Response of an antenna to arbitrary incident fields

In a variety of situations it is necessary to determine the response of an antenna to incident fields other than pure plane waves. Usually this is accomplished by the application of the principle of superposition to the antenna reciprocity relation for plane wave incidence. An alternative approach is to represent the incident field as a superposition of an infinite set of spherical free space modes and find the response with the aid of the corresponding antenna scattering matrix representation. Although the two formulations must necessarily be equivalent an explicit connection between them does not appear to have been previously established. This is done in this paper.