Submitted to: Proceedings International Seminar on Nondestructive Evaluation Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 9, 2005
Publication Date: March 23, 2006
Citation: Trabelsi, S., Nelson, S.O. 2006. Near-field measurements of dielectric properties of granular materials with microstrip antennas for microwave-sensing applications. Research in Nondestructive Evaluation. 17(1). Interpretive Summary: Development of cost-effective microwave sensors for determining moisture content and bulk density of grain and seed requires robust calibration methods and affordable microwave components. Both issues are addressed in this paper. A calibration method is, by definition, identifying correlations between the measured electrical properties of the material and the physical properties of interest such as moisture content and bulk density. In this paper, effectiveness of a calibration method for simultaneous and independent determination of moisture content and bulk density from measurement of the dielectric properties is tested for wheat and soybeans. The dielectric measurements were taken very close to the antennas in the near field with a pair of inexpensive flat antennas. The standard errors of calibration for moisture and density compared very well with those derived from dielectric measurements under ideal electromagnetic conditions with a pair of sophisticated and expensive horn/lens antennas. The paper highlights two important aspects for the development of microwave sensors: the robustness of the calibration method and the feasibility of dielectric measurements in the near field with inexpensive antennas. These findings will promote development of microwave sensors for nondestructive, real-time determination of moisture content and bulk density of grain and seed. These are important in managing the harvest, transport, storage, and processing of these products for preventing losses due to spoilage and providing safe feed and food products for the benefit of consumers.
Technical Abstract: Near-field microwave measurements of the dielectric properties of wheat and soybeans with a pair of single-element microstrip antennas at 5.8 GHz and 24 degrees C are compared to those taken in the far field with a pair of focused-beam horn/lens antennas. Also, effects of near-field measurements with microstrip antennas on the accuracy of determining bulk density and moisture content from measured dielectric properties are examined. Determinations with both measurement systems compared favorably, indicating promise for such measurements with inexpensive planar antennas.