A Review of Wearable AntennaN. H. M. Rais1, P. J. Soh1, F.Malek1, S. Ahmad1, N.B.M. Hashim1, P.S Hall2

1School of Computer and Communication, University Malaysia Perlis (UniMAP),No. 12 & 14, Jln Satu, Kompleks Pengajian UniMAP Seberang Ramai,

02000 Kuala Perlis, Perlis, Malaysia.nh_husna@yahoo.com.my, pjsoh@unimap.edu.my, shahadah@unimap.edu.my, baya@unimap.edu.my

2The University of BirminghamSchool of Engineering

Electronic, Electrical and Computer EngineeringEdgbaston, Birmingham, B15 2TT UK

P.S.Hall@bham.ac.uk

Abstract- Utilization of wearable textiles in the antennasegment has been seen on the rise due to the recentminiaturization of wireless devices. A wearable antennais meant to be a part of the clothing used forcommunication purposes, which includes tracking andnavigation, mobile computing and public safety. Thisliterature review intend to disclose this unconventionalantenna technology and provides readers with thebackground of the wearable antenna that would includeabout specification of the antenna, material for theantenna and analysis that must be done to design properwearable antennas. All the designs presented are of therecent development in wearable technology.Keywords conductive textile antenna, wearable antennas, on-body communication

I. INTRODUCTION

In recent years, body-centric wirelesscommunication becomes an important part of fourthgeneration mobile communication systems (4G). Insupporting the increasing interest in antennas andpropagation research for body communication systems, theIEEE 802.15 standardization group has been established tostandardize applications intended for on-body, off-body orin-body communication. Body-centric communications takesits place firmly within the sphere of personal area networks(PANs) and body area networks (BANs). One of theapplications the on-body communications describe thelink between body mounted devices communicatingwirelessly, while off-body communication defines the radiolink between body worn devices and base units or mobiledevices located in surrounding environment. Finally, in-bodycommunication is communication between wireless medicalimplants and on body nodes [1].

One of the dominant research topics in antennas forbody-centric communications is wearable, fabric-basedantennas. Commonly, wearable antenna requirements for allmodern application require light weight, low cost, almostmaintenance-free and no installation. There are number ofspecialized occupation segments that apply body centriccommunication systems, such as paramedics, fire fighters,and military. Besides, wearable antennas also can be appliedfor youngsters, the aged, and athletes for the purpose ofmonitoring.

Designing textile antenna requires the knowledgeon electromagnetic properties such as permittivity, and losstangent of the textile material. Conductive textile such asZelt, Flectron and pure copper polyester taffeta fabrics areregularly used as the radiating element while non-conductivetextile such as silk, felt and fleece are used as substrates.Electromagnetic properties for these textiles are not readilyavailable. Measurement of the electromagnetic properties oftextile substrate done in [2] using a transmission/reflectionwaveguide method. Important permittivity and loss tangentvalue were included in the simulation.

This work intends to present recent types of antennaand other considerations that have to be investigated,including suitable material selection, fabrication methodsand analysis required for a wearable antenna design.

II. TYPES OF WEARABLE ANTENNA

A. Conventional Wearable DesignsConventional antenna designs which include planar

dipoles, monopoles, planar inverted-Fs (PIFAs), andmicrostrip patches were used in recent research for wearableantennas design. Microstrip antennas are planar and thesecan be manufactured onto a printed circuit board (PCB). Thismade them a practical antenna type due to their low cost, andeases for fabrication. In [3], Salonen explored the planarinverted-F antenna (PIFA) design as a wearable antenna thatintended to be placed on the sleeve of clothing. PIFAs arelike quarter wave monopole antennas, which folded structureis parallel with the ground plane, as showed in Figure 1.

Figure 2 shows one possible location placement ofthe antenna, on the sleeve of clothing. The effect of theantennas ground plane to the direction of the strongestradiation was also shown. The ground plane formed a shieldfor human so the radiation will not radiate towards humanbody. In other words, the ground plane functioned as areflector for radiation. Using the same antenna designconcept, the design of a wearable flexible planar inverted-Fantenna (FlexPIFA) for Bluetooth operated system wasintroduced in [4]. The antenna was designed to be attachedon a human arm and used flexible substrate material with0.236 mm thickness, a dielectric constant of 3.29, and a losstangent of 0.0004.

Antennas requirements that influence the antennaselection in [5] are low profile, operated in the frequency

range of 100 MH 500 MHz, omni-directional coverage inthe azimuth plane, produces wideband return loss andpossesses a vertical or circular polarization. Omni-directionalradiation pattern is desired for a wearable antenna, in whichit will be suitable for mobile devices and smart clothing.Besides that, omni-directional radiation pattern should bedesigned to have minimal/no side lobes, which can harm thehuman body.

Fig. 1: Construction of the PIFA [3].

Fig. 2: Possible placement of the PIFA antenna [3].

B. Textile Antenna DesignsThe work in [6], claimed to be the first textile antenna

with circular polarization. Circular polarization is needed toensure that the antenna is reliable in applications where thewearer is mobile. As a result of this dynamism, theorientation of the body changes continuously. A circularpolarization wave radiates energy in both the horizontal andvertical planes and all planes in between [7] so however thebody orientation changes, the antenna still can receivesignals. Substrate used in the design is polymide spacerfabric with 6 mm thickness and has a permittivity of 1.5. Aconductive material a nickel plated woven textile wasused as the antenna patch and the ground plane. Theconductive textile used possesses sheet resistance of less than??/square to keep losses at a minimum. In order to connectthe 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. Atextile antenna for protective clothing in [8] was alsodesigned with circular polarization to improve reception inthe real life application. The design produced a circularpolarization by placing the feed point of the patch, therebyexciting the two orthogonally polarized TM01 and TM10modes.

Electromagnetic band gap (EBG) is one of the mostrapidly advancing sectors in the arena of electromagneticresearch. Using an EBG structure for the ground planeresembles a perfect magnetic conductor. As a consequence,an electric current can radiate efficiently near the EBGground plane. This concept used by Zhu in [9], in designinga dual-band, body worn antenna. Figure 4 shows the doublesquare dual band EBG. Based on the S11 measurement resultin Figure 5, creating the EBG layer at the ground planeimproved the return loss of the antenna to be approximatelyabout the same (at 15 dB) for all the resonant frequencies. Itwas also shown that the surface current of the antenna isbalanced, thus increasing the antennas efficiency.

A textile-based antenna in [10] was designed toprovide a wireless short range communication in body andpersonal area network. It was made entirely out of textilematerial. The aperture coupled feeding mechanism was usedfor the design, helped in increasing the bandwidth comparedto other classic planar antenna feeds [11].

Fig.3: Antenna with 50? line feed fabricated on a PCB [6].

Fig. 4: Fabric dual-band EBG substrate [9]

Fig.5: S11 measurement result [9]

II. DESIGNING WEARABLE ANTENNAS

A. Conductive materialElectro-textiles are conductive fabric constructed by

interpolating conductive metal/polymer threads with normalfabric. Characteristics of these fabrics, which are wearable,durable and flexible, made it suitable to be integrated intoclothing [12]. Ivo Locker in [13] discussed the requirementsfor conductive fabrics in designing textile antennas. Theconductive textile was desired to have a low and stableelectrical resistance (???/Square) to minimize losses.Flexibility of the material was also needed so that theantenna can be deformed.

Another researcher in [14] used a flexible material sothat it can be wrapped around an arm as shown in Figure 6.The material used was woven conductive fabric type, havinga 0.05?/square of surface resistance and 0.125 mmthickness. The material selection is a critical step whendesigning an antenna, in order to be robust and suited forcertain applications. The work in [15], which used an aramid

Fig.6: shows the drapability of the wearable antena [14].

woven fabric as the material, is flame resistant and suitablefor integration into fire fighter garment. In [16], a highlyconductive metalized Nylon fabric was used as theconductor. Its three metalized layers (NI/Cu/Ag) providedhigh conductivity while the surface resistivity is0.03?/square. Besides that, the material also providedflexibility and protection against corrosion, which will besuitable when applied in a highly corrosive environment.

B. Fabrication methodThe fabrication techniques, which will be partially

determined by the materials used in designing a textileantenna, is also another important consideration, in definingand determining the overall cost of the design. This isbecause different material used in the antenna designrequires different fabrication methods. The work in [17]explored different fabrication methods carried out tofabricate the same dimension of microstrip patch antenna.The use of copper tape was identified as the simplesttechnique, as it can directly be applied to the substrates, andhas no extra fabrication process.

Besides that, a more flexible fabrication technique wasto use a conductive spray technique, which can be applied toany textile material [17]. The spray, which is a mixture ofcopper with gases under pressure, can be used to obtain aconductive layer on the textile surfaces exposed to the spray.Figure 7 shows the microstrip patch fabricated using coppertape, woven copper thread and conductive spray.

Researchers in [18], on the other hand, constructed an E-shaped patch antenna using copper tape as the conductingelement, and felt fabric as the substrate. The copper tape wascut according to the dimension of the E-shaped patchantenna and mounted on the felt fabric. Manufacturingprocess of the textile UWB antenna using high conductivemetalized Nylon in [19] was difficult and had to be donecautiously. Dimension of the antenna must be retained whilebeing attached to the substrate using adhesive that not affectsthe electrical properties of the textile material. A SMA jackwas connected to the textile antenna using conductive two-component glue

Fig. 7: Fabricated textile patch antennas. From left to right; applying coppertape, woven copper thread and conductive spray [17].

IV. ANALYSIS REQUIRED FOR WEARABLEANTENNAS

Generally, the measurements required for conventionalantenna design are return loss, radiation pattern, gain andefficiency. However, conventional planar antennas areflattened, which makes it unnecessary to investigate itsbending characteristic. On the contrary, a wearable antennarequires other factors to be taken into careful consideration toguarantee the performance of the antenna in a body-worncontext. This section will include other measurements thathave to be carried out in examining a wearable antennadesign.

Fig. 8 Computed SAR distributions at 2.2 GHz [22].

A. SAR modelingPublic concern regarding the health effects of radiation

and legal requirements around the world have urgedengineers and researchers to always consider the amount ofpower absorbed by the human body. Therefore, specificabsorption 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 (InternationalCommission on Non-Ionizing Radiation Protection [21])2W/kg for any 10g of tissue. In [22], a torso modelconstructed from CT and MRI image of real human bodywas employed in the SAR modeling. The model was used tostudy the antenna performance when the antenna placed wason the upper portion of the human body. Figure 8 is anexample of simulated SAR distribution at 2.2 GHz. From thesimulated result, the SAR distribution was given for 1 Wattdelivered power and the colour bar showed relative SARvalue in dBi.

B. Measurement with different bendingMeasurements for flexible wearable antenna have to be

done with different bending position. This is to ensure theantenna performance in real life applications is up to mark,especially when the antenna is applied to rounded parts ofthe body, such as an arm. In [13], S11 measurements werecarried out using different bending conditions of the antenna.The antenna attached around a plastic cylinder as shown inFigure 9 was measured to investigate this bendingcharacteristic. Based on the analysis, the resonance shiftedtowards the lower frequencies and the bandwidth becamesmaller when bent, independent of the bending direction. Thesmaller the bending is, the lower the frequency it became.Investigators in [10] have also found out similarmeasurement trends when analyzing the bendingcharacteristics.

One of the methods to overcome this was that theantenna had to be designed with a wide frequencybandwidth. This is so that if the frequency really did shift,

Fig. 9: Measurement setup of the antenna [13].

the antenna will still be able to operate within the desiredfrequency range. Tanaka in [14] investigated this through hismeasurement report for return loss when the H-plane and E-plane of the antenna were bent. The bending conditions weredifferentiated using degrees. 00 indicated that the antenna isflattened, while a 900 bending was indicated when the H-plane was bent into a V-shape at the center of the microstripantenna. A 1800 bending indicated that the antenna was bentinto U-shape. Similar bending conditions also apply to the E-plane of the antenna.

C. On body measurementsBeside stand-alone antenna measurements, where the

antenna was measured without presence of human body, on-body measurements have to be carried out as well, in order toascertain the performance of the antenna at different on-bodypositions. Positions of wearable antennas will potentiallydiffer, depending on the application of the antenna. Wearableantennas might be designed to be placed on the chest, arm,back of the body and etc. In [17], the fabricated antenna wasmeasured in free space, on human chest and on the humanarm. In [15] researchers also went as far as including humanbody for the measurement. From these previousinvestigations, it was found out that the antenna placed onthe back of the body as shown in Figure 10, is the moststable location that will reduce the change of bodyorientation compared other parts such as the arm.

Fig.10: Measurement setup for on-body measurement [15].

V. CONCLUSIONFrom the review, it is concluded that there are

several additional aspects to be taken into account whendesigning a wearable antenna, in comparison to aconventional antenna design. It showed that there exists aspectrum of potential materials that could be used indesigning wearable antennas. SAR analyses, measurementswith different antenna bending and on body measurementshave to be done in order to obtain an antenna design thatmeets the wearable antenna specification. Wearable antennasare promising, and boast a great future alongside thedevelopment of the rapidly growing wireless communicationtechnology.

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