Diffraction Pattern of a Circular Aperture at Short Distances

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Abstract
The diffraction pattern of electromagnetic waves was studied at distances from zero to five wave-lengths from a circular aperture. Microwaves of 12.8-cm wave-length were employed. The aperture was an iris diaphragm that could be varied between one and six wave-lengths in diameter. The beam was incident normally upon the plane of the aperture from a 4 ft parabolic reflector 24 feet away. The sharpest diffraction patterns were in the plane of the aperture. Measurements were made of the intensity of radiation in the electric and magnetic planes through the axis. Individual plots were made of intensities along the diameter and along lines parallel to the diameter at steps of quarter wave-lengths from the diameter. Measurements were also made of the intensities at fixed points along the axis as the diameter of the iris was varied from one to six wave-lengths. It was observed that Fresnel zone theory could be employed as a rule of thumb in predicting the intensity along the axis even in the illogical case of predicting the intensity at the center of the aperture. Checks were made against Kirchhoff's theory.

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