Stuart O. Nelson

REVIEW AND ASSESSMENT OF RADIO-FREQUENCY AND MICROWAVE ENERGY FOR STORED-GRAIN INSECT CONTROL

Fundamental principles of radio-frequency (RF) and microwave dielectric heating are presented, with a basic consideration of differential or selective absorption of energy from RF and microwave fields that might be applicable for stored-grain insect control. Experimental findings of the past 50 years are reviewed, with respect to entomological and physical factors affecting the response of insects exposed to RF and microwave electromagnetic fields in grain and grain products. Practical aspects of RF and microwave energy application for stored-grain insect control are considered, and it is concluded that such use is highly unlikely without some new discovery of a nonthermal lethal mechanism that might be exploited for this purpose.

Dielectric properties of agricultural products-measurements and applications

The nature of the variation of dielectric properties with frequency, temperature, and product density is discussed. Techniques for measurement of dielectric properties are briefly reviewed, and graphical data on the dielectric properties of grain and soybeans as functions of moisture content, frequency, temperature, and bulk of density are presented. Applications in the electrical measurement of the moisture content and in the dielectric heating of these materials are discussed. >

Free-space measurement of dielectric properties of cereal grain and oilseed at microwave frequencies

Principles of dielectric property measurement by microwave free-space transmission measurements are presented, and the important sources of errors in such measurements are discussed. A system, including a vector network analyser, horn/lens antennas, holder for grain and oilseed samples and a radiation absorbing enclosure that was used for such measurements is described, and the techniques and procedures followed to obtain reliable permittivity data for wheat, shelled corn (maize) and soybeans are outlined. Data illustrating linear relationships between microwave attenuation and phase shift per unit sample thickness, each divided by the bulk density of the granular materials, and frequency and moisture content are presented graphically. The linear dependence of calculated permittivity components, dielectric constant and loss factor, on bulk density is also shown, and permittivity components for wheat, corn and soybeans are listed for reference at frequencies from 5 to 17 GHz at different densities and moisture levels at about 23 °C. Permittivity values are also listed for the same three commodities, adjusted to a medium density value through use of the Landau and Lifshitz, Looyenga dielectric mixture equation, for the total range of moisture contents at 10 GHz and at the same temperature.

Frequency and moisture dependence of the dielectric properties of hard red winter wheat

Abstract Measurements are described by which the dielectric properties of hard red winter wheat were determined over a frequency range from 250 Hz to 12·1 GHz for moisture contents between 2·7% and 23·8% w.b. Values for the dielectric constant, loss factor, loss tangent, and conductivity and their dependence upon frequency and moisture content are presented graphically. Additional graphs illustrate changes in the nature of the dependence of the dielectric constant and loss factor on moisture content as frequency changes. Ranges in variation of the dielectric constant and loss factor among lots of 7 varieties of hard red winter wheat are shown graphically. Data are also presented on the bulk density and kernel density of wheat as influenced by moisture content.

Dielectric Properties of Materials for Microwave Processing—Tabulated♦♦

ABSTRACTA brief description is given of the dielectric dispersion and relaxation as a function of frequency and temperature. Important aspects of the behavior of dielectric mixtures with respect to frequency, temperature, and composition are summarized and referenced. The dielectric constant and loss factor of many materials are tabulated as functions of frequency, temperature, moisture content, and composition. Materials are classified as Agricultural, Biological, Foods, Forest Products, Leather, Rubber, and Soils and Minerals. The data were collected for use in the application of microwaves to non-communication uses.

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.

Dielectric spectroscopy of watermelons for quality sensing

Dielectric properties of four small-sized watermelon cultivars, grown and harvested to provide a range of maturities, were measured with an open-ended coaxial-line probe and an impedance analyser over the frequency range from 10 MHz to 1.8 GHz. Probe measurements were made on the external surface of the melons and also on tissue samples from the edible internal tissue. Moisture content and soluble solids content (SSC) were measured for internal tissue samples, and SSC (sweetness) was used as the quality factor for correlation with the dielectric properties. Individual dielectric constant and loss factor correlations with SSC were low, but a high correlation was obtained between the SSC and permittivity from a complex-plane plot of dielectric constant and loss factor, each divided by SSC. However, SSC prediction from the dielectric properties by this relationship was not as high as expected (coefficient of determination about 0.4). Permittivity data (dielectric constant and loss factor) for the melons are presented graphically to show their relationships with frequency for the four melon cultivars and for external surface and internal tissue measurements. A dielectric relaxation for the external surface measurements, which may be attributable to a combination of bound water, Maxwell–Wagner, molecular cluster or ion-related effects, is also illustrated. Coefficients of determination for complex-plane plots, moisture content and SSC relationship, and penetration depth are also shown graphically. Further studies are needed for determining the practicality of sensing melon quality from their dielectric properties.

Microwave Permittivities of Fresh Fruits and Vegetables from 0.2 to 20 GHz

Procedures are described for microwave permittivity measurements taken on 23 kinds of common fresh fruits and vegetables with an open-ended coaxial-line probe used in conjunction with a microwave network analyzer. Plots of the dielectric constant and loss factor at 41 frequencies between 200 MHz and 20 GHz are illustrated for apple, lemon, carrot, cucumber, and avocado, and condensed data for all, along with moisture content, tissue density, and total soluble solids, are tabulated at six frequencies covering the same range. Although specific values differ, the dielectric constant decreases steadily with increasing frequency, dropping more rapidly at frequencies above 5 GHz. Values for the loss factor decrease as frequency increases above 200 MHz to a broad minimum in the 1- to 3-GHz region and then increase again as the frequency approaches 20 GHz. The dielectric behavior of the fruit and vegetable tissues appears to be influenced by ionic conductivity and bound water relaxations at the lower frequencies and by free water relaxation at the higher end of the frequency range.

Density-independent functions for on-line microwave moisture meters: a general discussion

Density-independent calibration functions are a suitable solution for problems involving variation of bulk density in on-line determination of the moisture content in particulate materials by microwave techniques. Foundations of three of these functions are briefly reviewed and their frequency and temperature dependences are shown for wheat. Their effectivenesses for prediction of the moisture content are compared and optimum conditions for a cost-effective moisture meter are discussed.