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ARS Home » Research » Publications at this Location » Publication #137437
Title: MICROWAVE DIELECTRIC PROPERTIES OF CEREAL GRAIN AND SEED

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

Submitted to: American Society of Agricultural Engineers
Publication Type: Proceedings
Publication Acceptance Date: 7/1/2002
Publication Date: 7/29/2002
Citation: TRABELSI, S., NELSON, S.O. MICROWAVE DIELECTRIC PROPERTIES OF CEREAL GRAIN AND SEED. AMERICAN SOCIETY OF AGRICULTURAL ENGINEERS, ST. JOSEPH, MI. 2002. ASAE PAPER NO. 026069.

Interpretive Summary: Because the moisture content of grain and seed is the most important characteristic determining time for harvest, suitability for transport or storage, and is important in determining the selling price of these commodities, research is continuing on developing principles for more accurate and more reliable moisture measurements by electronic grain moisture meters. Grain moisture meters currently in the grain trade sense capacitance of samples of grain at radio frequencies between about 1 and 20 MHz. Research has shown some advantages for microwave frequencies, which are a thousand times higher, in measuring the moisture content of grain and seed. For development of microwave grain moisture meter principles, the electrical characteristics known as dielectric properties need to be determined at microwave frequencies. This paper reports improvements in the accuracy of measurements useful for determining these microwave dielectric properties of grain. Better accuracy in determining the dielectric properties provides better accuracy in moisture determination. The new findings will be useful in developing new grain moisture meters operating at microwave frequencies. Improved moisture information about grain lots, provides managers of grain harvesting, handling, storage, and processing systems better data on which to base their decisions. This will lead to better quality products and prevention of losses due to spoilage of grain.

Technical Abstract: Dielectric properties of wheat, corn and soybeans were measured with a free-space-transmission technique at room temperature between 2.0 and 18.0 GHz over wide ranges of bulk density and moisture content of practical interest. For better accuracy and optimum use of the vector network analyzer (VNA) dynamic range, a pair of horn/lens antennas was used; and for each moisture level, the sample thickness was selected to keep the attenuation between 10 dB and the maximum attenuation measurable by the VNA; and time-domain gating was applied to the main response. Also, the sample was placed in a tunnel shaped enclosure of radiation-absorbing material to isolate it from surroundings. Variation of the dielectric properties with bulk density is shown at 10.0 GHz. Also, variations of the dielectric properties divided by bulk density as a function of moisture content are reported at 5.0, 10.0 and 15.0 GHz.