The Hydrogen Epoch of Reionization Array Dish II: Characterization of Spectral Structure with Electromagnetic Simulations and its science Implications

TL;DR

This study uses electromagnetic simulations to analyze HERA antenna spectral characteristics, demonstrating that reflections and resonances have limited impact on its ability to detect 21 cm signals from the epoch of reionization.

Contribution

It provides a detailed electromagnetic characterization of HERA antennas, showing their effectiveness despite structural reflections and resonances, informing EoR science capabilities.

Findings

Reflections between feed and dish are below -40 dB at 150 ns, minimally affecting EoR constraints.

Resonances in the feed's cylindrical skirt introduce power at large delays, slightly reducing sensitivity.

Spectral response remains smooth enough for delay filtering to effectively contain foregrounds.

Abstract

We use time-domain electromagnetic simulations to determine the spectral characteristics of the Hydrogen Epoch of Reionization Arrays (HERA) antenna. These simulations are part of a multi-faceted campaign to determine the effectiveness of the dish's design for obtaining a detection of redshifted 21 cm emission from the epoch of reionization. Our simulations show the existence of reflections between HERA's suspended feed and its parabolic dish reflector that fall below -40 dB at 150 ns and, for reasonable impedance matches, have a negligible impact on HERA's ability to constrain EoR parameters. It follows that despite the reflections they introduce, dishes are effective for increasing the sensitivity of EoR experiments at relatively low cost. We find that electromagnetic resonances in the HERA feed's cylindrical skirt, which is intended to reduce cross coupling and beam ellipticity, introduces significant power at large delays ($-40$ dB at 200 ns) which can lead to some loss of measurable Fourier modes and a modest reduction in sensitivity. Even in the presence of this structure, we find that the spectral response of the antenna is sufficiently smooth for delay filtering to contain foreground emission at line-of-sight wave numbers below $k_\parallel \lesssim 0.2$ $h$Mpc$^{-1}$, in the region where the current PAPER experiment operates. Incorporating these results into a Fisher Matrix analysis, we find that the spectral structure observed in our simulations has only a small effect on the tight constraints HERA can achieve on parameters associated with the astrophysics of reionization.