  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 $a=3.6\pm1.3$, 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.
