Proximity Cards ...

4.1 INTEGRATED ELECTRONIC AND AUTOMATED FARE PAYMENT SYSTEMS

... "There are two techniques that eliminate the requirement for physical contact between card and reader. The most prevalent of these techniques uses radio frequency inductive coupling. Since RF inductive coupling does not require physical contact between circuits being coupled, but only requires that the circuits be in proximity, cards using this type of coupling are called proximity cards. This technique employs an induction coil in the card read-write unit to generate an RF magnetic field that couples to another induction coil embedded in the card. The RF magnetic field provides power to the card's circuitry, and is modulated to carry signals to the card as well. The card must be equipped with a small power conditioning system to extract needed power from the magnetic field, regulate it, and provide it to the card's circuitry as long as the card is in the vicinity of the read-write unit. The card transmits signals back to the read-write unit by means of the same coil used to receive signals, or by means of a separate coil or antenna. " ...