Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 60

−

I

+ Magnetic flux

lines

Figure 4.1 Lines of magnetic flux are generated around every current-carrying conductor

+

I

+

H

H

−

I

−

Figure 4.2 Lines of magnetic flux around a current-carrying conductor and a current-carrying
cylindrical coil

Table 4.1 Constants used

Constant Symbol Value and unit

10 − 12 As/Vm Electric field constant ε 0 8 . 85 ×

10 − 6 Vs/Am

Magnetic field constant 1 . 257 × µ 0

Speed of light c 299 792 km/s

10 − 23 J/K Boltzmann constant k 1 . 380 662 ×

In a straight conductor the field strength H along a circular flux line at a distance
r is constant. The following is true (Kuchling, 1985):

1
2 πr

H = ( 4 . 2 )

4.1.1.1 Pathof field strengthH ( x ) in conductor loops

So-called 'short cylindrical coils' or conductor loops are used as magnetic antennas to
generate the magnetic alternating field in the write/read devices of inductively coupled
RFID systems (Figure 4.3).