The following instructions will increase the range of an Apple airport base station 30 to 100% depending on the materials in your home and placement, etc. I added a Lucent full wave 2.4GHz antenna (lucent #848072633). This also improves speed on clients that have to downshift in order to get a better connection. Begin by removing the three screws in the bottom cover. Open it up and see the image below (Note my wire routing for reassembly.)
Remove the three yellow screws in the EMI shield (above) and carefully lift the electronics package out (below). Do not open the EMI cage.
On the end of the airport card (which looks remarkably like a Lucent WaveLAN Silver card, exactly like, one might say) there is a small round black plastic plug covering the SMB connector which mates with the antenna (below). Connect the Lucent antenna here after routing the coax cable through the frame. The black box on the end of the card contains a 1/4th wavelength 2.4 GHz antenna.
As shown below, you will need to cut a small notch in the plastic case to allow the wire out. I used a small rat tail file. Be careful not to scratch the paint on the inside of the plastic or it'll show in the outside. Try not to snort any of the plastic shavings.
Carefully reassemble the unit and route the coaxial antenna wire. Make sure to reconnect any cables you disconnected. Place the antenna as high as you can. Mine is next to the ceiling. You are finished!