A large coaxial reflection cell for broadband dielectric characterization of coarse-grained materials

Abstract

Knowledge of the frequency-dependent electromagnetic properties of coarse-grained materials is imperative for the successful application of high frequency electromagnetic measurement techniques for near and subsurface monitoring. This paper reports the design, calibration and application of a novel one-port large coaxial cell for broadband complex permittivity measurements of civil engineering materials. It was designed to allow the characterization of heterogeneous material with large aggregate dimensions (up to 28 mm) over a frequency range from 1 MHz–860 MHz. In the first step, the system parameters were calibrated using the measured scattering function in a perfectly known dielectric material in an optimization scheme. In the second step, the method was validated with measurements made on standard liquids. Then the performance of the cell was evaluated on a compacted coarse-grained soil. The dielectric spectra were obtained by means of fitting the measured scattering function using a transverse electromagnetic mode propagation model considering the frequency-dependent complex permittivity. Two scenarios were systematically analyzed and compared. The first scenario consisted of a broadband generalized dielectric relaxation model with two Cole–Cole type relaxation processes related to the interaction of the aqueous phase and the solid phase, a constant high frequency contribution as well as an apparent direct current conductivity term. The second scenario relied on a three-phase theoretical mixture equation which was used in a forward approach in order to calibrate the model. Both scenarios provide almost identical results for the broadband effective complex relative permittivity. The combination of both scenarios suggests the simultaneous estimation of water content, density, bulk and pore water conductivity for road base materials for in situ applications.

Device for the Field Measurements of Frequency-Dependent Soil Properties in the Frequency Range of Lightning Currents

“…Peak-to-peak sensitivity was calculated as ±N times 2.3 V divided by 4096, where ±N corresponds to the noise measured in counts, 2.3 V is the ADC input range, 4096 is 2 12 (the ADC has 12 bits resolution). Note that the noise was measured at ADC input, where the signal is amplified by the voltage probe and the analog processing circuit 1.5.…”

“…There is a considerable number of works dedicated to measurements of the complex permittivity of soils, certain minerals, and other similar materials [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] .…”

“…Most of the works, however, conduct measurements only with small samples 1-14 , which does not represent large volumes of soil (needed for the grounding) as long as most soils are usually inhomogeneous. Besides, a significant portion of works does not cover the frequency range of the lightning currents [5][6][7][8][9][10][11][12][13] . Summarizing, most of the existing studies solve problems unrelated to the groundings and lightning protection, and they cannot be utilized for the in situ measurements of the frequencydependent soil properties for the frequency range from several kHz (or lower) to several MHz.…”

Determination of the Porosity Distribution during an Erosion Test Using a Coaxial Line Cell

“…To do so, a one port coaxial transmission line with a sealing system was used. The analysis used is similar to the one proposed in Reference [27]. Saturated glass beads with different sizes were characterized in terms of the complex permittivity.…”

“…In this configuration, the electromagnetic field is more specified since it is concentrated between the inner and outer conductor. Coaxial arrangements have been commonly used for the dielectric characterisation of liquids [25] and soils [26,27]. The soil samples are either prefabricated or prepared directly in the cell.…”