Understanding spectral artefacts in SKA-LOW 21-cm cosmology experiments: the impact of cable reflections

The Cosmic Dawn marks the first star formations and preceded the Epoch-of-Reionization, when the Universe underwent a fundamental transformation propelled by the radiation from these first stars and galaxies. Interferometric 21-cm experiments aim to probe redshifted neutral hydrogen signals from these periods, constraining the conditions of the early Universe. The SKA-LOW instrument of the Square Kilometre Array telescope is envisaged to be the largest and most sensitive radio telescope at m and cm wavelengths. The latest Aperture Array Verification Systems feature 7m coaxial transmission lines connecting the Low Noise Amplifiers to optical transmitters at the front of the analogue-receiving chain. An impedance mismatch between these components results in a partially reflected electromagnetic signal, which introduces chromatic aberrations in the instrument bandpass. This causes power from the foreground signals to appear at higher delays, potentially contaminating the EoR window, a region at which the 21-cm signal should be detectable. We present an end-to-end simulation pipeline for SKA-LOW using a composite sky model combining radio foregrounds from The GLEAM Survey, Haslam 408MHz, and a 1.5cGpc 21-cm brightness temperature cube generated with the 21cmSPACE simulator. Iterating a parametric approach, we derive a model for the scattering parameters of a coaxial transmission line in terms of its specifications and bulk material properties. Assuming identical cables of length ≤ 15.0m with impedance mismatch ≤ 10Ω confines the reflection to k-modes below the EoR window. However, we demonstrate that even a 0.1 across all accessible k-modes.