Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 59

Physical Principles
of RFID Systems

The vast majority of RFID systems operate according to the principle of inductive cou-
pling . Therefore, understanding of the procedures of power and data transfer requires
a thorough grounding in the physical principles of magnetic phenomena. This chapter
therefore contains a particularly intensive study of the theory of magnetic fields from
the point of view of RFID.
Electromagnetic fields — radio waves in the classic sense — are used in RFID
systems that operate at above 30 MHz. To aid understanding of these systems we
will investigate the propagation of waves in the far field and the principles of radar
technology.
Electric fields play a secondary role and are only exploited for capacitive data
transmission in close coupling systems. Therefore, this type of field will not be dis-
cussed further.

4.1 Magnetic Field

4.1.1 Magnetic field strength H

Every moving charge (electrons in wires or in a vacuum), i.e. flow of current, is
associated with a magnetic field (Figure 4.1). The intensity of the magnetic field can
be demonstrated experimentally by the forces acting on a magnetic needle (compass)
or a second electric current. The magnitude of the magnetic field is described by the
magnetic field strength H regardless of the material properties of the space.
In the general form we can say that: 'the contour integral of magnetic field strength
along a closed curve is equal to the sum of the current strengths of the currents within
it' (Kuchling, 1985).

I =

H · d s

( 4 . 1 )

We can use this formula to calculate the field strength H for different types of
conductor. See Figure 4.2.