Near field vs far field
Up close, reader and tag act like a transformer. Far away, the tag is a mirror.
Close to an antenna — within about λ/2π (wavelength divided by 2π) — the field is mostly stored energy sloshing back and forth, not a travelling wave. That's the near field. Past it, you're in the far field, where a proper radio wave has broken free.
#Near field: inductive coupling (LF, HF)
At 13.56 MHz the near field reaches about 3.5 m; at 125 kHz it's hundreds of metres. Every LF and HF tag is read deep inside it. The reader's coil and the tag's coil behave like the two windings of a transformer:
- the reader's field powers the tag;
- the tag talks back by switching a load on and off, which the reader sees as tiny changes in its own coil (load modulation).
The magnetic field falls off with roughly the cube of distance, which is why these tags work at a few centimetres and then suddenly don't.
#Far field: backscatter (UHF)
At 915 MHz the near field ends around 5 cm, so UHF tags live in the far field. The tag's antenna catches the reader's wave, harvests power, and answers by changing how strongly it reflects the wave back — like signalling with a mirror. That's backscatter, and it's why UHF reaches metres.
Into the weeds: Why "λ/2π"?
It's where the reactive (stored) and radiating parts of a small antenna's field are equal in strength. It isn't a hard wall, just the point where one starts to dominate the other.