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Accurate antenna beam models are critical for radio observations aiming toisolate the redshifted 21cm spectral line emission from the Dark Ages and theEpoch of Reionization and unlock the scientific potential of 21cm cosmology.Past work has focused on characterizing mean antenna beam models using eithersatellite signals or astronomical sources as calibrators, butantenna-to-antenna variation due to imperfect instrumentation has remainedunexplored. We characterize this variation for the Murchison Widefield Array(MWA) through laboratory measurements and simulations, finding typicaldeviations of order +/- 10-20% near the edges of the main lobe and in thesidelobes. We consider the ramifications of these results for image- and powerspectrum-based science. In particular, we simulate visibilities measured by a100m baseline and find that using an otherwise perfect foreground model,unmodeled beamforming errors severely limit foreground subtraction accuracywithin the region of Fourier space contaminated by foreground emission (the"wedge"). This region likely contains much of the cosmological signal, andaccessing it will require measurement of per-antenna beam patterns. However,unmodeled beamforming errors do not contaminate the Fourier space regionexpected to be free of foreground contamination (the "EOR window"), showingthat foreground avoidance remains a viable strategy.

Beamforming Errors in Murchison Widefield Array Phased Array Antennas and their effects on Epoch of Reionization Science