Properties and limitations of linear models for mutual coupling in 21 cm arrays
Abstract
Mutual coupling between elements of low-frequency arrays is a leading systematic for 21 cm cosmology, leaking bright foregrounds into otherwise clean regions of the power spectrum. Many mitigation models describe coupling as a linear combination of uncoupled signals re-radiated from neighboring feeds, but data suggest additional mechanisms. Inspired by antenna scattering theory, we decompose coupling into an antenna mode, constructed from translated isolated responses and expected to be removable by linear combination models, and a structural mode, the remainder. Full-wave simulations of HERA-like feeds and dishes in a 37-element hexagonal array show that the antenna mode accounts for only about 1% of the beam perturbation power, suggesting limited scope for mitigation with linear models. Two effects nevertheless lead to substantially better suppression. First, correlating beams and weighting them by the sky changes the modes' relative importance: correcting the antenna mode can remove more than half of the coupling power in simulated visibilities. Second, relaxing the coefficient relationships imposed by re-radiation allows linear corrections to remove structural-mode coupling too. Beam-derived corrections can reduce coupling power by over an order of magnitude in our noiseless simulations of perfectly redundant arrays. Fitting directly to known isolated visibilities reduces residuals further on the fitted field, but this target is generally unavailable in observations. Practical correction therefore requires additional constraints to separate the unknown sky from coupling while preserving the cosmological signal. We introduce an extension of holographic back-projection to coupled beams, providing an intuitive picture of these perturbations in the aperture plane.
Related
- Identifying narrow-band mutual coupling resonances in SKA-low stations using frequency-independent current basis functions
- Statistical properties of the convergence due to weak gravitational lensing by non-linear structures
- Parameter Estimation for Eccentric Supermassive Black Hole Binaries with Pulsar Timing Arrays
- Testing Source Dependence in Gravitational-Wave Tests of General Relativity with Mutual Information
- LIGHTS: Limitations of far-infrared tracers for Galactic cirrus mitigation in the era of ultra-deep imaging
BibTeX
@misc{mackay2026properties,
title = {{Properties and limitations of linear models for mutual coupling in 21 cm arrays}},
author = {Vincent MacKay and Robert Pascua and Eleanor Rath and Jacqueline N. Hewitt and Tyler Cox and Nicholas Kern and Joshua S. Dillon and Aaron Parsons and Eloy de Lera Acedo and Nicolas Fagnoni},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.22197},
eprint = {2610.22197},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.22197},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Properties and limitations of linear models for mutual coupling in 21 cm arrays
AU - Vincent MacKay
AU - Robert Pascua
AU - Eleanor Rath
AU - Jacqueline N. Hewitt
AU - Tyler Cox
AU - Nicholas Kern
AU - Joshua S. Dillon
AU - Aaron Parsons
AU - Eloy de Lera Acedo
AU - Nicolas Fagnoni
PY - 2026
DA - 2026/10/15
PB - arXiv
DO - 10.48550/arxiv.2610.22197
UR - https://arxiv.org/abs/2610.22197
AB - Mutual coupling between elements of low-frequency arrays is a leading systematic for 21 cm cosmology, leaking bright foregrounds into otherwise clean regions of the power spectrum. Many mitigation models describe coupling as a linear combination of uncoupled signals re-radiated from neighboring feeds, but data suggest additional mechanisms. Inspired by antenna scattering theory, we decompose coupling into an antenna mode, constructed from translated isolated responses and expected to be removable by linear combination models, and a structural mode, the remainder. Full-wave simulations of HERA-like feeds and dishes in a 37-element hexagonal array show that the antenna mode accounts for only about 1% of the beam perturbation power, suggesting limited scope for mitigation with linear models. Two effects nevertheless lead to substantially better suppression. First, correlating beams and weighting them by the sky changes the modes' relative importance: correcting the antenna mode can remove more than half of the coupling power in simulated visibilities. Second, relaxing the coefficient relationships imposed by re-radiation allows linear corrections to remove structural-mode coupling too. Beam-derived corrections can reduce coupling power by over an order of magnitude in our noiseless simulations of perfectly redundant arrays. Fitting directly to known isolated visibilities reduces residuals further on the fitted field, but this target is generally unavailable in observations. Practical correction therefore requires additional constraints to separate the unknown sky from coupling while preserving the cosmological signal. We introduce an extension of holographic back-projection to coupled beams, providing an intuitive picture of these perturbations in the aperture plane.
ER -