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Title: FREE-SPACE MEASUREMENT OF DIELECTRIC PROPERTIES OF CEREAL GRAIN AND OILSEED AT MICROWAVE FREQUENCIES

Author
item TRABELSI, SAMIR - UNIV GEORGIA
item Nelson, Stuart

Submitted to: Measurement Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2003
Publication Date: 3/26/2003
Citation: TRABELSI, S., NELSON, S.O. FREE-SPACE MEASUREMENT OF DIELECTRIC PROPERTIES OF CEREAL GRAIN AND OILSEED AT MICROWAVE FREQUENCIES. MEASUREMENT SCIENCE AND TECHNOLOGY. 2003. v. 14. p. 589-600.

Interpretive Summary: Moisture content of grain and oilseed is the most important characteristic determining the proper time for harvest and suitability for safe storage. The electrical characteristics of grain and seed, known as dielectric properties, are highly correlated with moisture content. Therefore, electrical moisture meters that provide rapid moisture determination for samples of these commodities have been developed for use in the grain and seed trade. However, there is need for instruments that can reliably sense the moisture content of grain and seed on-line for applications on harvesting and grain handling equipment. The problem in on-line applications is that the packing density of the granular materials can fluctuate in moving grain and cause large errors in moisture measurement. Measurements at microwave frequencies have shown promise for sensing grain and seed moisture content independent of density variations, and such measurements are being investigated for on-line moisture monitoring applications. Success of the technique depends on accurate determination of the dielectric properties of the granular materials. This paper deals with principles of measuring the microwave dielectric properties of grain and oilseed and identifies sources of error and methods for minimizing such errors of measurement. It presents dielectric properties data for corn, wheat, and soybeans and shows their relationships with bulk density and with moisture content. These studies are important in providing guidance for continuing research on microwave measurements for moisture sensing in grain and seed that will provide the basis for development of practical instruments that will be useful in reducing losses and improving the quality of such commodities for the benefit of producers, processors, and consumers.

Technical Abstract: Principles of dielectric properties measurement by microwave free-space transmission measurements are presented, and the important sources of errors in such measurements are discussed. A system, including a vector network analyzer, horn/lens antennas, holder for grain and oilseed samples, and a radiation absorbing enclosure, is described that was used for such measurements, and the techniques and procedures followed to obtain reliable permittivity data for wheat, shelled corn (maize) and soybeans are outlined. Data illustrating linear relationships between microwave attenuation and phase shift per unit sample thickness, each divided by bulk density of the granular materials, and frequency and moisture content are presented graphically. The linear dependence of calculated permittivity components, dielectric constant and loss factor, on bulk density is also shown, and permittivity components for wheat, corn, and soybeans, are listed for reference at frequencies from 5 to 17 GHz at different densities and moisture levels at about 23 C. Permittivity values are also listed for the same three commodities, adjusted to a medium density value through use of the Landau and Lifshitz, Looyenga dielectric mixture equation, for the total range of moisture contents at 10 GHz and at the same temperature.