  The frequency widening of pulsar profiles is commonly attributed to lower
frequencies being produced at greater heights above the surface of the pulsar;
so-called radius-to-frequency mapping. The observer's view of pulsar emission
is a 1D cut through a 3D magnetosphere: we can only see that emission which
points along our line of sight. However, by comparing the frequency evolution
of many single pulses positioned at different phases, we can build up an
understanding of the shape of the active emission region. We use single pulses
observed with the Giant Metrewave Radio Telescope to investigate the emission
region of PSR J1136+1551 and test radius-to-frequency mapping. Assuming that
emission is produced tangential to the magnetic field lines and that each
emission frequency corresponds to a single height, we simulate the single pulse
profile evolution resulting from the canonical conal beam model and a fan beam
model. Comparing the results of these simulations with the observations, we
conclude that the emission region of PSR J1136+1551 is better described by the
fan beam model. The diversity of profile widening behaviour observed for the
single pulses can be explained by orthogonally polarized modes propagating
along differing frequency-dependent paths in the magnetosphere.
