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This paper is the first in a series of papers describing the impact ofantenna instrumental artefacts on the 21-cm cosmology experiments to be carriedout by the low frequency instrument (SKA1-LOW) of the Square Kilometre Arraytelescope (SKA), i.e., the Cosmic Dawn (CD) and the Epoch of Reionization(EoR). The smoothness of the passband response of the current log-periodicantenna being developed for the SKA1-LOW is analyzed using numericalelectromagnetic simulations. The amplitude variations over the frequency rangeare characterized using low-order polynomials defined locally, in order tostudy the impact of the passband smoothness in the instrument calibration andCD/EoR Science. A solution is offered to correct a fast ripple found at 60~MHzduring a test campaign at the SKA site at the Murchison Radio-astronomyObservatory, Western Australia in September 2015 with a minor impact on thetelescope's performance and design. A comparison with the Hydrogen Epoch ofReionization Array antenna is also shown demonstrating the potential use of theSKA1-LOW antenna for the Delay Spectrum technique to detect the EoR.

Spectral performance of SKA Log-periodic Antennas I: Mitigating spectral artefacts in SKA1-LOW 21-cm cosmology experiments