A comparative evaluation of patch resonators layouts for moisture measurement in historic masonry units

Monitoring water content in cultural heritage materials through non-invasive and easy-to-use TDR/TDT (time-domain reflectometry / time-domain transmission) measurement systems has the potential to enhance conservation/restoration activities. In this study, two different probes for TDR/TDT technology, Square Patch Resonator (SPR) and Split Ring Resonator (SRR), were compared. The two probes were tested on two different materials (leccese and gentile stone), each one of them at three different levels of water content. All tests were conducted ten times with two different external impressed forces (hand pressure and 2 kg). Resonant frequencies (fr), fractional bandwidth (FBW), and standard deviation (σ) were evaluated to assess accuracy and repeatability in water content estimation. Results revealed optimum performance, FBW lower than 7 % and higher repeatability (σ < 5 MHz) with external mass, for the SRR probe. Our results reveal the possibility of monitoring the moisture level in the stone through a microwave tool based on a split ring and TDT device.