Electromagnetic scattering by coated convex surfaces and wedges simulated by approximate boundary conditions.

Abstract

Asymptotic/high-frequency solutions are developed for analyzing the non-specular scattering mechanisms associated with coated convex surfaces and edges simulated by approximate boundary conditions. In particular, the standard impedance boundary conditions (SIBCs) and the second order generalized impedance boundary conditions (GIBCs) are employed for a characterization of the edge diffraction, creeping wave and surface diffracted wave contributions. To study the creeping wave and surface diffracted wave mechanisms, rigorous UTD (uniform geometrical theory of diffraction) diffraction coefficients are developed for a convex coated cylinder simulated with SIBCs and GIBCs. The ray solutions obtained remain valid in the transition region and reduce uniformly to those in the deep lit and shadow regions. A uniform asymptotic solution is also presented for observations in the close vicinity of the cylinder. The diffraction coefficients for a convex cylinder are obtained via a generalization of the corresponding ones for the circular cylinder. To validate the asymptotic/high-frequency solution integral equations are derived for both E and H-polarization and solved numerically using the method of moments. Results are presented for a single and three layered coated convex cylinder. Some insights are also provided on the accuracy of the employed GIBCs versus SIBCs for application to curved surfaces. To characterize the scattering by impedance wedges illuminated at skew incidence, diffraction coefficients are derived from an approximate solution of the governing functional difference equations. This solution exactly recovers the known ones for an impedance half plane or an arbitrary wedge at normal incidence and to validate it for other wedge angles a moment method code was used. Finally, to test the usefulness of the approximate skew incidence impedance wedge diffraction coefficient for three dimensional structures, equivalent currents are derived in the context of PTD for a finite length impedance wedge of arbitrary internal angle. These are incorporated in a standard general purpose physical theory of diffraction (PTD) code and results are presented for a number of different impedance structures.