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I have a circularly polarized patch antenna for an RFID reader, with a gain of 8dBi and bandwidth from 914MHz to 919MHz and an axial of 0.34dB. I have read that using a CP RFID patch antenna means that the RFID tag will still be detected no matter the orientation of the RFID relative to the CP RFID patch antenna. But when testing this I see that there is a difference in terms of the detection of the RFID tag when I change the orientation of the RFID from horizontal orientated to vertically orientated. I connected the antenna to the reader and then to my computer where the software shows the retrieved data from the RFID tag.

For example, I placed two tags at a distance of 1.5m away from the reader's antenna at the same height in free space (outdoors). One RFID tag was horizontally orientated while the second RFID tag was vertically orientated. When testing these tags the horizontally orientated RFID tag showed to more stable because the reader's antenna picked it up continuously and I could see the data from the RFID tag using the software. However, when testing the vertically orientated RFID tag I noticed that the vertically orientated was barely detected by the reader's antenna, in that one second, the data appeared on the computer and next minute the data was not there. This does not make sense because the CP polarized suppose to detect the RFID tag regardless of its orientation. However, I did notice that when I brought the vertically orientated RFID closer to the reader's antenna the antenna started to pick the RFID tag in a more stable manner. Why does this change in performance exist?

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    \$\begingroup\$ You're going to need to expand on some of the terms you've used. "when testing this" - what does that mean? what kind of test? "a difference in terms of detection" - what does this mean? what kind of difference? "stability in terms of detection" - what kind of stability? Also, are you testing in an anechoic chamber? In a lab room? Outdoors? Pls rewrite your question and add much more detail. \$\endgroup\$ – scorpdaddy Apr 22 '20 at 14:50
  • \$\begingroup\$ How are the tags held in position during the test? Also you should provide a sketch of the setup showing the relative positions and orientations of the tags vs the antenna. Your description of vertical and horizontal orientation is not specific enough. \$\endgroup\$ – scorpdaddy Apr 22 '20 at 15:21
  • \$\begingroup\$ @scorpdaddy what is the reason for asking for the type of mount used and how does this influence your answer? \$\endgroup\$ – Joey Apr 22 '20 at 17:51
  • \$\begingroup\$ In some testing metal cables have distorted the shape of the e-field. \$\endgroup\$ – scorpdaddy Apr 23 '20 at 12:13
  • \$\begingroup\$ Do the mounts have a tilt mechanism to ensure that their object is exactly vertical? \$\endgroup\$ – scorpdaddy Apr 23 '20 at 12:19
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The antenna as an axial ratio of 0.34dB which means this antenna is not truly circularly polarized. A true circularly polarized antenna will have an axial that is unity (0dB). Therefore the antenna in the question is very close to unity but it is not truly circularly polarized. This axial ratio shows the antenna has an elliptical polarization (the minor and major axis are not equal).

"A circularly polarized field is made up of two orthogonal E-field components of equal amplitude (and 90 degrees out of phase)".

Since the two E-field components are not equal because of the elliptical polarization the E-field strength that reaches the RFID will be different. Therefore, in this case, the vertically orientated RFID tag might only be receiving the minor axis E-field, while the horizontally orientated receives the major axis E-field. Hence the variance in the detection with respect to the orientation of the RFID tag.

With regards to the RFID tag been detected when it is closer. Keep in mind that the intensity of the radiation is inverse to the distance. So the further you move away from the source the weaker the intensity gets and the closer you move towards to the source the stronger the intensity becomes. Also, it could be that as you move closer the minor axis E-field is now above the threshold to detect the RFID tag but as you move further away remember the minor E-field will drop and will always be less than the major axis E-field.

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  • \$\begingroup\$ @DaveTweed can somebody confirm if this is correct (moderator)? \$\endgroup\$ – Joey Apr 22 '20 at 20:10

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