  The critical component of radio astronomy radiometers built to detect
redshifted 21-cm signals from Cosmic Dawn is the antenna element. We describe
the design and performance of an octave bandwidth cone disc antenna built to
detect this signal in the band 40 to 90 MHz. The Cosmic Dawn signal is
predicted to be a wideband spectral feature orders of magnitude weaker than sky
and ground radio brightness. Thus, the engineering challenge is to design an
antenna at low frequencies that is able to provide with high fidelity the faint
cosmological signal, along with foreground sky, to the receiver. The antenna
characteristics must not compromise detection by imprinting any confusing
spectral features on the celestial radiation, ground emission or receiver
noise. An innovation in the present design is making the antenna electrically
smaller than half wavelength and operating it on the surface of a sufficiently
large water body. The homogeneous and high permittivity medium beneath the
small cone-disc antenna results in an achromatic beam pattern, high radiation
efficiency and minimum unwanted confusing spectral features. The antenna design
was optimized in WIPL-D and FEKO. A prototype was constructed and deployed on a
lake to validate its performance with field measurements.
  Index Terms: Antenna measurements, radio astronomy, reflector antennas.
