Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 69

Coupling coefficient k = f(x)

0.25

r 1
r r 3

0.2

0.15

0.1

0.05

10 − 3

1 × 0.01 0.1 1

Distance x (m)

Figure 4.10 Graph of the coupling coefficient for different sized conductor loops. Transponder
antenna: r Transp = 2 cm, reader antenna: r 1 = 10 cm, r 2 = 7 . 5cm, r 3 = 1cm

because in this case the conductor loops are in the same place and are exposed to exactly
the same magnetic flux ψ .
In practice, however, inductively coupled transponder systems operate with coupling
coefficients that may be as low as 0.01 ( < 1%) (Figure 4.10).

4.1.6 Faraday's law

Any change to the magnetic flux generates an electric field strength E i . This char-
acteristic of the magnetic field is described by Faraday's law .
The effect of the electric field generated in this manner depends upon the material
properties of the surrounding area. Figure 4.11 shows some of the possible effects
(Paul, 1993):

• Vacuum: in this case, the field strength E gives rise to an electric rotational field .

Periodic changes in magnetic flux (high frequency current in an antenna coil)
generate an electromagnetic field that propagates itself into the distance.

•

Open conductor loop: an open circuit voltage builds up across the ends of an almost
closed conductor loop, which is normally called induced voltage . This voltage
corresponds with the line integral (path integral) of the field strength E that is
generated along the path of the conductor loop in space.