Hi,
A couple of years back, I used an HF RFId reader module from TI (The one that was there before s6700, although I forget the name). It worked on 13.56MHz and we were able to achieve a distance of 1 ft in reading a passive Tag-It RFId tag by placing it between two antennas that were 2 feet apart.
The antennas were 2ftx2ft rectangles (approx) and needed to be matched to 50Ohms. We did this using a mechanism called gamma matching that is recommended in the TI website's RFId antenna cook book. We confirmed it was match by measuring the reflected power to be almost zero by a directional power meter.
My prob is that,all this suggests that the method of communication between the tag and the reader is something other than inductive coupling, as you need the reflected power to be there for inductive coupling to work, so that the reader antenna can figure out whether the load at the passive tag was modulated or not. Inductive coupling is what 13.56MHz RFId is supposed to be using as I've read it in many places in the internet.
But, the act of matching for 50Ohms, and thus making sure that there is no reflected power makes it impossible to have data communicated backwards through weak transformer coupling as indicated by inductive coupling, as now no matter whether the load tag absorbed or not the power is radiated out anyway.
Can anyone provide an answer as to what exactly TI HF RFId reader modules have; inductive coupling or backscatter? I could not find it in any of TI documents.
And pls correct me if my reasoning is wrong somewhere.
Thanks
Janaka