Design of all-textile wearable antennas for on-body communications

Abstract

Recently, wireless body area networks (WBANs) have received considerable attention owing to their extensive potential applications including wireless medical, health care, military, and entertainment services. An antenna should be able to generate a field that propagates along the body surface to provide a good on-body-to-on-body communication link among body-mounted devices. A vertical monopole antenna is a good candidate for such an application. However, a vertical monopole antenna is not suitable for on-body utilization owing to its impractical height. Therefore, a low-profile antenna with a monopole-like radiation characteristic is required for on-body-to-on-body communication. In addition, the wearable antenna requirements are applications-specific. The common requirements for many applications are that the antenna should be lightweight, inexpensive, low-maintenance, without setup requirements, and robust, i.e., able to withstand damage from obstacles. To satisfy the above requirements, two types of antennas are proposed in this dissertation for proper on-body communications. First, an all-textile TM21 higher order mode circular patch antenna for on-body-to-on-body communications is proposed. The proposed antenna is compact and comprises a coaxially center-fed circular patch with two shorting vias to generate the TM21 higher order resonance mode for a monopole-like radiation characteristic. The antenna is fabricated by using a conductive textile, conductive thread, and a felt substrate. The bandwidth can fully cover the 2.45 GHz industrial, scientific, and medical (ISM) band while achieving a low-profile configuration with a height of only 0.008λ0 at 2.45 GHz and a compact size with a radius of 0.16λ0 at 2.45 GHz. The effects of antenna deformation due to bending and the human body are analyzed when the antenna is placed on a two-thirds muscle-equivalent phantom and a full-scale human model. The performance of the antenna is minimally affected by antenna deformation and the human body owing to the TM21 higher order mode excitation and its monopole-like radiation pattern. However, this antenna has maximum radiation along the θ=60° plane because of the reflection by the phantom, and therefore, it has low radiation gain in the body surface direction (θ=90°). To generate a more tightly bounded surface wave, an all-textile circular patch antenna with a corrugated ground for the guided wave along the body surface is designed as a preliminary design. The designed antenna consists of a center-coupled-fed circular patch and a thread-corrugated ground. The thread-corrugated ground, made of conductive sewing threads, acts as an inductively reactive surface to support a body surface TM mode wave. The antenna with a thickness of 7 mm (0.14λ0 at 6 GHz) operates on the 6 GHz band. Considering a practical on-body application, the antenna is fabricated using all-textile materials such as conductive fabric, conductive threads, fabric substrate, and conductive epoxy. The measured 10 dB return loss bandwidth of the antenna on the phantom is 9.5%, which ranges from 5.77–6.34 GHz. The gain enhancement on the phantom surface (θ=90°) is 8.65 dB from −8.1 dBi for the antenna without a corrugated ground to 0.55 dBi for the antenna with a thread-corrugated ground. While the antenna with a corrugated ground establishes the gain enhancement on the phantom surface, it still has too large a size to be used for practical applications. To miniaturize the size of the antenna with corrugation, designed as preliminary design, a textile antenna with an electromagnetic band-gap (EBG) structure for body surface wave enhancement is proposed in the last step. The proposed antenna is composed of a center-fed circular patch to generate the TM01 mode and an EBG structure near the patch to guide the TM wave along the horizontal surface (θ=90°). Considering wearability, the antenna is fabricated using only textile materials such as conductive textiles, conductive thread, and a leather substrate, and it has an enhanced radiated field along the surface direction in the 5.8 GHz ISM band for on-body communication. The proposed antenna has a diameter of 1.98λ0 and a thickness of 0.058λ0 at 5.8 GHz. Thus, the proposed antenna is suitable for health self-monitoring wearable devices. The proposed design techniques for the wearable low-profile antenna with monopole-like radiation and the patch antenna with corrugation/EBG structures for body-surface wave enhancement can be applied to antennas for wearable on-body monitoring devices. More research on antenna performance, under practical operating environments such as dry, wet for wash, need to be investigated further in the near future.