Microwave Nondestructive Testing of Wood Anisotropy and Scatter

A microwave-focused beam transmission measurement is identified as a good candidate for fast, accurate, and affordable industrial wood testing. In this paper, the transmission measurement setup, in its various forms, is used to study wood´s properties, considering the wood as anisotropic heterogeneous and multiphase dielectric. The depolarization of a linearly polarized plane wave in an anisotropic media is considered first. It is used for grain angle detection for arbitrary grain inclination in 3-D space. A good correlation with visually inspected grain angle values is obtained. A scattering experiment is performed, measuring the transmission through the wood when the transmitting and the receiving antenna axes are at the right angle. The results indicate that the annual ring arrangement strongly influences scattering in the sideways direction, while other investigated parameters (defects, gradual density variation, and bulk density) show poor correlation with the measured scattering coefficient.