MICROWAVE ELECTROMAGNETIC NONDESTRUCTIVE TESTING OF WOOD IN REAL-TIME

As wood is lossy and anisotropic, a linearly polarized electromagnetic wave incident on the wood suffers attenuation, phase change and depolarization. A microwave measurement system has been developed for measuring the parameters of the electromagnetic field transmitted through dimension lumber. From these, the physical parameters (moisture content, specific gravity and grain angle) are determined.;From the measured electrical parameters, the physical parameters are determined for several wood species over a wide range of moisture contents. The amplitude and phase along the major polarization axis were found to be essentially independent of the grain angle for angles less than 20(DEGREES), so that the amplitude and phase are sufficient to uniquely determine the moisture content and specific gravity. Then the depolarization index can be found, which, together with the measured polarization angle, can be used to find the grain angle. This procedure considerably simplifies the testing problem.;Another major achievement of this work is that the measured electrical parameters can be used to calculate the two-dimensional complex dielectric tensor of the test medium. This is the first time that such a tensor has been quantified. The off-diagonal elements of the tensor were found to be very sensitive functions of the anisotropy, particularly the moisture content in the case of wood. This will undoubtedly play a key role in future nondestructive testing.;Besides nondestructive testing, this instrumentation system has many other applications such as microwave vector holography, near-field antenna measurements, inverse scattering and navigation.;The 4.81 GHz microwave system used is able to map field distributions in real-time as the wood moves through the field. It uses a phase-insensitive homodyne (coherent) detector to measure the round-trip field scattered from an electrically modulated scattering dipole. The scatterer is mechanically spun to permit measuring the polarization characteristics of the transmitted field. Thus, the backscattered signal is amplitude modulated at the frequency of electrical modulation (10-20 kHz), and amplitude and phase modulated at the mechanical spinning frequency (300 Hz). The homodyne detected signal is proportional to the square of the E-field component along the instantaneous axis of the spinning dipole. The amplitude, phase and polarization are measured simultaneously and independently in real-time, so the field is uniquely determined.