Siemens SIMATIC RF600 System Manual page 160

Antennas
6.2 Guidelines for selecting RFID UHF antennas
Selection of circular polarized antenna
If the orientation of the transponder is unknown, or if an alternating orientation can be
expected, the transmit and receive antennas must have circular polarization.
When selecting a circular antenna, the polarization purity must be observed in addition to the
polarization direction. A differentiation is made between left-hand and right-hand circular
polarization (LHCP and RHCP). The two types cannot be combined in the same system. On
the other hand, selection of the polarization direction is insignificant if the antenna system of
a transponder is linearly polarized. With actual antennas, elliptical polarization is
encountered rather than the ideal circular polarization. A measure of this is the ratio between
the large and small main axes of the ellipse, the axial ratio (AR), which is frequently specified
as a logarithm.
Axial ratio
Ideal
Real
Figure 6-6
160
Circular polarization of antenna system and transponder
AR
0 dB
2-3 dB
System Manual, 10/2009, J31069-D0171-U001-A8-7618
SIMATIC RF600