We have studied Very Long Baseline Array ( VLBA ) polarimetric observations of 8 sources including quasars and BL Lacs at 12 , 15 , 22 , 24 and 43 GHz and high frequency rotation measure ( RM ) maps are presented .
We find typical values for the RM in the VLBI core of several thousand rad/m ^ { 2 } , which are higher than values in the literature at lower frequencies .
Assuming a dependence of the form RM \propto \nu ^ { a } , we obtain an average value of a = 3.6 \pm 1.3 , which is larger than that expected by theoretical considerations .
Rotation measures are detected in the jet of only two sources and we find that only 0906+430 ( and possibly 1633+382 ) show indications of a robust gradient .
We discuss the Faraday–corrected polarization properties of the sources .
Our interpretation supports the presence of helical magnetic fields with new , unresolved , components affecting the intrinsic direction of polarization close to the base of the jet of some objects .