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

TL;DR

This paper introduces a novel sparse array design that significantly reduces focal shift and improves focusing accuracy in near-field applications, enabling stable beam scanning with uniform energy distribution.

Contribution

It proposes an optimized sparse array configuration that mitigates focal shift and enhances focusing stability during beam scanning, validated through simulations.

Findings

Reduced focal shift with optimized sparse arrays

More uniform energy distribution at focal points

Accurate and stable focusing during scanning

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.