  We present the results from the analysis of 26 extragalactic radio sources of
type FRII which were observed with the VLA at 5 GHz and around the 1.4 GHz
band. The sources were selected to have redshifts in the range $ 0.3<z<1.3$,
radio powers between $6.9 \times 10^{26} {\rm WHz^{-1}}<P_{151 {\rm MHz}}<1.3
\times 10^{28} {\rm WHz^{-1}}$ and angular size $\theta \ge 10''$. We found
that the depolarisation and the rms variations in the rotation measure
increased with redshift. The flux values obtained from the observations were
used to derive by means of analytical modelling the jet--power, density of the
central environment, age of the source and its lobe pressure and the results
were then compared with the observations. We find no significant correlations
with the density parameter suggesting that the depolarisation and the rms
variations in the rotation measure are indicative of the environment becoming
more disordered rather than denser. The age and size of a source are correlated
and both were found to be independent of redshift and radio--power. Jet--power
strongly correlated with the radio--power. The lobe pressure was found to be
anti--correlated with size which could explain why there are no sources beyond
a few Mpc in size. We found no significant correlation between size and density
which demonstrates that the sample is a fair representation of the population.
