  The standard model for radio supernovae considers that the observed
synchrotron radio emission arises from the high-energy shell that results from
the strong interaction between the expanding supernova ejecta and the
circumstellar medium. This emission is considered to be only partially absorbed
by ionized thermal electrons in the circumstellar wind of the progenitor star.
Based on a study of the radio light curves of the type II supernova SN1993J, we
present evidence of synchrotron self-absorption. Our modeling of the radio
light curves requires a large initial magnetic field, of about 30 Gauss, and
the existence of an (initially) highly-relativistic population of electrons. We
also show that while at early epochs the dominant absorption mechanism is
external absorption by thermal electrons, at late epochs and long wavelengths
the dominant absorption mechanism is synchrotron self-absorption. Consequently,
the spectral turnover takes place at much shorter wavelengths than expected in
the standard model, and at long wavelengths (greater than about 90cm at current
epochs) the flux predictions depart substantially from those of the standard
model.
