Dispersion Errors in Time Domain Reflectometry

Measurement of moisture content in a range of materials commonly uses time domain reflectometry and a transmission line surrounded by the material under test. The approach uses propagation time on the transmission line as a surrogate for permittivity and hence moisture content. However frequency dispersion that is caused by radiation from the transmission line can induce errors when using conventional methods for calculating the propagation delay. We show a relative permittivity discrepancy of 0.6 when measuring a material with a permittivity of 9. We observe that empirical calibration may remove this error, describe a adaptation of the conventional tangent method for calculating propagation time, and propose a theoretical approach.