= Whole-aperture Fraunhofer distance of a coherent array
{title2=$R\gg b^2/\lambda$}
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 $b^2/(\lambda R)\ll1$ 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.
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