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This paper determines the radiation characteristics of a semi-flush-mounted cardioid-pattarn antenna on a theoretical basis. The antenna is similar to one described by Clapp in a report of the Electronic Research Laboratory of the University of California. By judicious use of image theory in a radial transmission line formed by extending the antenna surfaces to infinity, the approximate fields of the principal polarization in the antenna are determined. If these fields are terminated in the aperture by a magnetic current sheet, the exterior radiation fields are obtained by integration over the aperture, according to the well-known equivalence theorem. Experimental patterns are given for comparison.