Electronic – UHF RFID: Why are read range (forward) and backscatter range curves different


I have made two measurements with a UHF RFID tag (pic below)

Dual-Frequency Tag

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).
Read Range (forward)

Backscatter Range

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)formula

Read range: formula

With: Pmax,EIRP = 3.28W

Effective backscatter power of tag when answering to a command:

Backscatter range:formula

with formula

Read range for DogBone NXP UCODE7 label:

Read Range DogBone NXP UCODE7

Best Answer

You don't give much detail on your experimental setup or indeed what you've exactly measured, and you mention these figures are calculated.

In the first case I think you're measuring the ability of your transmitting antenna to couple energy into the receiving antenna, and you get the classic two peaks you'd expect with two loosely coupled [somewhat] tuned antennas.

If I understand your second graph what you're showing is the sensitivity of your antenna and receiver to receive backscattered signal, so this is the merely response of your receiving antenna and amplifier. There's no coupled system and you get a single peak corresponding to the response of your [somewhat] tuned antenna.

PVC and Plywood do not have permittivity near one, it's more like 3. Your other materials (apart from the ferrite) are mostly air and have a permittivity of near 1. (depending on the type of PVC, PVC can also interact in odd ways at microwave frequencies)

-- edit --

Actually you can see the permittivity effect quite nicely, for your received signal (2nd graph) it lowers the tuned frequency, and for the coupled graph you can see it affects the position of only one peak (the slight motion of the second being due to compositing it with the rapidly falling tail).