Abstract

Abstract : The overestimation of power density by the Fraunhofer (far-field) approximation to the more exact Fresnel (near-field) diffraction integral is determined as a function of Fresnel number at arbitrary on-axis field points for rectangular and circular aperture illuminations of uniform amplitude and phase. For square and circular apertures, the overestimation is 2.8 percent and 1.3 percent, respectively, at a Fresnel number of 1/8 (corresponding to the far field boundary distance D-sq/ lambda) and 518 percent and 147 percent, respectively, at a Fresnel number of 1. Keywords: Antenna aperture; Aperture distribution of uniform intensity and phase circular aperture; Constructive interference; Destructive interference; Diffraction; Fresnel kirchhoff integral; Fresnel number; Optical aperture; Rectangular aperture.

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