Application of a dagger probe for soil dielectric permittivity measurement by TDR

The most basic parameter of soil is volumetric water content ( VWC ). This parameter is typically determined indirectly, based on other parameters. Currently, a widely used method for indirect VWC determination is a method which is based on soil dielectric permittivity ( DP ) measurement, using the well-known Topp formula. The paper presents a novel probe for soil VWC and electrical conductivity ( EC ) measurement employing the above method. The new probe is going to be adopted on a mobile plant-watering machines used in precise agriculture e.g. for plants watering, thus it features a robust mechanical design and allows for instantaneous readout of VWC and EC values. The design of the probe mimics a dagger with three flat conductors forming a short-circuited coplanar waveguide with the space between the conductors filled with a mineral cold-curing resin. The resin filling the space between the bars improves the mechanical stiffness of the probe and ensures constant electrical parameters, which increases the measurement accuracy. In the first step, Ansys HFSS software was used to perform electromagnetic (EM), numerical simulations for the proposed solution, in order to determine the optimal electrical parameters of the probe. Next, a probe prototype was made to carry out laboratory tests. The measurements were performed with the use of a vector network analyzer (VNA) in the frequency range (3.74 MHz − 3 GHz). The measured complex reflection coefficients were transformed into the time domain with the use of the inverse Discrete Fourier Transform (IDFT). Based on the time distance between the reflections of an electric pulse traveling along the probe's sensing element, bulk dielectric permittivity of soil surrounding the probe can be calculated. A linear relation between the square root of DP and the pulse propagation time was obtained. Also, the probe was calibrated for bulk electrical conductivity measurements.