  We report on Expanded Very Large Array (EVLA) observations of the Type IIb
supernova 2011dh, performed over the first 100 days of its evolution and
spanning 1-40 GHz in frequency. The radio emission is well-described by the
self-similar propagation of a spherical shockwave, generated as the supernova
ejecta interact with the local circumstellar environment. Modeling this
emission with a standard synchrotron self-absorption (SSA) model gives an
average expansion velocity of v \approx 0.1c, supporting the classification of
the progenitor as a compact star (R_* \approx 10^11 cm). We find that the
circumstellar density is consistent with a {\rho} \propto r^-2 profile. We
determine that the progenitor shed mass at a constant rate of \approx 3 \times
10^-5 M_\odot / yr, assuming a wind velocity of 1000 km / s (values appropriate
for a Wolf-Rayet star), or \approx 7 \times 10^-7 M_\odot / yr assuming 20 km /
s (appropriate for a yellow supergiant [YSG] star). Both values of the
mass-loss rate assume a converted fraction of kinetic to magnetic energy
density of {\epsilon}_B = 0.1. Although optical imaging shows the presence of a
YSG, the rapid optical evolution and fast expansion argue that the progenitor
is a more compact star - perhaps a companion to the YSG. Furthermore, the
excellent agreement of the radio properties of SN 2011dh with the SSA model
implies that any YSG companion is likely in a wide, non-interacting orbit.
