Impedance considerations for inductive coupling in the EMVCo test environment

Contactless Payment incorporates the Proximity card specification for the physical-layer of the air interface. The physics is based on the coupling of resonant loop antenna circuits in the inductive near-field. Two main effects determine the near-field communication channel in the 13.56 MHz ISM frequency band: A strong decrease of H-field with increasing distance between reader and card, and a shift of resonance, or in fact a change of impedance. Understanding this impact on the reader antenna impedance as a function of coupling with the card, which also incorporates a certain impedance, is key to understanding all specified aspects of the physical-layer at the air interface. We investigate this impedance for the EMVCo test environment under operating conditions, with direct measurement and using analytical equations. We provide equivalent circuit element values for the specified test devices and also impedance values for positions in the operating volume.