Wearable Antennas for Diverse Body-Centric Communications Modalities: On-Body, Off-body & In-Body Channels
Files
(Thesis)
Date
2023
Authors
Yin, Xiao Yang
Editors
Advisors
Fumeaux, Christophe
Chen, Shengjian Jammy
Chen, Shengjian Jammy
Journal Title
Journal ISSN
Volume Title
Type:
Thesis
Citation
Statement of Responsibility
Conference Name
Abstract
Wireless Body Area Networks (WBANs) as part of the Internet-of- Things (IoT), revolve around wireless communications related to the human body. Wearable antennas are one of the crucial components of WBAN and have in recent years steadily progressed in their radiation performance. Though operability of wearable antennas has been addressed in the contemporary literature, this thesis is devoted to further exploring on-body antenna materials, shapes and structures to improve wearability. It is without doubt an art to transform conventional antenna configurations into their wearable counterparts with appropriate mechanical and electromagnetic characteristics. The overarching aim is to maintain the radiation performance of wearable antennas, while in a physical sense improving their wearing comfort and robustness levels. Driven by the aforementioned design objectives, wearable antenna design topics targeting novel materials, new antenna topologies and advanced fabrication techniques are of great interest. This provides the motivation of this thesis, including background reviews of body-centric wireless technologies and development of three principal antenna configurations, which integrate closely onto the body or in clothing accessories for different body-centric channels. The discussion in the improvement of antenna communications efficiency is prioritized throughout the thesis, together with considerations about particular properties of wearable antennas, such as mechanical flexibility and/or deployment simplicity. In this context, this dissertation starts with a review of the breakthroughs and challenges of wearable antenna design in WBAN applications. Specifically, state-of-theart wearable antenna designs and established human body models in WBAN are presented, along with characterization of the individual requirements for various communication modalities. This pre-research work provides firm foundations for the novel antenna designs proposed in the rest of the thesis. As a first contribution of this thesis, three button-like antennas, which match the shape of clothing fastener accessories and simultaneously play a role of radiation modules are developed with unique characteristics. Firstly, a dual-band dual-mode button antenna is proposed, as a combination of a crossed-dipole configuration to generate broadside circular polarization (CP) and a top-hat monopole configuration to achieve omnidirectional vertical polarization. This experimentally-validated button antenna combines functions of off-body and on-body communications in a single compact antenna. Secondly, another button antenna offering an end-fire radiation pattern is proposed, which constructs an equivalent Huygens source by fully exploiting its ground plane structure. Thirdly, a rubber-based dual-mode button antenna is presented to explore the application of natural materials in wearable antenna design. The non-rigid rubber feature leads to a comfortable button antenna which exhibits a directional end-fire pattern for on-body communications and a broadside radiation pattern for off-body communications, simultaneously. In a second part of the thesis, a wristband antenna for off-body communications is presented, which involves 3D-printing technology and meandered mesh structures. The antennas’ grid shapes with meandering patterns are investigated through materialaware and efficiency-driven methods, with the aim of fulfilling breathability and flexibility for direct on-skin operation. Based on the required specifications for biological sensing applications, a wristband rectenna directly connected to a rectifier circuit is elaborated. The relevant design procedure takes into account critical factors for specific Wireless Power Harvesting (WPT) scenarios, including variations of load impedance and available input power density. The carefully-selected conjugate matching condition between the antenna and the rectifier circuit realizes a most balanced power transfer within the whole expected working range of this WPT system. The long-term experimental results in a biological sensing system demonstrate that the resulting highly efficient wearable rectenna allows 24/7 continuous medical monitoring without the need of changing battery. In the last main part of this thesis, a mass-produced commercial product namely an ECG pad, is integrated into the antenna structure to provide a tight on-body attachment and thus enhance the reliability of in-body communications, i.e., communications from the surface of the skin to an electronic implant in the body. This ECG-pad-based multi-band wearable antenna possesses a high into-body transmission efficiency, reduced back radiation, and enhanced stability in antenna performance due to the tight attachment to the skin surface provided by the ECG pad. To accurately measure this compact ECG-based antenna concept, issue of return currents affecting the measurement process of compact or electrically small antennas is also tackled by using a dedicatedly designed RF choke based on RF transformers. Thanks to the RF transformers, the RF choke exhibits a very wide operation bandwidth as well as a compact geometry. This choke is based on a back-to-back configuration which allows measurement for unbalanced to unbalanced coaxial connections. Generally, all the results in this thesis suggest that, antenna configurations which are integrated into clothing fixtures or fit ergonomically onto the human body, are a pathway to provide an unobtrusive and comfortable wearing experience while maintaining excellent radiation performance. Structures based on traditional PCB materials, thermoplastic polyurethane (TPU) materials and 3D-printing techniques, conductive textile materials, natural rubber materials or commercial ECG pads have been investigated in an attempt to implement wearable antennas with nearly imperceptible wearing experience.
School/Discipline
School of Electrical and Mechanical Engineering
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Mechanical Engineering, 2023
Provenance
This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legals.