This document describes how the Knogo RF system is suspected to work. (clothing stores and such) This information hasn't been confirmed, however careful analysis of signals in the area of these systems gives a reasonable indication as to how they work. This data is provided for informational purposes only, and no guarantee is made to the accuracy of the contained information.
A careful examination of the tags in this system reveal that they are basically an antenna frame with a diode strategically mounted on it. RF analysis of a tag reveals that it acts as a frequency multiplier, generating a 1830 Mhz peak when driven with a 915 Mhz signal. This is somewhat confirmed by showing that frequency multiplier circuitry that is active in the 900 Mhz band will trigger this type of alarm system.
This system can be made very inconspicuous due to the range of the radio waves. Design of the tags (diode in parallel with coil) renders tags very immune to Electrostatic Discharge damage. Since this system relies on microwave devices, there is a fairly good resistance to false triggering, although some cheap RF circuitry can generate false signals. Tags are reasonably small and light, reducing the hassle to customers.
The tags rely on driving the diode into conduction, causing some possible problems in situations where the field is reduced, causing possible problems with pick rates. Tags use packaged components (small glass encapsulated diode) which can appreciably increase cost.