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Textile Antennas for Body Area Networks: Design Strategies and Evaluation Methods

Ping Jack Soh1 and Guy A. E. Vandenbosch2

1 Advanced Communication Engineering Centre, School of Computer and Communication Engineering, Universiti Malaysia Perlis, Arau, Perlis, Malaysia

2 ESAT-TELEMIC Research Division, Department of Electrical Engineering, KU Leuven, Leuven, Belgium

1.1 Introduction

Due to the increasing demand for multi-functional, multi-band wireless operation and consumer-centric technology, textile antennas have been receiving growing attention [1]. Future wearable systems should be unobtrusive, flexible, and operating with minimal degradation in proximity to the human body. These antennas have to meet the bandwidth, efficiency, and safety requirements, while being consistent with low-cost manufacturing techniques. Moreover, in wearable applications, flat surfaces cannot be guaranteed. Thus, an important antenna requirement is its ability to work with good robustness against environmental, positional, and location changes when being worn, besides complying with medical and safety regulations.

Body-centric antennas are crucial in catering for various current and future wireless standards. Among others, wearable antennas could assist medical monitoring for hospitalized, homebound, or outpatients [2, 3]. They could be applied in emergency service communication and public safety support (e.g., firefighters) [4-6]. They could also provide flexibility in assisting communication in search, rescue, and location-tracking alerts, especially in hazardous environments [7, 8]. There is also the possibility that they will become popular in consumer electronics in the near future, applied for communication [9], positioning, and navigation for recreational purposes [10].

Wearable antennas are electrical radiators being made flexible enough to be worn and to work in the proximity of a user's body. Since it is ergonomically more suitable that a wearable antenna for Wireless Body Area Networks (WBAN) applications is flexible and made to conform to the body, it is only natural that textiles be used to achieve these requirements compared to conventional metallic structures, for example, rigid copper plates or tapes which are worn. However, degradation of the antenna performance when worn on the human body has been one of the major deterrents in its successful implementation, be it in terms of frequency detuning, bandwidth reduction, and efficiency degradation or radiation distortion [11]. In other words, ideally, a wearable antenna must be designed to be immune enough for an on-body operation. Moreover, a flexible antenna made from textile is regarded as a realistic candidate due to the advancements in conductive textiles and the ergonomic properties that it is able to offer. Since these textiles are either newly introduced or have been traditionally used for other purposes, for example, electromagnetic interference (EMI) shielding or grounding, one of the important and yet challenging aspects of this work is to properly characterize their electrical properties at the intended frequencies.

In this chapter, firstly, a brief overview of the types of textiles (conductive and non-conductive) is given. Next, the characterization procedure using a commercial setup is explained prior to the proposal of a systematic antenna fabrication procedure. Finally, this chapter also describes the evaluation methods used for the fabricated antenna prototypes, that is, in terms of reflection coefficient, radiation characteristics, efficiency, and specific absorption rates (SAR).

1.2 Textile Materials and Antenna Fabrication Procedure

Textile antenna prototyping materials generally consist of two textile types, conducting and non-conducting. The former is typically used to form antenna conductive elements (radiator, ground plane, shorting wall, etc.), whereas the latter is used to form the substrate, spacer, etc. For example, in the case of a Planar Inverted-F Antenna (PIFA) topology, conductive textiles are used as its radiator, ground plane, and shorting wall, whereas felt or fleece is used as the substrate. The properties of several popular commercial off-the-shelf textiles can be found in [4] and [12-14], and will be explained in the following sections.

1.2.1 Conductive Textiles/Foils

Initially, flexible antennas are prototyped using copper foil, as this is flexible and a rough representation of a textile antenna. However, the introduction of electrically conductive acrylic adhesives which are reasonably homogeneous in terms of surface resistivity/conductivity and mechanically stable has eased the fabrication of antennas made using commercial off-the-shelf textiles. These materials can be used as the radiating or grounding element for a textile antenna, and are required to be highly conductive, with surface resistivities (Rs) of less than 0.05 O/sq.

Three of the more popular conducting materials for wearable antennas are described as follows and are shown in Figure 1.1

1. Copper foil tape: a 0.035 mm thick foil, coated with a 0.03 mm thickness.
2. ShieldIt Super [15]: a ripstop, woven polyester textile coated with copper and nickel. Its thickness is 0.17 mm and its estimated weight is 230 g/m2.
3. Pure copper polyester taffeta fabric (PCPTF) [16]: plain woven and coated using pure copper. It has a thickness of 0.08 mm and an estimated weight of 80 g/m2.

Figure 1.1 Conductive textiles: (a) ShieldIt Super [15] and (b) PCPTF [16]. Used with permission from LessEMF.com.

The parameter of prime importance is the equivalent conductivity of the textile used. This parameter ultimately determines the losses, and consequently, the efficiency and gain of the antennas. The homogenized conductivities were calculated based on the surface resistances provided by the manufacturer. The thickness of the conductive textile can be chosen depending on the application and location of the on-body deployment. For example, if the antennas are needed for health monitoring of the aged, thinner and low-cost materials may be chosen as they might be less exposed to harsh environments compared to the use of such antennas in military applications.

1.2.2 Non-conductive Textiles

The substrates used to fabricate wearable textile antennas are generally chosen to enable ease of integration onto the users' clothing. Upon the selection of a suitable textile substrate, it is important that its permittivity and loss tangent are characterized via measurements. Note that material properties for the same substrate might differ due to (small) variations in the fabrication process. For example, it is found that for felt [17], values given in literature are within the following ranges: er in between 1.18 [18] and 1.45 [19], and dtan in between 0.004 [18] and 0.025 [14]. This illustrates that a proper measurement of felt characteristics is still necessary. On the other hand, measurements using an in-house developed technique based on the cavity method yielded a relative permittivity of er = 1.45, and dtan = 0.044 at 2.45 GHz. Another example is the polyester fleece [20], a soft napped insulating fabric made from synthetic fibers. Such material is typically used to manufacture loss tangent jackets, hats, pants, sweaters, gym clothes, and high-performance outdoor clothing [21]. Its reported permittivity ranges from 1.04 [7, 22] to 1.25 [23], and none of the publications reported the characterization of its loss tangent.

1.2.3 Textile Antenna Fabrication Procedure Using Commercial Textiles

Fabrication starts by cutting a single piece of textile or copper foil according to the designed and optimized shape. Then, the textile is folded over and fastened on the felt fabric to form the antennas. Depending on the antenna topology, a sufficiently large hole for the insertion of a 50 O SMA connector is needed. This is mainly to avoid shorting contacts between the radiator and the ground plane, once soldering or conductive epoxy is applied. Fastening copper foil and ShieldIt textile to the substrate is simpler, as both materials come with adhesive reverse sides. ShieldIt comes with a heat-activated reverse side adhesive, which is firm and typically enables it to be secured accurately onto substrates. On the other hand, it should be noted that the contact between the copper foil adhesive and the non-woven felt substrate degrades with time, thus the use of additional non-conductive adhesive is highly recommended. Although this might slightly affect the overall material thickness and its electrical properties, more adverse effects are expected as the original adhesive degrades and as the copper foil edges peel off and lift up, allowing the existence of air gaps. PCPTF, on the other hand, has to be stitched onto the substrate along its perimeter. This almost always results in an uneven conductive surface around the antenna perimeter, at locations where the conductive surface is sewn to the substrate layer. Besides, manual stitching, if not carefully done, could also result in misalignments. Thus it is also recommended that a small amount of non-conductive adhesive be applied prior to the stitching process. Meanwhile, to ensure proper electrical connection between PCPTF textile and the SMA connector, a silver conductive epoxy model 8331 from MG Chemical is used. Fabrication using this simple...