Submitted to: Measurement Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/2006
Publication Date: 2/27/2007
Citation: Trabelsi, S., Nelson, S.O. 2007. Investigating effect of near-field free-space permittivity measurements on accuracy of bulk density and moisture content determination in grain. Measurement Science and Technology. Interpretive Summary: Because moisture content in cereal grains and oilseeds is so important in determining their suitability for short-term or long-term storage, rapid and reliable moisture measurement methods are important. Electrical moisture meters have been developed that are used to determine moisture content whenever these commodities are sold and often when they are harvested, transported and processed. These moisture meters function because of correlations between the grain and seed moisture content and their electrical characteristics, called dielectric properties, which can be sensed with electric fields generated by the moisture testing instruments. Improvement in the accuracy of such moisture testers is needed for many applications. Measurements of the dielectric properties of grain and seed have indicated potential advantages of microwave frequencies, which are much higher frequencies than those used by current moisture meters. The method uses a transmitting antenna, a receiving antenna and instrumentation for determining the attenuation and phase change of the microwaves in traversing a layer of grain or seed. For the development of practical microwave grain moisture meters, low-cost antennas are needed. The research being reported compares results in measuring the microwave dielectric properties of wheat and soybeans with very economical, compact single-patch microstrip antennas and with costly and bulky broadband horn/lens antennas in a laboratory setup that provides highly accurate measurements. The comparison showed that the small patch antennas gave results comparable to those obtained with the expensive laboratory system, so moisture content and bulk density can be obtained with similar accuracy with the economical patch antennas. This is an important step in the development of techniques for economical and practical microwave sensing of moisture content in grain, which will provide better moisture information for management of grains and oilseeds for the prevention of losses and preservation of high quality products for consumers.
Technical Abstract: Near-field free-space microwave transmission measurements on wheat with a pair of inexpensive single-patch microstrip antennas at 5.8 GHz and 23 degrees C are compared to those taken in the far field with a pair of focused-beam horn/lens antennas. Different calibration methods were used to predict bulk density and moisture content in wheat from measurement of the attenuation, phase shift, and dielectric properties of a layer of material placed between the two antenna systems. Computation of corresponding standard errors of calibration (SEC) for bulk-density and moisture-content determination indicates that predictions from near-field measurements with microstrip antennas compare well with those derived from far-field measurement with a pair of horn/lens antennas.