Effect of applied pressure on patch resonator-based measurements of moisture level for cultural heritage materials

In this paper, preliminary results of variations as a function of applied pressure in the reflection coefficient of a planar patch resonator, placed in contact with cultural heritage stone materials, will be presented. The general aim of the experimental project is to correlate the resonant frequency of the planar sensor, for the different pressures applied to the resonator, with the different levels of water content $\theta_{v}$ of the tested stone material. In fact, in previous works, it has been demonstrated that by placing a planar resonator in contact with the considered stone sample, it is possible to associate the resonant frequency of the resonator with the moisture content of the stone sample, through reflection scattering parameter measurements. In previous studies, however, the level of applied pressure is not standardized and controlled. An application of an external force could improve the repeatability and increase the detectability of the first resonance peak. The current study shows a negligible resonant frequency shift among measurements with different applied pressures at the same water content $\theta_{v}$ level, but a significant change regarding Q factor. Moreover, applying an external force on the patch, the first resonance peak can be identified more easily, thanks to an increase in the Q factor.