Diffraction Errors in an Optical Measurement at Radio Wavelengths

Abstract

The errors arising in the measurement of antenna patterns by means of a lens are examined. The effect of the lens aperture is separated from the effect of lens aberrations and only the effect of the aperture is discussed. Theoretical solution of the problem is obtained for a uniformly illuminated aperture and for short electromagnetic horns. The theory is found to be in agreement with experimental measurements. The phenomenon of mutilation due to the aperture is examined with the help of both computed results and experimental measurements. The systematic errors due to mutilation have been found and have been shown as precision limits. An extrapolation of the data suggests that a lens forty wavelengths wide can be used for the precision measurement of the patterns of antennas ten to twenty wavelengths wide. The technique is entering its range of usefulness at a wavelength of 3.2 cm and should increase in value as the wavelength is decreased.