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APERTIF beams take shape

Submitter: Stefan Wijnholds, Wim van Cappellen
Description: A nice feature of RT5, the Westerbork dish used to test the APERTIF prototype system, is that it is part of the Westerbork array. This allows us to study the shapes of the beams produced by the phased array feed (PAF) system in great detail by correlating the PAF signals with the signals from two regular feeds. This picture shows the co- and cross-polarization amplitudes for the central compound beam produced by 56 PAF elements measured on 3C147 at 1.36 GHz. The compound beam was formed using a bi-scalar beamforming scheme. In this beamforming scheme, the 29 elements for one polarization and the 27 elements for the other polarization are beamformed separately assuming that their polarimetric responses are perfectly orthogonal. Of course, since nothing in real life is perfect, this is not the case, as can be seen in these plots.

The top row shows the response to co- and cross-polarized radio waves of the first set of elements, the bottom row shows the same results for the second set of elements. The color scales are in dB with respect to the peak of the co-polarized patterns. This shows that the cross-polarization in the field center is about -25 dB and that it peaks near the first null in the co-polarized pattern. The cloverleaf pattern is very familiar to those who have worked with the Westerbork telescope. This result is very good news for the APERTIF project, since it shows that the PAF system can produce well behaving beams on the sky with similar characteristics as the beams we have been working with for almost four decades.
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