A system is disclosed for detecting the presence of an article. The system includes
a transmitter for radiating a first electromagnetic signal at a predetermined primary
frequency and a resonant tag secured to the article. The resonant tag generates
a second electromagnetic signal in response to receiving the first electromagnetic
signal. The second electromagnetic signal has components at the primary frequency
and at a predetermined secondary frequency different from the primary frequency.
The system also includes a receiver for receiving the second electromagnetic signal
and a computer connected to an output of the receiver. The computer processes the
received second electromagnetic signal and generates an output signal when the
secondary frequency is detected in the second electromagnetic signal.
