Microwave grading of lumber

Microwave measurements of the electrical properties of wood (Douglas fir) at various frequencies, moisture contents and grain orientations were made. Based on these measurements, a frequency of 35 GHz (nominal) was chosen for the development of a microwave apparatus for stress grading of lumber as it gave the best compromise between resolution and sensitivity. The design and implementation of a homodyne system incorporating a dual balanced mixer is described. The system is capable of measuring the co-polar, cross-polar and RF phase (of the co-polar) signals of a linearly polarized electromagnetic wave transmitted through wood. The performance of the system is tested by measuring samples of Douglas fir lumber boards which were later subjected to tests for strength (modulus of rupture). The results indicate excellent qualitative correlation between the microxtfave measurements and the strength of the lumber boards.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofGraduat