  I define 3 samples of extragalactic radio sources of type FRII, containing 26
objects in total. The control sample consists of 6C and 7C sources with
radio-luminosities of around 10E27 W\Hz at 151 MHz and z~1. The other samples
contain 3CRR sources with either comparable redshifts but radio luminosities
about a decade larger or with comparable radio-luminosities but redshifts
around z~0.4. These samples are used to investigate the possible evolution of
depolarisation and rotation measure properties with redshift and radio
luminosity independently. I use the \citet{kat97} model with 3 flux
measurements to estimate the density of the source environment, source age and
jet power. Depolarisation and variations in the rotation measure were found to
be insensitive to changes in the density of the source environment. This
indicates that depolarisation and variations in the rotation measure are not
density indicators but map how the disorder in the magnetic field changes with
redshift. The model is then used on a larger set of FRII sources taken from the
complete 3CRR, 6CE and 7C III samples. I find that the density of the source
environment is a strong function of the source radio-luminosity but is
insensitive to changes in redshift. The jet power is also a strong function of
radio-luminosity but also a weak function of redshift. This indicates that the
mass of the black-hole powering FRII sources is epoch dependent. In general, I
find that the source environment is dominated by changes in the
radio-luminosity but is relatively unaffected by changes in cosmic epoch.
