  A radio antenna is primarily designed to convert electromagnetic waves into
electrical current and vice versa. However, a part of the incident wavefield is
scattered due to structural effects and reflected power at the antenna's
electrical port. Because the latter depends on the load impedance, an antenna
can also be referred to as a loaded scatterer. Its interaction with
electromagnetic waves is defined by means of absorption and scattering
cross-sections (ACS and SCS). When immersed in a diffuse field such as the one
generated within a reverberation chamber (RC), the impact of the load antenna
results from averaged properties over incident angles. One fundamental quantity
is the antenna quality factor (Q-factor) directly related to the averaged ACS.
Current formulations of Q-factor are based on different power budget analysis
which do not enable the consideration of wave interferences between the ingoing
and outgoing fields. Additionally, the structural component is always neglected
in the current formulations. In this paper, we introduce a rigorous formulation
of the antenna Q-factor which takes into account the aforementioned effects.
The antenna is modeled using the scattering-matrix theory which linearly links
the ingoing and outgoing waves in terms of spherical harmonics expansion. The
derived theory is validated using several numerical simulations based on a
Method-of-Moment code. Then, the capability to take advantage of this model to
retrieve antenna properties from multiple Q-factor estimations in an RC is
demonstrated. All results are compared to the existing Q-factor formulations.
