On the Resolution of Near-Field Focused Arrays

A key design metric for near-field focused antennas is its resolution, which is the spatial extent over which the fields decay by 3 dB. While the resolution for far-field focused antennas is widely appreciated, a detailed understanding of the resolution of near-field focused antennas has not been presented. A unique requirement of near-field focused antennas is the requirement to characterize both transverse and axial resolutions. We begin by revisiting the classical scalar Fresnel-Kirchhoff’s diffraction integral and the assumptions involved in obtaining a closed-form solution of the integral. These closed-form solutions are then analyzed in depth to reveal the dependency of the resolution on the array size, focal length, and the frequency (wavelength) of operation. These observations are validated via a representative phased array consisting of infinitesimal dipoles which radiates vector fields. This simulation model serves as the basis to assess the domain of applicability of the closed-form solutions and reveals insightful dependencies of the near-field resolution on the geometry and frequency. This model is further employed to study the impact of focal-spot scanning on the resolution of near-field focused antennas.