RFID Detector for 13.56 MHz

As we all know, checkout gates in highstreet stores will faithfully sound an alarm if you walk out with an item not ‘cleared’ by checkout staff, i.e., paid for. This article describes a sensitive detector that will equally faithfully produce a sound in response to pulses picked up from an 13.56 MHz RFID tag transmitter, be it large or small, ‘portal’ or ‘portable’.
The system of passive RFID recognition has been in use for decades already mainly in large department and fashion stores like C&A. Traditionally, these systems work at frequencies designated for ISM (industrial, scientific, medical) use.