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| \sim { \nu } ^ { a } with a = 1.6 \pm 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 \pm 1 ) \cdot 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 .