  Some radio pulsar profiles (in particular those of millisecond pulsars
contain wide emission structures which cover large intervals of pulse phase.
Local distortions of an average curve of polarisation angle (PA) can be
identified in such profiles, and they are often found to be associated with
absorption features or narrow emission components. The features may be
interpreted as a convolution of a lateral profile of an emitter with a
microscopic radiation pattern of a non-negligible angular extent. We study a
model which assumes that such an extended microbeam of the X-mode curvature
radiation is spreading the radiation polarised at a fixed position angle within
an interval of pulse phase. The model is capable of interpreting the strongly
dissimilar polarisation of double notches in PSR B1821-24A (for which we
present new polarisation data from the Nancay Radio Telescope) and PSR
J0437-4715. It also explains a step-like change in PA observed at the
bifurcated trailing component in the profile of J0437-4715. A generic form of
the modelled PA distortion is a zigzag-shaped wiggle, which in the presence of
the second polarisation mode (O mode) can be magnified or transformed into a W-
or U-shaped deflection of a total net PA. The model's efficiency in
interpreting dissimilar polarisation effects provides further credence to the
stream-based (fan-beam) geometry of pulsar emission. It also suggests that the
microbeam width may not always be assumed negligible in comparison with the
angular scale of emissivity gradients in the emission region.
