  Drifting subpulse patterns in pulsar signals are frequently interpreted in
terms of a model in which a rotating ring of sparks on the polar cap gives rise
to emission from regions of the magnetsophere connected to the sparks by
dipolar magnetic field lines. The spacing of drift-bands in time depends on the
circulation rate of the polar cap pattern, but to first order the longitudinal
phase dependence of the subpulse modulation should obey a frequency-independent
relation determined by the geometrical configuration in a similar manner to the
polarization position angle. We present here observations at 272-1380 MHz of
PSR B0320+39 and PSR B0809+74, both of which show nearly linear drift in two
longitude regions, separated by a region of reduced modulation and accompanied
by a large step in the phase of the subpulse pattern. We show that the
observation of Bartel et al. (1981) that the subpulse spacing for PSR B0809+74
was 1.8 times greater at 102.5 MHz than at 1720 MHz is most likely an artifact
of the phase step, which is only present at high frequencies. The phase steps
can be understood as a consequence of observing overlapping offset images of
the polar cap spark pattern. We also detected more complicated,
frequency-dependent behaviour that would require that the images do not simply
differ by rotation about their centers. Detailed modelling of non-axisymmetric
refraction or distorted magnetic fields is suggested as a means of pursuing an
explanation for this phenomenon.
