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The most promising near-term observable of the cosmic dark age prior towidespread reionization (z~15-200) is the sky-averaged \lambda 21 cm backgroundarising from hydrogen in the intergalactic medium. Though an individual antennacould in principle detect the line signature, data analysis must separateforegrounds that are orders of magnitude brighter than the \lambda 21 cmbackground (but that are anticipated to vary monotonically and gradually withfrequency). Using more physically motivated models for foregrounds than inprevious studies, we show that the intrinsic "spectral smoothness" of theforegrounds is likely not a concern, and that data analysis for an idealantenna should be able to detect the \lambda 21 cm signal after deprojecting a~5th order polynomial in log(\nu). However, we find that the foreground signalis corrupted by the frequency-dependent response of a real antenna. Thefrequency dependence complicates modeling of foregrounds commonly based on theassumption of spectral smoothness. Much of our study focuses on theLarge-aperture Experiment to detect the Dark Age (LEDA), which combines bothradiometric and interferometric measurements. We show that statisticaluncertainty remaining after fitting antenna gain patterns to interferometricmeasurements does not compromise extraction of the \lambda 21 cm signal for arange of cosmological models after fitting a 7th order polynomial toradiometric data. Our results generalize to most efforts to measure thesky-averaged spectrum.

Foreground Model and Antenna Calibration Errors in the Measurement of the Sky-Averaged \lambda 21 cm Signal at z~20