A passive tag (Figure 3-6) does not have its own power source; it has no battery
on-board. The tag obtains power fromradio waves received fromthe interrogator.
The amount of power thus received is very small, just enough to energize its
IC. Therefore, passive tag functionalities are limited. Due to a lack of enough
power, it cannot support an active transmitter to communicatewith the interrogator.
The good thing about the lack of transmitter, however, is that passive tags
do not contribute to radio noise. To communicate with the interrogator, passive
tags operating at low and high frequencies use inductive coupling, while those
operating above a high frequency range use radiative coupling. (See Chapter 2
for more about these coupling methods.) Inductively coupled tags have a read
range of a few inches to about 2 feet, while radiatively coupled tags have read
range of up to 20 feet.