Frequency Dependence of the Complex Permittivity and Its Impact on Dielectric Sensor Calibration in Soils

The capacitance (CAP) method and time domain reflectometry (TDR) are two popular electromagnetic techniques used to estimate soil water content. However, the frequency dependence of the real and imaginary part of the permittivity complicates sensor calibration. The frequency dependence can be particularly significant in fine‐textured soils containing clay minerals. In this work, we applied both the CAP method and TDR to a nondispersive medium (fine sand) and a strongly dispersive medium (bentonite). The measurements were conducted for a range of water contents. Results using a network analyzer showed that the frequency dependence of the real permittivity of the bentonite was particularly strong below 500 MHz. Above this frequency, the real permittivity of the bentonite was mainly a function of the water content. The TDR predicted apparent permittivity in the bentonite was below the CAP predicted real permittivity at low water contents. This was attributed to the dispersive nature of the bentonite combined with the high frequency of operation of TDR (up to 3 GHz in dry soil). The CAP sensor (frequency of 100–150 MHz) overestimated the real permittivity of the bentonite at high water contents. An electric circuit model proved partially successful in correcting the CAP data by taking the dielectric losses into account. The TDR signal became attenuated at higher water contents. It seems worthwhile to raise the effective frequency of dielectric sensors above 500 MHz to benefit from the relatively stable permittivity region at this frequency.