Interrogation Zone of Readers

In the calculations above the implicit assumption was made of a homogeneous magnetic field H parallel to the coil axisx. A glance at Figure 4.23 shows that this only applies for an arrangement of reader coil and transponder coil with a common central axisx. If the transponder is tilted away from this central axis or displaced in the direction of theyorzaxis this condition is no longer fulfilled.

If a coil is magnetised by a magnetic fieldH, which is tilted by the angleϑ in relation to the central axis of the coil, then, in very general terms, the following applies:

u_0ϑ=u0·cos(ϑ) (4.40)

whereu0is the voltage that is induced when the coil is perpendicular to the magnetic field. At an angleϑ =90^◦, in which case the field lines run in the plane of the coil radiusR, no voltage is induced in the coil.

As a result of the bending of themagnetic field lines in the entire area around the reader coil, here too there are different anglesϑ of the magnetic fieldH in relation to the transponder coil.

This leads to a characteristicinterrogation zone(Figure 4.24, grey area) around the reader antenna.

Areas with an angle ϑ=0^◦ in relation to the transponder antenna – for example along the coil axisx, but also to the side of the antenna windings (returning field lines) – give rise to an optimal read range. Areas in which the magnetic field lines run parallel to the plane of the transponder coil radiusR – for example, exactly above and below the coil windings – exhibit a significantly reduced read range. If the transponder itself is tilted through 90^◦ a completely different picture of the interrogation zone emerges (Figure 4.24, dotted line). Field lines that run parallel to theR-plane of the reader coil now penetrate the transponder coil at an angleϑ =0^◦and thus lead to an optimal range in this area.

2R

2R^T

x Reader antenna

Transponder antenna

Figure 4.23 Cross-section through reader and transponder antennas. The transponder antenna is tilted at an angleϑ in relation to the reader antenna

Magnetic field line H

Transponder coil Transponder coil

cross-section Reading range with

parallel transponder coil Reading range with

vertical transponder coil

Reader antenna (cross-section)

Figure 4.24 Interrogation zone of a reader at different alignments of the transponder coil

Up to this point we have considered the characteristics of inductively coupled systems primarily from the point of view of the transponder. In order to analyse in more detail the interaction between transponder andreader in the system, we need to take a slightly different view and first examine the electrical properties of the reader so that we can then go on to study the system as a whole.

Figure 4.25 shows the equivalent circuit diagram for a reader (the practical realisation of this circuit configuration can be found in Section 11.4). Theconductor loopnecessary to generate the magnetic alternating field is represented by the coilL1. The series resistorR1corresponds with the ohmic losses of the wire resistance in the conductor loopL1. In order to obtain maximum current in the conductor coilL1 at the readeroperating frequency fTX, aseries resonant circuit with the resonant frequencyfRES=fTXis created by the serial connection of the capacitorC1. The resonant frequency of the series resonant circuit can be calculated very easily using the Thomson equation (4.25). The operating state of the reader can be described by:

fTX=fRES= 1 2π√

L1·C1

(4.41) Because of the series configuration, the total impedanceZ1 of the series resonant circuit is the sum of individual impedances, i.e.:

Z1=R1+j ωL1+ 1 j ωC1

(4.42) At the resonant frequencyfRES, however, the impedances ofL1andC1cancel each other out.

In this case the total impedanceZ1is determined byR1 only and thus reaches a minimum.

j ωL1+ 1 j ωC₁ =0

ω=2π·fRES

⇒Z1(fRES)=R1 (4.43)

Theantenna current i₁ reaches a maximum at the resonant frequency and is calculated (based upon the assumption of an ideal voltage source whereRi=0) from the source voltageu0 of the transmitter high level stage, and the ohmic coil resistanceR1.

u₀

Figure 4.25 Equivalent circuit diagram of a reader with antennaL1. The transmitter output branch of the reader generates the RF voltageu0. The receiver of the reader is directly connected to the antenna coilL1

0 200 400 600 800

u₁(f) uC₁(f)

Voltage step-up at series resonance

Frequencyf(Hz) VoltageuL1,uC1 / V

1× ^{107 1.1}× ^{107 1.2}× ^{107 1.3}× ^{107 1.4}× ^{107 1.5}× ^{107 1.6}× ^{107 1.7}× ¹⁰⁷

Figure 4.26 Voltage step-up at the coil and capacitor in a series resonant circuit in the frequency range 10– 17 MHz (fRES=13.56 MHz,u0=10 V(!),R1=2.5,L1=2µH,C1=68.8 pF). The voltage at the conductor coil and series capacitor reaches a maximum of above 700 V at the resonant frequency. Because the resonant frequency of the reader antenna of an inductively coupled system always corresponds to the transmission frequency of the reader, components should be sufficiently voltage resistant

i1(fres)= u0

Z1(fRES) = u0

R1

(4.44) The two voltages, u1 at the conductor loopL1, anduC1 at the capacitorC1, are in antiphase and cancel each other out at the resonant frequency because currenti1 is the same. However, the individual values may be very high. Despite the low source voltageu0, which is usually just a few volts, figures of a few hundred volts can easily be reached atL1 andC1. Designs forconductor loop antennas for high currents must therefore incorporate sufficient voltage resistance in the components used, in particular the capacitors, because otherwise these would easily be destroyed by arcing. Figure 4.26 shows an example of voltage step-up at resonance.

Despite the fact that the voltage may reach very high levels, it is completely safe to touch the voltage-carrying components of the reader antenna. Because of the additional capacitance of the hand, the series resonant circuit is rapidly detuned, thus reducing the resonance step-up of voltage.