A passive tag is an RFID tag that does not have its own power supply. Instead, it uses RF energy from the RFID reader for the power needed to reply to the reader. Due to the lower power, passive tags have shorter ranges than other tags, but are generally smaller, lighter, and cheaper than other tags.
An RFID reader must first query a passive tag, sending electromagnetic waves that form a magnetic field when they "couple" with the antenna on the RFID tag. Consistent with any applicable authorization, authentication, and encryption, the tag will then respond to the RFID reader, sending via radio waves the data stored on it.
The reply signal from a passive tag, which is also known as the backscattered signal, has only a fraction of the power of the reader's signal. This limited power significantly restricts the operating range of the passive tag. It also means that passive tags can only support data processing of limited complexity. On the other hand, passive tags typically are cheaper, smaller, and lighter than other types of RFID tags, which are advantages for many RFID applications.
- Guidelines for Securing Radio Frequency Identification (RFID) Systems, Glossary, at B-2.
- "Overview" section: Radio Frequency Identification: Applications and Implications for Consumers, at 6.