Radiation characteristics are established for a turnstile antenna in a cylindrical shield. The antenna consists of perpendicularly crossed wires driven by a sinusoidal current distribution, harmonic in time, and differing in phase by 90° between members of the cross. The shield is assumed to be a perfectly conducting cylindrical can open at one end and its diameter is taken to be of the order of a fraction of the driving wavelength. The axis of the shield is perpendicular to the plane of the antenna, passing through its center. The excitation of an infinitely long circular wave guide by the above current distribution is considered first, and conditions under which only the dominant (TE11) mode is important are determined. The essential radiation problem can then be regarded as one in which a semi‐infinite circular guide, excited by aTE11mode, radiates into free space. This problem has been formally solved by Levine and Schwinger and, using their results, values of the reflection coefficient and gain function have been computed by the National Bureau of Standards, Institute for Numerical Analysis, University of California, Los Angeles. The results of this theory are compared with experiment and with the Kirchhoff method.