Measuring RF and microwave permittivities of stored-grain insects

The dielectric permittivities of bulk samples of adult rice weevils were measured over the frequency range from 0.2 to 20 GHz at temperatures from 10 to 65°C with an open-ended coaxial-line probe, network analyzer, and a sample temperature control assembly designed for the measurements. Repeated measurements were highly variable, because mean sample bulk densities did not accurately reflect effective densities of the bulk rice weevil samples in the small volume of sample sensed by the coaxial-line probe. Density corrections based on earlier permittivity measurements on bulk rice weevil samples at 9.4 GHz, at known sample densities, removed much of the variability. The corrections utilized the linear relationship between the cube root of the dielectric constant and bulk density, which permitted estimates of the weevil body permittivities to be obtained with the Landau & Lifshitz, Looyenga equation for dielectric mixtures. Estimated dielectric constants and loss factors of the insects from averages of eight different measurement sequences are presented graphically for temperatures from 15 to 65°C