https://www.irda.org/what-is-rfid-block … is-it-used

I have carefully excavated some different library book security tags to extract the actual functional thin sheet (printed circuit) that does the job, Each has an aerial (antenna) coil and a two contact chip.  I gather that the chip receives a signal from the interrogating transmitter, rectifies it to get sufficient power to run the chip which sends back via the same antenna a signal coded with the identifier ( a number ?)

It appears that the commonly found RFID frequency devices can be interrogated  at a modest distance, and it is advisable to store  bank cards between covers of an RF blocking material.  My research some time back, and I think I may have posted in the old forum, showed that the majority of the blocking was done by reflection of the interrogating signal at the outer surface of a metallic sheet, and only  in part by absorbsion within the bulk of the cover. It is possible that the blocking covers should be polished, although I am not at all sure about that.

I hope this post is informative and useful.