Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 114

Therefore, the field strength E at a certain distance r from the radiation source can
be calculated using equation (4.61). P EIRP is the transmission power emitted from the
isotropic emitter:

P EIRP · Z F
4 πr 2

E = ( 4 . 65 )

4.2.4 Polarisation of electromagnetic waves

The polarisation of an electromagnetic wave is determined by the direction of the
electric field of the wave. We differentiate between linear polarisation and circular
polarisation . In linear polarisation the direction of the field lines of the electric field E
in relation to the surface of the earth provide the distinction between horizontal (the
electric field lines run parallel to the surface of the earth) and vertical (the electric
field lines run at right angles to the surface of the earth) polarisation .
So, for example, the dipole antenna is a linear polarised antenna in which the electric
field lines run parallel to the dipole axis. A dipole antenna mounted at right angles to
the earth's surface thus generates a vertically polarised electromagnetic field.
The transmission of energy between two linear polarised antennas is optimal if
the two antennas have the same polarisation direction. Energy transmission is at its
lowest point, on the other hand, when the polarisation directions of transmission and
receiving antennas are arranged at exactly 90 ◦ or 270 ◦ in relation to one another (e.g.
a horizontal antenna and a vertical antenna). In this situation an additional damping
of 20 dB has to be taken into account in the power transmission due to polarisation
losses (Rothammel, 1981), i.e. the receiving antenna draws just 1/100 of the maximum
possible power from the emitted electromagnetic field.
In RFID systems, there is generally no fixed relationship between the position of
the portable transponder antenna and the reader antenna. This can lead to fluctuations
in the read range that are both high and unpredictable. This problem is aided by the
use of circular polarisation in the reader antenna. The principle generation of circular
polarisation is shown in Figure 4.59: two dipoles are fitted in the form of a cross.
One of the two dipoles is fed via a 90 ◦ ( λ/ 4) delay line. The polarisation direction of

H

E

j =
90 °

H

E

E

H

(a) Vertical polarisation (b) Horizontal polarisation (c) Circular polarisation

Figure 4.59 Definition of the polarisation of electromagnetic waves