Effective Frequency and Attenuation Measurements of Glass Beads/Magnetite Mixtures by Time-Domain Reflectometry

Samples with different percentages (5–30%) of magnetite and different ranges of grain size are analysed by time-domain reflectometry (TDR). The passband, the attenuation factor α and the effective frequency are derived from the TDR measurements. The passband was obtained from the second edge of the sample by eliminating the DC component and applying a Fourier transform to the residual signal. The bandwidth of the resulting spectral shape decreases as the magnetite percentage increases. The second edge was also used to evaluate the effective attenuation factor αe, using multiple-reflection theory, terminated at the second reflection. The attenuation values α can be estimated independently by means of the electromagnetic parameters obtained using an LCR meter. To compare α and αe, an effective TDR frequency νe is introduced. The agreement between the values obtained from TDR and LCR-meter techniques supports the reliability of the attenuation derived from the second TDR reflection, and that computed from the effective frequency. The shapes of the passbands and the αe and νe values for magnetite/glass-beads mixtures, which simulate dry soils with increasing iron oxides content, are reported and discussed.