Radio antenna design for sky-averaged 21 cm cosmology experiments: the
  REACH case
  Following the reported detection of an absorption profile associated with the
21~cm sky-averaged signal from the Cosmic Dawn by the EDGES experiment in 2018,
a number of experiments have been set up to verify this result. This paper
discusses the design process used for global 21~cm experiments, focusing
specifically on the Radio Experiment for the Analysis of Cosmic Hydrogen
(REACH). This experiment will seek to understand and compensate for systematic
errors present using detailed modelling and characterization of the
instrumentation. There is detailed the quantitative figures of merit and
numerical modelling used to assist the design process of the REACH dipole
antenna (one of the 2 antenna designs for REACH Phase I). This design process
produced a 2.5:1 frequency bandwidth dipole. The aim of this design was to
balance spectral smoothness and low impedance reflections with the ability to
describe and understand the antenna response to the sky signal to inform the
critically important calibration during observation and data analysis.
