Backscattering Overview and RFID
In these kinds of systems, &quot;antenna&quot; is a bit of a misnomer:
instead, consider the two coils of the reader and the tag as primary and secondary windings of a transformer. A simple measurement on how much power can be pushed through the transformer allows the reader to determine the &quot;short-circuit-ness&quot; of the tag side, to stay within your picture.