I found  this until I was looking for a way to increase Wi-Fi signal of my wireless router. It is called Windsurfer  and it is very easy to make. I suggest you to try it before buying professional antenna. All you need is :

1. Picture of the windsurfer diagram here: Winsurfer diagram or this file
2. Print it on paper
3. Cut it base on the line of the diagram
4. Put aluminium/tin foil on the paper reflector

Here are some videos :


Windsurfer on Linksys