vap11n mount an external antenna how?
Those who know please tell.
Find an old satellite dish and mount the whole unit, unmodified, at the focal point.
There's no easy way to tap a coaxial cable into the RF signal on that board, nor would it be very effective since it is just the SoC "barefoot" with no amplifier chips. It's not meant for high performance.
What are you trying to do? Better overall signal or "beaming" a signal to/from one place?
to catch a good signal from 300m, line of sight
Definitely a dish then. Though there are purpose built setups that are not very expensive and likely to outperform anything you'd modify based around that little low-power AP.
Watch out for interference when you try to use 2.4 GHz outdoors.