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A pair of narrow slots crossed at right angles and located at the proper point in the broad wall of a rectangular waveguide will radiate a circularly polarized wave. Some of the results of a study of the properties of such slots is presented. The study was undertaken with the aim of obtaining information useful in design of a circularly polarized linear array. Some of the properties of the slot pairs are as follows: 1) They are inherently matched, independent of the slot length. 2) When the slot arms are made resonant, approximately 75 per cent of the incident power is radiated, with a vswr of 1.12. 3) When fed from one end of the waveguide, the slots radiate right-hand circular polarization; from the other end, left-hand. In using these slots in linear arrays the power radiated is varied by changing slot length, since position and orientation are fixed by the requirements of circular polarization. The slots must be separated by a guide wavelength, so waveguide loading or other complicated schemes must be used to reduce the slot spacing.