How to design backsactter modulation in RFID...

http://www.rfid-handbook.de/download...-testcard.html
http://www.rfid-handbook.de/rfid/measurements.html
http://www.rfid-handbook.de/downloads/index.html

Just put a transistor between the two antenna ports and short them to change the impedance

Very simple to do, you'll just have to be careful when selecting components or laying out the transistor