  Most of the current Global 21-cm experiments include ground screens that help
moderate effects from the Earth. In this paper, we report on a possible
systematic artifact within the ground plane that may produce broad absorption
features in the spectra observed by these experiments. Using analytical
approximations and numerical modeling, the origin of the artifact and its
impact on the sky-averaged spectrum are described. The publicly released EDGES
dataset, from which a 78 MHz absorption feature was recently suggested, is used
to probe for the potential presence of ground plane resonances. While the lack
of a noise level for the EDGES spectrum makes traditional goodness-of-fit
statistics unattainable, the rms residual can be used to assess the relative
goodness of fits performed under similar circumstances. The fit to the EDGES
spectrum using a model with a simple 2-term foreground and three cavity-mode
resonances is compared to a fit to the same spectrum with a model used by the
EDGES team consisting of a 5-term foreground and a flattened Gaussian signal.
The fits with the physically motivated resonance and empirical flattened
Gaussian models have rms residuals of 20.8 mK (11 parameters) and 24.5 mK (9
parameters), respectively, allowing us to conclude that ground plane resonances
constitute another plausible explanation for the EDGES data.
