Low-Cost Cell Based on Symmetric Stripline for Soil Permittivity Measurement in the Frequency Range of 0.1–1 MHz TO 5–7 GHz

The paper presents results of finite element modeling, development of and experiments with the measuring cell based on a symmetric stripline for measuring coarse-grained soil permittivity. The wave impedance of the measuring cell section intended for filling with soil, is about 80 Ω to expand the frequency range. This allows reducing the width of the central strip and increasing the critical frequency, which cause the higher-order modes. Cell sections with the transfer from SMA connectors to measuring section are filled with a solid dielectric. The distance between outer conductors and the central strip width in these sections, are linearly increased to the size of the measuring section to provide the wave impedance of 50 Ω. The wave impedance growth in the measuring section is considered in the soil complex permittivity calculations. The complex permittivity is measured for five calibration liquids with the static permittivity of 2.27 (transformer oil) to 78.5 (water) and three soil samples with different moisture. It is shown that acceptable values of measurement error can be obtained if the real part of the complex permittivity does not exceed 23–25 units at a frequency of ~1 GHz.