Whole-aperture Fraunhofer distance of a coherent array ($R\gg b^2/\lambda$)

The relevant aperture size $b$ spans the entire coherent array, including the separated elements, rather than one element or a single baseline increment. A quadratic propagation-phase error is small only when $b^2/(\lambda R)\ll 1$ under the paraxial approximation. The often-used boundary $2b^2/\lambda$ is a convention for an allowed phase error, not a sharp physical transition. Individual-element patterns still supply an angular envelope in the far field.