in aqueous media remains challenging due to the strong dielectric losses introduced by dissolved ions. This work presents a resonator-based microwave sensing system designed for non-invasive characterization of such lossy media, demonstrated using ammonium chloride (NH4Cl) as a representative analyte. The system employs an electric-LC resonator tag coupled to two different reader configurations, a gap-coupled transmission line (GCTL) and a standard transmission line (TL). Changes in ionic concentration modify the effective permittivity of the surrounding medium, which in turn perturbs the resonator’s electromagnetic loading. These perturbations manifest as measurable variations in the resonant response, allowing the reader to sense the medium entirely through wireless coupling without requiring physical contact. Based on wideband dielectric assessments of the material samples, 1000 MHz was selected as the optimal operating frequency balancing sensitivity and signal attenuation. Experimental and simulation studies show that the resonance amplitude varies monotonically with ionic content, exhibiting exponential behavior for the GCTL and a logarithmic dependence for the TL reader with R2= 0.99. Leveraging these predictable behaviors, we introduce a three-point calibration methodology that parameterizes the full response curve from only three measurements. This approach drastically reduces computational overhead while maintaining high accuracy (error