A novel technique for reflection coefficient measurement in precision cosmology

Abstract

The detection of the global 21-cm signal from the Cosmic Dawn and Epoch of Reionisation remains a challenge to experiments worldwide. Emitted at a rest-frame frequency of 1420.405~MHz, this signal from the early Universe is redshifted to 40-200~MHz with a maximum brightness temperature of a few 100~mK. Efforts to detect this sky-averaged signal include experiments such as the Shaped Antenna measurement of the background RAdio Spectrum (SARAS) and Probing ReionizATion of the Universe using Signal from Hydrogen (PRATUSH). Detecting this faint signal requires precise calibration of the antenna, which includes a high-precision measurement of its reflection coefficient. This measurement must be performed \textit{in situ} at the observation site, as the antenna characteristics vary significantly with the environment. PRATUSH, a space-based radiometer, faces the additional challenge of structural distortions induced by thermal cycling, necessitating multiple measurements of the reflection coefficient. This work highlights the development of an \textit{in situ} Vector Network Analyser, which utilises a novel noise source-based calibration scheme and a cross-correlation spectrometer to perform magnitude and phase measurements of the complex reflection coefficient of the antenna. Further, we demonstrate the performance of the designed network analyser using independent measurements from a precision network analyser and reflection coefficient modelling of the device under test. We find the level of non-smooth calibration systematics, which need critical control for 21-cm signal detection, to be about $10^{-5}$. Finally, we study the impact of reflection coefficient correction on sky measurements, highlighting its usability for precision 21-cm observations.