## ABSTRACT

This chapter examines electromagnetic diffraction from a circular hole in a perfectly conducting spherical shell. Several aspects of this problem are interesting. First it is the simplest three-dimensional conducting structure (of finite extent) that incorporates a cavity and edges. On the other hand, it is the archetypical open resonator with a high Q-factor strongly dependent on aperture size. Finally, any solution that is uniform with respect to aperture size (or angle measure of the spherical cap) also provides the basis for a comprehensive analysis of spherical reflector antennas. The full-wave solution developed in this chapter enables us to analyse accurately a large spherical reflector antenna, of aperture size some hundreds of wavelengths.