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The importance of a waveguide linear array capable of producing many different radiation patterns has led to the development of a slot radiator with variable coupling. An array of such radiators can be used as a laboratory apparatus for the evaluation of aperture distributions, or as a ground-painting reconnaissance antenna operable at many different altitudes. The slot radiator consists of a longitudinal slot centered in the broad face of a rectangular waveguide. An adjustable iris excites the slot by introducing controlled asymmetry in the waveguide fields. The theory of operation and the characteristics of this variable coupling slot are presented. These characteristics are shown in curves usable for design purposes. A detailed discussion is given of the technique used in determining the range of conductances of this slot. The control of coupling available is demonstrated by measured radiation patterns on a 12-element array in which the conductance values are varied to obtain sidelobe ratios from 10 to 34 db. The method used in obtaining the aperture distributions for these radiation patterns is also presented.