  We characterised and studied, in the radio band, a sample of candidates of
high Rotation Measure (RM). These point-like objects show a strong
depolarisation at 21cm. This feature suggests the presence of a very dense
medium surrounding them in a combination of a strong magnetic field. This work
aims at selecting and studying a sample of radio sources with high RM, thus to
study their physical conditions and their status with respect to their
surrounding medium. We want to understand if any connection is present between
the AGN hosting galaxy medium with some evolutionary track and/or some
restarting phase of the AGN itself. Multifrequency single-dish observations
were performed with the 100-m Effelsberg telescope to define the initial
sample, to characterise the SED of the final sample (30 targets) and to
determine their RM in the 11 to 2 cm wavelength range. From the observations,
the SED together with polarisation information, i.e. the fractional
polarisation and the polarisation angle, have been determined. Three different
object types were revealed from the SEDs analysis: Older, GPS-like and Mixed.
For each of the targets, the rotation measure has been found and the
depolarisation has been modelled. No significant correlation have been found
between the depolarisation behaviours and the SEDs, while a correlation has
been found between sources with mixed SED (with an old component at low
frequency and compact components at high frequencies) and high values of the
rotation measure (with values in the rest frame larger than 1000 rad/m^2). This
work helps us to define and identify a sample of sources with high RM. From the
analysis we can conclude that the sources showing a restarting phase at high
frequency (with a Mixed SED), are characterised by a really dense and/or a
magnetised medium that strongly rotates the polarisation angle at the different
frequencies, leading to a high RM.
