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.