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• We propose and test a portable, low-cost reflectometric setup for moisture measurement for cultural heritage application. • A tailored load cell is developed and integrated to increase the reproducibility of measurements. • The application of steady and controlled force decreases the uncertainty of measurements. • The portable device shows high compatibility with other experiments in the literature. • Linear graduation curves between relative humidity and resonant frequency are reported. A low-cost and portable reflectometric system was described to correlate the water content inside building material with the resonance frequency of a planar probe. An inexpensive planar patch resonator was realized, and a high sensitivity load cell was developed to investigate the external force effect on the resonance frequency shift. Regarding the material under test, four stones commonly employed in Italy Cultural Heritage buildings were used: Pietra Gentile, Leccese, Carparo, and Tuff. Measurements were carried out at different water content values and were performed using a portable vector network analyzer. Two different trials were carried out ten times for each moisture level: with an unknown handheld force exerted on the probe and using the load cell to ensure a repeatable force. Results showed that a repeatable applied force guaranteed more reproducible measurements. The achieved humidity/frequency calibration curves can be used, in practice, for non-invasive on-site water content monitoring of historic structures. [ABSTRACT FROM AUTHOR]