A cavity model for a spherical inverted-F antenna (SIFA) is proposed using a custom curvilinear coordinate system. The coordinate transformation maps the spherical structure into an equivalent rectangular topology to which conventional cavity method procedures can be applied. The wave equation under this transformation is solved to predict the modal characteristics, input impedance, and radiation behavior of the antenna. Analytically-obtained results are compared to simulated models and the measured results from one fabricated model. \ 1963-2012 IEEE.