The effects of the antenna power pattern uncertainty within a global 21 cm experiment

Experimental 21 cm cosmology aims to detect the formation of the first stars during the cosmic dawn and the subsequent epoch of reionization by utilizing the 21 cm hydrogen line transition. While several experiments have published results that begin to constrain the shape of this signal, a definitive detection has yet to be achieved. In this paper, we investigate the influence of uncertain antenna-sky interactions on the possibility of detecting the signal. This paper aims to define the level of accuracy to which a simulated antenna beam pattern is required to agree with the actual observing beam pattern of the antenna to allow for a confident detection of the global 21 cm signal. By utilising singular value decomposition, we construct a set of antenna power patterns that incorporate minor, physically motivated variations. We take the absolute mean averaged difference between the original beam and the perturbed beam averaged over frequency ($\Delta D$) to quantifying this difference, identifying the correlation between $\Delta D$ and antenna temperature. To analyse the impact of $\Delta D$ on making a confident detection, we utilize the REACH Bayesian analysis pipeline and compare the Bayesian evidence $\log \mathcal{Z}$ and root-mean-square error for antenna beams of different $\Delta D$ values. Our calculations suggest that achieving an agreement between the original and perturbed antenna power pattern with $\Delta D$ better than -35 dB is necessary for confident detection of the global 21 cm signal. Furthermore, we discuss potential methods to achieve the required high level of accuracy within a global 21~cm experiment

Receiver design for the REACH global 21-cm signal experiment

We detail the the REACH radiometric system designed to enable measurements of the 21-cm neutral hydrogen line. Included is the radiometer architecture and end-to-end system simulations as well as a discussion of the challenges intrinsic to highly-calibratable system development. Following this, we share laboratory results based on the calculation of noise wave parameters utilising an over-constrained least squares approach demonstrating a calibration RMSE of 80 mK for five hours of integration on a custom-made source with comparable impedance to that of the antenna used in the field. This paper therefore documents the state of the calibrator and data analysis in December 2022 in Cambridge before shipping to South Africa.Comment: 30 pages, 19 figure

Numerical modelling and design of wideband electromagnetic structures at radio frequencies: Applications in cosmology and digital communications

This thesis discusses the use of computational electromagnetic simulation technology to simulate various physical scenarios for analysis and design. Digital communications continue to dominate modern communication, and require increasing bandwidth to transfer the volume of information in use in today's society. One method for allowing these expansions is to improve the transmission through currently existing infrastructure, through improved impedance matching. The use of the surface wave mode of transmission is also considered to help improve data throughput, with a focus on the impedance of the required launchers. Simulated measurements for these impedances are calculated, with reference to their physical origins. Also examined within this thesis is the application of computational electromagnetics to the global 21 cm experiment REACH. This experiment aims to detect a signal five orders of magnitude below the foreground signal, and so requires highly accurate and precise understanding of the radiometer instrument. Both the design and analysis of aspects of the REACH dipole radiometer are considered throughout this thesis. While simulation of physical scenarios is to some degree accurate, it is inevitable that uncertainties will arise from simplifications made in computational models. So in the context of a dipole antenna's directivity pattern, and corresponding antenna temperature, the use of a parameterized sum of basis functions is considered to remodel the directivity of a physically perturbed antenna. Through the use of physically based basis functions and per frequency fitting, a rebuild accuracy within 0.1% of directivity is shown. The effect of likely physical deviations in a ground plane is considered in the case of the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) dipole radiometer. The physical deviations considered include the presence of soil, multiple and single dips in the ground plane. These alterations often induce uncertainties above 1 K, a level which would obscure a global 21 cm signal detection. Also noted is the impact of the addition of serrations to the edge of a square ground plane, with the replacement of large wavelike chromatic fluctuations with smaller pockmark type deviations. Finally, I describe the implementation of a quantified figure of merit based design method of radio frequency electromagnetic situations. This is used for the design of the REACH global 21 cm dipole radiometer. The figures of merit considered encompass the impedance of the antenna in addition to the chromaticity of its directivity pattern and are combined in such a way as to allow even comparison between these important aspects of the antenna