DIELECTRIC PROPERTIES OF WOOD AND HARDBOARD: VARIATION WITH TEMPERATURE, FREQUENCY, MOISTURE CONTENT, AND GRAIN ORIENTATION

The dielectric constant and loss tangent of white oak, Douglas-fir, and four commercial hardboards were measured at frequencies from 20 Hertz (Hz) to 50 Megahertz (MHz), moisture conditions from ovendry to complete saturation, temperatures from -20 to +90° C., ,and, for the natural wood, with the electric field alined with three principal structural orientations. The hardboard was measured with the electric field perpendicular to the faces of the board. The behavior of all materials studied was qualitatively similar. The dielectric constant increased with increasing moisture content or temperature, and decreased with increasing frequency. Magnitudes ranged from about 2.0 for cold, dry material at high frequencies to near 1 million for warm, water-soaked wood at low frequencies (parallel to the grain). Loss tangent showed maximum and minimum values under various conditions. The frequencies at which extremes occurred generally were higher as either moisture content or temperature increased. The loss tangent ranged from about 0.01 to about 100. A theory based on physically plausible assumptions was developed, and provided adequate description of the observed influence of frequency on dielectric properties of wood. The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the U.S. Department of Agriculture of any product or service to the exclusion of others that may be suitable. DIELECTRIC PROPERTIES OF WOOD AND HARDBOARD: VARIATION WITH TEMPERATURE, FREQUENCY, MOISTURE CONTENT, AND GRAIN ORIENTATION By WILLIAM L. JAMES, Physicist Forest Products Laboratory, 1 Forest Service U.S. Department of Agriculture