Diffraction Patterns in Circular Apertures Less Than One Wavelength in Diameter

Abstract

Measurements were made of the diffraction patterns of circular apertures from 0.2 to 1.0 wavelength in diameter when a plane polarized electromagnetic wave was incident normally upon them. Microwaves of wavelengths 16 cm and 32 cm were employed. The intensity of radiation relative to that of the unperturbed beam was determined at points along the electric and magnetic diameters of the apertures. The data disagreed with calculations from Young's circuital form in two major ways: (1) A sharp increase in intensity was observed at the ends of the electric diameter, (2) a surprising peak in intensity at the centers of apertures near one-half wavelength in diameter was 50 percent greater than calculations from the integral. A more complete interpretation of Young's theory of diffraction would include multiple reflections from the edges. The high intensity at the center of the aperture a half-wavelength in diameter was qualitatively accounted for by multiple reflections from the edges near the ends of the magnetic diameter.