• To absorb enough electromagnetic energy to activate the bonded chip

• To be an efficient reflector for the electromagnetic radiation emitted by the reader

At HF (13.56 MHz), a planar spiral with 5-7 turns over a credit-card-sized form factor can be used to provide ranges of tens of cm. These coils are less costly to produce than LF coils, since they can be made using lithographic techniques rather than by wire winding, but two metal layers and an insulator layer are needed to allow for the crossover connection from the outermost layer to the inside of the spiral where the integrated circuit and resonance capacitor are located.