On-body near field confining spherical helical antennas

This research attempts to address the need for a small and inconspicuous antenna as part of a Wireless Body Area Network for continuous health monitoring. A balanced antenna, which avoids the need for a ground plane, worn as a pendant or clipped to a pocket or belt, was considered a convenient solution. Optimising the design using a genetic algorithm indicated that efficient air-cored examples would need to be about the size of a tennis ball. To reduce the size, the air core needed to be replaced with a high permittivity material. This would require a shell-like structure to support the wire antenna elements and contain the dielectric material in granular form, rather than the cellular foam used in early examples. By then, 3D printing had become commonplace and inexpensive. Printing a shell with grooves to retain the wire elements and space inside for the granular dielectric was helpful, but 3D printing also enabled further improvements. Firstly, the antenna could easily be made ellipsoidal, holding considerably more dielectric material than a sphere with the same radius. Secondly, a further layer of dielectric material could be retained outside the antenna elements. This kept the antenna away from the body, so the radius of the reactive near field of the antenna was never encroached upon and de-tuning did not occur. However, the original proprietary granular materials did not pack well into these intricate antenna structures and trapped considerable amounts of air. Therefore, a process of pressure agglomeration was used to turn mixtures of two fine powders, one with low permittivity, the other with high permittivity, into dielectrics of any value permittivity in between. When compressed, the mixture is solid and completely fills the space it is forced into. This technique, along with the calculating formulae and the measurement and packing techniques used were published as an IET journal Paper. A revised genetic algorithm was used to select the best combination of the above additional features for the final antenna design. Having successfully reduced the size of the antenna, then the balun also needed to be reduced in size. The final tests on the antenna were performed in an anechoic chamber, and proved satisfactory. The “free space” characteristics were obtained in the usual way. The “on-body” characteristics were obtained with the antenna in contact with a phantom representing a wearer’s chest. The phantom had been developed using the above dielectric powders in a 3D printed frame and had passed basic tests to show that its performance was close to that of a live human upper chest.