  Studying the interaction between AGN jets and lobes and their surrounding
environment is important in order to understand how they transfer energy to
their environment as well as determining the intrinsic physical properties of
the sources themselves. This paper presents broadband VLA polarization and
Faraday rotation observations of the radio galaxy Coma A (3C 277.3) from 1 to 4
GHz, including archival VLA observations at 4.9 and 15 GHz. Through broadband
polarization model-fitting, we find that an external Faraday screen with a
turbulent magnetic field provides an appropriate description to most of the
data. By combining the polarization and Faraday rotation results with previous
H$\alpha$ observations, we identified the H$\alpha$-emitting gas as the Faraday
screen responsible for the observed Faraday depolarization. We were able to
derive the magnetic field strength in the H$\alpha$-emitting gas, finding
typical field strengths of $\sim1$ $\mu$G, which is consistent with studies of
the intra-group medium local to other radio galaxies. However, we find a highly
depolarized region of the southern lobe coincident with a H$\alpha$ filament
that has a field strength comparable to the equipartition field strength in the
radio lobe (i.e. $\gtrsim$36 $\mu$G). This implies that the H$\alpha$ filament
is internal to the radio emitting plasma. Such clear examples of internal
Faraday depolarization are rare, thus providing another key insight into the
evolution of radio galaxies and their ability to provide significant feedback
on the local gas that would otherwise cool and form stars.
