Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 64

4

x = 10 cm

x = 20 cm

x 30 cm

1.5 A/m (ISO 14443)

=

3

2

1

0

0 0.2 0.4 0.6 0.8 1

Radius R (m)

Figure 4.5 Field strength H of a transmission antenna given a constant distance x and variable
radius R ,where I = 1Aand N = 1

The optimal radius of a transmission antenna is thus twice the maximum desired
read range. The second zero point is negative merely because the magnetic field H of
a conductor loop propagates in both directions of the x axis (see also Figure 4.3).
However, an accurate assessment of a system's maximum read range requires
knowledge of the interrogation field strength H min of the transponder in question (see
Section 4.1.9). If the selected antenna radius is too great, then there is the danger that
the field strength H may be too low to supply the transponder with sufficient operating
energy, even at a distance x = 0.

4.1.2 Magnetic flux and magnetic flux density

The magnetic field of a (cylindrical) coil will exert a force on a magnetic needle.
If a soft iron core is inserted into a (cylindrical) coil — all other things remaining
equal — then the force acting on the magnetic needle will increase. The quotient
I × N (Section 4.1.1) remains constant and therefore so does field strength. However,
the flux density — the total number of flux lines — which is decisive for the force
generated (cf. Pauls, 1993), has increased.
The total number of lines of magnetic flux that pass through the inside of a cylin-
drical coil, for example, is denoted by magnetic flux . Magnetic flux density B is
a further variable related to area A (this variable is often referred to as 'magnetic
inductance B in the literature') (Reichel, 1980). Magnetic flux is expressed as:

= B · A

( 4 . 8 )