A Microwave Resonant Sensor for Concentration Measurements of Liquid Solutions

This paper presents the design, fabrication, and characterization of a microwave resonator as a tool for concentration measurements of liquid compounds. The sensing device is a rectangular waveguide cavity tuned at 1.91 GHz, which exploits the fundamental TE₁₀₁ mode in a transmission-type configuration. The coupling structure is optimized by means of a finite element code so as to achieve a high Q-factor. According to the type of substance inside the mixture, its concentration is conveniently related to changes of the S₂₁ scattering parameter (transmission coefficient) in terms of: 1) resonance frequency; 2) 3-dB bandwidth; and 3) amplitude at the resonance frequency. Experimental tests on liquid solutions in controlled conditions are presented to evaluate the performance of the device.