I have added a high gain WiFi antenna but my WiFi datarate has not improved?

This has come up quite frequently:

I have added a high gain WiFi antenna but my WiFi datarate has not improved?

The answer is simply a high gain 2.4GHz antenna is not necessarily the best solution for a weak received signal strength solution especially in an in-building application. A few reasons include:
(i) Multipath fading
(ii) FCC limits on ISM band radiated power
(iii) Omnidirectional vs Directional high gain antennas

What we have found to be the best (in terms of price/performance) is to daisy-chain routers. The beauty of this is that we can not only setup the second router to operate in a non-overlapping frequency spectrum (just a reminder there are 3 non-overlapping spectrums in the U.S channel 1, 6 and 11) but we can also isolate collision domains.

Just a quick reminder: WiFi media access method is CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance what this means is that if mobile station detects that the medium/air-interface is in use it defers transmission to avoid a collision. This deference is manifested as a drop in datarate. By having multiple non-overlapping frequency channels the possible of collision hence the probability of deference is reduced. The other advantage has to do with the fact that we actually create a second network and the second router actual "routes" network traffic independent of the first (oversimplified description )

See High Gain WiFi Antennas and WiFi Repeaters/ Amplifiers