  We present radio observations of the afterglow of the bright gamma-ray burst
GRB980703 made between one day and one year after the burst. These data are
combined with published late-time radio measurements and existing optical,
near-infrared (NIR) and X-ray observations to create a comprehensive broadband
dataset for modeling the physical parameters of the outflow. While a
wind-stratified medium cannot be ruled out statistically, it requires a high
fraction of the shock energy in the electrons, and so is not favored on
theoretical grounds. Instead, the data are consistent with a fireball model in
which the ejecta are collimated and expanding into a constant density medium.
The radio data cannot be fit with an isotropic shock but instead require a jet
break at ~ 3.5 days, not seen at optical wavelengths due to the presence of a a
bright host galaxy. The addition of the full radio dataset constrains the
self-absorption frequency, giving an estimate of the circumburst density of n ~
30 cm^-3, a value which differs substantially from previous estimates. This
result is consistent with the growing number of GRB afterglows for which
broadband modeling yields n ~= 0.1-100 cm^-3, with a typical value ~ 10 cm^-3.
