Average polarization degree as a function of the viewing angle θ_obs calculated for different combinations of toroidal and chaotic magnetic field components. The thick line on the top corresponds to a purely toroidal magnetic field (f = 1), the consecutive lines are calculated with steps Δf = 0.1 down to f = 0 corresponding to a purely chaotic magnetic field. The shock structure is the same as in the models shown in Figs. 1 and 2. The vertical line indicates the jet opening angle of Θ_j = 5°.