MMA Memo 260 A Comparison of Zoom Arrays with Circular and Spiral Symmetry

There are advantages in making the intermediate arrays of the MMA/LSA which ll the 3km plain so-called 'zoom' arrays. In such arrays the antenna pads are arranged according to a scale-free self-similar distribution. By moving antennas from inner pads to outer ones the array gradually expands keeping a self-similar uv coverage. So far two types of zoom array have been proposed, one based on a de nite shape, a three-armed logarithmic spiral (Conway 1998), and another assuming a circularly symmetric inverse-square law density of pads (Webster 1998). In this memo we make a rst comparison of these two geometries. In the arrays tested we nd small but signi cant advantages of the spiral geometry over the circularly symmetric ones; both in terms of uv coverage and how well the geometry stays self-similar as the array zooms. Although better untested circularly symmetric con gurations may exist the advantages we see for the spiral appear to be intrinsic; coming fundamentally from its three-fold symmetry and the 'open' nature of this pattern. Despite these small di erences we argue that both circular and spiral zoom arrays have better capabilities than the series of set array con gurations that have dominated array con guration planning up to now.