sources of systematic errors. Additional details on the analysis of systematic contributions and the hardware design can be found in the EDGES memorandum series[^1].

![ \[f\_edges\_model\] Example of redshifted 21 cm contribution (solid) to $T_{sky}$ based on the model described in § \[s\_edges\_limits\] with $\Delta T_{21}=25$ mK, $z_r = 8$, and $\Delta z = 0.6$. The residuals are also shown for a seventh-order polynomial fit to a simulated spectrum between 130 and 190 MHz with (dash) and without (dot) the redshifted 21 cm contribution. The foreground contribution was modeled for the plot using $\beta=2.5$, and $T_{gal}(150$ MHz$) = 250$ K.](f2.eps){width="20pc"}

Site Selection
--------------

Some of the contributions to the systematic uncertainty listed above can be addressed by careful selection of the observing site. Avoiding terrestrial transmitters (primarily from FM radio and television stations) is the most serious problem. Even at distances of hundreds or thousands of kilometers, tropospheric ducting and scattering (troposcatter), sporadic E propagation in the ionosphere, and reflections from meteors are all capable of transferring a significant amount of power from Earth-based transmitters. The background produced by the integrated effect of many distant transmitters may have significant spectral structure above the expected redshifted 21 cm level. For example, 