Determination of dielectric constant of wood under microwave frequencies- Method of test
1993 الطبعة

IS 13621 (1993) recommends conducting dielectric constant measurements of wood within the microwave frequency range of 1 GHz to 15 GHz, with an optimal testing frequency set at 10 GHz. This range facilitates accurate, non-destructive evaluation of wood’s dielectric properties, which are influenced by moisture, density, and wood-water interactions. The measurement setup typically uses microwave benches operating in S, J, or X bands. Frequency is precisely determined by adjusting the cavity frequency meter to identify dips on the VSWR meter, followed by calibration to record the exact frequency.

The determination of wood's dielectric constant as per IS 13621 involves microwave measurements within a waveguide setup. Key parameters include the propagation constant in the dielectric medium (K), wavelength in air (λg), specimen length (t), waveguide width (a), and the measured shift in signal minima due to the specimen insertion (Δ). The dielectric constant (ε) is calculated using specific formulas that incorporate these parameters and intermediate functions derived from tangent tables. The methodology ensures non-destructive, reliable estimation of the dielectric constant, essential for assessing wood properties.

The test apparatus for evaluating the dielectric constant under IS 13621 consists of a microwave bench equipped with several key instruments: a Voltage Standing Wave Ratio (VSWR) meter to monitor standing wave patterns; a variable attenuator to control microwave power; a cavity frequency meter featuring a cylindrical tunable cavity perpendicular to the waveguide for frequency measurement; a slotted section with a probe to detect voltage maxima and minima; and a waveguide specimen holder designed with an adjustable low-loss contacting choke type short to securely position the wood specimen. This configuration ensures precise and accurate dielectric measurements.

Moisture content has a significant impact on the dielectric constant measurements of wood. Since water exhibits a high dielectric constant (approximately 80), an increase in moisture within the wood elevates its overall dielectric constant substantially compared to dry wood, which typically ranges between 2 and 5. This increase enhances the wood’s capacity to store electrical energy, affecting the interaction with microwave signals during testing. Accurate control or accounting for moisture content is therefore critical to obtain reliable dielectric constant values.

Specimen preparation according to IS 13621 involves placing the wood sample in contact with a short-circuit plate inside a waveguide section equipped with an adjustable low-loss choke-type short. The shift in signal minima position is recorded before and after specimen insertion. The specimen dimensions, including length and width matching the waveguide, are measured, and moisture content is determined by weighing before and after oven drying at 103 ± 2°C until a constant mass is achieved. Test reports must comprehensively document the timber species, grain orientation relative to the electric field, moisture content percentage, operating frequency, and the calculated dielectric constant to ensure consistency and reproducibility.