Measuring Dielectric Properties for Sensing Foreign Material in Peanuts

Free-space dielectric properties measurements at 10 GHz with four pairs of low-cost printed Yagi-Uda antennas are proposed for detecting foreign material in unshelled peanuts, including sticks, peanut shells, peanut raisins, and stones. The four-element array design of the Yagi-Uda antenna allowed better averaging over the entire sample volume and improved the measurement resolution. The antenna gain is about 9.7 dBi. The beam width of each antenna is about 18° and 38° for the E-plane and H-plane at 3 dB, respectively. The attenuation and phase shift were measured for each pair of antennas by controlling automatically the microwave switches. Measurement repeatability of the system was confirmed by using different thicknesses of polyethylene dielectric slab samples. The standard deviation was less than 0.5 dB for the attenuation and less than 5.0° for phase measurements. Sensitivity of each antenna pair for unshelled peanuts and different foreign materials, each loaded in a small volume (4.4 cm × 4.4 cm × 12.1 cm), was tested by using a partitioned sample container. It was shown that a difference in attenuation was more than 1.5 dB for different types of foreign materials and also for unshelled peanut samples with a 1% moisture content difference. Prediction of moisture content in cleaned unshelled peanut samples agreed well with the standard oven-drying method (standard error of calibration = 0.51%). The dielectric properties of foreign materials mixed with cleaned unshelled peanut samples were measured through the attenuation and phase shift. The measurement repeatability of each antenna and all four pairs of antennas is presented.