The optical transmission through a subwavelength hole in a metal film is usually very small, and the beam radiated from its exit aperture is very broad. However, the transmission may be increased by orders of magnitude, and the output beam sharply narrowed, when the tiny hole is surrounded by a properly designed periodic structure, which may take the form of an array of grooves or indentations on the metal surface. We have shown that these dramatic effects are due to the excitation of a leaky surface plasmon mode by the periodic structure on the metal film. Following this understanding, we introduce a simple but effective leaky wave antenna model, which we use to further explain and to quantify these dramatic effects. Particular attention is given to optimizing the structure to achieve maximum radiation at broadside, which offers a significant challenge in view of the open stopband in precisely the broadside direction.