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The object of this investigation is a study of the electromagnetic field produced by a diffraction antenna in the form of a circular gap made in a conducting plane. An e.m.f. is applied across the gap. The method of investigation is based on the classical diffraction theory by Fresnel and Kirchhoff. The expressions for E and H are obtained and applied to the calculation of electric field intensity at a great distance, and the directive patterns are plotted. The current distribution in the screen is then studied and the expression for the gap admittance is obtained. The surface current density appears as a sum of an infinite number of partials or wave modes. The radiation conductance rises step-wise with increasing radius like the radiation conductance of an oscillating sphere excited at the equator.