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A recently developed phased-array feed configuration offering significant advantages over the conventional space, corporate, and series feed techniques is described. This technique, referred to as the "flat feed," allows power division for monopulse sum and difference pattern illumination functions in a feed depth of less than a half-wavelength with low loss. The technique used to extract energy from the power divider, which consists in part of a radial transmission line, results, in its simplest configuration, in a circular grid of antenna elements. Relations governing the circular grid array geometry design are derived which relate the angular locations of attenuated grating lobes to the spacing between the rings of radiating elements. Experimental -band hardware, built to prove the feed technique, is described. It includes a multimode launcher with measured coupling between circular wavegulde sum ( ) and difference ( ) modes of less than -37 dB; a seven-ring 1:195 radial power divider measured across a 10-percent band to have insertion loss of 0.1 dB and rms phase and amplitude deviations of less than and 0.47 dB; a 144-element array whose measured sum and difference beam radiation patterns are compared with calculated patterns for scan angles out to and whose sum port VSWR, measured across a 10-percent band, was under 1.8:1 with the array steered to broadside, and under 1.5:1 for other scan angles out to .