Three-Dimensional Helicoidal Antennas for Near-Field Communication Wireless Charging in Smart Ring Applications: Design, Modelling, and Comparison to Representative Planar Reference Antennas

The deployment of embedded near-field communication (NFC) wearable devices continues to grow with the Internet of Things (IoT). Constraints on bill of materials (BOM) cost, representing all the components of a device, and available space lead manufacturers to seek alternatives to implement wireless charging. This article introduces an antenna solution to respond to requirements of NFC charging applications for smart ring form factors. The proposed study includes electrical parameter modelling, estimation of inductance, resistance, quality factor, and coupling coefficient together. This 3D helicoidal antenna operates in quasi-magnetostatic mode. The different pairs are then compared to existing representative planar reference antennas in simulation. The results are confirmed with experimental validation for the best case, worst case, and typical case. First, the article presents the different antennas produced. It defines the pairs of antennas and their respective coupling coefficients. The methodology to simulate and measure losses is also presented. Then, the article compares a representative planar reference with a prototype. The proposed methodology is validated through a comparison of simulations performed with identical S-parameters. This comparison demonstrates both the robustness of the model and the end-to-end charging performance of the prototype. Finally, the article presents a comparative simulation at 2 W and 3 W for all prototype pairs. All antennas provide improved end-to-end power transfer performance, achieving simulated efficiencies up to 64.2%, representing a maximum absolute improvement of 26.2 percentage points, with an average absolute improvement of 22 percentage points in the end-to-end efficiency compared to pre-existing representative planar reference antennas at the same operating power.