Thickness-independent measurement of the permittivity of thin samples in the X band

A method is proposed for measuring the complex permittivity of thin materials. The method allows us to simultaneously measure the permittivity and thickness of the material, allowing us in particular to obtain a meaningful estimate of the average thickness of natural materials. The measuring system consists of a section of rectangular waveguide, operating in the X band and modified to accommodate the material under measurement, and two standard impedance loadings (short-circuit and matched load). Two reflection coefficient values, measured by a vector network analyser at the input of the waveguide sample-holder, allow the determination of the parameters of interest by means of a numerical procedure employing a genetic algorithm. The method has been tested on certified dielectric materials and preliminary results are presented concerning paper and thin cardboard.