GENERALIZED IMPEDANCE BOUNDARY-CONDITIONS IN SCATTERING

Approximate boundary conditions (ABC's) can be used to simulate the material properties of a surface, and can be very useful in simplifying the analytical and numerical solution of scattering problems. One of the simplest of these is the standard impedance condition, but in an effort to improve the accuracy, generalized impedance boundary conditions (GIBC's) are now being considered. By virtue of the derivatives, the conditions are less local in character, and the additional degrees of freedom can be employed to better simulate the material properties. For the special case of a coated surface which is planar or of large radius, methods are described for specifying the conditions, and their accuracy is examined as a function of the thickness and properties of the coating. The new conditions have been used in a number of analytical and numerical solutions of scattering problems, but difficulties may arise if the surface has an edge or other spatial discontinuity. In particular, additional information must be furnished to make the solution unique and satisfy reciprocity. Numerical applications are discussed as well as the extension to curved surfaces.