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United States Department of Agriculture

Agricultural Research Service


Location: Quality and Safety Assessment Research Unit

Title: A planar transmission-line sensor for measuring the microwave permittivity of liquid and semisolid biological materials

item Roelvink, Jochem
item Trabelsi, Samir
item Nelson, Stuart -

Submitted to: IEEE Transactions on Instrumentation and Measurement
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 4, 2013
Publication Date: N/A

Interpretive Summary: Dielectric properties of materials are electrical properties that determine how materials interact with electric fields such as those of high-frequency and microwave electromagnetic energy. Therefore, the dielectric properties of materials determine how rapidly they will heat in microwave ovens and lower radio-frequency dielectric heating equipment. Dielectric properties are also important in low power applications, such as the rapid measurement of moisture content in grain and other commodities. Therefore it is often important to measure the dielectric properties of materials at the frequencies of interest in any application. In this article, a two-standard calibration procedure for a relatively new type of sensor for microwave permittivity, or dielectric properties, of biological materials is described, which consists of two lengths of coplanar transmission line or waveguide against which the material to be measured is placed in contact. Coplanar means that both conductors of the transmission line are in the same plane, which is on the surface of a printed-circuit board. Fringing electric fields from the planar transmission line conductors extend into the material to be measured, and the measured characteristics of the planar transmission lines can be mathematically related to the dielectric properties of the material. This article discusses the influence of the various parameters of the planar transmission lines and their influence on the accuracy of the measurement of the dielectric properties of the material in contact with the waveguide. The new calibration technique for these planar lines provides high accuracy in the determination of the dielectric properties of materials as proven with measurements on six liquids and four gelatin samples, which compared closely with values determined by commercial coaxial-line probe measurements. This coplanar waveguide sensor was designed for measurement of the dielectric properties of biological material such as poultry meat. Suitable correlations between dielectric properties of poultry meat and its quality attributes can thus be used for development of rapid quality sensors. The information presented is of interest to engineers and scientists in developing instruments that can provide important tools for improving agricultural production, product maintenance and quality preservation, and marketing for the benefit of growers, processors and consumers.

Technical Abstract: A planar transmission-line configuration for rapid, nondestructive, wideband permittivity measurements of liquid and semisolid materials at microwave frequencies is described. The transmission-line propagation constant of the proposed configuration is determined with the multiline technique from scattering parameter measurements of two lines with a known length difference. Guidelines for the appropriate selection of the line dimensions and a technique for resolving the phase ambiguity associated with the multiline technique are presented. The material permittivity is related to the propagation constant by closed-form expressions that allow the permittivity to be determined by a relatively simple numerical inversion procedure. The proposed sensor has the advantage of not requiring the sample to have uniform edges, nor that they be perpendicular to the planar line. The technique used for determining the line propagation constant has the advantage of not requiring the shunt any transmission-line discontinuities at the sample faces to be explicitly modeled. To illustrate the technique, a coplanar waveguide transmission-line sensor was fabricated and used to measure the permittivity of six liquids and four gelatin samples from 1 to 5 GHz. These measurements agree very well with coaxial-line probe measurements, demonstrating the accuracy of the proposed configuration. The sensitivity of the results to dimensional tolerances is demonstrated.

Last Modified: 7/25/2014
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