Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 115

the electromagnetic field generated in this manner rotates through 360 ◦ every time the
wave front moves forward by a wavelength. The rotation direction of the field can be
determined by the arrangement of the delay line. We differentiate between left-handed
and right-handed circular polarisation.
A polarisation loss of 3 dB should be taken into account between a linear and a
circular polarised antenna; however, this is independent of the polarisation direction
of the receiving antenna (e.g. the transponder).

4.2.4.1 Reflection of electromagneticwaves

An electromagnetic wave emitted into the surrounding space by an antenna encounters
various objects. Part of the high frequency energy that reaches the object is absorbed
by the object and converted into heat; the rest is scattered in many directions with
varying intensity.
A small part of the reflected energy finds its way back to the transmitter antenna.
Radar technology uses this reflection to measure the distance and position of distant
objects (Figure 4.60).
In RFID systems the reflection of electromagnetic waves ( backscatter system, mod-
ulated radar cross-section ) is used for the transmission of data from a transponder to
a reader. Because the reflective properties of objects generally increase with increas-
ing frequency, these systems are used mainly in the frequency ranges of 868 MHz
(Europe), 915 MHz (USA), 2.45 GHz and above.
Let us now consider the relationships in an RFID system. The antenna of a reader
emits an electromagnetic wave in all directions of space at the transmission power
P EIRP .The radiation density S that reaches the location of the transponder can easily
be calculated using equation (4.61). The transponder's antenna reflects a power P S that
is proportional to the power density S and the so-called radar cross-section σ is:

P S = σ · S ( 4 . 66 )

The reflected electromagnetic wave also propagates into space spherically from
the point of reflection. Thus the radiation power of the reflected wave also decreases
in proportion to the square of the distance (r 2 ) from the radiation source (i.e. the
reflection). The following power density finally returns to the reader's antenna:

P S
4 πr 2

σ
4 πr 2

P EIRP
4 πr 2

σ
4 πr 2

P EIRP · σ
( 4 π) 2

S Back = = S · = · = ( 4 . 67 )

· r 4

P S

P EIRP

Figure 4.60 Reflection off a distant object is also used in radar technology