I maintain a work network that at one point bridges a gap between two buildings wirelessly, while also descending from the 4th floor to ground level. The AP is at the top and connects to the internet connection, and the client bridge at ground level extends the network into the second building. My estimate of the distance from the high-up AP to the client bridge router is about 25m, with perfect uninterrupted line of sight. Please dig this fabulous illustration I have scanned:
"NO" represents units in the first building which don't need to receive any wireless from my network. I don't care if they do, but it isn't necessary to attempt to provide it to them. The little joystick-looking things are the current positions of the two wifi routers described in this question.
They both run dd-wrt and while the upstairs AP is responsible for both providing wireless to the unit it is in and also providing it to the client bridge downstairs, the downstairs client bridge immediately forwards the connection to a wired lan so it has no rebroadcasting responsibilities; it only has to maintain a good wireless connection with the AP upstairs. Therefore the upstairs AP has two antennae, one oriented towards the interior of the unit it is in and one oriented towards the downstairs client bridge across the gap. Both antennae are just the little standard antennae that routers ship with. The client bridge downstairs has a single (cheap, waterlogged and in need of replacement) outdoors antenna of the patch form factor with its flat surface approximately oriented towards the upstairs AP.
Currently, the network link between the upstairs AP and the downstairs client bridge maxes out at around 18Mbit/s on a good day. It seems to me that with the correct antennae and orientation they could do 54, especially if I bring the second AP antenna outside and use antennae with a little more directionality.
What is the optimal antenna form factor for creating this 25m line-of-sight link, without overkill? Please include details such as how to know if a particular model is sufficiently weatherproofed since they both should be outdoor antennae. Please be very specific, if you don't mind.