Frequency Ranges and Radio Licensing Regulations - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 164

5.1.12 Selection of a suitable frequency for inductively

coupled RFID systems

The characteristics of the few available frequency ranges should be taken into account
when selecting a frequency for an inductively coupled RFID system. The usable field
strength in the operating range of the planned system exerts a decisive influence on
system parameters. This variable therefore deserves further consideration. In addition,
the bandwidth (mechanical) dimensions of the antenna coil and the availability of the
frequency band should also be considered.
The path of field strength of a magnetic field in the near and far field was described
in detail in Section 4.2.1.1. We learned that the reduction in field strength with
increasing distance from the antenna was 60 dB/decade initially, but that this falls to
20 dB/decade after the transition to the far field at a distance of λ /2 π . This behaviour
exerts a strong influence on the usable field strengths in the system's operating range.
Regardless of the operating frequency used, the regulation EN 300 330 specifies the
maximum magnetic field strength at a distance of 10m from a reader (Figure 5.3).
If we move from this point in the direction of the reader, then, depending upon the
wavelength, the field strength increases initially at 20 dB/decade. At an operating fre-
quency of 6.78MHz the field strength begins to increase by 60 dB/decade at a distance
of 7.1m — the transition into the near field. However, at an operating frequency of
27.125MHz this steep increase does not begin until a distance of 1.7m is reached.

Magnetic field strength = f(f)

200

125 kHz H1 (x)

H2 (x)
H3 (x)

6.75 MHz

150

27.125 MHz

100

65 dB A/m

@ 125 kHz

µ

50

42 dB µ A/m @6.78, 13.56, 27.125 MHz

0

− 5 0.01

0.1 1

Distance (m)

10 100

Figure 5.3 Different permissible field strengths for inductively coupled systems measured at
a distance of 10m (the distance specified for licensing procedures) and the difference in the
distance at which the reduction occurs at the transition between near and far field lead to marked
differences in field strength at a distance of 1m from the antenna of the reader. For the field
strength path at a distance under 10 cm, we have assumed that the antenna radius is the same
for all antennas