Passive RFID transponder timing system
The "questioning" antenna provides the power to the RFID tag that contains a circuit to provide the tag ID back.  The whole system can be thought of like a transformer.  The number of wire turns in the "questioning" antenna and the number of wire turns in the tag have to be at a particular ratio so the tag gets the proper voltage.  It is going to be a rather involved undertaking.  You also have to take frequency and power into consideration.  The little devices used to open doors with these tags require less power than something that requires reading at a distance because you usually have to "touch" these cards to the readers.

Here is some reading to get you started:
http://edn.com/design/wireless-networking/4341980/Optimizing-read-range-in-RFID-systems