Thickness-Independent Measurement of Grain Moisture Content by Attenuation and Corrected Phase Shift of Microwave Signals at Multiple Optimized Frequencies

Inadequate drying and overdrying of grains due to the absence of timely and reliable information on moisture content (MC) aggravates grain postharvest loss. In this study, we investigate the real-time acquisition of MC using microwave free-space transmission measurements. A multifrequency swept signal is used to acquire the attenuation and phase-shift spectrum (2.00–10.00 GHz) of the paddy samples (9.553%–29.633% w.b.) for six thicknesses (1–6 cm). Based on the four rules that are followed in the phase-shift measurement, a phase-shift correction algorithm without any restrictions on the sample thickness is proposed to solve the phase-shift ambiguity that occurs when the sample thickness exceeds the wavelength. To choose the most effective frequencies, 17 candidate frequency subsets are generated by the recursive feature elimination algorithm, and the optimal frequency set, containing eight individual frequencies, is selected by using the Friedman and Nemenyi post hoc tests. The sample thickness and microwave characteristics are used as the input variables to establish the prediction models to achieve a thickness-independent measurement of the MC, and the support vector machine model yielded the best performance ( $R^2$ = 0.992, RMSE = 0.555%, and MAE = 0.398%). The results of this study should encourage future research on the real-time acquisition of reliable MC information in food processing and agriculture-related industries.