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In this paper, we present a novel model for the design of micrometric optical antennas. The accuracy of the model allows to evaluate the near field generated by a micrometric aperture in parallel-plate waveguides. The model starts from the equivalent source distribution of magnetic current excitation of a rectangular aperture and it proceeds by employing the multipole expansion of the Green's function applied to the aperture, including the analysis of the electromagnetic field in proximity of metallic discontinuities. The presented method introduces the novel principle of simultaneous transverse resonance diffraction which determines the order of singularity for different edges at optical frequencies. The theory is validated by numerical simulations and experimental results yielding information about the accuracy of the radiated field solution. The approach is suitable for inclusion in standard electromagnetic simulators.