Radio frequency RF resonant systems have been employed for many years for anti-pilferage purposes. Such systems are commonly referred to as electronic article surveillance or EAS systems. The most popular type of EAS system which is typical of the industry is the swept RF system, consisting of a transmitter and receiver and their associated antennas. The swept system uses a transmitter that is FM modulated, typically with a low frequency sinusoidal sweep signal, and that transmits continuously. The receiver, spaced some distance away, continuously monitors the swept signal that is radiated within the detection zone. A resonant circuit on a tag or the like introduced into the detection zone produces a periodic (synchronous with the sweep frequency) distortion that is detected by the receiver and triggers an alarm. These systems are also referred to as field-disturbance sensors since the resonant circuit disturbs the field within the detection zone.
Another type of EAS system, the pulsed system, typically places the transmitter and receiver into a single antenna structure. The transmitter and receiver may be connected to their own loop antennas within the structure, or they may share a single antenna. In both cases, the antennas can be configured as single or multiple loop, depending upon the specific application. In a pulsed system, the transmitter is not typically FM modulated, nor does it transmit continuously. The transmitter is only on for a very short duration (pulsed) at a periodic rate and a relatively high peak intensity. The receiver looks for any RF energy within the detection zone immediately after each transmitted pulse. This RF energy should be present only if there was a resonant circuit within the detection zone that was excited by the transmitted RF pulse.
The most popular pulsed systems were originally designed to work primarily with reusable "hard tag" RF resonant circuits also known as "targets" or "tags". These targets are made of a coil of wire and a fixed capacitor, encapsulated typically into a molded plastic package. Although popular in certain applications, the "hard tag" is not the primary target used by retailers today.
The most common target today is the printed and etched RF label. Pulsed systems that are designed to operate using the etched label present a new set of challenges that must be overcome. The pulsed system relies upon the bandwidth of the transmitted RF energy pulse to be able to excite any targets produced within their manufacturing tolerances. While the hard tag has a typical frequency tolerance of 2-4% in production, the printed and etched labels may vary in frequency up to 5-7%. Additionally, the printed labels are smaller in size and have a lower "Q" factor than the hard tag. Since the size (bigger is better) and the "Q" factor (higher is better) affects the amount of energy that the target acquires during the transmitted RF energy pulse, overall system performance will suffer significantly when merely substituting the printed label for the hard tag.
The easiest and least expensive method to recover the performance level when using printed labels is to 1) widen the pulsed transmitter spectrum bandwidth by narrowing the transmitted pulse on-time to cover the increased frequency tolerances of the printed labels, and 2) compensate for the lost tag energy due to the decrease in size and "Q" by increasing the transmitted pulse intensity or amplitude.
As if these challenges aren't enough, a major problem can result from using a pulsed system with a deactivatible printed label. Most of the printed and etched tags produced today are processed to be deactivatible. The most common method of deactivating printed labels is by shorting its internal capacitor plates using a relatively low intensity RF energy field. The energy emitted from the antenna of a pulsed system could deactivate deactivatable printed labels up to 10 inches from the antenna. This could seriously impair the merchant from confirming a shoplifting event.
The only time that the EAS system alarm should sound is when a "live" label passes through the detection zone. Normally labels are disabled or "deactivated" at the time of purchase utilizing a separate deactivating device located adjacent the cash register. Should the EAS alarm sound, this is an indication that there is a live tag, either because the patron is shoplifting or the tag was not properly deactivated. Occasionally, the EAS system may also generate a "false" alarm even though the tag has been properly deactivated. A common procedure when an alarm sounds is to ask the suspected shoplifter to walk back through the system. If he has a tagged article that was not purchased legitimately, the alarm should sound again, and the merchant can take whatever action he deems necessary. If, however, the label was deactivated by the transmitter itself shortly after creating the alarm, the merchant can not confirm the event, and the shoplifter would go free.
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