Derivation of Uniform PO Diffraction Coefficients Based on Field Equivalence Principle Ken-ichi Sakina, Suomin Cui, and Makoto Ando Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, Tok yo, 152-8552 Japan SUMMARY A novel approach for asymptotic reduction of physi- cal optics (PO) integration is proposed for two-dimensional line source diffraction from a half-sheet. The field equiva- lence principle provides alternative integration surfaces not on the original half-sheet but on the geometrical shadow (SB) and reflection (RB) boundaries, where analytical in- tegration leads to the well-known Fresnel-type uniform PO diffraction coefficient of UTD type. The superiority of the uniform diffraction coefficient to those of other types is explained in terms of the location of the integration surfaces and is demonstrated numerically. © 2001 Scripta Technica, Electron Comm Jpn Pt 2, 84(2): 5462, 2001 Key words: PO; PO diffraction coefficient; field equivalence principle. 1. Introduction Physical optics (PO) [1] is a high-frequency tech- nique in which the total induced currents J are approxi- mated in the sense of geometrical optics (GO). The PO currents J PO thus defined are then integrated over the sur- face to give finite fields everywhere, including geometrical boundaries and caustics in focusing systems. PO has been widely applied to the pattern analysis of reflector antennas. In PO, the scattering fields are obtained by evaluating the surface radiation integrals of J PO , which is performed numerically in general. The asymptotic evaluation of these surface integrals [2, 3] leads us to line integral repre- sentations or closed-form expressions of fields, which greatly contributes not only to reducing the computation time but also to mechanism extraction of PO [4]. In general, the asymptotic reductions of PO surface integrals such as the geometrical theory of diffraction (GTD) become infi- nite at geometrical boundaries and caustics. To eliminate these difficulties, several uniform expressions have been proposed. In Ufimtsevs physical theory of diffraction (PTD) [57], PO currents are improved by adding another component called fringe wave currents J FW . Many works about the evaluation of surface integrals for diffraction from a half-sheet are developed in the spectral domain [811]. Efforts to achieve surface to line integral reduction have included both an exact approach based on the Helm- holtzHuygheng principle [1215] and asymptotic ap- proaches such as the high-frequency approximation [1621]. The asymptotic and local expressions are quite different from the exact and global ones and sometimes have the advantage that the former is applicable to a much wider class of scatterers based on local features of the diffraction phenomena. In two-dimensional (2D) problems of half-sheet dif- fraction illuminated by a line source, the edge contribution of the PO integral is asymptotically expressed in terms of PO diffraction coefficients. PO diffraction coefficients of the classical Keller type are nonuniform at geometrical boundaries, such as shadow and reflection boundaries (SB/RB) [1618]. Two types of uniform expressions to cope with these difficulties are available. The coefficients of the first kind were derived by directly applying the uniform asymptotic evaluation to integration on a half-sheet © 2001 Scripta Technica Electronics and Communications in Japan, Part 2, Vol. 84, No. 2, 2001 Translated from Denshi Joho Tsushin Gakkai Ronbunshi, Vol. J83-C, No. 2, February 2000, pp. 118127 54