4
2007 Loughborough Antennas and Propagation Conference COMPACT UWB WEARABLE ANTENNA B. Sanz-Izquierdo, J.C. Batchelor and M.I. Sobhy Department of Electronics, The University of Kent Canterbury, Kent, CT2 7NT, UK. Tel: +44 1227827004, Fax: +44 1227456084 b.azkn.ac .uk, tchlo t ac.uk Abstract This paper reports a novel UWB antenna suitable for wireless on body applications. The antenna is based on previous studies of wearable button antennas and uses the circular metal nail employed in some buttons to create the radiating element of the antenna. The visible part of the antenna was fabricated using PTFE, although other dielectric or plastic material could be used. The novel antenna is intended for textile jackets where the button structure can be easily camouflaged and disregarded as a plastic button. The antenna is fed by a microstrip line and denim was used as the substrate as well as the superstrate that hides the feeding. The antenna achieves the 3.1GHz to 10.6GHz bandwidth required for the ultra-wide-band technology. The radiation patterns are those of a planar circular monopole antenna with no significant deleterious effect of the dielectric structure on the far-field patterns. Introduction Wearable electronics have started to appear in the commercial market in recent years. Some of the most fashionable applications are volume control for mp3 player [1] and step sensors with transmitter in shoe trainers [2]. Antennas are considered an essential component in this new trend of wearable electronics where wireless can avoid the use of physical connections between wearable devices. Ultra-wide-band is an emerging technology which promises high-speed data transmission at low cost for short-range communications [3]. UWB technology is seen as a good candidate for wireless on body networks with research in [4] having characterized on-body propagation channels. Several antennas have been developed for wearable antennas in form of flexible metal patches on a textile substrate i.e. [5-6]. More recently, the authors of this manuscript have developed antennas that can be regarded as metallic button structures [6-9]. In [9] a metallic button antenna for ultra-wide-band communications was presented with the shape of a conical monopole on a textile substrate. The UWB antenna showed good-omnidirectional radiation pattern, however, the dimension of the antenna were significantly higher than existing metallic buttons. The antenna to be discussed in this paper is shown in Fig. 1. The antenna uses the same button structure principle; however, the metallic structure is only used for the nail that attaches the button to the clothing while the rest of the button is designed on PTFE. The resulting structure is over 50% smaller in surface area and height than the antenna in [9]. However, the equivalent antenna of the novel structure is a planar monopole which is more dependent on the human body proximity [12] and would present higher radiation towards the human body. Antenna design: The main dimensions of the novel UWB wearable antenna are shown in Fig. 2 and Table I. The antenna is made up of a square ground plane on a denim textile substrate, a 50ohm microstrip line, a metallic disc connected to the microstrip line and a PTFE button structure fixed to the metal disc. The measured permittivity of the denim substrate was relative epsilon = 1.7 (approx.). The overall size of the metal ground was 36 by 20.8mm and there was a gap of 8.7mm between the edges of the ground plane and the 16mm lower disc of the antenna. A second layer of denim textile was placed on the top of the microstrip line to hide the feeding on the clothing. 1-4244-0776-1/07/$20.00 (C2007 IEEE 2 -3 April 2007. Loughborough, UK. 121

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Page 1: [IEEE 2007 Loughborough Antennas and Propagation Conference - Loughborough, UK (2007.04.2-2007.04.3)] 2007 Loughborough Antennas and Propagation Conference - Compact UWB Wearable Antenna

2007 Loughborough Antennas and Propagation Conference

COMPACT UWB WEARABLE ANTENNA

B. Sanz-Izquierdo, J.C. Batchelor and M.I. SobhyDepartment ofElectronics,

The University ofKentCanterbury, Kent, CT2 7NT, UK.

Tel: +44 1227827004, Fax: +44 1227456084b.azkn.ac .uk, tchlo t ac.uk

AbstractThis paper reports a novel UWB antenna suitable for wireless on body applications. The antenna is based onprevious studies of wearable button antennas and uses the circular metal nail employed in some buttons tocreate the radiating element of the antenna. The visible part of the antenna was fabricated using PTFE,although other dielectric or plastic material could be used. The novel antenna is intended for textile jacketswhere the button structure can be easily camouflaged and disregarded as a plastic button. The antenna is fedby a microstrip line and denim was used as the substrate as well as the superstrate that hides the feeding. Theantenna achieves the 3.1GHz to 10.6GHz bandwidth required for the ultra-wide-band technology. Theradiation patterns are those of a planar circular monopole antenna with no significant deleterious effect of thedielectric structure on the far-field patterns.

IntroductionWearable electronics have started to appear in the commercial market in recent years. Some of the mostfashionable applications are volume control for mp3 player [1] and step sensors with transmitter in shoetrainers [2]. Antennas are considered an essential component in this new trend of wearable electronics wherewireless can avoid the use of physical connections between wearable devices.Ultra-wide-band is an emerging technology which promises high-speed data transmission at low cost forshort-range communications [3]. UWB technology is seen as a good candidate for wireless on body networkswith research in [4] having characterized on-body propagation channels.Several antennas have been developed for wearable antennas in form of flexible metal patches on a textilesubstrate i.e. [5-6]. More recently, the authors of this manuscript have developed antennas that can beregarded as metallic button structures [6-9]. In [9] a metallic button antenna for ultra-wide-bandcommunications was presented with the shape of a conical monopole on a textile substrate. The UWBantenna showed good-omnidirectional radiation pattern, however, the dimension of the antenna weresignificantly higher than existing metallic buttons.The antenna to be discussed in this paper is shown in Fig. 1. The antenna uses the same button structureprinciple; however, the metallic structure is only used for the nail that attaches the button to the clothingwhile the rest of the button is designed on PTFE. The resulting structure is over 50% smaller in surface areaand height than the antenna in [9]. However, the equivalent antenna of the novel structure is a planarmonopole which is more dependent on the human body proximity [12] and would present higher radiationtowards the human body.

Antenna design:The main dimensions of the novel UWB wearable antenna are shown in Fig. 2 and Table I. The antenna ismade up of a square ground plane on a denim textile substrate, a 50ohm microstrip line, a metallic discconnected to the microstrip line and a PTFE button structure fixed to the metal disc. The measuredpermittivity of the denim substrate was relative epsilon = 1.7 (approx.). The overall size of the metal groundwas 36 by 20.8mm and there was a gap of 8.7mm between the edges of the ground plane and the 16mmlower disc of the antenna. A second layer of denim textile was placed on the top of the microstrip line to hidethe feeding on the clothing.

1-4244-0776-1/07/$20.00 (C2007 IEEE

2 -3 April 2007. Loughborough, UK.

121

Page 2: [IEEE 2007 Loughborough Antennas and Propagation Conference - Loughborough, UK (2007.04.2-2007.04.3)] 2007 Loughborough Antennas and Propagation Conference - Compact UWB Wearable Antenna

2007 Loughborough Antennas and Propagation Conference

Figure 1. Photo of the UWB wearable antenna. Left, top view with top textile layer and thedielectric button structure. Centre, Middle layer showing the circular radiating structurewith the connector. Right, lower structure of the antenna with the flexible ground plane.

Th MetalTh disc

Dt Do Db

i l- ~~~~~~~~~~~~~~~~~~~~~~~~~~~PPTFE

Metal Lg Lcground Ws-

Microstripline

Textilematerial

WgFigure 2. Antenna Elevation with principal dimensions

Table I. Principle antenna dimensionsParameter Size (mm) Parameter Size (mm)

Dt 15.0 Lg 20.8Db 16.0 Lc 29.5Do 5.0 Th 7.0

|Wg 36.0 H textile 1.5

122

4-.,

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Page 3: [IEEE 2007 Loughborough Antennas and Propagation Conference - Loughborough, UK (2007.04.2-2007.04.3)] 2007 Loughborough Antennas and Propagation Conference - Compact UWB Wearable Antenna

2007 Loughborough Antennas and Propagation Conference

Results:The measured and simulated Return Loss (SI,) curves in air space are given in Fig.3. The measured returnloss was obtained using an HP 8722 ES network analyzer with coaxial calibration and CST MicrowaveStudio was used for the simulations. The calculated and measured -1OdB SI, bandwidth achieved more thanthe 3-10.6 GHz needed for the UWB communication system with good agreement between the simulatedand measured results.

0

-2

-4- Measured

-6 Simulated

-10

-16

-182 3 4 5 6 7 8 9 10 11 12

frequency (GHz)

Figure 3. Measured and computed S11 curves for dual band button antenna.

The measured x-y radiation patterns for the button antenna on air are shown in Fig.4. Good omni-directionality can be observed with measured gains of 2.6dBi at 3.5 GHz, 1.9dBi at 6 GHz and 3.4dBi at9 GHz. The antenna was designed to be mounted on a textile jacket with a typical separation from the humanbody between 5 to 10mm. The proximity of the human body is expected to have some effect on the antennainput matching and radiation pattern.

ConclusionsA novel compact wearable antenna for Ultra-wide-band applications have been presented. The antenna ismade up of a square metallic ground plane, a microstrip feed line, a circular metallic disc and a buttonstructure made of PTFE material. The antenna achieves sufficient -1OdB bandwidth matching to cover the3.1GHz to 10.6GHz UWB band. Good omni-directional patterns are measured in the x-y plane withmeasured gains between 2.6dBi and 3.4dBi for the UWB frequency band. The antenna is intended to beembedded on a wearable jacket with some effect expected on the input matching and radiation patterns of theantenna.

AcknowledgmentsThe authors would like to thank Nigel Simpson, Terry Rockhill and Clive Birch for helping in the fabricationand measurement of the antennas.

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2007 Loughborough Antennas and Propagation Conference

References[1] http://www.kenpojacketforipod.com/mens/demo.swf[2] http://www.nike.com/nikeplus/#overview.[3] http://www.see.ed.ac.uk/-ccc/research.html Proc. IEEEVehicular Tech. Conf (VTC2004-Fall), LosAngeles, CA. (2004)[4] A. Alomainy; Y.Hao; C.G. Parini; P.S Hall, "Comparison between two different antennas for UWB on-body propagation measurements", IEEE Antennas and Wireless Propagation Letters Volume 4, 2005Page(s):.31 - 34.[5] P.Salonen and H. Hurme, "A novel fabric WLAN antenna for wearable applications", IEEE Antennas

and Propagation Society International Symposium, vol: 2, June 2003, pp. 700-703.[6] M.Tanaka and J.-H. Jang, "Wearable microstip antenna", IEEE ntennas and Propagation

SocietyInternational Symposium, vol: 2, June 2003, pp. 704-707.[7] B.Sanz-Izquierdo, F. Huang and J.C. Batchelor, "Dual Band Button Antennas for WearableApplications", IWAT2006, White Plains, New York, March 6-8.[8] B.Sanz-Izquierdo, F. Huang, J.C.Batchelor "Wearable Dual Band Button Antennas", LoughboroughAntennas & Propagation Conference (LAPC), Loughborough April 2006, pp. 33-35.[9] B.Sanz-Izquierdo, M.I.Sobhy, J.C.Batchelor "UWB Wearable Button Antenna", European Conferenceon Antennas and Propagation EuCAP06, Nice, France, p. 131, 2006 November

90 90

270

(a) 3.5 GHz (b) 6.0 GHz

270

(b) 9.0 GHz

Figure 4. Measured y-z plane radiation patterns of the UWB wearable Antenna on air.Co-polarisation Cross polarisation

124

2 -3 April 2007. Loughborough, UK.