Characteristics - Siemens SIMATIC RF600 System Manual

Antennas
6.4 Guidelines for selecting RFID UHF antennas
Polarization
Definition
The polarization parameter describes how the electromagnetic wave is radiated by the
antenna. A distinction is made between linear and circular polarization. With linear polarization,
a further distinction is made between vertical and horizontal polarization.
Specifications
UHF transponders usually have a receive characteristic similar to that of a dipole antenna
which is linearly polarized. Horizontal or vertical polarization is then present depending on the
transponder mounting.
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
190 System Manual, 06/2010, J31069-D0171-U001-A10-7618
Draft Version 02.06.2010
AR
0 dB
2-3 dB
SIMATIC RF600