Andrzej W. Kraszewski

ANA Calibration Method for Measurements of Dielectric Properties

Routine network analyzer calibration procedures in measurements of the dielectric properties of materials using an open-ended coaxial line probe are frequently inadequate and limit the accuracy of measurements. A calibration method, which makes use of liquids whose properties are well known, is proposed to alleviate this limitation. It is shown that even one liquid used as a standard in place of a matched load greatly improves the accuracy of measurements. Theoretical relationships and experimental results as well as some practical suggestions related to the application of this method are given.

Phase-shift ambiguity in microwave dielectric properties measurements

Phase measurements of the transmission coefficient are important when used for the dielectric characterization of materials. They are required for industrial material monitoring applications, where the phase is correlated with parameters such as moisture content and density, which need to be continuously determined, However, when the thickness of the material under test is greater than the wavelength in the material, a phase problem is encountered. Two methods are proposed to solve this problem. The first is based on the selection of the appropriate material thickness; the second requires the use of measurements at two frequencies. Advantages and limitations of both methods are discussed, and numerical validations are given for particulate materials.

A microwave method for on-line determination of bulk density and moisture content of particulate materials

A new method for simultaneous and independent on-line determination of bulk density and moisture content in particulate materials by measurements of the relative complex permittivity is proposed. The bulk density is determined, based on a representation in the complex plane of relative complex permittivity normalized to bulk density. For moisture content determination, a new density-independent function, exclusively dependent on the dielectric properties, is used. Results obtained from measurements on wheat over broad ranges of microwave frequencies, temperatures, densities, and moisture contents are presented.

Using cereal grain permittivity for sensing moisture content

A brief history of cereal grain moisture measurement by sensing the electrical properties of grain is presented. The basic principles are also described for using radio-frequency (RF) and microwave dielectric properties, or permittivity, of grain for sensing moisture through their correlation with moisture content. The development of density-independent functions of the permittivity is explained. The findings of recent research are summarized, which indicate that reliable density-independent moisture content determinations can be realized by measurements on grain at RF and microwave frequencies. Development of these techniques will provide useful instruments for on-line monitoring of moisture content in flowing grain to manage moisture in grain, prevent spoilage in storage and transport, improve processing, and provide information important for yield determinations in precision agriculture applications.

TEMPERATURE DEPENDENCE OF THE DIELECTRIC PROPERTIES OF WHEAT

ABSTRACT Temperature dependent dielectric properties data were collected on Stacy soft red winter wheat at temperatures from 0 to 50° C and moisture contents ranging from 8.2 to 23.4%, wet basis. Measurements by two impedance analyzers with a shielded, open-circuited, coaxial-line sample holder were transformed by the invariance-of-the-cross-ratio technique into dielectric properties data in the frequency range from 0.1 to 100 MHz. Measurements compared well with values reported in the literature.

RF Energy Deposition in a Heterogeneous Model of Man: Near-Field Exposures

The electric field strength was measured in a full-scale heterogeneous model of man exposed in the near field of resonant dipoles. The model was comprised of skull, spinal cord, rib cage, all other major bones, brain, lung, and muscle tissue. Electrical properties of these simulated tissues were the same as respective live tissue properties at test frequencies of 160, 350, and 915 MHz. The rates of energy absorption were calculated on the basis of the measured field strengths and tissue conductivities. Patterns of the energy deposition are compared for two orientations of the antennas with respect to the body. Also the results for the heterogeneous model are compared to data for homogeneous model having average tissue electrical properties.

On the Measurement Accuracy of the Tissue Permittivity in Vivo

An open-ended coaxial line and a computer-controlled network analyzer have been extensively used for measuring the tissue permittivity in vivo at radio and microwave frequencies. An analysis is presented of measurement errors resulting from calibration and random errors of the network analyzer [1],[2]. The uncertainties in our system are ¿¿ = 0.05 dB and ¿¿ = 0.3°. Key Words-Radio frequency, Microwave permittivity in vivo, Automated network analyzer.

ADVANCES IN SENSING GRAIN MOISTURE CONTENT BY MICROWAVE MEASUREMENTS

The history of information development on the electrical properties of grain and their use for sensing moisture content is briefly reviewed. Basic principles of grain moisture sensing by microwave measurements are presented, including techniques for moisture content determination independent of bulk density. Recent research findings on densityindependent microwave determination of moisture content in grain are briefly summarized. Three functions of the grain dielectric properties or the wave parameters are identified from which moisture content can be determined independent of bulk density, and the performance of these density-independent functions is compared for sensing moisture content in hard red winter wheat. Results show that standard errors of performance of about 0.3% moisture content or less can be achieved by using these functions with microwave measurements at frequencies between 11 and 18 GHz. Results of these and other cited studies indicate that measurement techniques for density-independent moisture sensing in grain should provide a sound basis for development of microwave moisture meters for on-line applications.

DETERMINING PHYSICAL PROPERTIES OF GRAIN BY MICROWAVE PERMITTIVITY MEASUREMENTS

Potential use of the complex permittivity measured at microwave frequencies for indirect nondestructive determination of physical properties of grain is discussed. Examples of extraction of bulk density and moisture content from permittivity measurements are shown for wheat and corn over wide ranges of frequency and temperature. Calibration equations for both entities are given along with their standard errors of performance (SEP). The bulk density of wheat and corn, ranging from 720 to 880 kg/m3 and 695 to 830 kg/m3, respectively, can be determined without prior knowledge of the sample moisture content or its temperature with, on average, SEP of 7.8 kg/m3 and 12.9 kg/m3, respectively. The moisture content of wheat and corn, ranging from 10.6 to 19.2% and 9.0 to 19.2%, wet basis, respectively, can be determined at a given temperature independent of density with SEP of less then 0.27% and 0.46%, respectively. One of the attractive features of permittivity-based methods is that they can be applied regardless of the measurement technique. One identified function of the complex permittivity provides moisture content for both wheat and corn with the same calibration.

Energy Deposition in a Model of Man: Frequency Effects

A computer-controlled scanning system and implantable, nonperturbing electric field probes were used to measure spatial distributions of the electric field in a full scale homogeneous model of a human body. The measurements were performed at three frequencies (160, 350, and 915 MHz) in the far-field and in the near-field of resonant dipoles. The specific absorption rate (SAR) distributions and the averages for body parts and the whole body are analyzed as functions of frequency. In the far-field, the SAR decreases exponentially in the direction of wave propagation in the torso at all frequencies, and large gradients of the SAR are observed along the body main axis, particularly for the E polarization. At 160 and 350 MHz high local SARs are produced in the neck. It appears that for plane wave exposures the ratio of the peak SAR to the whole-body average SAR does not exceed 20. In the near-field, large SAR gradients are also produced, and the ratios of the peak spatial SAR to the whole-body average SAR vary from about 30 to 250 depending on the frequency and polarization. It is suggested that for near-field exposures the whole-body average SAR is not a proper dosimetric measure, and the SAR averaged over any 0.1 of the tissue volume is recommended instead.