Studies of the cosmic microwave background (CMB) have shown the structure of the universe at the start of this interval, and deep optical surveys such as the Sloan Digital Sky Survey show the structure after this time interval, but there is little data available from the time when the first generation of stars and the earliest black holes were appearing.

Assuming that reionization occurs at z = 6 to 8, one would expect to see this spectral line redshifted into a frequency range around 150 to 200 MHz.

Warm hydrogen will emit 21 cm (rest frame) photons, increasing the sky brightness above what the CMB provides.

[3] This frequency range includes broadcast signals for FM radio, television, and many other terrestrial sources.

Another major technical challenge is emission from foreground sources such as quasars and ionized gas within the Milky Way.

This emission is expected to be four to five orders of magnitude stronger than the recombination epoch signals HERA will try to detect.

[5] In contrast, the antennas in the HERA array are deployed in a hexagonal tiling pattern, which provides a large number of identical baselines.

[7] When completed, the array will consist of 350 antennas (318 in a densely-packed hexaconal core 300 meters across, and 32 in more distant outriggers).