Near field and Far field in EM radiation, it is generally considered that if $D \ll \lambda$, $D \ll r$ and $r \gg \lambda$, it is radiation zone ($D$ is the antenna's length). I can't seem to see how that fits the Fraunhofer distance $\sim \frac{D^2}{\lambda r}$, in which the far field is considered when $\frac{D^2}{\lambda r}\ll 1$. Seeing as $D\ll\lambda,r$, I can't seem to see how that implies $r\gg \lambda$. I hope I was clear in my confusion.
*Is there even any connection between the two? The Fraunhofer expression is obtained from diffraction.
 A: I suspect this is a matter of convention, which differs for radar engineering from that used in visible or near-visible engineering.
The Near/Far labels for visible frequencies are based on the Fraunhofer vs. Fresnell approximations being a good match to the exact solution.
But radar folks tend to call it "Far Field" as soon as the wavefront curvature becomes inconsequential to the system in question.  This is rather a different distance from the distance where Fraunhofer equations take over.
A: If you assume that the characteristic size of the source is $D$ and $D\ll \lambda$ then you are immediately assuming an essentially "point-like" source whose radiation is very broad. In this case the Rayleigh formula $r_{far} \approx \frac{2D^2}{\lambda}$ for the Fraunhofer region is not very useful for it also says that $r_{far} \ll \lambda$  and a wavelength away you are already "there", which is what you would expect for a point source with its almost spherical wave emissions anyhow.
The Rayleigh formula is useful when $\lambda \le D$ in which case there is some measurable difference between $\lambda$ and $r_{far}$ and it is applicable when across the aperture of the source field defined by the characteristic size $D$ not only the amplitude variation is small but also the phase variation of the incident wave is smooth (essentially linear, so it looks like if it is a directional wavefront) as it passes through the aperture. The best example for this is a waveguide feeding a large and long horn or a narrow paraxial collimated beam is incident onto a lens. The resulting diffraction field on the other side is the antenna radiation pattern and far away (Rayleigh distance - Fraunhofer region) it is the Fourier Transform of the aperture field.
The restriction on the smoothness of the aperture phase excludes super-directivity from this analysis.
