  The lowest frequency band (70 - 450 MHz) of the Square Kilometre Array will
consist of sparse aperture arrays grouped into geographically-localised
patches, or stations. Signals from thousands of antennas in each station will
be beamformed to produce station beams which form the inputs for the central
correlator. Two-stage beamforming within stations can reduce SKA-low signal
processing load and costs, but has not been previously explored for the
irregular station layouts now favoured in radio astronomy arrays. This paper
illustrates the effects of two-stage beamforming on sidelobes and effective
area, for two representative station layouts (regular and irregular gridded
tile on an irregular station). The performance is compared with a single-stage,
irregular station. The inner sidelobe levels do not change significantly
between layouts, but the more distant sidelobes are affected by the tile
layouts; regular tile creates diffuse, but regular, grating lobes. With very
sparse arrays, the station effective area is similar between layouts. At lower
frequencies, the regular tile significantly reduces effective area, hence
sensitivity. The effective area is highest for a two-stage irregular station,
but it requires a larger station extent than the other two layouts. Although
there are cost benefits for stations with two-stage beamforming, we conclude
that more accurate station modelling, and SKA-low configuration specifications,
are required before design finalisation.
