  As radio polarised emission from astrophysical objects traverse through
foreground magnetised plasma, the physical conditions along the lines of sight
are encrypted in the form of Rotation Measure (RM). We performed broadband
spectro-polarimetric observations of high Rotation Measure (|RM| >~ 300 rad
m-2) sources away from the Galactic plane (|b| > 10 deg) selected from the NVSS
RM catalogue. The main goals are to verify the NVSS RM values, which could be
susceptible to n{\pi}-ambiguity, as well as to identify the origin of the
extreme RM values. We show that 40 % of our sample suffer from n{\pi}-ambiguity
in the NVSS RM catalogue. There are also hints of RM variabilities over ~20
years epoch for most of our sources, as revealed by comparing the RM values of
the two studies in the same frequency ranges after correcting for
n{\pi}-ambiguity. At last, we demonstrate the possibility of applying
QU-fitting to study the ambient media of AGNs.
