Impact of Antenna Structure and Orientation on Forward-Modelled Global 21 cm Signal Recovery

Abstract The redshifted 21 cm absorption trough from cosmic atomic hydrogen is one of the most promising probes of the early Universe, but its detection is challenged by bright foregrounds and instrumental systematics. In this work we quantify the impact of antenna mismodelling on the recovery of a simulated 21 cm signal within a fully Bayesian, forward-modelled data analysis pipeline. We show that discrepancies between simulated and modelled antenna beams lead to frequency-dependent errors in antenna temperature that can bias parameter inference. In particular, we demonstrate that orientation mismatches at the level of ∼0.25○ can significantly bias recovered signal parameters in typical observing scenarios. However, we also show that Bayesian evidence can be used to infer antenna orientation within this precision by scanning over model realisations. For structural mismodelling, we find that broadband recovery of all signal parameters requires accurate beam knowledge, but that partial recovery remains possible. In particular, signal frequency and width can be robustly recovered under restricted frequency bands even when the antenna structure is imperfectly modelled, while signal depth remains highly sensitive to beam errors. These results quantify the level of beam knowledge required for forward-modelled global 21 cm experiments and highlight the importance of observing strategy and antenna design in mitigating beam–sky coupling systematics.