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Abstract-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 applied to the measurement of wood properties, considering wood as an anisotropic, heterogeneous, 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 three-dimensional 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 the scattering in the sideways direction, while other investigated parameters (defects, gradual density variation, bulk density) show poor correlation with measured scattering coefficient.

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In order to determine more accurate indicators of wood structure obtained by microwave sensing and improve our understanding of plane wave propagation through this complex material, we have undertaken a permittivity survey and experimentally investigated scattering of a plane wave, measuring its transmission over two non-parallel surfaces of a rectangular lumber sample. This novel non-destructive testing technique offers results which may significantly contribute in a more accurate propagation modeling and industrial wood quality testing.

This paper presents a novel approach to microwave non-contact, non-destructive testing of wood, investigating additional indicators of wood structure, in particular the significance of wave polarization in transmission measurements. Contribution of wave polarization to wood characterization is studied, considering detection of rapid and gradual variations in wood structure, determination of moisture content and density distribution along the sample, as well as bulk properties detection. A set of microwave sensors in a collinear arrangement was used in two orthogonal linear polarizations. Significant findings are reported, in particular polarization dependence of moisture related attenuation, as well as improved defect detection accuracy when measurements in two orthogonal linear polarizations are considered. In addition, sorting wood samples into categories is considered and the advantages of this approach are presented.