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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 calibration technique for measuring the complex permittivity of materials with planar transmission lines

Authors
item Roelvink, Jochem -
item Trabelsi, Samir

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: March 6, 2013
Publication Date: April 4, 2013
Citation: Roelvink, J., Trabelsi, S. 2013. A calibration technique for measuring the complex permittivity of materials with planar transmission lines. IEEE International Instrumentation and Measurement Technology Conference. p.1445-1448.

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 simple two-standard calibration procedure for determining the permittivity of a material from the propagation constant measured with planar transmission lines is presented. The suitability of this procedure is demonstrated by using a coplanar waveguide transmission-line sensor to measure the permittivity of five liquids, covering a wide range of complex permittivity. The results agree closely with expected values when the radiation and conductor losses are small. This technique allows the permittivity of materials to be measured without knowing the planar transmission-line dimensions apart from the line lengths.

Last Modified: 10/24/2014
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