Passive Microwave Substrate Integrated Cavity Resonator for Humidity Sensing

This paper presents an original passive microwave substrate integrated cavity resonator [substrate integrated waveguides (SIWs)] as an environment sensor for humidity detection. The proposed structures are based on a high quality factor (Q ~ 300) substrate integrated cavity resonator operating at 3.6 and 4.15 GHz. The detection principle is based on a frequency shift due to the permittivity variation of the humid air. This variation can be detected and used as the sensor indication. The frequency shift has been estimated analytically using the dielectric perturbation method for the resonator prototypes. The structure of the presented SIW resonators has been tested in the presence of humidity and shows sensitive characteristics in the range of 0%-80% relative humidity in accordance with the proposed model. A comparison of sensitivity performance between the new structure and other reported microwave components for environmental sensing is also presented. Measurements of repeatability and reliability for the proposed structure are discussed as well. As a new microwave component type, the proposed substrate integrated environmental sensor has the advantage of providing a new fabrication solution for radio frequency environmental sensing and greatly simplifies the sensor´s manufacturing processes and cost.