  We are using "broadband" (4.6 to 43 GHz) multi-frequency VLBA polarization
observations of compact AGN to investigate the 3-D structure of their jet
magnetic (B) fields. Observing at several frequencies, separated by short and
long intervals, enables reliable determination of the distribution of Faraday
Rotation, and thereby the intrinsic B field structure. Transverse Rotation
Measure (RM) gradients were detected in the jets of 0954+658 and 1418+546,
providing evidence for the presence of a helical B field surrounding the jet.
The RM in the core regions of 2200+420 (BL Lac), 0954+658 and 1418+546 display
different signs in different frequency-intervals (on different spatial scales);
we suggest an explanation for this in terms of modest bends in a helical B
field surrounding their jets. In future, polarization observations with a
combination of VSOP-2 at 8, 22 and 43 GHz and ground arrays at frequencies with
corresponding resolution will help map out the distributions of Faraday
rotation, spectral index and the 3-D B field structure both across the jet and
closer to the central engine, providing strong constraints for any jet B field
models.
