When detecting UHF RFID tags multi-path signal reception can be a significant source of error due to reflections and cancellations of signals. The multi-path signal is either additive or subtractive in nature. In order to minimize the error due to multi-path, we need to understand the multi-path behavior and corresponding signal characteristics.

Multi-path is the signal that is coming to the antenna element, not directly from the RFID tag, but reflected by the objects in the vicinity of the beam.

Electromagnetic propagation is depicted as an electric field vector orthogonal to a magnetic field vector. The wave propagation is described by the Poynting vector traveling perpendicular to both the electric and magnetic fields.

Every electromagnetic wave is polarized and the polarization may be vertical, horizontal, elliptical or circular. Polarization is defined by the direction of the electric field vector with respect to the wave’s direction of propagation.

When UHF signals are reflected off objects their polarization may change. Also, the signal strength of the reflected signal depends upon the reflecting object and the number of reflections.

If the polarization of the tag's signal is always known, be it horizontal or vertical, a linear reader antenna placed in the appropriate plane would be optimum for signal reception. However, if reflections and unknown tag orientation are at issue it would be more prudent to choose a circularly polarized antenna. The circular polarized elements can receive signals in any linear polarization, however the signal strength is not as strong as a proper oriented linear antenna would yield.

When using a Left Hand Circular Polarized (LHCP) UHF antenna the RFID signal transmitted by the reader is left hand circularly polarized, which means the projection of the tip of the electric field vector upon any fixed plane intersecting, and normal to, the direction of propagation, describes a circle with a thread of a left-hand screw.

Dynasys offers a selection of UHF antennas for Gen 2 readers. A typical UHF portal may require a number of antenna elements of differing polarizations to assure complete tag coverage in all orientations. The proper selection of antennas is highly dependent on the exact requirements of each specific RFID tag application.

Call your Dynasys RFID Sales Engineer for assistance in selecting the proper components for your application.