  The phenomenon of sub-pulse drifting is an important single-pulse phenomenon
that can potentially provide important insights into the elusive radio emission
mechanism in pulsars. We analyze the frequency behaviour of the single pulses
of B0818-41, observed from 300 to 500 MHz (Band 3 of the uGMRT), and compare it
to the evolution of the average profile to place constraints on the geometry of
the pulsar's emission beam. We show that a circular carousel of discrete
beamlets, where each beamlet has radial symmetry, is not consistent with the
observed behaviour, and describe an alternative, consistent range of possible
elliptical carousel geometries. We also combine the uGMRT data with some
archival MWA observations and several other published profiles to characterize
the profile evolution across a frequency range spanning ~170 MHz to ~1.4 GHz.
