Field Guide · protocol

Also known as: RFID, radio-frequency identification

RFID (radio-frequency identification) is contactless identification in which a reader energizes a nearby tag over radio and the tag replies with its stored ID.1 The defining feature at the RF layer is that most tags are passive: they carry no battery and instead harvest power from the reader’s field, then answer either by load modulation in the near field or by backscatter in the far field. RFID is the workhorse of access badges, inventory, toll tags, and animal microchips, and it is the family NFC grew out of.

reader tag (passive) carrier powers the tag → ← backscatter / load-modulated reply (ID)
The reader supplies a continuous carrier that both powers a passive tag and carries its reply back as a load-modulated or backscattered signal.

Overview

RFID spans three broad band regimes with genuinely different physics. LF and HF tags sit in the reader’s near field and couple inductively, like loosely linked transformer windings; the tag replies by varying the load on its coil, which the reader senses as a small change in its own field. UHF tags sit in the far field and reply by backscatter — switching the impedance of their antenna so they reflect more or less of the reader’s incident wave, encoding data in that modulated reflection.

Technical characteristics

Band Frequency Coupling Typical range Examples
LF 125 / 134 kHz Inductive (near field) ~cm Animal ID, access fobs
HF 13.56 MHz Inductive (near field) ~cm–1 m Smart cards, NFC, ISO 14443/15693
UHF 860–960 MHz Backscatter (far field) ~1–10 m EPC Gen2 inventory, toll tags

Readers commonly encode commands with amplitude/on-off keying so the tag keeps receiving power during the modulation, while tag replies use subcarrier ASK or phase changes on the backscattered signal.

History

Passive backscatter identification traces back to World War II IFF and to Harry Stockman’s 1948 work on communication by reflected power. Commercial RFID grew through the late 20th century, and the EPCglobal Gen2 UHF standard (later ISO 18000-63) standardized supply-chain tagging in the 2000s.2

Deployment

RFID is everywhere unglamorous: access control, library and retail inventory, passports and transit cards (HF), electronic toll collection and warehouse tracking (UHF), and pet microchips (LF). It underpins much of the physical Internet of Things, with NFC as its consumer-facing HF subset.

Decoding it with GopherTrunk

RFID is out of scope for GopherTrunk, which decodes trunked land-mobile voice, not short-range identification. LF/HF RFID lives near the reader’s coil and is not a free-space signal a scanner tunes across a band; UHF backscatter is dominated by the reader’s own carrier and needs a reader-style transceiver, not a passive receiver. GopherTrunk implements none of the RFID air interfaces.

Sources

  1. Radio-frequency identification — Wikipedia, for the LF/HF/UHF band structure, passive tags, and coupling methods. ↩

  2. Backscatter — radio frequency — Wikipedia, for the reflected-power reply mechanism used by UHF tags. ↩

See also