  We discuss our VLA and VLBI observations of supernova 1986J, which is
characterized by a compact radio-bright component within the expanding shell of
ejecta. No other supernova (SN) has such a central component at cm wavelengths.
The central component therefore provides a unique probe of the propagation of
radio signals at cm wavelengths through the ejecta of a young SN. Such a probe
is important in the context of Fast Radio Bursts (FRB), which, in many models,
are thought to be produced by young magnetars or neutron stars. The FRB signals
would therefore have to propagate through the expanding SN ejecta. Our
observations of the Type II SN 1986J show that the ejecta would remain opaque
to cm-wave emission like FRBs for at least several decades after the explosion,
and by the time the ejecta have become transparent, the contribution of the
ejecta to the dispersion measure is likely small.
