A Review of Wearable Antenna N. H. M. Rais1, P. J. Soh1, F.Malek1, S. Ahmad1, N.B.M. Hashim1, P.S Hall2 1
School of Computer and Communication, University Malaysia Perlis (UniMAP), No. 12 & 14, Jln Satu, Kompleks Pengajian UniMAP Seberang Ramai, 02000 Kuala Perlis, Perlis, Malaysia.
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The University of Birmingham School of Engineering Electronic, Electrical and Computer Engineering Edgbaston, Birmingham, B15 2TT UK
[email protected] Abstract- Utilization of wearable textiles in the antenna
segment has been seen on the rise due to the recent miniaturization of wireless devices. A wearable antenna is meant to be a part of the clothing used for communication purposes, which includes tracking and navigation, mobile computing and public safety. This literature review intend to disclose this unconventional antenna technology and provides readers with the background of the wearable antenna that would include about specification of the antenna, material for the antenna and analysis that must be done to design proper wearable antennas. All the designs presented are of the recent development in wearable technology. Keywords – conductive textile antenna, wearable antennas, onbody communication
I.
INTRODUCTION
In recent years, body-centric wireless communication becomes an important part of fourth generation mobile communication systems (4G). In supporting the increasing interest in antennas and propagation research for body communication systems, the IEEE 802.15 standardization group has been established to standardize applications intended for on-body, off-body or in-body communication. Body-centric communications takes its place firmly within the sphere of personal area networks (PANs) and body area networks (BANs). One of the applications – the on-body communications – describe the link between body mounted devices communicating wirelessly, while off-body communication defines the radio link between body worn devices and base units or mobile devices located in surrounding environment. Finally, in-body communication is communication between wireless medical implants and on body nodes [1]. One of the dominant research topics in antennas for body-centric communications is wearable, fabric-based antennas. Commonly, wearable antenna requirements for all modern application require light weight, low cost, almost maintenance-free and no installation. There are number of specialized occupation segments that apply body centric communication systems, such as paramedics, fire fighters, and military. Besides, wearable antennas also can be applied for youngsters, the aged, and athletes for the purpose of monitoring.
Designing textile antenna requires the knowledge on electromagnetic properties such as permittivity, and loss tangent of the textile material. Conductive textile such as Zelt, Flectron and pure copper polyester taffeta fabrics are regularly used as the radiating element while non-conductive textile such as silk, felt and fleece are used as substrates. Electromagnetic properties for these textiles are not readily available. Measurement of the electromagnetic properties of textile substrate done in [2] using a transmission/reflection waveguide method. Important permittivity and loss tangent value were included in the simulation. This work intends to present recent types of antenna and other considerations that have to be investigated, including suitable material selection, fabrication methods and analysis required for a wearable antenna design. II. TYPES OF WEARABLE ANTENNA A. Conventional Wearable Designs Conventional antenna designs which include planar dipoles, monopoles, planar inverted-Fs (PIFAs), and microstrip patches were used in recent research for wearable antennas design. Microstrip antennas are planar and these can be manufactured onto a printed circuit board (PCB). This made them a practical antenna type due to their low cost, and eases for fabrication. In [3], Salonen explored the planar inverted-F antenna (PIFA) design as a wearable antenna that intended to be placed on the sleeve of clothing. PIFAs are like quarter wave monopole antennas, which folded structure is parallel with the ground plane, as showed in Figure 1. Figure 2 shows one possible location placement of the antenna, on the sleeve of clothing. The effect of the antenna’s ground plane to the direction of the strongest radiation was also shown. The ground plane formed a shield for human so the radiation will not radiate towards human body. In other words, the ground plane functioned as a reflector for radiation. Using the same antenna design concept, the design of a wearable flexible planar inverted-F antenna (FlexPIFA) for Bluetooth operated system was introduced in [4]. The antenna was designed to be attached on a human arm and used flexible substrate material with 0.236 mm thickness, a dielectric constant of 3.29, and a loss tangent of 0.0004. Antenna’s requirements that influence the antenna selection in [5] are low profile, operated in the frequency
range of 100 MH – 500 MHz, omni-directional coverage in the azimuth plane, produces wideband return loss and possesses a vertical or circular polarization. Omni-directional radiation pattern is desired for a wearable antenna, in which it will be suitable for mobile devices and smart clothing. Besides that, omni-directional radiation pattern should be designed to have minimal/no side lobes, which can harm the human body.
A textile-based antenna in [10] was designed to provide a wireless short range communication in body and personal area network. It was made entirely out of textile material. The aperture coupled feeding mechanism was used for the design, helped in increasing the bandwidth compared to other classic planar antenna feeds [11].
Fig. 1: Construction of the PIFA [3]. Fig.3: Antenna with 50
line feed fabricated on a PCB [6].
Fig. 2: Possible placement of the PIFA antenna [3].
B. Textile Antenna Designs The work in [6], claimed to be the first textile antenna with circular polarization. Circular polarization is needed to ensure that the antenna is reliable in applications where the wearer is mobile. As a result of this dynamism, the orientation of the body changes continuously. A circular polarization wave radiates energy in both the horizontal and vertical planes and all planes in between [7] so however the body orientation changes, the antenna still can receive signals. Substrate used in the design is polymide spacer fabric with 6 mm thickness and has a permittivity of 1.5. A conductive material – a nickel plated woven textile – was used as the antenna patch and the ground plane. The conductive textile used possesses sheet resistance of less than /square to keep losses at a minimum. In order to connect the textile antenna with the SMA connector, a 50 impedance line was fabricated on a printed circuit board (PCB). Construction of the textile is shown in Figure 3. A textile antenna for protective clothing in [8] was also designed with circular polarization to improve reception in the real life application. The design produced a circular polarization by placing the feed point of the patch, thereby exciting the two orthogonally polarized TM01 and TM10 modes. Electromagnetic band gap (EBG) is one of the most rapidly advancing sectors in the arena of electromagnetic research. Using an EBG structure for the ground plane resembles a perfect magnetic conductor. As a consequence, an electric current can radiate efficiently near the EBG ground plane. This concept used by Zhu in [9], in designing a dual-band, body worn antenna. Figure 4 shows the double square dual band EBG. Based on the S11 measurement result in Figure 5, creating the EBG layer at the ground plane improved the return loss of the antenna to be approximately about the same (at –15 dB) for all the resonant frequencies. It was also shown that the surface current of the antenna is balanced, thus increasing the antenna’s efficiency.
Fig. 4: Fabric dual-band EBG substrate [9]
Fig.5: S11 measurement result [9]
II.
DESIGNING WEARABLE ANTENNAS
A. Conductive material Electro-textiles are conductive fabric constructed by interpolating conductive metal/polymer threads with normal fabric. Characteristics of these fabrics, which are wearable, durable and flexible, made it suitable to be integrated into clothing [12]. Ivo Locker in [13] discussed the requirements for conductive fabrics in designing textile antennas. The conductive textile was desired to have a low and stable electrical resistance ( /Square) to minimize losses. Flexibility of the material was also needed so that the antenna can be deformed. Another researcher in [14] used a flexible material so that it can be wrapped around an arm as shown in Figure 6. The material used was woven conductive fabric type, having a 0.05 /square of surface resistance and 0.125 mm thickness. The material selection is a critical step when designing an antenna, in order to be robust and suited for certain applications. The work in [15], which used an aramid
IV. ANALYSIS REQUIRED FOR WEARABLE ANTENNAS
Fig.6: shows the drapability of the wearable antena [14].
woven fabric as the material, is flame resistant and suitable for integration into fire fighter garment. In [16], a highly conductive metalized Nylon fabric was used as the conductor. Its three metalized layers (NI/Cu/Ag) provided high conductivity while the surface resistivity is 0.03 /square. Besides that, the material also provided flexibility and protection against corrosion, which will be suitable when applied in a highly corrosive environment. B. Fabrication method The fabrication techniques, which will be partially determined by the materials used in designing a textile antenna, is also another important consideration, in defining and determining the overall cost of the design. This is because different material used in the antenna design requires different fabrication methods. The work in [17] explored different fabrication methods carried out to fabricate the same dimension of microstrip patch antenna. The use of copper tape was identified as the simplest technique, as it can directly be applied to the substrates, and has no extra fabrication process. Besides that, a more flexible fabrication technique was to use a conductive spray technique, which can be applied to any textile material [17]. The spray, which is a mixture of copper with gases under pressure, can be used to obtain a conductive layer on the textile surfaces exposed to the spray. Figure 7 shows the microstrip patch fabricated using copper tape, woven copper thread and conductive spray. Researchers in [18], on the other hand, constructed an Eshaped patch antenna using copper tape as the conducting element, and felt fabric as the substrate. The copper tape was cut according to the dimension of the E-shaped patch antenna and mounted on the felt fabric. Manufacturing process of the textile UWB antenna using high conductive metalized Nylon in [19] was difficult and had to be done cautiously. Dimension of the antenna must be retained while being attached to the substrate using adhesive that not affects the electrical properties of the textile material. A SMA jack was connected to the textile antenna using conductive twocomponent glue
Fig. 7: Fabricated textile patch antennas. From left to right; applying copper tape, woven copper thread and conductive spray [17].
Generally, the measurements required for conventional antenna design are return loss, radiation pattern, gain and efficiency. However, conventional planar antennas are flattened, which makes it unnecessary to investigate its bending characteristic. On the contrary, a wearable antenna requires other factors to be taken into careful consideration to guarantee the performance of the antenna in a body-worn context. This section will include other measurements that have to be carried out in examining a wearable antenna design.
Fig. 8 Computed SAR distributions at 2.2 GHz [22].
A. SAR modeling Public concern regarding the health effects of radiation and legal requirements around the world have urged engineers and researchers to always consider the amount of power absorbed by the human body. Therefore, specific absorption rate (SAR) by wireless devices has been defined. The two most commonly used SAR limit are those of IEEE [20] 1.6W/kg for any 1g of tissue, and ICNIRP (International Commission on Non-Ionizing Radiation Protection [21]) 2W/kg for any 10g of tissue. In [22], a torso model constructed from CT and MRI image of real human body was employed in the SAR modeling. The model was used to study the antenna performance when the antenna placed was on the upper portion of the human body. Figure 8 is an example of simulated SAR distribution at 2.2 GHz. From the simulated result, the SAR distribution was given for 1 Watt delivered power and the colour bar showed relative SAR value in dBi. B. Measurement with different bending Measurements for flexible wearable antenna have to be done with different bending position. This is to ensure the antenna performance in real life applications is up to mark, especially when the antenna is applied to rounded parts of the body, such as an arm. In [13], S11 measurements were carried out using different bending conditions of the antenna. The antenna attached around a plastic cylinder as shown in Figure 9 was measured to investigate this bending characteristic. Based on the analysis, the resonance shifted towards the lower frequencies and the bandwidth became smaller when bent, independent of the bending direction. The smaller the bending is, the lower the frequency it became. Investigators in [10] have also found out similar measurement trends when analyzing the bending characteristics. One of the methods to overcome this was that the antenna had to be designed with a wide frequency bandwidth. This is so that if the frequency really did shift,
REFERENCES 1. 2. 3. 4. Fig. 9: Measurement setup of the antenna [13].
the antenna will still be able to operate within the desired frequency range. Tanaka in [14] investigated this through his measurement report for return loss when the H-plane and Eplane of the antenna were bent. The bending conditions were differentiated using degrees. 00 indicated that the antenna is flattened, while a 900 bending was indicated when the Hplane was bent into a V-shape at the center of the microstrip antenna. A 1800 bending indicated that the antenna was bent into U-shape. Similar bending conditions also apply to the Eplane of the antenna. C. On body measurements Beside stand-alone antenna measurements, where the antenna was measured without presence of human body, onbody measurements have to be carried out as well, in order to ascertain the performance of the antenna at different on-body positions. Positions of wearable antennas will potentially differ, depending on the application of the antenna. Wearable antennas might be designed to be placed on the chest, arm, back of the body and etc. In [17], the fabricated antenna was measured in free space, on human chest and on the human arm. In [15] researchers also went as far as including human body for the measurement. From these previous investigations, it was found out that the antenna placed on the back of the body as shown in Figure 10, is the most stable location that will reduce the change of body orientation compared other parts such as the arm.
5. 6. 7. 8. 9. 10.
11. 12.
13. 14. 15. 16. 17. 18. 19. 20.
Fig.10: Measurement setup for on-body measurement [15].
V. CONCLUSION From the review, it is concluded that there are several additional aspects to be taken into account when designing a wearable antenna, in comparison to a conventional antenna design. It showed that there exists a spectrum of potential materials that could be used in designing wearable antennas. SAR analyses, measurements with different antenna bending and on body measurements have to be done in order to obtain an antenna design that meets the wearable antenna specification. Wearable antennas are promising, and boast a great future alongside the development of the rapidly growing wireless communication technology.
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22.
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