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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.

The measurement of the radio background at long wavelengths has received a great deal of attention in observational cosmology over past decade. The Cosmic Microwave Background (CMB) has imprinted on it the history of the cosmological evolution of the primordial neutral hydrogen. The interaction of the CMB and the neutral hydrogen resulted in 21 cm spin flip transitions that has manifested itself in the form of faint spectral signature and are now redshifted to long radio wavelengths. A successful detection of these signatures requires extremely wideband measurements with a 1:10⁁5 accuracy that could be accomplished with smart system engineering and accurate theoretical description of the system that is connected to its electrical performance. We herein present the measurement technique and the result of the radio background measurements by a single element experiment “Shaped Antenna measurements of the background RAdio Spectrum” (SARAS). We also present the design and development of a multi element experiment “Hydrogen Probe of the Epoch of REIONization” (HYPERION) for precision measurements of the radio background at long wavelengths.