A FULLY FABRIC WEARABLE ANTENNA FOR HIPERLAN/2 APPLICATIONS S.Sankaralingam1, Sanghamitra Dasgupta2, Sandip Sankar Roy3, Kaushik Mohanty4 and Bhaskar Gupta5

1,2,4,5 Department of Electronics and Tele-Communication Engineering Jadavpur University, Kolkata – 700 032 INDIA

3National Remote Sensing Centre ISRO, Hyderabad – 500625 INDIA

slingam.nec@gmail.com1, sanghamitra_0@rediffmail.com2, sandipsankarroy@gmail.com3,mohanty_kaushik@yahoo.com4, gupta_bh@yahoo.com5

Abstract- Electronics may soon be integrated into

textiles in our near environment. These “Interactive Electronic Textiles” or “Smart Clothes” will benefit many wireless communication applications, leading to the development of body centric networks. Rapid progress in wireless communication promises to replace wired-communication networks in the near future in which antennas play a vital role. This paper deals with design, development and evaluation of a fully fabric wearable antenna suitable for HiperLAN/2 (5.8 GHz band) applications. This antenna yields promising results and demonstrate the use of textile materials as substrates and smart clothes as conducting layers for the design and development of wearable microstrip antennas.

I. INTRODUCTION Wearable computing and wearable electronics are

seen as the next step in integration of electronic devices into everyday human life. In recent years, both 2.45 GHz (WLAN) and 5.8 GHz (HiperLAN/2) bands are used for wireless network applications. HiperLAN/2, which stands for High Performance Radio Local Area Network, is a Wireless LAN standard developed by the Broadband Radio Access Networks (BRAN) division of the European Telecommunications Standards Institute (ETSI). HiperLAN/2 technology operates in the 5 GHz frequency band using Orthogonal Frequency Division Multiplexing (OFDM) and offers many features [1] including high speed transmission (data rate up to 54 Mbps), power saving, mobility support, security support and increased range of coverage (up to 50 m). HiperLAN/2 band ranges from 5.725 GHz – 5.875 GHz with 5.8 GHz as its centre frequency. This HiperLAN/2 band is aimed for communication between all kinds of wireless devices. These mobile wireless systems contain several subsystems and antenna is an essential one among them. Therefore wearable antenna plays a paramount role in optimal design of any wearable system. For convenience of the user, the wearable antenna needs to be hidden and

of low profile. This requires a possible integration of the antenna elements within everyday clothing. Microstrip patch is a suitable candidate for any wearable application, as it can be made conformal for integration into clothing [2]. Copper based wearable antennas with rectangular [3] – [4] and circular [5] – [6] geometries, intended for WLAN applications, have already been reported by the authors of this manuscript. Wearable antennas utilizing conductive fabrics, for WLAN applications, have also been reported by many researchers [7] - [8]. However, there is no much work reported on wearable antennas meant for HyperLAN applications in the literature. Therefore, this paper presents the design, development and assessment of a fully fabric microstrip wearable antenna with circular geometry for HiperLAN/2 applications. The antenna under investigation makes use of Zelt conductive fabric for all its conducting parts and polyester insulating fabric for the dielectric substrate material. Simulated and experimental results on performance characteristics of this wearable antenna like resonant frequency, return loss, impedance bandwidth, gain and radiation pattern are presented. The rest of the paper has been organized as follows: Section II explains the steps involved in the antenna design procedure. Section III describes the electromagnetic modeling and fabrication of antenna. Both simulated and experimental results on the performance characteristics of the wearable antenna under investigation are presented in Section IV. Concluding remarks of this research work are offered at the end of this paper in Section V.

II. ANTENNA DESIGN PROCEDURE The design specifications of the wearable antenna chosen are as follows: resonant frequency (fr) = 5.8 GHz, substrate permittivity ε = 1.44, thickness of the substrate (h) = 2.85 mm, loss tangent (tan δ) = 0.01. The resonant frequency (fr110) for the dominant mode of propagation, in the case of a circular patch antenna, is written as [9]:

f .

п µε .2пa√ε (1)

where v0 is the velocity of light in free space. The resonant frequency of eqn. (1) does not take into account fringing. Fringing makes the patch look electrically larger. So, a correction is introduced by using an effective radius ae to replace the actual radius a. The formula [9] for effective radius is given by, a a 1 h

п εln п

h 1.7726 (2)

where h is the thickness or height of the substrate. The modified equation of the resonant frequency for the dominant mode TM is given below: f .

п √ε (3)

A first-order approximation to the solution of (2) for a is to find ae using (3) and to substitute it into (2). This leads to a F 1 h

пε F ln пFh

1.7726 (4)

where F . √ε (5)

and thickness h is in cm.

III. MODELING AND FABRICATION OF ANTENNA

The modeling of the HiperLAN antenna is performed using the Method of Moments (MoM) based IE3D simulator [10] from Zeland Software Inc., USA. An infinite ground plane is assumed so as to (i) avoid back lobes in the radiation pattern of the antenna (ii) reduce the diffraction and scattering effects at the edges of the ground plane and to (iii) minimize the undesirable effects of surface waves. The conductive parts of the antenna are made up of Zelt fabric whereas the substrate material is the polyester fabric of required thickness. Zelt fabric is more durable, tear resistant and can conform to any shape. It can be cut and sewn like ordinary fabric to make protective clothing. The surface resistivity of Zelt is 0.01 ohms/sq and it can withstand temperature up to 700 C. Its thickness is 0.1613 mm. The value of radius a for this patch antenna is optimized as

11.4 mm by doing rigorous simulations. This wearable patch antenna is excited by means of a coaxial feed. While modeling the coaxial probe feed to patch, the inner and outer diameters of the probe are taken as 1.3 mm and 4.1 mm respectively corresponding to a standard 50 ohm SMA connector. The feed position is optimized to get good matching characteristics (50 ohm impedance) at the centre frequency. It is located at a distance of 5.4 mm from the centre of the circular patch towards its edge. The ground plane size is taken as 120 mm X 120 mm. The size of the insulating fabric material is taken to be equal to that of ground plane. The insulating fabric pieces are stacked and stitched properly to get required thickness. While assembling the antenna elements, the Zelt conductive fabric is sewn on the dielectric fabric material with thread and due care is taken such that there is no air gap between the insulating fabric material and the conducting parts of the antenna. Fig. 1 shows the photograph of the antenna developed and the coordinate system used in the analysis.

IV. RESULTS ON PERFORMANCE

CHARACTERISTICS OF WEARABLE ANTENNA

A. Return Loss Characteristics Simulations and measurements as well are carried

out over the frequency range of 5.2 GHz to 6.2 GHz for the antenna developed. Fig. 2 shows the simulated and measured S11 plots of the HiperLAN/2 antenna under investigation. As depicted by the simulation results, this wearable antenna resonates at a frequency of 5.8 GHz and exhibits a -10dB return loss bandwidth of 558.6 MHz. The measurements are done using a vector network analyzer (Model # 5071 B) from Agilent Technologies. Initially, the network analyzer is calibrated using the 2 port Ecal [11] module, bearing model # 85092C for an operating frequency ranging from 5.2 - 6.2 GHz, which provides excellent accuracy. The fabricated structure is measured for a resonant frequency of 5.91 GHz with an impedance bandwidth of 353 MHz and having a return loss of -23.76 dB at the resonant frequency as shown in fig 2. There is a close match between simulated and measured values of resonant frequencies in this case as the deviation between these values is only 1.9%, which is very much acceptable for practical applications.

(a)

(b) Fig. 1 (a) Photograph of the fabricated wea

(b) Coordinate system As far as the impedance bandwidth i

is observed that the measured value bandwidth is lesser than the simulatedmay be due to variations in inducoffered by the coaxial probe. Howevervalue of impedance bandwidth coveHiperLAN/2 band and therefore this adesigned and employed for High PerfoArea Network applications.

Fig. 2 Return loss plot of circular microstrip w

arable antenna

is concerned, it of return loss

d value, which tive reactance

r, the measured ers the entire antenna can be ormance Local

wearable antenna

A. Far-field Radiation CharaSimulated total far-field radelectromagnetically modeled principal planes of ф = 00 obtained at its simulated resonaGHz. Fabricated circular micsubjected to far-field radiation paat its measured resonant freSimulated and measured far-fiecircular shaped wearable antenfigures 3 (a) – (c).

(a) Simulated

(b) Measured (Azimu

acteristics: diation patterns of

antenna, in both and ф = 900, are

ant frequency of 5.8 crostrip antenna is attern measurements quency (5.91GHz). eld patterns of this nna are shown in

uth)

(c) Measured (Elevation)

Fig. 3 (a)-(c) Radiation patterns of circular shaped wearable HiperLAN antenna

Referring to radiation patterns of the fabricated antenna, it is understood that the values of discrimination between co-polar and cross polar components in azimuth and elevation planes are 23.44 dB and 28.7 dB respectively. These values are reasonably good for practical applications. Simulated values of 3dB beam-width in the ф = 00 and ф = 900

planes of this antenna are 71.380 and 78.310 respectively. Measured 3 dB beam-width values in azimuth and elevation planes, as obtained from the corresponding radiation pattern plots, are 700 and 730 respectively.

C. Gain, Directivity and Efficiency

Simulations are done for a range of frequencies from 5.5 GHz to 6.0 GHz in order to find antenna parameters like gain, directivity and radiating efficiency. Gain of the antenna in the same frequency range is measured using gain-comparison method [12]. Variations of simulated gain, measured gain and simulated directivity as functions of frequency for the investigated antenna are plotted in figure 4. The measured value of gain is greater than estimated value, as it is measured in the laboratory environment and not in the RF shielded anechoic chamber. The simulated radiating efficiency plot of the antenna under study is shown in fig. 5. These promising performance characteristics of the wearable antenna are tabulated in Table 1. The characteristics exhibited by the developed antenna are very useful for practical considerations.

Fig.4 Gain and Directivity as functions of frequency

Fig. 5 Efficiency plot of wearable antenna

TABLE 1 Performance characteristics of wearable antenna

Parameter Simulated Measured

Gain (dBi) 7.67 11.04

Directivity (dBi) 10.66 -

Efficiency (%) 80.10 74.0

V. CONCLUSIONS In this paper a circular shaped wearable patch

antenna has been designed, developed and tested in order to get its impedance and radiation characteristics. The following conclusions may be drawn from this experimental work. Firstly, microstrip antenna is a suitable candidate for wearable applications, as it can be built using fabric substrate materials. The antennas of this type are very versatile and it is easy to make them operate at various frequency bands. In addition, the well known techniques [9] of improving bandwidth and obtaining different polarizations, adopted for microstrip patch antennas are readily suitable for wearable antennas

too. It may be concluded that these textile patch antennas may eventually replace patch antennas on standard PCB substrates for various applications. Further investigations are required to study the effects of antenna bending on performance characteristics of wearable antennas due to human body movements.

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lan2.html#ch1 [2] Pekka Salomen and Heli Hurme, “A Novel Fabric WLAN

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[3] S.Sankaralingam and Bhaskar Gupta, “Performance Study of a Bluetooth antenna for wearable applications” Proc. of Int. Symposium on Antennas and Propagation (ISAP), pp. 975-978, Bangkok, Oct. 2009

[4] S. Sankaralingam and Bhaskar Gupta, “Use of Fabric Materials as Substrates for Antennas for ISM applications” Proc. of Int. conf. on Communication Technologies and VLSI Design, pp. 106-109, Vellore, Oct. 2009

[5] S. Sankaralingam and Bhaskar Gupta, “A circular disk microstrip WLAN antenna for wearable applications,” Proc. of INDICON 2009, pp. 513-516, Ahmedabad, Dec. 2009

[6] S. Sankaralingam and Bhaskar Gupta, “A textile antenna for WLAN applications” Proc. of ELECTRO 2009, pp. 397-400, Varanasi, Dec. 2009

[7] Qiang Bai, Richard Langley, “Wearable EBG antenna bending,” 3rd European Conference on Antennas and Propagation, pp. 182-185, 2009

[8] Hertleer C., Rogier H., Vallozzi L., Van Langenhove V., “A Textile antenna for off-body communication integrated into protective clothing for fire-fighters,” IEEE Tansactions on Antennas and Propagation, vol. 57, issue 4, pp. 919-925, 2009

[9] Ramesh Garg, Prakash Bhartia, Inder Bahl and Apisak Ittipiboon, Microstrip Antenna Design Handbook, Artech House Publishers, 2001

[10] IE3D Electromagnetic Simulator from Zeland Software Inc., USA, 2004 , Release 10.2

[11] Internet resource, Applying Error Correction to Network Analyzer Measurements, Agilent application note 5965-7709E, March 27, 2002, Available: http: //www. home.agilent.com

[12] Constantine A. Balanis, “Antenna Theory: Analysis and Design”, 2nd edition, John Wiley & Sons (Asia) Pte Ltd., Singapore, pp 722-736, 1997