  We 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 powers of around 10e27 W\Hz at 151 MHz and redshifts of around z=1. The
other samples contain 3CRR sources with either comparable redshifts but radio
powers about a decade larger or with comparable radio powers but redshifts
around z = 0.4. We use these samples to investigate the possible evolution of
their depolarisation and rotation measure properties with redshift and radio
power independently. We used VLA data for all sources at 4800 MHz and two
frequencies within the 1400 MHz band, either from our own observations or from
the archive. We present maps of the total intensity flux, polarised flux,
depolarisation, spectral index, rotation measure and magnetic field direction
where not previously published. Radio cores were detected in twelve of the
twenty-six radio sources. Fourteen of the sources show a strong
Laing-Garrington effect but almost all of the sources show some depolarisation
asymmetry. All sources show evidence for an external Faraday screen being
responsible for the observed depolarisation. We find that sources at higher
redshift are more strongly depolarised. Rotation measure shows no trend with
either redshift or radio power, however variations in the rotation measure
across individual sources increase with the redshift of the sources but do not
depend on their radio power.
