Super Broadband Antenna Element of a Low-Frequency Radio Telescope. 1. Construction Principles

Abstract

The requirements to a super broadband antenna element of a radio telescope for frequencies lower than 200 MHz are discussed, as well as the construction principles and possibility for matching such a passive short horizontal dipole with a high frequency path of a radio telescope antenna array. It is shown that despite abrupt change of the input impedance of a dipole, it is possible to match it at high sky temperature with high frequency path if VSWR is no worse than 3 in the frequency band 12.5 ÷ 60 MHz with antenna sensitivity worsening less than by 0.5 dB in this band with respect to a half-wave lossless antenna matched with the high frequency path. This can be achieved due to an additional attenuator connected in series with the dipole input impedance that results in its active component matching within 12.5 to 60 MHz. The coaxial line lengths with the common central core are used as a dipole arm. This allows the coupling coefficient between coaxial line lengths (aperiodic circuits) in the interval [0.8 1.0] and essential rejection of the reactive component of the antenna element input impedance in the wide frequency range.