If I have a stack of papers each with an RFID tag in the corner, is there a way for an RFID reader to only read the RFID tag on the top piece of paper, without the RFID tags below interfering?
OK I said it was impossible. Now I'll tell you how to do it!
to make these directional you need to increase the size of the lower plate of the capacitor such that when such a tag is above another tag of the same type the lower plate couples with the tag belows lower and upper plates de-tuning it. thus leaving only the uppermost tagke in a working state.
the large green circle is the top capacitor plate, the large brown circle is the lower plate, the slots reduce the shorted turn effect of such a large plate inside the coil
the small circle is a join between the brown and green layers
Because the capacitance effect needs to work through a sheet of paper the capacitopr will probably need a fairly thick dielectric for the effect to be significant, or perhaps a foil sticker can be placed on the back of the paper.