Antenna systems for 21 cm cosmology

Measurements of the fine scale spectral distortion in the radio background spectrum below 200 MHz due to redshifted 21 cm radiation of the cosmological neutral hydrogen (referred to as red-shifted 21 cm signal hereafter) has achieved extraordinary importance in present day cosmology. Detection of this spectral distortion at long wavelength can provide the first radio constraints on the astrophysics that directed the thermal history of the Universe. Amplitude of these spectral distortions are, however, at least five orders of magnitude smaller than the mean brightness of the radio background at these wavelengths. This poses the highest measurement challenges at long wavelength radio astronomy and provides the motivation for innovating unique antenna and radio telescope systems. We present a qualitative discussion on the antenna system requirements for detecting the redshifted 21 cm spectral distortions.