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ARS Home » Southeast Area » Athens, Georgia » U.S. National Poultry Research Center » Quality and Safety Assessment Research Unit » Research » Publications at this Location » Publication #292943
Title: Determination of complex permittivity from propagation constant measurement with planar transmission lines

Author
item Roelvink, Jochem
item Trabelsi, Samir
item NELSON, STUART - Collaborator

Submitted to: Measurement Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2013
Publication Date: 7/1/2013
Citation: Roelvink, J.T., Trabelsi, S., Nelson, S.O. 2013. Determination of complex permittivity from propagation constant measurement with planar transmission lines. Measurement Science and Technology. 24(10):1-8.

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 five liquids of known dielectric properties. 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 new two-standard calibration procedure is outlined for determining the complex permittivity of materials from the propagation constant measured with planar transmission lines. Once calibrated, a closed-form expression for the material permittivity is obtained. The effects of radiation and conductor losses are accounted for in the calibration. The multiline technique, combined with a recently proposed planar transmission line configuration, is used to determine the line propagation constant. An uncertainty analysis is presented for the proposed calibration procedure that includes the uncertainties associated with the multiline technique. This allows line dimensions and calibration standards to be selected that minimize the total measurement uncertainty. The use of air and water as calibration standards gives relatively small measurement uncertainty. Permittivity measurement results for five liquids, covering a wide permittivity range, agree very closely with expected values from 0.5 – 5 GHz.