Characterization and Modeling of Embroidered NFC Coil Antennas for Wearable Applications

This paper proposes an accurate model for yarn-based embroidered coil antenna used in Near Field Communication (NFC) applications. Traditionally, the inductance of a coil-shaped antenna is calculated using the Wheeler's equation, while accurate enough for metallic coil geometries, the results from Wheeler's equation have more than 20% errors when being applied to yarn-based embroidered antennas. We have discovered that this discrepancy is related to the parasitic capacitance, which is caused by the subtle gaps along twisted fibers. Experiments demonstrate that the parasitic capacitance has significant impact on the overall performances of the coil antenna. In this paper, an equivalent model for yarn-based coils is proposed to take into account of the parasitic capacitance and antenna geometrical characteristics. Compared with traditional Wheeler's equation, the inductance calculated from this proposed model is within 5% from the measured value. Additionally, the electromagnetic performance of the embroidered coil is resilient to mechanical deformation. When being used as a coupling antenna, it can harvest energy and power-up the NFC-based platform for body-area-network applications.