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Most UHF RFID applications are implemented with readers that use circularly polarized antennas. With this approach, the application is less sensitive to the orientation of the tag. Most commercially available passive UHF RFID tags, on the other hand, are based on a dipole antenna that is linearly polarized. As a result, the tag can be read equally well from any orientation with the exception of the ends of the tag antenna facing the reader antenna.
All tags though are not pure dipoles, and characterizing their performance with linear polarization might not give a truthful description of their performance in a circularly polarized field.
In this test, we look at the orientation sensitivity of an inlay tag with linearly and circularly polarized waves. The tag under test is the UPM Web shown in Figure 1. The tag is not a typical dipole tag, though its main polarization axis seems to be horizontal. However, the tag antenna also has vertical segments. So let’s first characterize it with an orientation sensitivity measurement with horizontal linear polarization.
The orientation sensitivity measurement is performed with the Tagformance system in a small anechoic chamber at 866 MHz with 0 degrees corresponding to the tag facing the antenna from the front, as shown in Figure 1. The orientation axis is vertical, and goes through the center of the tag.
The results of the horizontal polarization test are shown in Figure 2. The sensitivity of the tag from the front is approximately -11 dBm, and since the tag is symmetrical, we get the same sensitivity from the back. With 90 degree and 270 degree orientations, i.e. when the ends of the tag antenna are facing the test antenna, the tag is unreadable even at full power. The result corresponds very well to the radiation pattern of a dipole antenna.
As the next step, let’s test the tag with vertical polarization. In this test, the tag orientation, and the orientation axis remain the same. But now the test antenna generates vertical polarization. The results of this test are shown in Figure 3. The results are a little surprising at first. With vertical polarization, the tag is unreadable from the front and back, but has a sensitivity of approximately -6 dBm at 90 and 270 degrees. After giving it some thought, the results make a lot sense though. If we look at the tag’s two vertical segments, when we try to power up the tag from the front, equal voltages are induced on each segment. But since the segments are at equal distances from the antenna, the voltages are in the same phase, thus causing no differential voltage over the tag IC. But at 90 degrees, there is a phase difference, and thus a voltage is caused over the IC.
Finally, let’s test the same tag with circular polarization. Linear polarized antenna is replaced with a circular polarized antenna. Now everything else remains the same, but the test field is right hand circularly polarized. The results of this test are shown in Figure 4. Once again, the results show a radiation pattern resembling that of a dipole antenna. Now, it seems to be tilted approximately 30 degrees. Let’s first look at the 0 degrees and 180 degrees. At these angles, the sensitivity is still more or less -11 dBm. Thus, only the horizontally polarize component of the radiation has an effect here. Similarly, at 90 and 270 degrees, the sensitivity is -6 dBm, so only the vertical component has an effect here.
But why the tilted radiation pattern? Since the tag gathers energy from both polarization axes, the radiation pattern is a superposition of the two polarizations. Depending on the orientation angle, the voltages across the tag IC induced by the two polarizations may either combine constructively or destructively. In this case, there are again angles where the tag is unreadable, approximately at 60 degrees and 240 degrees. At these angles, the voltages caused by the two polarization cancel each other out. On the other hand, at angles of 150 degrees and 330 degrees, the voltages combine to boost the performance.
There is a case for testing tags with both linear and circular polarization. Linear polarization reveals more detailed information about the functionality of the tag antenna with regard to transmitted waves in each polarization axis. When designing a tag, this helps to verify that the tag works as it is designed to.
When looking at application level performance, just one linear polarization access does not tell the whole story about how well the tag performs. It should be noted though, that not all antennas that claim to be circular, are exactly that. So when characterizing a tag, the measurement antenna should be chosen with care. However, when evaluating the performance of a system comprising of a reader antenna and a tag, it might make sense to perform with the test with that specific antenna.