Incremental diffraction coefficients at the edge of a circular horn (CH) are derived on the basis of the Incremental Theory of Diffraction (ITD). The resulting distributed diffracted field contributions are used to predict mode radiation in the near zone, as well as to the reciprocal problem of EM coupling of a spherical wave into the CH. Second order, incremental diffraction contributions are also included to preserve accuracy at aspects below the aperture plane. For simplicity purposes, the analysis is restricted to the scalar case, with hard boundary conditions on the walls. Thus, guidelines are provided for the more general EM case.