  We confirm our recent prediction of the "pitchfork" foreground signature in
power spectra of high-redshift 21 cm measurements where the interferometer is
sensitive to large-scale structure on all baselines. This is due to the
inherent response of a wide-field instrument and is characterized by enhanced
power from foreground emission in Fourier modes adjacent to those considered to
be the most sensitive to the cosmological H I signal. In our recent paper, many
signatures from the simulation that predicted this feature were validated
against Murchison Widefield Array (MWA) data, but this key pitchfork signature
was close to the noise level. In this paper, we improve the data sensitivity
through the coherent averaging of 12 independent snapshots with identical
instrument settings and provide the first confirmation of the prediction with a
signal-to-noise ratio > 10. This wide-field effect can be mitigated by careful
antenna designs that suppress sensitivity near the horizon. Simple models for
antenna apertures that have been proposed for future instruments such as the
Hydrogen Epoch of Reionization Array and the Square Kilometre Array indicate
they should suppress foreground leakage from the pitchfork by ~40 dB relative
to the MWA and significantly increase the likelihood of cosmological signal
detection in these critical Fourier modes in the three-dimensional power
spectrum.
