Informing antenna design for sky-averaged 21-cm experiments using a simulated Bayesian data analysis pipeline

Abstract: Global 21cm experiments aim to measure the sky averaged HI absorption signal from cosmic dawn and the epoch of reionisation. However, antenna chromaticity coupling to bright foregrounds can introduce distortions into the observational data of such experiments. We demonstrate a method for guiding the antenna design of a global experiment through data analysis simulations. This is done by performing simulated observations for a range of inserted 21cm signals, then attempting to identify the signals with a data analysis pipeline. We demonstrate this method on five antennae that were considered as potential designs for the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH); a conical log spiral antenna, an inverted conical sinuous antenna and polygonal-, square- and elliptical-bladed dipoles. We find that the log spiral performs significantly better than the other antennae tested, able to correctly and confidently identify every inserted 21cm signal. In second place is the polygonal dipole antenna, which was only unable to detect signals with both very low amplitudes of 0.05K and low centre frequency of 80MHz. The conical sinuous antenna was found to perform least accurately, only able to detect the highest amplitude 21cm signals, and even then with biases. We also demonstrate that, due to the non-trivial nature of chromatic distortion and the processes of correcting for it, these are not the results that could have been expected superficially from the extent of chromatic variation in each antenna.