A wide-frequency dielectric spectrum can provide meaningful information that is intimately related to the properties of soils. A small-sized sensor allows the assessment of local dielectric characteristics of soils and is conducive for long-term monitoring as it can be buried in the soil sample without causing significant perturbations. Considering these advantages, the objective of this study is to explore the broadband measurement of complex permittivity ranging from kHz up to GHz using a slim-form open-ended coaxial probe with an outside diameter of 2.2 mm. The measurements are conducted in three different frequency ranges, i.e., high frequency (HF, 500 MHz ∼ 20 GHz), medium frequency (MF, 10 MHz ∼ 1 GHz), and low frequency (LF, 1 kHz ∼ 15 MHz), in view of pertinent aperture admittance models, feasible measurement principles, and required calibrations for system biases. These considerations are described and discussed in turn from the high-frequency measurements to the low-frequency tests. Verification of the associated measurement techniques in the three operating frequency regimes is provided by experiments on pure ethanol and methanol liquids as well as on sodium chloride solutions of different concentrations. These testing techniques are also applied to measure the broadband dielectric spectrum of kaolinite sediment. Simple analyses focusing on multiple relaxations, which are associated with bulk water polarization, bound water polarization, and spatial polarization are presented.

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