  We report on VLA measurements between 1 and 45 GHz of the evolving radio
spectral energy distribution (SED) of SN 1986J, made in conjunction with VLBI
imaging. The SED of SN 1986J is unique among supernovae, and shows an inversion
point and a high-frequency turnover. Both are due to the central component seen
in the VLBI images, and both are progressing downward in frequency with time.
The optically-thin spectral index of the central component is almost the same
as that of the shell. We fit a simple model to the evolving SED consisting of
an optically-thin shell and a partly-absorbed central component. The evolution
of the SED is consistent with that of a homologously expanding system. Both
components are fading, but the shell more rapidly. We conclude that the central
component is physically inside the expanding shell, and not a surface hot-spot
central only in projection. Our observations are consistent with the central
component being due to interaction of the shock with the dense and
highly-structured circumstellar medium that resulted from a period of
common-envelope evolution of the progenitor. However a young pulsar-wind nebula
or emission from an accreting black hole can also not be ruled out at this
point.
