For the distances you are trying to attain you may want to consider a directional antenna instead of an omni directional antenna.
For a omni directional antenna think of the wifi radio waves being transmitted from the source coming out straight up in a mushroom cloud pattern.
For a directional antenna imagine concentrated wifi radio waves extending out into a baseball bat pattern.
If you aim the baseball bat (directional antenna) at the mushroom (wifi transmitting source) It will touch the source whereas another mushroom (omni directional antenna) sitting near the source will not contact it.
Hope that makes sense.
I use a Hawking HA12W
12dbi directional antenna and have no problem getting at least 50% signal from a wifi source 1/2 mile away. Of course that is with a line of sight source.