Philip G. Bartley

Measuring RF and microwave permittivities of adult rice weevils

The dielectric permittivities of bulk samples of adult rice weevils were measured over the frequency range from 0.2 GHz to 20 GHz at temperatures from 10/spl deg/C to 65/spl deg/C with an open-ended coaxial-line probe, network analyzer, and a sample temperature control assembly designed for the measurements. Repeated measurements were highly variable, because mean sample bulk densities did not accurately reflect effective densities of the bulk rice weevil samples in the small volume of sample sensed by the coaxial-line probe. Density corrections based on earlier permittivity measurements on bulk rice weevil samples at 9.4 GHz, at known sample densities, removed much of the variability. The corrections utilized the linear relationship between the cube root of the dielectric constant and bulk density, which permitted estimates of the weevil body permittivities to be obtained with the Landau, Lifshitz, and Looyenga equation for dielectric mixtures. Estimated dielectric constants and loss factors of the insects from averages of seven different measurement sequences are presented graphically for temperatures from 15/spl deg/C to 65/spl deg/C.

Measuring frequency- and temperature-dependent permittivities of food materials

The permittivities or dielectric properties of food materials are of vital importance in understanding the behavior of these materials when they are exposed to electromagnetic fields in the process of microwave cooking or in other processes involving RF or microwave dielectric heating. Understanding these properties is also important in quality sensing by RF and microwave instruments. The most prominent example is instruments designed for rapidly sensing or measuring the moisture content of cereal grains and other food materials. An open-ended coaxial-line probe was used with sample temperature control equipment, designed for use with the probe, to measure permittivities of some liquid, semisolid, and pulverized food materials as a function of frequency and temperature. Graphical data for the dielectric constant and loss factor of homogenized macaroni and cheese, ground whole-wheat flour, and apple juice illustrate the diverse frequency- and temperature-dependent behavior of food materials, and the need for measurements when reliable permittivity data, are required. The materials were selected because interest had been expressed by others in their dielectric properties.

Open-ended coaxial probe permittivity measurements on pulverized materials

The open-ended, coaxial-line probe technique can be used to obtain estimates of the permittivities of some solid dielectrics over broad ranges of frequency by measurements on powdered or pulverized samples, but certain limitations must be recognized. Such a probe was used with a network analyzer to estimate the permittivities of coal and limestone from reflection coefficients measured on pulverized samples. The bulk density of the pulverized samples for the coaxial probe measurements was determined from auxiliary, single-frequency permittivity measurements on the samples at known bulk densities, and the permittivities of the solid materials over the frequency range from 0.2 GHz to 20 GHz were then estimated by computations based on the Landau and Lifshitz, Looyenga dielectric mixture equation and solid material densities.

Improved Free-Space S-Parameter Calibration

An improved method for performing a full two-port s-parameter calibration in free-space is presented. The proposed calibration technique computes the error coefficients from measurements made on an empty fixture and a measurement made on a metal plate of known thickness. Time-domain gating was employed. This technique requires fewer and simpler standards than the existing TRL and TRM calibration techniques. Permittivity calculated from measurements, calibrated using this technique, made on a material sample appear to be superior to results published using the TRL and TRM calibration technique

Determining moisture content of wheat with an artificial neural network from microwave transmission measurements

An artificial neural network (ANN) was used to determine the moisture content of hard, red winter wheat. The ANN was trained to recognize moisture content in the range from 10.6% to 19.2% (wet basis) from transmission coefficient measurements on samples of wheat. The measurements were made at 8 microwave frequencies (10 GHz to 18 GHz) on wheat samples of varying bulk densities (0.72 g/cm/sup 3/ to 0.88 g/cm/sup 3/) at 24/spl deg/C. The trained network predicted moisture content (%) with a mean absolute error of 0.135 (compared with oven-dried measurements).

Coaxial dielectric sensor for cereal grains

A system for measuring the dielectric properties of cereal grains from 25 to 350 MHz with a coaxial sample holder is presented. A signal-flow graph model was used to determine the permittivity of several polar alcohols from the full two-port S-parameter measurements. The system was calibrated with measurements on air and decanol and verified with measurements on octanol, hexanol, and pentanol. The standard error for the polar alcohols used for verification was 2.3% for the dielectric constant and 7.6% for the dielectric loss factor. Although measurements were taken on static samples, the sample holder is designed to accommodate flowing grain.

Moisture determination with an artificial neural network from microwave measurements on wheat

An artificial neural network (ANN) was used to determine the moisture content of hard red winter wheat. The ANN was trained to recognize moisture content in the range from 10.6% to 19.2% (wet basis) from transmission coefficient measurements on samples of wheat placed between two radiating elements. The measurements were made at 8 microwave frequencies (10 to 18 GHz) on wheat samples of varying bulk densities (0.72 to 0.88 g/cm/sup 3/) at 24/spl deg/C. The trained network predicted moisture content (%) with a mean absolute error of 0.135.

Flow-through coaxial sample holder design for dielectric properties measurements from 1 to 350 MHz

A system for measuring the dielectric properties of cereal grains from 1 to 350 MHz with a coaxial sample holder is presented. A signal-flow graph model was used to determine the permittivity of several polar alcohols from the full two-port S-parameter measurements. At the lowest frequencies 1-25 MHz where the phase measurements are less accurate, a lumped-parameter model was used to predict the dielectric loss factor values. The system was calibrated with measurements on air and decanol and verified with measurements on octanol, hexanol, and pentanol. The standard error for the polar alcohols used for verification was 2.3% for the dielectric constant and 7.6% for the dielectric loss factor. Although measurements were taken on static samples, the sample holder is designed to accommodate flowing grain.

A new technique for the determination of the complex permittivity and permeability of materials

A technique is proposed for determining the complex permittivity and permeability of material samples placed in a transmission line sample holder. The proposed technique is based on fitting the material properties to polynomials. The technique is immune to several of the shortcomings of traditional techniques, in particular the popular technique proposed by Nicolson and Ross. Examples of determining the material properties of several materials are given.

Dimensional analysis of a permittivity measurement probe

Open-ended coaxial-line probes provide a convenient means of determining the dielectric properties of many materials over a relatively wide frequency range. Because of this, much attention has been given to understanding the interaction of the probe and the material which it is inserted into. In this paper, a dimensional analysis was performed on a generalized open-ended coaxial-line probe. Applying the Buckingham /spl Pi/-theorem revealed that the admittance of the probe/dielectric interface, scaled by the frequency, is a function of a single dimensionless variable. This fact greatly simplifies the modeling of the probe. The problem is reduced from fitting a model of two variables, frequency and permittivity, to one dimensionless variable. In addition, the dimensional analysis also revealed that the same results hold for any permittivity measurement probe where the admittance of the probe is a function of permittivity, frequency, and any number of linear dimensions.