Theoretical and experimental study on air humidity measurement using temperature-compensated microwave coaxial resonant cavity

Abstract Humidity as an important physical parameter, requires real-time and accurate monitoring of its levels, which is crucial for manufacturing, environmental monitoring, and the medical industry. To this end, this study proposes a λ /2 microwave coaxial resonant cavity with temperature compensation and applies it as a sensing device for humidity measurement. First, based on the effective medium theory, a dielectric constant calculation model for humid air was established, and its dielectric properties were analyzed. Subsequently, a comprehensive theoretical measurement model was developed by integrating microwave coaxial resonant cavity theory. Through parameter scanning, the simulation modeling of the resonant cavity was optimized, determining the characteristic dimensions that enable the cavity to achieve high energy absorption efficiency and significant modal isolation characteristics, thereby verifying the global optimality of the structural parameters. Finally, temperature characteristic experiments were conducted to correct measurement errors caused by thermal expansion effects. Air humidity measurement experiments were performed to validate the performance of the designed resonant cavity. The experimental results demonstrate that the resonant cavity achieves a maximum sensitivity of 234 kHz/%RH within the 20%RH–90%RH range, with an overall average sensitivity of 53.34 kHz/%RH. Additionally, it exhibits advantages such as fast response and stable measurement, enabling online monitoring of air humidity.