Fundamental Operating Principles
This chapter describes the basic interaction between transponder and reader, in particular the power supply to the transponder and the data transfer between transponder and reader. For a more in-depth description of the physical interactions and mathematical models relating to inductive coupling or backscatter systems please refer to Chapter 4.

1-Bit Transponder
A bit is the smallest unit of information that can be represented and has only two states: 1 and 0. This means that only two states can be represented by systems based upon a 1-bit transponder: ‘transponder in interrogation zone’ and ‘no transponder in interrogation zone’. Despite this limitation, 1-bit transponders are very widespread – their main field of application is in electronic anti-theft devices in shops (EAS , electronic article surveillance).

An EAS system is made up of the following components: the antenna of a ‘reader’ or interrogator, the security element or tag, and an optional deactivation device for deactivating the tag after payment. In modern systems deactivation takes place when the price code is registered at the till. Some systems also incorporate an activator, which is used to reactivate the security element after deactivation (Gillert, 1997). The main performance characteristic for all systems is the recognition or detection rate in relation to the gate width (maximum distance between transponder and interrogator antenna).

The procedure for the inspection and testing of installed article surveillance systems is specified in the guideline VDI 4470 entitled ‘Anti-theft systems for goods – detection gates. Inspection guidelines for customers’. This guideline contains definitions and testing procedures for the calculation of the detection rate and false alarm ratio. It can be used by the retail trade as the basis for sales contracts or for monitoring the performance of installed systems on an ongoing basis. For the product manufacturer, the ’Inspection guidelines for customers’ represents an effective benchmark in the development and optimisation of integrated solutions for security projects (in accordance with VDI 4470).

Radio Frequency
The radio frequency (RF) procedure is based upon LC resonant circuits adjusted to a defined resonant frequency fR. Early versions employed inductive resistors made of wound enamelled copper wire with a soldered on capacitor in a plastic housing (hard tag). Modern systems employ coils etched between foils in the form of stick-on labels. To ensure that the damping resistance does not become too high and reduce the quality of the resonant circuit to an unacceptable level, the thickness of the aluminium conduction tracks on the 25-µm-thick polyethylene foil must be at least 50 µm (J¨orn, 1994). Intermediate foils of 10 µm thickness are used to manufacture the capacitor plates.

The reader (detector) generates a magnetic alternating field in the radio frequency range. If the LC resonant circuit is moved into the vicinity of the magnetic alternating field, energy from the alternating field can be induced in the resonant circuit via its coils (Faraday’s law). If the frequency fG of the alternating field corresponds with the resonant frequency fR of the LC resonant circuit the resonant circuit produces a sympathetic oscillation. The current that flows in the resonant circuit as a result of this acts against its cause, i.e. it acts against the external magnetic alternating field (see Section 4.1.10.1). This effect is noticeable as a result of a small change in the voltage drop across the transmitter’s generator coil and ultimately leads to a weakening of the measurable magnetic field strength. A change to the induced voltage can also be detected in an optional sensor coil as soon as a resonant oscillating circuit is brought into the magnetic field of the generator coil.

The relative magnitude of this dip is dependent upon the gap between the two coils (generator coil – security element, security element – sensor coil) and the quality Q of the induced resonant circuit (in the security element).

The relative magnitude of the changes in voltage at the generator and sensor coils is generally very low and thus difficult to detect. However, the signal should be as clear as possible so that the security element can be reliably detected. This is achieved using a bit of a trick: the frequency of the magnetic field generated is not constant, it is ‘swept’. This means that the generator frequency continuously crosses the range between minimum and maximum. The frequency range available to the swept systems is 8.2MHz ± 10% (J¨orn, 1994).

Whenever the swept generator frequency exactly corresponds with the resonant frequency of the resonant circuit (in the transponder), the transponder begins to oscillate, producing a clear dip in the voltages at the generator and sensor coils (Figure 3.3). Frequency tolerances of the security element, which depend upon manufacturing tolerances and vary in the presence of a metallic environment, no longer play a role as a result of the ‘scanning’ of the entire frequency range.

Because the tags are not removed at the till, they must be altered so that they do not activate the anti-theft system. To achieve this, the cashier places the protected product into a device – the deactivator – that generates a sufficiently high magnetic field that the induced voltage destroys the foil capacitor of the transponder. The capacitors are designed with intentional short-circuit points, so-called dimples. The breakdown of the capacitors is irreversible and detunes the resonant circuit to such a degree that this can no longer be excited by the sweep signal.

Large-area frame antennas are used to generate the required magnetic alternating field in the detection area. The frame antennas are integrated into columns and combined to form gates. The classic design that can be seen in every large department store is illustrated in Figure 3.4. Gate widths of up to 2 m can be achieved using the RF procedure. The relatively low detection rate of 70% (Gillert, 1997) is disproportionately influenced by certain product materials. Metals in particular (e.g. food tins) affect the resonant frequency of the tags and the coupling to the detector coil and thus have a negative effect on the detection rate. Tags of 50 × 50 mm must be used to achieve the gate width and detection rate mentioned above.

The range of products that have their own resonant frequencies (e.g. cable drums) presents a great challenge for system manufacturers. If these resonant frequencies lie within the sweep frequency 8.2MHz ± 10% they will always trigger false alarms.