Wideband array developments for planned and future radio astronomy antennas

The radio astronomy community is engaged in the development of the Square Kilometre Array (SKA). The SKA will have 10-100 times improved sensitivity and 10⁵ times improved survey speed compared to existing radio telescopes. Besides dishes, Aperture Arrays (AA) are a promising concept: they potentially offer a very large field of view in theory only limited by the computing power and they hold promise for multioctave frequency coverage. Provided that e.g. polarimetric and calibration issues can be solved, this approach is the preferred solution for low frequency astronomy, with the SKA AA-low system, specified to run from 70 to 450 MHz, and the AA-mid system designed for 400-1420 MHz. This contribution will address some key issues to be solved using input from demonstrators.