Sparse Arrays Enable Near-Field Constant-Distance Focusing with Reduced Focal Shift

Abstract: In near-field beam focusing for finite-sized arrays, focal shift is a non-negligible issue. The actual focal point often appears closer to the array than the predefined focal distance, significantly degrading the focusing performance of finite aperture arrays. Moreover, when the focus point is scanned across different locations, the degradation becomes even more pronounced, leading not only to positional deviation but also to substantial energy loss. To address this issue, we revisit the problem from the perspective of communication degrees of freedom. We demonstrate that a properly designed sparse array with optimized element spacing can effectively mitigate focal shift while enabling stable control of the focusing height during beam scanning. Simulation results based on dipole antennas with different polarizations and patch antennas validate our findings. Notably, with optimized inter-element distances, the energy distribution across focal points becomes nearly uniform, and highly accurate focusing positions are achieved.