What is RFID?

You probably have been using a radio frequency identification device for years, in the form of a card you press against a box to unlock a door. The box emits an RF signal, generally in the 50-kilohertz to 2.5-gigahertz frequency range, that creates an electromagnetic field spanning a few inches. When you place your card nearby, the field induces a current in an antenna that's printed inside the card; the current powers up a microchip containing a few bits of data—in this case, probably just your name and ID number. The antenna returns a modulated signal to the reader, which then shunts the card data to a computer that compares it with a value on file. Such a system can keep a history of when you enter the premises and limit your access to certain doors.