Source: http://lifehacker.com/5964111/diy-wi+fi-antenna-cheaply-extends-your-wireless-network
DIYer Danilo wanted to share his internet access across multiple apartments, and traditional methods just weren't doing the trick. He wanted to try out a bigger antenna, but before buying one, cooked up a DIY option himself to see how well it worked. All i took was a bit of copper wiring, some aluminum foil, and some tupperware (to keep it protected from the elements, since it sat outside). The result was so good that he didn't even need to buy an antenna. You'll need to be able to solder, but if that's already a skill you have under your belt, this'll probably be a quick and easy project. Hit the link to see more.
Direttiva biquad per i 2,4Ghz (Translated) | Danilo Larizza via Hack a Day