Nondestructive measurement of complex permittivity by a microwave technique: detection of contamination and food quality

In this paper, we interested of wideband measurement system that permits to characterize high- and low-loss materials according to physical and chemical properties of cheddar cheese and beef. The proposed measurements system, spanning a frequency range of 1–18 GHz, is constituted by one port junction of a small diameter (a=0.8$ a=0.8 $ mm, b=2.7$ b=2.7 $ mm) coaxial probe and a standard rectangular waveguide. Theoretical reflection coefficient is obtained using the generalized equivalent circuit method that takes into account most of the involved physical phenomena. Thus, the effect of higher order modes as well as the dominant modes is considered. The complex permittivity of material is extracted by minimizing the difference between the theoretical and measured reflection coefficient. The complexity of the electromagnetic analysis does not allow to find an analytical relation between these two parameters. A computer code based on genetic algorithm is developed to numerically solve the inverse problem. To validate this program, we perform measurements on standard materials of known permittivity as air and FR4 (Flame Resistant 4) in the 1–18 GHz range.