  In this paper we apply and discuss a method for the determination of the
magnetic field ($H$) evolution in supernova remnants (SNRs) from radio
luminosity at given frequency $\nu$ to diameter ($L_\mathrm{\nu}-D$)
correlation. We assumed that $H$ evolves as $H\propto D^{-\delta}$, where $D$
is the diameter of the remnant.
  Value $\delta\approx1.2$ is obtained under the equipartition assumption from
the equations for revised equipartition calculation (REC) and by using the data
sample from the nearby starburst galaxy M82. We try to investigate whether or
not SNRs in M82 are in the equipartition state. This is done by comparison of
our empirically obtained $\delta$ with the theoretical value expected for the
equipartition conditions. Inconsistence between the value obtained for
equipartition conditions and adiabatic expansion $(\delta=1.5)$ and the value
empirically obtained herein, can be explained mainly by the influence of
sensitivity selection effects which tend to flatten the slope of the $L_\nu-D$
relations for extragalactic samples.
