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Aperture Arrays for the SKA: Dense or Sparse?

We briefly consider some design aspects of aperture arrays for use in radio astronomy, particularly contrasting the performance of dense and sparse aperture arrays. Recent insights have emerged in the final design phase of LOFAR which suggest that sparse aperture arrays have the best prospects for cost-effective performance at radio frequencies below about 500 MHz; exceeding those of both dense aperture arrays and parabolic reflectors by an order of magnitude. Very attractive performance, of 10,000 - 20,000 m2/K, can be achieved with a sparse design that covers the 70 - 700 MHz range with two antenna systems that share receiver resources. Cost-effective systems of this type represent only a modest increment in system complexity over that being deployed in LOFAR and are achievable with today's technology.