Inaccuracies in numerical calculation of scattering near natural frequencies of penetrable objects

Calculated scattering from a nonconducting cylindrical ring shows great variability with small changes in the material or geometric parameters within certain ranges of these parameters. The observations are explained by a resonance phenomenon in which the operating frequency is found to lie close to a complex natural frequency of the scattering object. This resonance is a real, observable effect predicted by analytical solutions and not the spurious numerical resonances which have been widely discussed and which one wishes to suppress. Attempts have been made to reproduce near-resonance scattering results using the method of moments and finite-difference-time-domain (FDTD) codes. These have failed despite the use of widely accepted discretization densities. Thus, the existence of such resonances requires additional care when interpreting computed results for scattering from similar nonconducting objects have electric and/or magnetic properties