  We present Faraday Rotation Measure (RM) properties of seven active galactic
nuclei (AGN), observed with the NRAO VLA at three epochs in 2012-2014. Data was
taken at 1.4, 2.2, 5.0, 8.2, 15.4, 22.4, 33.5 and 43.1 GHz quasi simultaneously
in full polarization mode. For the first time RMs were calculated in a range of
wavelengths covering more than one order of magnitude: from 21 cm up to 6 mm.
We measured RM for each source and showed a tendency to increase its value
toward high frequencies according to the law |RM|~f^a with a=1.6+/-0.1. For
0710+439, we observed an increase over the frequency range of 4 orders of
magnitude and measured one of the highest RM ever, (-89+/-1)*10^3 rad/m^2.
Analysis of different epochs shows variations of the value and the sign of RM
on short and long time-scales. This may be caused by changing physical
conditions in the compact regions of the AGN jets, e.g. strength of magnetic
field, particle density and so on.
