  We present multi-frequency radio observations from the Very Long Baseline
Array (VLBA) of selected AGN that seem to have a B-field structure with a
central "spine" of B-field orthogonal to the jet and a longitudinal B-field
near one or both edges of the jet. Two explanations for this structure have
been discussed in the literature: shocks making the central orthogonal field
combined with a jet-medium interaction causing the longitudinal "sheath", or
both components produced by a helical jet magnetic field. One way to
investigate this structure is to look for gradients in the Faraday Rotation
across the jet. We will discuss results for 0333+321, 1150+812 and 2037+511
providing evidence for the latter picture.
