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?