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United States Department of Agriculture

Agricultural Research Service

Research Project: SENSING METHODS & INSTRUMENTATION FOR RAPID DETERMINATION OF MOISTURE CONTENT... AND OTHER QUALITY ATTRIBUTES OF GRAINS, SEEDS, & NUTS

Location: Quality and Safety Assessment Research Unit

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

Authors
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: December 31, 2012
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 relatively new type of sensor for the microwave permittivity, or dielectric properties, of biological materials is described that consists of a length 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 coplanar transmission line conductors extend into the material to be measured, and the measured characteristics of the coplanar waveguide can be mathematically related to the dielectric properties of the material. This article discusses the influence of the various parameters of the coplanar waveguide and their influence on the accuracy of the measurement of the dielectric properties of the material in contact with the waveguide. 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: An accurate technique for determining the permittivity of biological materials with coplanar waveguide transmission line is presented. The technique utilizes closed-form approximations that relate the material permittivity to the line propagation constant. A thru-reflect-line calibration procedure is employed. The propagation constant is determined from scattering parameter measurements of two samples with different lengths. Results for the permittivity of five samples, having mechanical and dielectric properties similar to those of biological materials, are presented and shown to be in good agreement with coaxial-line probe measurements. Results for the sample-face discontinuity admittance are also presented. A brief discussion of some practical aspects of coplanar waveguide sensors is provided.

Last Modified: 11/26/2014
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