Single-Sided Wood Moisture Nondestructive Testing Using Free-Space Microwave Reflection

Moisture-induced deformation, cracking, and biological deterioration threaten wooden cultural heritage, making reliable, nondestructive moisture assessment essential for preventive conservation. The microwave method has been recognized as an emerging method with advantages of noncontact, fast and high accuracy. However, conventional microwave transmission-based moisture detection methods require access to both sides of a specimen, hindering the inspection of fixed components with inaccessible rear surfaces. This study investigates single-sided, non-contact wood moisture assessment using the principle of free-space microwave reflection. A three-dimensional full-wave antenna–wood model identified significant moisture-dependent variations in a valley near 1.1 GHz and its adjacent peak. Their magnitude difference was defined as a valley-depth feature, with an optimized 100 mm standoff selected to balance linearity and sensitivity. The sensing system was developed based on the optimized experimental parameters from the simulation. And the calibration test showed a linear relationship between valley depth and relative permittivity (R2 = 0.98). Pine and paulownia specimens below 30% moisture content were measured in two polarization orientations, yielding short-term repeatability standard deviations less than 0.013 dB. Independent validation achieved the lowest errors for paulownia with the electric field perpendicular to the grain, with RMSE = 1.27% and MAE = 0.99%. An in-situ trial on a historic wooden door demonstrated field deployment feasibility and measurable spatial and temporal variations in the reflection response. These findings support the potential of the method for single-sided, non-contact wood moisture assessment.