A wideband sparse fractal array antenna for low frequency radio astronomy

We discuss the use of a sparse fractal phased array antenna to be used in an aperture synthesis array with over forty of such antenna stations. The multi-beam phased array approach offers a solution that potentially allows effective suppression of man-made signals from the received sky noise as to detect astronomical objects that are even 70 dB weaker. Key element is an active short dipole above a ground plane of which the effective collecting area is about a quarter of the wavelength squared. This implies that the maximum collecting area of a sparse array antenna is reached for a frequency where the average separation between the short dipoles is about half a wavelength. We propose a fractalised ring structure that can by appropriate tapering maintain an almost constant beam-width over more than a decade bandwidth and still have about 50% aperture efficiency. To reduce the grating lobes in a basically "regular" structure, some randomisation in receptor positions is proposed. It is shown that adaptive nulling with subspace techniques hardly influences the shape of the array\´s main beam even in case the null is created in a residual grating lobe. The Netherlands Foundation for Research in Astronomy, in close cooperation with the Naval Research Labs in the United States, have established a joint project called LOFAR, an acronym for Low Frequency Array, to realise this novel instrument.