This standard specifies a laboratory method for determining the dielectric constant and dielectric loss factor of wood at microwave frequencies, specifically in the X-band (around 9.375 GHz). The method is based on measurements of wavelength and attenuation in a short-circuited waveguide containing the wood sample. This property is critical for applications involving microwave processing of wood, such as drying, gluing, and modification.
Determination of dielectric constant of wood under microwave frequencies- Method of test
Quick Reference Values
Test Frequency9.375 ± 0.015 GHz (X-Band)
Specimen Thickness (d)Exactly equal to one or more half-guide wavelengths inside the dielectric
Specimen Cross-sectionTo fit snugly inside the waveguide (22.86 mm x 10.16 mm for WR-90)
Standard Conditioning Temperature27 ± 2 °C
Standard Conditioning Relative Humidity65 ± 5 percent
Key Formulas
ε' = ( (λg / 2a)² + (λg / λc)² ) / ( 1 + (λg / λc)² ) — Formula for Dielectric Constant
ε'' = ( (λg / 2a)² * β₂ * d * ( 1 + tan²(2πx₀/λg) ) ) / ( π * ( 1 + (λg / λc)² ) * tan²(2πx₀/λg) ) — Formula for Dielectric Loss Factor
Practical Notes
Moisture content is the single most critical variable affecting the dielectric properties of wood; it must be carefully controlled and measured.
The test specimen must be machined with high precision to fit snugly into the waveguide, as air gaps will cause significant measurement errors.
This is a highly specialized test for research and specific industrial applications, not a routine quality control test for timber in general construction.