by the first luminous sources, and then climbs to $T_{21}\approx25$ mK at the beginning of the reionization epoch before falling back to $T_{21}\approx0$ mK at the end of reionization, the global mean redshifted 21 cm spectrum may contain up to three relatively sharp features between $50\lesssim\nu\lesssim200$ MHz that would not be represented well by a power-law profile. For the large solid angles of a single antenna beam, the mean redshifted 21 cm signal should vary little from one location to another on the sky. @2006MNRAS.371..867F and [@2004ApJ...608..611G] have calculated example global mean redshifted 21 cm spectra for various assumptions of stellar formation histories.

The specific approach employed with EDGES to exploit these expected differences in spectral characteristics in order to overcome the difficulty in separating the foreground and signal contributions in the measured spectrum is to limit the scope of the experiment to test for discontinuous features in the spectrum, since these would necessarily be due to the rapid transitions in the redshifted 21 cm brightness temperature and not the spectrally smooth foregrounds. In particular, the frequency response of the system is designed to test for fast reionization only (and not the transitions that might arise at higher redshifts from 