Effects of surface curvature and beam defocusing on the characterization of conductor-backed absorbing materials using a free-space system

Algorithms that extract the permittivity and permeability of conductor-backed absorbing materials from data obtained using a free-space system often assume plane-wave excitation and planar materials to improve the computational efficiency, even though the surfaces are usually curved and focus beam systems are used for interrogation. It is thus important to assess the effects that these assumptions have on the accuracy of the extracted material parameters. A two-dimensional model is used to determine the conditions under which the error in the extracted parameters exceeds that expected from measurement uncertainty. It is found for typical absorbers that errors introduced by the modelling assumptions are significant when the surface curvature radius is less than about 10 wavelengths or the beam is defocused by a quarter to a half of a wavelength. These results should prove useful for the design of practical free-space systems.