  Characteristic modes on infinite periodic structures are studied using
spectral dyadic Green's functions. This formulation demonstrates that, in
contrast to the modal analysis of finite structures, the number of radiating
characteristic modes is limited by unit cell size and incident wavevector
(i.e., scan angle or phase shift per unit cell). The reflection tensor is
decomposed into modal contributions from radiating modes, indicating that
characteristic modes are a predictably sparse basis in which to study
reflection phenomena.
