Antenna characterization relating voltage sources to radiated fields is an important part of any electromagnetic measurement, with applications ranging from two-way communication systems to full-wave inverse scattering. Here, we derive a two-antenna model relating the multipole expansion of the fields radiated and received by antennas to S-parameter measurements between them. We also show how to determine the antenna model parameters from numerical simulation. Finally, we validate the model experimentally using two rectangular waveguides. This model is based on the source-scattering matrix used in near-field antenna measurements [2–4], where we have extended it for direct S-parameter measurements.