Degree of polarization for a range of rest-frame viewing angles θ^′ onto axial-symmetric helical magnetic fields in a hollow cylindrical geometry. The estimated range of θ^′ for 3C 111 is highlighted as gray-shaded region with two independent estimates as white, dashed lines based on inclination angles stated in the literature. Negative values of p correspond to an integrated EVPA perpendicular to the jet axis, positive values result in parallel EVPAs. The figure shows a range of intrinsic pitch angles ψ^′ with steps of10^° between 20^°–90^° from the bottom to the top. This example corresponds to the analytical expression provided for a particle distribution N(E) d E ~ E^p dE with p = 3 in Eq. (21) by Lyutikov et al. (2005).