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Elliptical LWA Station Configurations Optimized to Minimize Side Lobes.

An elliptical station, elongated in the north-south axis to compensate for projected foreshortening for sources in the far north and far south, may be desirable to produce a more circular field of view over as much of the sky as possible (Clarke 2007). Here, we present elliptical station configurations that are each optimized to minimize sidelobes across the entire sky. Each design is characterized by the minimum allowed spacing between elements, and we used values of 3.5, 4.0 and 5.0 meters. Because the final size of the station is still not settled, we consider three possibilities for the major axis (100, 110 and 120m). With three possible sizes and three possible minimum element spacings, a total of nine potential station configurations are presented. Various properties of each design are examined including maximum sidelobe levels and phased-beam axis ratios. We find that the maximum side lobe increases with increased minimum spacings, while the effects of station size depend on the minimum dipole spacing. This side-lobe optimization does not take into account the effects of mutual coupling, which is far too computationally intensive to be feasible in the context of an iterative optimization scheme. 1. Overview A circular design for the perimeter of an LWA station would produce a circular station beam at zenith, but off-zenith would be elongated in the azimuthal direction by a factor of 1/cos(ZA) where ZA is the zenith angle. The elongation is minor at low zenith angles, but close to the horizon it can be quite large. The regions of the sky where it is most critical to observe near the horizon are towards the north and south. The north because sources near the celestial north pole never “leave” that region from our vantage point. The south because sources at far southern declinations only reach a high enough elevation above the horizon when they are near transit, which is always toward the south from the LWA site. Any source near the horizon at the east or west will reach a much higher elevation at some point in the day. For this reason, we concentrate on the station beam shape along the north-south azimuth or, in other words, as a function of source declination when a source is at its transit. In Figure 1 we plot the phased-beam axis ratio (the ratio of the major axis to the minor axis of the station beam ellipse) at transit for a circular station as a function of source declination for the range of