Andrzej Wilczek

A FDR Sensor for Measuring Complex Soil Dielectric Permittivity in the 10–500 MHz Frequency Range

Mechanical details as well as electrical models of FDR (frequency domain reflectometry) sensors for the measurement of the complex dielectric permittivity of porous materials are presented. The sensors are formed from two stainless steel parallel waveguides of various lengths. Using the data from VNA (vector network analyzer) with the connected FDR sensor and selected models of the applied sensor it was possible obtain the frequency spectrum of dielectric permittivity from 10 to 500 MHz of reference liquids and soil samples of various moisture and salinity. The performance of the analyzed sensors were compared with TDR (time domain reflectometry) ones of similar mechanical construction.

A TDR-Based Soil Moisture Monitoring System with Simultaneous Measurement of Soil Temperature and Electrical Conductivity

Elements of design and a field application of a TDR-based soil moisture and electrical conductivity monitoring system are described with detailed presentation of the time delay units with a resolution of 10 ps. Other issues discussed include the temperature correction of the applied time delay units, battery supply characteristics and the measurement results from one of the installed ground measurement stations in the Polesie National Park in Poland.

Determination of Soil Pore Water Salinity Using an FDR Sensor Working at Various Frequencies up to 500 MHz

This paper presents the application of a frequency-domain reflectometry (FDR) sensor designed for soil salinity assessment of sandy mineral soils in a wide range of soil moisture and bulk electrical conductivity, through the determination of soil complex dielectric permittivity spectra in the frequency range 10–500 MHz. The real part of dielectric permittivity was assessed from the 380–440 MHz, while the bulk electrical conductivity was calculated from the 165–325 MHz range. The FDR technique allows determination of bulk electrical conductivity from the imaginary part of the complex dielectric permittivity, without disregarding the dielectric losses. The soil salinity status was determined using the salinity index, defined as a partial derivative of the soil bulk electrical conductivity with respect to the real part of the soil complex dielectric permittivity. The salinity index method enables determining the soil water electrical conductivity value. For the five sandy mineral soils that have been tested, the relationship between bulk electrical conductivity and the real part of dielectric permittivity is essentially linear. As a result, the salinity index method applied for FDR measurements may be adapted to field use after examination of loam and clayey soils.

Application of a Coaxial-Like Sensor for Impedance Spectroscopy Measurements of Selected Low-Conductivity Liquids

The paper presents a coaxial-like sensor operating in the 20 Hz–2 MHz frequency range used to determine the electrical properties of selected liquids of low electrical conductivity. Examined materials included low-concentrated aqueous solutions of potassium chloride, sodium chloride and trisodium citrate, which are common food additives. Impedance spectra of the measurement cell filled with particular liquids were obtained and analyzed using the electrical equivalent circuit approach. The values of physical quantities and parameters describing the equivalent circuit components, including a constant phase element, were calculated for each sample. The applied sensor was also calibrated for electrical conductivity measurements up to 8 mS/m. The constant phase element parameters differed among the studied solutions and concentrations. This may provide a basis for a detection method of small amounts of compounds, such as food additives in low-concentrated aqueous solutions. To demonstrate the potential of the presented method, samples of purchased mineral water and a flavored drink containing various additives were tested.

A Time-Domain Reflectometry Method with Variable Needle Pulse Width for Measuring the Dielectric Properties of Materials.

Time-domain reflectometry (TDR) methods used for measuring the dielectric properties of materials mostly utilize step or needle electrical pulses of constant amplitudes and shapes. Our novel approach enables determining the dielectric relaxation time of a sample using the analysis of the amplitudes of reflected pulses of two widths, in addition to bulk dielectric permittivity and electrical conductivity commonly obtained by the TDR technique. The method was developed for various values of electrical conductivity and relaxation time using numerical simulations of a five-rod probe placed in a material with complex dielectric permittivity described by the Debye model with an added electrical conductivity term. The characterization of amplitudes of two pulses of selected widths was done with regard to the dielectric parameters of simulated materials. The required probe parameters were obtained solely from numerical simulations. Verification was performed for the probe placed in aqueous KCl solutions with 14 different electrical conductivity values. The determined relaxation time remained roughly constant and independent of electrical conductivity. The obtained electrical conductivity agreed with the reference values. Our results indicate that the relaxation time, dielectric permittivity and electrical conductivity of the tested solutions can be simultaneously determined using a simple analysis of the amplitude and reflection time of two needle pulses of different widths.

Detection of Atmospheric Water Deposits in Porous Media Using the TDR Technique

Investigating the intensity of atmospheric water deposition and its diurnal distribution is essential from the ecological perspective, especially regarding dry geographic regions. It is also important in the context of monitoring the amount of moisture present within building materials in order to protect them from excessive humidity. The objective of this study was to test a constructed sensor and determine whether it could detect and track changes in the intensity of atmospheric water deposition. An operating principle of the device is based on the time-domain reflectometry technique. Two sensors of different plate volumes were manufactured. They were calibrated at several temperatures and tested during field measurements. The calibration turned out to be temperature independent. The outdoor measurements indicated that the upper limits of the measurement ranges of the sensors depended on the volumes of the plates and were equal to 1.2 and 2.8 mm H2O. The respective sensitivities were equal to 3.2 × 10−3 and 7.5 × 10−3 g·ps−1. The conducted experiments showed that the construction of the designed device and the time-domain reflectometry technique were appropriate for detecting and tracing the dynamics of atmospheric water deposition. The obtained outcomes were also collated with the readings taken in an actual soil sample. For this purpose, an open container sensor, which allows investigating atmospheric water deposition in soil, was manufactured. It turned out that the readings taken by the porous ceramic plate sensor reflected the outcomes of the measurements performed in a soil sample.

Salinity index determination of porous materials using open-ended probes

The relations among soil water content, bulk electrical conductivity and electrical conductivity of soil solution can be described by a number of theoretical and empirical models. The aim of the paper is to examine the performance of open-ended coaxial probes with and without a short antenna in determination of complex dielectric permittivity spectra, moisture and salinity of porous materials using the salinity index approach. Glass beads of 0.26 and 1.24 mm average diameters moistened to various water contents with distilled water and KCl solutions were used to model the soil material. Due to the larger sensitivity zone, only the probe with the antenna enabled determination of bulk electrical conductivity and salinity index of the samples. The relations between bulk electrical conductivity and dielectric permittivity of the samples were highly linear, which was consistent with the salinity index model. The slope of the relation between salinity index and electrical conductivity of moistening solutions closely matched the value for sand presented in literature.

0.05–3 GHz VNA characterization of soil dielectric properties based on the multiline TRL calibration

We present a methodology for characterization of soil relative dielectric permittivity in the frequency range 0.05–3 GHz. Soil samples are placed in a measurement cell constructed out of a EIA coaxial transmission line, and then measured with a calibrated vector-network-analyzer. From these measurements the relative dielectric permittivity is obtained by use of a modified Boughriet algorithm. In order to calibrate the vector-network-analyzer directly at the EIA coaxial-transmission-line measurement planes, we use the multiline through-reflect-line method. This method, while providing superior vector-network-analyzer calibration accuracy, is also easy to implement since it uses only transmission lines with known lengths and a single unknown highly-reflective termination. The implemented calibration method was compared to a simplified approach that uses the standard SOLT calibration in Type-N reference planes, and then accounts for the Type-N/EIA adapters by removing their electrical delay. Experimental results for teflon and soil samples with different moisture content and salinity confirmed the validity of our approach.

Aquametry in Agrophysics

Aquametry is a part of metrology that uses the available measurement techniques in the measurement of water content in solid, liquid and heterogeneous materials. A similar branch of metrology, called hygrometry, deals with determination of the water vapour con‐ tent in air and other gases (Kraszewski, 2001). The aim of agrophysics is to apply physical methods and techniques for studies of materials and processes which take place in agricul‐ ture. Possible test objects may therefore include soil, fruit, vegetables, intermediate and final products of the food industry, grain, oils, etc.

Evaluation of Apple Maturity with Two Types of Dielectric Probes

The observed dielectric spectrum of ripe apples in the last period of shelf-life was analyzed using a multipole dielectric relaxation model, which assumes three active relaxation processes: primary α-process (water relaxation) and two secondary processes caused by solid-water-ion interactions α’ (bound water relaxations), as well as β’ (Maxwell-Wagner effect). The performance of two designs of the dielectric probe was compared: a classical coaxial open-ended probe (OE probe) and an open-ended probe with a prolonged central conductor in a form of an antenna (OE-A-probe). The OE-A probe increases the measurement volume and consequently extends the range of applications to other materials, like granulated agricultural products, soils, or liquid suspensions. However, its measurement frequency range is limited as compared to the OE probe because, above 1.5 GHz, the probe with the antenna generates higher propagation modes and the applied calibrations and calculations are not sufficient. It was shown that data from measurements using the OE-A probe gave slightly stronger correlations with apples’ quality parameters than using the typical OE probe. Additionally, we have compared twelve multipole fitting models with different combinations of poles (eight three-pole and four two-pole models). It was shown that the best fit is obtained using a two-pole model for data collected for the OE-A probe and a three-pole model for the OE probe, using only Cole-Cole poles in both cases.