A frequency dependent FDTD surface impedance for scattering from coated PEC targets

One method for reducing the radar-cross section of objects such as aircraft and missiles is the application of a lossy coating. Computing scattering from targets coated with dielectric/magnetic materials is challenging for many computational methods due to the reduced wavelengths of an incident field inside the coating. These smaller wavelengths require finer sampling of the fields and thus more memory and computation 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 and tested in one-, two- and three-dimensions. The method requires knowledge of the frequency behavior of the complex permittivity and permeability and the thickness of the dielectric coating. The surface 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-Transformed and added to the FDTD code in special update equations for the fields at the surface of the coating. No computations are required inside the coatings, so the FDTD grid can be sized based on the free space wavelength. The result obtained is valid over the entire frequency range of interest assuming that the approximated surface impedance is a good match over the entire range. Comparisons with analytical solutions in one-dimension and measurements in three-dimensions show good agreement.