Broad-band, high-gain, low-frequency antennas for radio detection of earth-skimming tau neutrinos

A promising approach to detect high-energy tau neutrinos is through the measurement of
impulsive radio emission from horizontal air showers initiated in the Earth’s atmosphere. Observations
at frequencies between 30 and 80 MHz seem particularly promising — if high-gain antennas focused
at the horizon and blocking out as much as possible of the noisy sky are employed. Due to the large
wavelengths, however, designing an antenna with the required properties is highly non-trivial at such
low frequencies. In this article, we explore suitable antenna designs that provide the desired high
gain, possess a smooth beam, are insensitive to ground conditions, are easily impedance-matched
over the wide band, and are mechanically simple for deployment in large numbers in inaccessible
terrain. In particular, we consider the “rhombus” antenna design for both horizontally and vertically
polarized radiation a very attractive option for tau neutrino detection efforts in the atmosphere
with the radio technique.