Tanais writes "This is a follow up to an article I did shoehorning an ordinary Lucent Card into an iBook's Airport slot -- that was published in the Make: blog here This time I decided to use the same Lucent card but with a pair of DIY "coil" antennae to boost the signal of the Airport enough to reach me through two stone walls of my home." - Link.
http://blog.makezine.com/archive/2006/06/airport_range_extending_p.html