The wire-mesh high-frequency modified Luneberg lens has numerous possibilities of application, among which are direction finding and azimuthal scanning with step-frequency ionospheric sounders. In these applications, where close feed spacing is desirable, numerous questions arise regarding the effect of the placement of feeds in the antenna aperture upon the lens radiation pattern in the absence of feeds. This paper describes an initial attempt to answer some of these questions. A geometrical configuration of feeds equally spaced around the periphery of the lens and the perturbing effect of this feed "grating" upon the lens radiation pattern without the feeds is considered.nA scale-model antenna was tested with an omnidirectional active feed and a semicircular grating of shorted quarter-wavelength monopoles with variable angular spacing. The scale antenna was tested near the apparent low-frequency cutoff, as this region allows measurement of scale patterns showing least lobing, and also because the feed grating has its most pronounced effect at these low frequencies. The lowest experimental frequency was chosen to be below the cutoff with the re¬maining frequencies chosen to be slightly above the cutoff. At the highest experimental frequency, the unperturbed lens begins to work well, with the side radiation being well attenuated from the main beam.nAn attempt was made to calculate the composite lens-grating radia¬tion pattern using geometrical optics and Fourier techniques without reverting to the more precise boundary-value problem. This was done to gain insight into the mechanisms involved in the lens blocking process. Within the assumptions made in the calculations, the measured results and the calculations are in general agreement. Backscatter through the lens for the feed spacings considered is shown to represent about 10 to 20 percent of the power entering the lens. Although the study presented in this memorandum is incomplete, it demonstrates the feasibility of using closely spaced feeds with the lens.
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