An Approximation of the Range Resolution in the Near-Field of a Distributed Antenna Array

Abstract

This letter presents a simple formula that approximates the range (or depth) resolution of a focused beam in the near-field of a narrowband antenna array. The formula depends only on the wavelength and the span of aspect angles across the array. A second simpler formula is then derived which directly relates the radial resolution with the Fraunhofer distance. This formula suggests that the Fraunhofer distance overestimates the near-field region. In particular, it suggests that the actual near-field, when measured in terms of range resolution, is proportional to the square root of the Fraunhofer distance.