On the design and characterisation of a microwave microstrip resonator for gas sensing applications

This study focuses on the microwave characterisation of a microstrip resonator aimed for gas sensing applications. The developed one- port microstrip resonator, consisting of three concentric rings with a central disk, is coupled to a 50-Ω microstrip feedline through a small gap. A humidity sensing layer is deposited on this gap by drop-coating an aqueous solution of Ag@α-Fe2O3 nanocomposite. The operation principle of the developed humidity sensor is based on the change of the dielectric properties of the Ag@α-Fe2O3 nanocomposite when the relative humidity is varied. However, it should be underlined that, depending on the choice of the sensing material, different target gases of interest can be detected with the proposed structure. The frequency-dependent response of the sensor is obtained using the reflection coefficient measured from 3.5 GHz to 5.6 GHz, with relative humidity ranging from 0 %rh to 83 %rh. The variation of the humidity concentration strongly impacts on the two resonances detected in the measured reflection coefficient. In particular, an increase of the humidity level leads to lowering both resonant frequencies, which can be used as sensing parameters for humidity monitoring purpose. An exponential function has been used to accurately model the two resonant frequencies as a function of the humidity.