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Foreground subtraction in global redshifted 21 cm measurements is limited byfrequency-dependent (chromatic) structure in antenna beam patterns. Chromaticbeams couple angular structures in Galactic foreground emission to spectralstructures that may not be removed by smooth functional forms. We reportresults for simulations based on two dipole antennas used by the Experiment toDetect the Global EoR Signature (EDGES). The residual levels in simulatedforeground-subtracted spectra are found to differ substantially between the twoantennas, suggesting that antenna design must be carefully considered.Residuals are also highly dependent on the right ascension and declination ofthe antenna pointing, with RMS values differing by as much as a factor of 20across pointings. For EDGES and other ground-based experiments with zenithpointing antennas, right ascension and declination correspond directly to thelocal sidereal time and the latitude of the deployment site, hence chromaticbeam effects should be taken into account when selecting sites. We introducethe "blade" dipole antenna and show, via simulations, that it has betterchromatic performance than the "fourpoint" antenna previously used for EDGES.The blade antenna yields 1-5~mK residuals across the entire sky after a 5-termpolynomial is removed from simulated spectra, whereas the fourpoint antennatypically requires a 6-term polynomial for comparable residuals. For bothantennas, the signal-to-noise ratio of recovered 21 cm input signals peaks fora 5-term polynomial foreground fit given realistic thermal noise levels.

Limits on Foreground Subtraction from Chromatic Beam Effects in Global Redshifted 21 cm Measurements