This paper compares two different representations of the dyadic Green's functions for circular cylindrical cavities, also valid at the source point. The representations are obtained by considering the cavity either as a circular waveguide (longitudinal representation) or as a radial waveguide (radial representation), terminated by conducting surfaces. The radial representation of dyadic Green's functions for circular cylindrical cavities is not available in the previous literature and is applied to study the field penetrating a space station module through apertures, including lateral ones, or due either to elementary sources or to wire antennas active in the module. We show that for the typical dimensions of spatial modules the radial representations give rise to more convergent series and therefore are more suitable for the computation of the cavity electromagnetic fields. The problem of obtaining the self and mutual impedances of two wire antennas located in the cavity is also discussed in general, and various results are reported for particular cases. Simplified cavity models can be effectively applied to the electromagnetic compatibility (EMC) characterization of space station modules.