  We report here the study, in the radio band, of a sample of candidates of
high Rotation Measure (RM). The point-like objects (at kpc scale) were selected
by choosing unpolarised sources from the NVSS which show significant linear
polarisation at 10.45 GHz. Assuming in-band depolarisation, this feature
suggests the presence of a very dense medium surrounding them in a combination
of a strong magnetic field. Further single-dish observations were performed
with the 100-m Effelsberg telescope to characterise the SEDs of the sample and
to well determine their RM in the 11 to 2 cm wavelength range. Besides, a
wideband (L, S, C and X band ) full polarisation observational campaign was
performed at the JVLA facility. It allows us to analyse the in-band RM for the
most extreme objects. Some Effelsberg results and analysis, and preliminary
JVLA results are presented. The observations reveal that sources with young,
newly growing, radio components at high frequency (i.e. GPS and HFP sources)
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.
