Spectral performance of Square Kilometre Array Antennas – II. Calibration performance

Abstract

We test the bandpass smoothness performance of two prototype Square Kilometre Array (SKA) SKA1-Low log-periodic dipole antennas, the SKALA2 and SKALA3 (`SKA Log-periodic Antenna'), and the current dipole from the Murchison Widefield Array (MWA) precursor telescope. Throughout this paper, we refer to the output complex-valued voltage response of an antenna when connected to a low noise amplifier (LNA), as the dipole bandpass. In Paper I (de Lera Acedo et al. 2017), the bandpass spectral response of the log-periodic antenna being developed for the SKA1-Low was estimated using numerical electromagnetic simulations and analyzed using low-order polynomial fittings and it was compared with the HERA antenna against the delay spectrum metric. In this work, realistic simulations of the SKA1-Low instrument, including frequency-dependent primary beams and array configuration, are used with a weighted least-squares polynomial estimator to assess the ability of prototype antennas to perform the SKA Epoch of Reionisation (EoR) statistical experiments. This work complements the ideal estimator tolerances computed for the proposed EoR science experiments in Trott & Wayth (2016), with the realised performance of an optimal and standard estimation (calibration) procedure. With a sufficient sky calibration model at higher frequencies, all antennas have bandpasses that are sufficiently smooth to meet the tolerances described in Trott & Wayth (2016) to perform the EoR statistical experiments, and these are primarily limited by an adequate sky calibration model, and the thermal noise level in the calibration data. At frequencies of the Cosmic Dawn (CD), which is of principal interest to SKA as one of the first next-generation telescopes capable of accessing higher redshifts, the MWA dipole and SKALA3 antenna have adequate performance, while the SKALA2 design will impede the ability to explore this era.