Coupling

Coupling

Coupling describes the physical mechanism by which electromagnetic energy is transferred between an RFID reader antenna and a tag antenna. The coupling type -- inductive or radiative -- is determined by the operating frequency and the distance between reader and tag relative to the wavelength.

Near-Field (Inductive) Coupling

At LF (125-134 kHz) and HF (13.56 MHz), the tag operates within the reader antenna's near-field region (within one wavelength, approximately 22 m at HF). Energy transfer occurs through magnetic induction, analogous to a transformer: the reader's coil antenna generates a magnetic field, which induces a current in the tag's coil antenna. The tag communicates back by load-modulating its coil impedance, which the reader detects as amplitude changes in its own antenna signal.

Near-field coupling provides reliable performance in the presence of liquids and metals (the magnetic field is less affected by these materials) but limits read range to approximately one coil diameter -- typically under 1 m for HF and under 0.5 m for LF.

Far-Field (Radiative) Coupling

At UHF (860-960 MHz) and microwave (2.4-5.8 GHz), the tag operates in the reader antenna's far-field region. The reader radiates electromagnetic waves that propagate through space. The tag antenna captures a portion of this radiated energy, and the Tag IC modulates the antenna's radar cross-section to reflect data back via backscatter.

Far-field coupling enables read ranges of 1-12 m for passive tags and 100+ m for active tags, but is more susceptible to detuning from nearby metals, multipath reflections causing null points, and orientation sensitivity.

Choosing the Right Coupling

Some UHF systems use near-field antennas to achieve short-range, orientation-insensitive reads at UHF frequencies -- combining the protocol advantages of EPC Gen2 with the coupling benefits of near-field physics.

See also: Backscatter | Near-Field Antenna | Read Range