Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 70

Flux change d Φ /dt

Conductor (e.g. metal surface)

Eddy current,
Current density S

Open conductor loop

U i

Nonconductor (vacuum),
Induced field strength E i
=> Electromagnetic wave

Figure 4.11 Induced electric field strength E in different materials. From top to bottom: metal
surface, conductor loop and vacuum

• Metal surface: an electric field strength E is also induced in the metal surface. This

causes free charge carriers to flow in the direction of the electric field strength.
Currents flowing in circles are created, so-called eddy currents . This works against
the exciting magnetic flux (Lenz's law), which may significantly damp the mag-
netic flux in the vicinity of metal surfaces . However, this effect is undesirable in
inductively coupled RFID systems (installation of a transponder or reader antenna
on a metal surface) and must therefore be prevented by suitable countermeasures
(see Section 4.1.12.3).

In its general form Faraday's law is written as follows:

d (t)
d t

u i = E i · d s =− ( 4 . 21 )

For a conductor loop configuration with N windings, we can also say that u i = N ·
d / d t . (The value of the contour integral

d s can be increased N times if the
closed integration path is carried out N times; Paul, 1993).
To improve our understanding of inductively coupled RFID systems we will now
consider the effect of inductance on magnetically coupled conduction loops.
A time variant current i 1 ( t ) in conduction loop L 1 generates a time variant magnetic
flux d (i 1 )/ d t . In accordance with the inductance law, a voltage is induced in the
conductor loops L 1 and L 2 through which some degree of magnetic flux is flowing.
We can differentiate between two cases:

∫ E i ·

• Self-inductance : the flux change generated by the current change d i n / d t induces a

voltage u n in the same conductor circuit.

• Mutual inductance : the flux change generated by the current change d i n / d t induces

a voltage in the adjacent conductor circuit L m . Both circuits are coupled by
mutual inductance.

Figure 4.12 shows the equivalent circuit diagram for coupled conductor loops. In an
inductively coupled RFID system L 1 would be the transmitter antenna of the reader.