  The spectral smoothness properties of the low-frequency array of the Square
Kilometer Array (SKA), namely SKA-Low, are an important issue for its
scientific objectives to be attainable. A large array of 256 log-periodic
dipole antennas, installed on top of a 42~m circular ground plane, will work as
an SKA-Low station in the frequency range 50-350 MHz. In this article, the
ground plane induced effects are examined in terms of antenna beam spectral
characteristics, while different antenna placements are considered. Results are
produced both at isolated antenna and at array level in the band 50-100 MHz, by
employing an approximate method for the speeding-up of array simulations. We
attempt to distinguish the ground plane effect from that of mutual coupling
among antennas, which appears to be more severe at specific frequencies, using
2 figures of merit. The Discrete Fourier Transform (DFT) components of gain
pattern ratios identify the fundamental spatial components of the ripple, while
the Envelope Correlation Coefficient quantifies the penalty to considering an
infinite ground plane.
