SCATTERING FROM COATED TARGETS USING A FREQUENCY-DEPENDENT, SURFACE IMPEDANCE BOUNDARY-CONDITION IN FDTD

One method for reducing the radar cross section of objects such as air craft and missiles is the application of a lossy coating, Computing sc attering from targets coated with dielectric/magnetic materials is cha llenging for many computational methods due to the reduced wavelengths of an incident field inside the coating, These smaller wavelengths re quire finer sampling of the fields, and thus more memory and computati on time. A technique for implementing this calculation without greatly increased memory requirements or computation times has recently been developed using a finite-difference, time-domain (FDTD) code which has been tested in one, two, and three dimensions. The method requires kn owledge of the frequency behavior of the complex permittivity and perm eability, and the thickness of the dielectric coating and is applicabl e to thin coatings when one or more reflections from the conducting su rface are significant. The impedance at the surface of the coating is computed based on the given information and then approximated using a summation of causal functions. The approximated impedance is Z-transfo rmed and added to the FDTD code in special update equations for the fi elds at the surface of the coating, No computations are required insid e the coatings so the FDTD grid can be sized based on the free-space w avelength, The result obtained is valid over the entire frequency rang e of interest, assuming that the approximated surface impedance is a g ood match over the entire range, Comparisons with measurements of a sc ale model coated missile show good agreement and almost no increase in resource requirements over a standard FDTD calculation for an uncoate d metal target.