Wideband antennas for precision spectral radiometers for cosmology

Precision cosmology has been made possible by differential microwave radiometers that have imaged the anisotropy in the cosmic microwave background radiation. Continued progress is expected from measurement of spectral distortions in this cosmic radiation. A key distortion in the global sky spectrum is in the 40-250 MHz band and expected from variations in the hydrogen spin temperature and ionization state during Cosmic Dawn and Epoch of Reionization when the first stars and galaxies form. Electrically-small shaped ultra-wideband antennas are described that have been designed for the detection of these global or all-sky spectral distortions; a key design feature is the making of the transfer function Maximally Smooth.