Measurement of wood grain angle using free-space microwave measurement system in 8-12 GHz frequency range

Microwave nondestructive testing (MNDT) methods are fast, contactless, accurate and continuous techniques for evaluation of moisture content, grain angle and density of the wood. MNDT techniques using free-space microwave measurement (FSMM) system involve measurement of reflection and transmission coefficients in free-space. Materials such as wood which are lossy and anisotropic causes a linearly polarized electromagnetic wave to be depolarized (elliptically polarized) upon transmission through the material. The FSMM system can be used to measure polarization angle (θ) and axial ratio (AR) of elliptically polarized wave transmitted by the wood specimen. Strength and stiffness along the grain direction are at least ten times greater than perpendicular to the grain for Malaysian wood. There is a critical need to know the wood grain angle for structural applications. The FSMM system consists of a pair of spot-focusing horn lens antennas, mode transitions, coaxial cables and a vector network analyzer. AR and θ of the elliptically polarized transmitted signal of Malaysian wood specimens were measured in the frequency range of 8 to 12 GHz for species such as Nyatuh and Light Red Meranti. Wood specimens (12 cm by 12 cm by lcm) were prepared such that they are at different grain angles. Results are reported for AR, θ and wood grain angle for different Malaysian wood specimens. The relationship between wood grain angle and measured θ is discussed.