  Cosmic Dawn, the onset of star formation in the early universe, can in
principle be studied via the 21cm transition of neutral hydrogen, for which a
sky-averaged absorption signal, redshifted to MHz frequencies, is predicted to
be {\it O}(10-100)\,mK. Detection requires separation of the 21cm signal from
bright chromatic foreground emission due to Galactic structure, and the
characterisation of how it couples to instrumental response. In this work, we
present characterisation of antenna gain patterns for the Large-aperture
Experiment to detect the Dark Ages (LEDA) via simulations, assessing the
effects of the antenna ground-plane geometries used, and measured soil
properties. We then investigate the impact of beam pattern uncertainties on the
reconstruction of a Gaussian absorption feature. Assuming the pattern is known
and correcting for the chromaticity of the instrument, the foregrounds can be
modelled with a log-polynomial, and the 21cm signal identified with high
accuracy. However, uncertainties on the soil properties lead to
\textperthousand\ changes in the chromaticity that can bias the signal
recovery. The bias can be up to a factor of two in amplitude and up to few \%
in the frequency location. These effects do not appear to be mitigated by
larger ground planes, conversely gain patterns with larger ground planes
exhibit more complex frequency structure, significantly compromising the
parameter reconstruction. Our results, consistent with findings from other
antenna design studies, emphasise the importance of chromatic response and
suggest caution in assuming log-polynomial foreground models in global signal
experiments.
