I have made two measurements with a UHF RFID tag (pic below)
My test setup consists of a reading device with a sensitivity of -70dBm and an antenna gain of 6dBi. I made a frequency sweep across 700 - 1200MHz bands, the device sends a query command (ISO 18000-6C/ISO 18000-63) at increasing power levels (power step = 0.1dBm up until 30dBm ERP) until the transponder responds. Path losses have been measured beforehand so after this frequency sweep, the read range and the backscatter range can be calculated (see figures below).
Now as far as I understood, the actual read range is limited by the lower value, in this case the actual read range would be the forward read range depicted in figure 1. I was able to verify this (with small differences in actual range) with a different reader setup.
What I do not understand is why the curves are fundamentally different. I would have expected them to be at least a little similar. There appear to be two resonance points in figure 1 while there is only one in figure 2.
Another point to consider is that plywood and the PVC plate seem to have a strong detrimental effect on the reading range even though the permitivity of those materials should be closer to 1.
Edit: There seems to be a smaller, rectangular loop on the tag, is one of the resonance points appearing because of that?
Edit#2: Read range forward and read range reverse (backscatter range) habe been calculated as follows (Path losses have been measured beforehand):
Power on tag (forward link)
With: Pmax,EIRP = 3.28W
Effective backscatter power of tag when answering to a command:
Read range for DogBone NXP UCODE7 label: