Overview and comparison of microwave noncontact wood measurement techniques

This article presents a comprehensive review of the literature on microwave wood testing, offering a critical comparison of reported methods and techniques. The purpose of this review is to establish current achievements and evaluate the potential for the application of microwave methods in industry. All reviewed articles are classified based on the way they deal with four key issues: propagation modeling, measurement technique, hardware implementation, and determination of wood properties. Currently used methods for propagation modeling are considered first, with particular attention given to the calculation of propagation factors along the principal directions, because these methods take into consideration wood anisotropy and allow grain angle detection. Measurement techniques are also considered in more detail, with a qualitative comparison of the two most advanced techniques: the focused beam technique and the modulated scattering technique. Reported hardware implementation and methods for determining the wood properties of interest from measured data are reviewed in the final sections. The technology is mature, but more effort is needed to reach a solution suitable for an industrial environment. This article aims to help researchers in this field to identify the remaining issues.