  Some recent publications by authors from the University of Maryland analyse
the fluctuations of multi-port model parameters in stochastic environments.
These authors use random matrix theory (RMT) for estimates concerning
eigenfunction expansions in cavities which show so-called ``wave chaos.'' A
universal relation between the variances of the fluctuations of multi-port
impedance parameters, of the same type as we had observed experimentally for
scattering parameters in mode stirred chambers, is analysed in detail. However,
contrary to our own observations, these recent papers claim that only the
impedance fluctuations satisfy universal variance ratios. In the present paper,
we detail our own approach to the problem which is based on electromagnetic
scattering theory. It is shown that universal variance ratios are obtained, for
any type of multi-port model, when the environment of the multi-port system is
described by a scattering operator, which is statistically isotropic in a sense
we make precise below. The only condition on the system is that the radiation
power correlation between the ports considered vanishes. This is to be compared
to the diagonality of the radiation impedance required for the Maryland results
to be valid.
