  The 21 cm hyperfine transition of neutral hydrogen offers a promising probe
of the large scale structure of the universe before and during the Epoch of
Reionization, when the first ionizing sources formed. Bright radio emission
from foreground sources remains the biggest obstacle to detecting the faint 21
cm signal. However, the expected smoothness of foreground power leaves a clean
window in Fourier space where the EoR signal can potentially be seen over
thermal noise. Though the boundary of this window is well-defined in principle,
spectral structure in foreground sources, instrumental chromaticity, and choice
of spectral weighting in analysis all affect how much foreground power spills
over into the EoR window. In this paper, we run a suite of numerical
simulations of wide-field visibility measurements, with a variety of diffuse
foreground models and instrument configurations, and measure the extent of
contaminated Fourier modes in the EoR window using a delay-transform approach
to estimating power spectra. We also test these effects with a model of the
HERA antenna beam generated from electromagnetic simulations, to take into
account further chromatic effects in the real instrument. We find that
foreground power spillover is dominated by the so-called "pitchfork effect", in
which diffuse foreground power is brightened near the horizon due to the
shortening of baselines. As a result, the extent of contaminated modes in the
EoR window is largely constant over time, except when the galaxy is near the
pointing center.
