Radio Frequency Identification (RFID) has been around in one form or another since World War II. Although it has been used in niche industrial sectors for many years, the increasing desire for greater efficiencies in supply logistics have really pushed the development and use of this technology.

An RFID system consists of a reader and transponders. Transponders (derived from the words "transmitter" and "responder") are attached to the items to be identified. They are often called "tags".

Fig.1: a basic RFID setup cons-ists of a reader (or interrogator) and transponder. Low frequency systems rely on inductive coup-ling to provide transponder power.

Just like a bar code, a transponder tag carries data about its host. When interrogated by a reader, it responds with that data over a radio frequency link. The transponder could be really simple, like those in clothing price tags, consisting of just an antenna and diode. When irradiated, the diode rectifies the incoming carrier and the frequency-doubled signal is radiated back to the reader which responds with an alarm if you try to leave the store without paying for the product.

These days, the generic term "RFID" is used to describe an entire range of dedicated short-range communication (DSRC) systems.

This article does not attempt to describe all RFID devices and technologies. Instead, we will focus exclusively on RFIDs used in identity tagging and closely associated areas. Let's begin by dividing the subject into two broad categories: active and passive transponders.