This paper reviews the approximation principle of Physical Optics in view of diffraction theory. Two key error factors are identified for PO, that is, 1) errors in edge diffraction coefficients and 2) fictitious penetrating rays. Improved methods named PO-AF and PTD-AF are proposed as the methods which suppress the fictitious penetrating rays from PO and PTD respectively. In deep shadow regions of the reflector antennas, PO-AF and PDT-AF approach to PO-EEC and UTD respectively, while the continuity is assured. The effectiveness is numerically demonstrated for two dimensional scatterers.

The equivalence between Aperture Field Integration Method (AFIM) and Physical Optical (PO) is discussed for polyhedron surfaces in this paper. The necessary conditions for the equivalence are summarized which demand complete equivalent surface currents and complete apertures. The importance of the exact expressions for both incident and reflected fields in constructing equivalent surface currents is emphasized and demonstrated numerically. The fields from reflected components on additional surface which lies on the Geometrical Optics (GO) reflection boundary are evaluated asymptotically. The analytical expression enhances the computational efficiency of the complete AFIM. The equivalent edge currents (EECs) for AFIM (AFIMEECs) are used to extract the mechanism of this equivalence between AFIM and PO.

Equivalence of physical optics (PO) and aperture field integration method (AFIM) in the full 360 observation angle is discussed for polyhedron approximate reflectors; the necessary conditions of integration surface in AFIM for the equivalence to PO are presented. In addition to the condition that complete equivalent currents consisting of both geometrical optics (GO) reflected fields from the reflector and direct incident fields from the feed source are used, the integration surface should cap the reflector perfectly and should be in the illuminated region of the GO reflected field. Validity of the conditions is numerically confirmed for a two-dimensional (2-D) strip reflector, 3-D corner reflectors and a 2-D polyhedron approximate reflector.

Physical optics(PO) and the aperture field integration method (AFIM) give accurate and similar field patterns near the first few sidelobes of reflector antennas. It is widely accepted that the use of AFIM is restricted to norrower angles than PO. In this paper, uniform equivalent edge currents of PO and AFIM are compared analytically and their equivalence in high frequency in discussed. It is asymptotically verified that the patterns by AFIM are almost identical to PO fields in the full 360angular region, provided that AFIM uses the equivalent surface currents consisting of two components, that is, the geometrical optics(GO) reflected fields from the reflector and the incident fields from the feed source, the latter of which are often neglected. Slightly weaker equivalence is predicted for cross polarization patterns. Numerical comparison of PO and AFIM confirms all these results, the equivalence holds not only for large but also for a very small refiector of the order of one wavelength diameter.