CHAPTER FOUR – DATA CAPTURE
55
EPC UHF Class1 Gen2 Tag Interfacing
Using the backscatter coupling with passive tags the power and signal is
transferred from the reader to the tag through their antennas in radio frequencies,
and the antenna of the reader gathers the RF waves that the tag scatters back.
The nature of radio waves is that the propagation path of the RF signals is varied
when radio waves encounter obstructions. Therefore, for example because of
reflection, you cannot be sure that the physical position of a particular tag is in the
direction where you are pointing the reader’s antenna, when your application
shows information from that particular tag on the screen. Having said that, the
antennas are designed for the UHF RFID reader on the PL3000 so that the
antenna performance is best at the front of the antenna.
In many instances, you may orient your PL3000 towards the RFID UHF tag(s)
so that the reader’s antenna (in particular a linear dipole antenna) is not physically
aligned with the tag’s antenna. Any misalignment between the antennas will result
in a polarization mismatch which reduces the efficiency of the communication
between the tag and the reader. Typically, you may encounter it in the form of
shorter operational distance.
The power level that the reader uses for tag communication has also an
influence on operational convenience of the reader. Depending on the application,
for example from time to time there can be too much RFID tags within range of
your reader as ‘background noise’. In that case, if the reader used less power for
tag reading, the situation would be more convenient. The UHF RFID reader on
your PL3000 can operate in two power level, which your program may change
through Nordic ID’s MHL, if need be. (The MHL documentation refers to this
property as ‘RF power control detect’ and ‘RF power control’.)
For EPC UHF Gen2 tags the PL3000 reader unit has two antenna designs: a
linear dipole antenna and a cross dipole antenna.
The read/write operations of the UHF RFID reader
with the linear dipole antenna are subject to a
polarization mismatch. However, you can achieve
better operational performance if you align the reader
antenna with the tag antenna when you are reading or
writing tags. For example, you may turn the PL3000
slowly so that the antenna plane turns in the direction to
the tag from a horizontal position to a vertical position
and back again as if you were using your PL3000 as a
screwdriver.
The adjacent figure depicts the PL3000 with the
linear dipole antenna that is in optimum alignment with
the RFID tag having the kind of physical
implementation.
Linear Dipole
UHF RFID Unit
