My first goal was to identify how much of a tag has to be “shielded” before the tag is no longer visible to the reader. I initially tried using cut-up pieces of a commercial RFID shielded credit card pouches to encase parts of a tag. I learned that it was most effective to cover the part of the tag closest to the chip.
Using the square Frog 3D tag from Smartrac, I experimented with shields of different sizes:
I used simple aluminum foil as the shielding material and placed it on disks of different diameters (from left to right: 25 mm, 35 mm, 40 mm, 45 mm 50 mm, 75 mm). Each with a hollow collar on top.
The tag was mounted to a disk that was 1 mm thick and had a central post that would easily pass through a shield’s collar. That allowed me to move the shield up and down on the post and position the shield at different distances from the tag.
The smallest shields would still allow the reader to see the tag at close distances – even with the shied all the way down on the post. The larger shields easily hid the tag at close distances but when the tag was placed further from the reader, the shield had to be moved a significant distance up the post before the reader was able to see the tag. I settled on an intermediate shield diameter of 45 mm. That diameter was large enough to prevent the tag from being read when touching the reader but only required a separation distance of a couple of millimeters at a distance of about 2 ft.
It’s disconcerting that the exposure distance is a function of the distance of the tag from the reader rather than a constant but probably to be expected based on how the power decreases as the square of the distance from the reader.
The shielding process appears to work consistently even when the face of the tag is pointed directly at the reader and the shield is located “behind” the tag.
Note that the shield never gets closer to the tag than 1 mm – the thickness of the tag plate.
Also note that I was able to successfully replace the aluminum foil (which was messy to mount to the shield) with aluminum tape – which is often used to join HVAC ducts. The tape fits flat on the shield plate with no wrinkles to create gaps when resting against the tag plate.
This concept became the core of my initial switch design.