  Accurate instrumental bandpass corrections are essential for the reliable
interpretation of spectral lines from targeted and survey-mode observations
with radio interferometers. Bandpass correction is typically performed by
comparing measurements of a strong calibrator source to an assumed model,
typically an isolated point source. The wide field-of-view and high sensitivity
of modern interferometers means that additional sources are often detected in
observations of calibrators. This can introduce errors into bandpass
corrections and subsequently the target data if not properly accounted for.
Focusing on the standard calibrator PKS B1934-638, we perform simulations to
asses this effect by constructing a wide-field sky model. The cases of ASKAP
(0.7-1.9 GHz), MeerKAT (UHF: 0.58-1.05 GHz; L-band: 0.87-1.67 GHz) and Band 2
(0.95-1.76 GHz) of SKA-MID are examined. The use of a central point source
model during bandpass calibration is found to impart amplitude errors into
spectra measured by the precursor instruments at the ~0.2-0.5% level dropping
to ~0.01% in the case of SKA-MID. This manifests itself as ripples in the
source spectrum, the behaviour of which is coupled to the distribution of the
array baselines, the solution interval, the primary beam size, the hour-angle
of the calibration scan, as well as the weights used when imaging the target.
Calibration pipelines should routinely employ complete field models for
standard calibrators to remove this potentially destructive contaminant from
the data, a recommendation we validate by comparing our simulation results to a
MeerKAT scan of PKS B1934-638, calibrated with and without our expanded sky
model.
