Power arriving through the antenna

How a sticker becomes a circuit

A passive RFID tag contains no battery. It is, at the physical level, a coil of wire connected to a chip, printed or etched onto a substrate thin enough to laminate into a label. When a reader broadcasts an alternating field — at 125 kHz for legacy proximity cards, at 13.56 MHz for the ISO/IEC 14443 and ISO/IEC 15693 families that underpin modern NFC — that coil intercepts the field and the induced current rectifies to a small DC supply. The chip wakes, executes its logic, and modulates its response back to the reader by varying the load on the coil, a technique called backscatter modulation. The reader detects the impedance shift. No battery changed hands; no external power was consumed by the tag.

This is why range collapses so sharply. The magnetic field available to the tag falls with roughly the cube of distance from the reader's antenna in the near-field regime that both NFC and low-frequency RFID operate in. The ISO/IEC 14443 standard specifies a nominal read range of up to 10 centimetres, and real deployments — transit cards, contactless payment, phone tap-to-pair — sit comfortably inside that. UHF passive RFID, operating around 860–960 MHz under ISO/IEC 18000-63, shifts into far-field propagation and can reach several metres, but requires a larger antenna footprint and a reader that radiates meaningfully more power.

The RF front end on the reader side is where the engineering concentrates. The reader must generate a stable, clean carrier, detect the faint backscatter signal sitting on top of it, and suppress its own transmitted signal enough to resolve that return — a dynamic-range problem in miniature. On the tag side, the front end is the entire device: the matching network, the rectifier, and whatever energy-storage capacitance keeps the chip alive through a modulation dip. Mismatched tuning, even a detuned coil caused by proximity to metal, can cut harvested power by an order of magnitude, which is why NFC tags on metal objects require a ferrite spacer layer.

NFC adds a second mode — active peer-to-peer, where both devices carry their own power — but the tap-and-go use case that defined the technology's adoption is always the passive one, reader powering tag. The NFC Forum, which maintains the technical specifications above the ISO physical layer, formalised four tag types mapping to different ISO standards; NFC Forum tag type definitions govern what a certified device must read and write.

The achievement, understated by its ubiquity, is that a circuit sophisticated enough to hold a unique identifier, enforce a cryptographic handshake, or update a counter draws its entire operating power from a radio wave. The antenna is both the energy source and the communications channel.

The ISO/IEC 14443 standard specifies a nominal read range of up to 10 centimetres, and real deployments — transit cards, contactless payment, phone tap-to-pair — sit comfortably inside that.

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The rest of what has to implement it

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