In numerical solution of radiation and scattering problems, the evaluation of the electromagnetic field produced by a set of induced currents lying on the surface of an object is almost always needed. There are some special cases where the a priori information about the nature of the surface and the type of illuminating source enables us to use either analytical or asymptotic methods for fast computation of the electromagnetic radiation. However, this is not the typical scenario in most applications, since the geometry of the object might be completely arbitrary and it may not be possible to represent the source in a convenient analytical form. Furthermore, an analytical representation of the induced current distribution is seldom ever available when a numerical technique, such as the Method of Moments (MoM) [1], is used to derive a solution for the current. In addition, the computation of the radiated field in the near-field zone might encounter difficulties because the Green's function is highly oscillatory in this region. A typical situation where the field needs to be evaluated at observation points very close to the object arises when we are interested in estimating the reaction between the fields produced by a set of currents, and the currents themselves. Such a procedure is typically needed for the computation of the self-terms in the reduced matrix when applying the Characteristic Basis Function Method (CBFM) [2],[3].
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