  Sparse sensor arrays offer a cost effective alternative to uniform arrays. By
utilizing the co-array, a sparse array can match the performance of a filled
array, despite having significantly fewer sensors. However, even sparse arrays
can have many closely spaced elements, which may deteriorate the array
performance in the presence of mutual coupling. This paper proposes a novel
sparse planar array configuration with few unit inter-element spacings. This
Concentric Rectangular Array (CRA) is designed for active sensing tasks, such
as microwave or ultra-sound imaging, in which the same elements are used for
both transmission and reception. The properties of the CRA are compared to two
well-known sparse geometries: the Boundary Array and the Minimum-Redundancy
Array (MRA). Numerical searches reveal that the CRA is the MRA with the fewest
unit element displacements for certain array dimensions.
