  The recently discovered high-energy transient Sw J1644+57 is thought to arise
from the tidal disruption of a passing star by a dormant massive black hole.
The long-term, bright radio emission of Sw J1644+57 is believed to result from
the synchrotron emission of the blast wave produced by an outflow expanding
into the surrounding medium. Using the detailed multi-epoch radio spectral
data, we are able to determine the total number of radiating electrons in the
outflow at different times, and further the evolution of the cross section of
the outflow with time. We find that the outflow gradually transits from a
conical jet to a cylindrical one at later times. The transition may be due to
collimation of the outflow by the pressure of the shocked jet cocoon that forms
while the outflow is propagating in the ambient medium. Since cylindrical jets
usually exist in AGNs and extragalactic jets, this may provide independent
evidence that Sw J1644+57 signals the onset of an AGN.
