How a passive tag wakes up
A passive radio-frequency identification (RFID) tag has no battery. Instead, its antenna captures a small amount of energy from a nearby reader’s radio signal. The tag’s chip converts that energy into enough electrical power to operate briefly—just long enough to respond.
The response is not usually a miniature radio broadcast. The chip changes how the antenna reflects the reader’s signal, a technique called backscatter. By switching that reflection in a pattern, the tag encodes its stored identifier. The reader detects those changes and decodes the number.
Why the reader matters
The reader supplies both the energy and the signal the tag uses to answer, so passive tags work only within range of a compatible reader. That range depends on factors such as the radio system, antenna design, and surroundings; metal and liquids can interfere with the signal.
Because the tag does not need a battery, it can be small, inexpensive, and usable for years without a power source to replace. The tradeoff is that it cannot report on its own: until a reader energizes it, the tag is effectively dormant. A scanner passing nearby briefly turns that dormant label into a readable one.
