Mechanism of transformation of the bidirectional PA distortion (the thin solid X-mode wiggle in panel a) into a one-directional W-shaped deflection of the total PA (thick solid). The Stokes vectors associated with the marked points are shown schematically in panel c). Those with small letters (a, b, c) are the sum of the corresponding X-mode vectors (A,B,C) with the quasi-orthogonal ∥-mode vector shown on the left. The transition to the W-shaped PA is mainly caused by the drop in the X-mode flux at the minima of double notches (short vectors A and B). Panel b) presents a similar sum for the case with no drop in polarised flux at A and B (all ⊥-mode vectors have the same length).