Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 106

B

Saturation

B

B r

Virgin curve

A

H c

H

Figure 4.50 Typical magnetisation or hysteresis curve for a ferromagnetic material

( − H ) does the flux density B fall further and finally return to zero. The field strength
necessary for this is termed the coercive field strength H C .
Ferrite is the main material used in high frequency technology. This is used in the
form of soft magnetic ceramic materials (low B r ), composed mainly of mixed crystals
or compounds of iron oxide (Fe 2 O 3 ) with one or more oxides of bivalent metals (NiO,
ZnO, MnO etc.) (Vogt. Elektronik, 1990). The manufacturing process is similar to that
for ceramic technologies (sintering).
The main characteristic of ferrite is its high specific electrical resistance, which
varies between 1 and 10 6 m depending upon the material type, compared to the range
for metals, which vary between 10 − 5 and 10 − 4 m. Because of this, eddy current losses
are low and can be disregarded over a wide frequency range.
The relative permeability of ferrites can reach the order of magnitude of µ r = 2000.
An important characteristic of ferrite materials is their material-dependent limit
frequency, which is listed in the datasheets provided by the ferrite manufacturer. Above
the limit frequency increased losses occur in the ferrite material, and therefore ferrite
should not be used outside the specified frequency range.

4.1.12.2 Ferriteantennas in LF transponders

Some applications require extremely small transponder coils (Figure 4.51). In transpon-
ders for animal identification, typical dimensions for cylinder coils are d × l = 5mm ×
0 . 75 mm. The mutual inductance that is decisive for the power supply of the transpon-
der falls sharply due to its proportionality with the cross-sectional area of the coil
( M ∼ A ; equation (4.13)). By inserting a ferrite material with a high permeability
µ into the coil ( M ∼ → M ∼ µ · H · A ; equation (4.13)), the mutual inductance
can be significantly increased, thus compensating for the small cross-sectional area of
the coil.
The inductance of a ferrite antenna can be calculated according to the following
equation (Philips Components, 1994):

L = µ 0 µ Ferrite · n 2

· A

( 4 . 59 )

l