An open-ended probe with an antenna for the measurement of the water content in the soil

Abstract

Knowledge of water content (WC) in soil is especially necessary in agriculture to assess the individual irrigation state of the plant. Therefore, there is a need to develop sensors able to measure WC in large volumes and in possibly wide frequency range, e.g. by modifications of the classical open-ended (OE) probes which are characterized by low penetration depth of electromagnetic waves. The objective of the research presented in this paper was to test the performance of an open-ended dielectric probe with an antenna (OE-A) in the frequency range 1 MHz–6 GHz for three mineral soils using vector network analyzer (VNA) one port (reflective) measurements. Firstly, numerical simulations of the probe using Ansys HFSS software were performed, followed by the probe calibration on reference materials and measurements of the soil samples to determine soil moisture. The simulation and measurement results of the real part of dielectric permittivity (ε') were in good agreement with literature data up to 350 MHz. ε'obtained for the tested soils was connected with their moisture and the relation between ε'and volumetric water content (θv) was determined and compared with the Topp’s equation. The obtained results proved that our probe can be used for the measurement of the WC in soils in the frequency range from 50 MHz to 350 MHz.