Bend Profiling of Textile Antenna for Body Centric Wireless Communication

Bend Profiling of Textile Antenna for Body Centric Wireless Communication

S. Shahid1, M. Rizwan1, M.A.B. Abbasi1, M.A. Tarar1, S.M. Abbas2

1 Department of Electrical Engineering, School of Electrical Engineering and Computer Science (SEECS), National University of Sciences and Technology (NUST), Islamabad, Pakistan

2 Department of Electrical Engineering, COMSATS Institute of Information Technology, Islamabad, Pakistan

{saleem.shahid, 11mseemrizwan, 11mseemabbasi, munir.tarar}@seecs.edu.pk, [email protected]

Abstract Body-centric wireless communication (BCWC) is assumed as core technology for healthcare and rescue applications which always needs a flexible and bendable antenna for near the body propagation. Multiple technologies terminated on a single antenna with suitable radiation and gain characteristics are the concern of antenna designers from few decades. A wide band antenna, covering variety of frequency bands used for BCWC, is realized and presented. Human body effects on antenna and bend profiling of antenna is analyzed. In addition to flexibility and bending characteristics, easy integration in human clothing is also considered. Copper tape is used as conductive part of the antenna while 1mm thin blue jean is used as dielectric substrate, with relative permittivity of 1.68 and loss tangent 0.01. Various meandering techniques were investigated to enhance the gain and bandwidth of the antenna without effecting antenna performance in near body bent conditions. Parametric analysis confirms the antenna resonance at 2.45GHz with impedance bandwidth of 2.15GHz ranging from 0.8 to 2.95GHz and peak gain up to 4.2dB. The fabricated antenna is tested using Agilent Vector Network Analyzer. Simulated and measured results, of this novel recto-circular antenna showed good agreement proving the propose antenna as a suitable candidate for near body applications.

Index TermsBody-centric wireless communication, Wide band, Flexible antenna, Recto-circular, Antenna bending

I. INTRODUCTION Antenna is always recognized as an important component

in wireless communication systems. Large bandwidth and compact size of an antenna are important for power efficient broadband wireless communication as it is replacing cables and providing flexibility in todays user demands. BCWC is emerging as front line technology to fully avail the benefits of the wireless communication in medical and military applications. It combines wireless body area networks (WBAN), wireless body sensor networks (WBSN) and wireless personal area networks (WPAN), due to which it is considered as an important part of fourth generation personal communication systems (PCS) [1]. A WBSN uses sensor or electronic devices which are implanted inside or mounted on the human body for different applications. WBAN is defined by IEEE 802.15.4 as a communication standard in which devices can be integrated on or around the human body for user benefit [2]. It has many applications such as personal

video and audio devices, safety, sports, fitness monitoring where antenna subsystems are placed close to human for near the body communication [3]. M.M. Khan et. al has presented a dual band power efficient antenna on FR4 epoxy substrate [4] whereas P.S. Hall et. al has modeled variety of narrowband antennas for on-body communication [5-6]. A dual band button antenna for WLAN (2.45 and 5.2GHz) applications was presented by J. Batchelor et. al [7]. A. Alomainy et. al presented various wearable antennas operating at 2.4GHz (ISM band) for on-body communications [8-10]. To the best of authors knowledge, a wideband flexible antenna needs to be designed which covers almost all the near body communication frequencies, also realizing the bend profiling of the antenna.

A wideband flexible substrate antenna is presented in this paper. The proposed antenna covers 800-900MHz band for on-body to on-body communication and 1.8GHz, 2.1GHz and 2.45GHz bands for on-body to off-body communication. The bend profiling of the antenna is also understood for various bent angles in one plane along transmission line. Omni directional radiation patterns have been achieved at 800MHz and relatively directional patterns at higher frequency, which is required for power efficient BCWC [4]. Rest of the paper is organized as; design and modeling of human body phantom and antenna is explained in Section II. Simulated and measured results and their comparison are presented in Section III while Section IV concludes the paper.

(a) (b)

Fig. 1. Antenna Hardware (a) Dimensions (b) Prototype 1744

II. PROPOSED ANTENNA DESIGN The antenna design and its dimensions are shown in Fig.

1(a) whereas antenna prototype is shown in Fig. 1(b). Proposed antenna is printed on blue jeans substrate with relative permittivity (r) 1.68 and loss tangent (tan) 0.01 having dimensions 80x80mm2 and thickness of 1mm. Copper tape is used as radiating sheet to maintain the flexibility of the conformal antenna.

A. Phantom Modeling The antenna needs to be designed in near body

environment for practical integration in BCWC systems. The human body phantom was designed in HFSS (High Frequency Structure Simulator) to carry out simulations in near-body environment. The average widths of skin, muscle and fats are considered while creating a model phantom for antenna simulation as designed in authors previous work [1]. The young male body model with antenna mounted at average distance of 5mm is shown in Fig. 2. The antenna and human body separation has no significant effect on the performance of antenna.

Fig. 2. Human phantom for near-body analysis

B. Antenna Modeling Different antenna modeling phases are shown in Fig. 3.

Antenna is designed in near body environment along with performance analysis at flat, 30o and 60o bend angles. The dimensions of transmission line are kept as 1.2x17.1mm2 for 50 impedance matching as show in Fig. 1(a) and Fig. 3. The design is created by combining rectangular and circular patches with rectangular dimensions 56x56mm2 and circular radius 25mm [11-13]. Using rectangular patch, benefits of radiation are acquired from radiating edges while non-radiating edges do not play any significant role. Circular patch is introduced to convert non-radiating edges of rectangular patch to radiating edges [8]. Partial grounding technique was also used for bandwidth enhancement but back lobes in radiation patterns were also mitigated by applying different meandering techniques [12].

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Fig. 3. Antenna design phases

Symmetry is maintained while uniting circular patch with a rectangular radiator [11-13], to provide equivalent radiating edges. Wide band resonance is achieved but significant frequency shifts has been witnessed antenna is bend at 30o and 60o angles. Here, it is important to mention that, by bending authors means the restructuring of antenna in any one plane. The return loss characteristics of first design phase are shown in Fig. 4.

Fig. 4. Simulated S11 for first design phase of the proposed antenna

The crescent shape slot of radius 20.84mm and width of 1.8mm is subtracted from patch to enhance the bandwidth and get the improved radiation patterns [12]. The frequency shifting at bending angles is still there but desired frequency band is realized in flat position. Return loss characteristics of the second design phase are shown in Fig. 5.

Fig. 5. Simulated S11 for second design phase of the proposed antenna

The quad-circular and semi-circular slots are introduced to create two virtual feed lines for upper and left part of the patch to increase current flow and enhance the bandwidth [8-9]. By the injection of virtual feeds, the radiating edges are decreased but it helps to provide extra current intensity to the upper part of the patch [13] whereas rectangular slit of dimensions 1.2x18.9mm2 is introduced for current blockage and to create extra radiating edges on vertical sides of the patch [7-10]. The frequency shift at 30o bend angle is avoided using these modifications in design. Return loss characteristics of third design phase are shown in Fig. 6.

The antenna is placed at average clothing thickness of 5mm from the body. The detuning in the performance of the antenna usually occurs when placed closed to the human body. This was avoided by using various meandering techniques but due to small size of the antenna, detuning is 1745

more significant due to which frequency band is slightly shifted when antenna is in bent condition. The decrease in return loss is also due to the influence of human body tissues on the antenna [15].

Fig. 6. Simulated S11 for third design phase of the proposed antenna

A thin slot of dimensions 1.12x17mm2 is subtracted near

the transmission line to tune the center frequency at 2.45GHz [9, 11-12]. The return loss of final design phase is shown in Fig. 7. At 2.44GHz S11 is -44dB in flat position whereas it is -33dB at 30o bend angle and -36dB at 60o bend angle with slight frequency shift.

Fig. 7. Simulated S11 for final design of the proposed antenna

III. RESULTS AND DISCUSSION

A. Reflection Coefficient (S11) The designed antenna is fabricated and measured for flat

position, 30o and 60o bend angles. The antenna resonates at 2.2 to 2.5GHz for different conditions but covering all the desired frequency bands all the time. The measured return loss for different bent profiling of the antenna is shown in Fig. 8. In bent scenarios, antenna is also resonating at 3.5GHz WiMAX band.

The bending of antenna at 30o also shows a comparatively higher resonance at frequency of 2.5GHz where the return loss is -42.58dB. This occupies a bandwidth of 2.82GHz from 1.4GHz to 4.22GHz. At higher angles of bending, the pattern of S-parameter is almost similar with frequency back shift. At 60 degree angle, significant resonance at 2.2GHz is achieved with the return loss of -31.11dB which makes this pattern slightly different from the patterns observed at 30o bending angle. At 30o bending, antenna shows larger impedance

bandwidth with averaged return loss of -15 dB whereas 60o angle of bending have a comparatively smaller impedance bandwidth along with the return loss above -10dB at some particular frequencies near 1GHz. However, in all these scenarios, the antenna performance is acceptable.

Fig. 8. Measured S11 of antenna for different bend angles

B. Radiation Patterns Antenna with omnidirectional radiation patterns along the

body surface is required for better on-body radio channel performance at lower frequencies of 800-900MHz. This is also necessary to achieve power-efficient body-centric wireless communication whereas directional patterns are desired for higher frequencies around 2GHz for off-body radio propagation. Close to omni-directional radiation patterns are achieved at 865MHz for on-body to on-body communication, slight defragmentation in patterns is observed for 30o and 60o bend angles. Mostly directional patterns, outward from body, are achieved at 1.8, 2.1 and 2.5GHz for on-body to off-body communication, as desired. Acceptable change in radiation patterns in observed for bending angles of 30o and 60o. Radiation patterns at different frequencies with bending effect on antenna radiation performance are shown in Fig. 9.

For wide band short-range on-body communication, antennas with suitable gain of above 0dB are desired. Near the human body, gain and radiation efficiency are frequency-dependent as electromagnetic properties of body tissues vary with frequency. When the WB antenna is placed in flat position, the gain reduces to 1.9B in the lower frequency band of 1.8GHz, while increases up to 4.2dB in the higher frequency band of 2.5GHz. The antenna placement distance (5mm) hasnt affected the effect gain for the on-body gain for WB antennas. Moreover, when the antenna is placed at bend angle of 30o, gain at 2.5GHz remains in the range of 2.9dB, but at bend angle of 60o, the gain reduces to 1.88 due to destructive interference caused by the lossy tissues of body. Antenna behaves oppositely at frequencies of 865MHz and 1.8GHz; gain at 30o bend angle is less than gain at 60o bend angle. The gain of antenna over the frequency of 0.7-2.9GHz at 5mm from the body is in the range of 0.25 to 4.2dB. Results show that as the bend angle of antenna increases, peak gain of the proposed antenna decreases due frequency detuning by high lossy human body tissues. 1746

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Fig. 9. Antenna radiation patterns at different frequencies, (a-c) 865MHz (d-f) 1.8GHz (g-i) 2.1GHz (j-l) 2.5GHz1747

It is also noticed that, the variation of on-body gain is higher when antenna is placed at lower bending angle while the variation is lower when antenna is bent at higher angles. The frequencies with corresponding gains at different bending angles are tabulated in Table I.

TABLE I. SIMULATED PEAK GAINS

Frequency / Bend Angle 865MHz 1.8GHz 2.1GHz 2.5GHz

0o 2.13 1.90 3.38 4.20 30o 0.35 1.27 1.86 2.90 60o 0.85 0.44 0.53 1.88

IV. CONCLUSION A compact and flexible on-body micro-strip antenna is

presented and discussed. The recto-circular patch antenna design, for the first time, has been realized for near the body scenario along with different bending angles of the textile antenna. Blue jeans with thickness 1mm and permittivity 1.68 is preferred for proposed antenna because of its low cost and easy integration in human clothing. The antenna resonates at 2.5GHz with additional bandwidth of 2.25 GHz ranging from 0.7GHz to 2.95GHz. Near body analysis of the textile antenna is conducted by placing antenna over a young male body phantom with 5mm separation between antenna and phantom. The different design phases towards optimized antenna design with bend profiling were discussed. Antenna at 30o and 60o bending angles is observed; the results show frequency shifting towards higher bands and variation in bandwidth. Omni directional radiation patterns at 865MHz and relatively directional patterns at 1.8, 2.1 and 2.5GHz have been achieved, which makes antenna suitable for power efficient BCWC. The gain of antenna remains in the range of 0.25 to 4.2dB even during antenna bending. The agreement between simulated and measures results marks this antenna feasible for a variety of wireless technologies like WLAN, Bluetooth, UMTS, ZigBee, DECT, WiMAX, Wi-Fi and RFID.

REFERENCES [1] S. Shahid, M. Rizwan, M.A.B. Abbasi, H. Zahra, S. M. Abbas and M.

A. Tarar, Textile Antenna for Body Centric WiMAX and WLAN Applications, 8th IEEE International Conference on Emerging Technologies (ICET 2012), pp. 196-200.

[2] F. Martelli, C. Buratti and R. Verdone, On the performance of an IEEE 802.15.6 Wireless Body Area Network, 11th European Wireless Conference on Sustainable Wireless Technologies (2011), pp. 1-6.

[3] S. Sankaralingam and B. Gupta, A textile antenna for WLAN applications, International Conference on Emerging Trends in Electronic and Photonic Devices & Systems, (ELECTRO '09), pp. 391-394.

[4] M.M. Khan, Q.H. Abbasi, A. Alomainy, C. Parini and H. Yang, Dual band and dual mode antenna for power efficient body-centric wireless communications, IEEE International Symposium on Antennas and Propagation (APSURSI), 3-8 July 2011, pp. 396 399.

[5] P.S. Hall, Y. Hao, Y.I. Nechayev, A. Alomainy, C.C. Constantinou, C.G. Parini, M.R. Kamruddin, T.Z. Salim, D.T. M. Hee, R. Dubrovka, A. Wadally, W. Song, A. Serra, P. Nepa, M. Gallo and M. Bozzetti, Antennas and propagation for on body communication systems, IEEE Antennas and Propagation Magazine, vol. 49, no 3, June 2007, pp. 41- 58.

[6] Y. I. Nechayev, P.S. Hall, Z. H. Hu, Characterization of narrowband communication channels on the human body at 2.45 GHz, IET Microwave Antenna and Propagation, vol. 4, no. 6, 2010, pp. 722 732.

[7] B. Sanz-Izquierdo, F. Huang, and J. C. Batchelor, Covert dual-band wearable button antenna, IEEE Electronics Letter, vol. 42, no. 12, 2006, pp. 3 4.

[8] A. Alomainy, Y. Hao and F. Pasveer, Numerical and Experimental Evaluation of a Compact Sensor Antenna for HealthCare Devices, IEEE Transactions on Medical Circuits and Systems, vol. 1, no .4, Dec. 2007, pp. 242-249.

[9] A. Alomainy, Y. Hao, A. Owadally, C.G. Parini, P.S. Hall, and C.C. Constantinou, Statistical analysis and performance evaluation for onbody radio propagation with microstrip patch antennas, IEEE Transactions on Antenna and Propagation, vol. 55, no. 1, 2007, pp. 245248.

[10] Y. Lu, Y. Huang, H.T. Chattha, Y.C. Shen and S.J. Boyes, An elliptical UWB monopole antenna with reduced ground plane effects, International Workshop on Antenna Technology (iWAT 2010), pp. 14.

[11] Constantine A. Balanis, Antenna Theory Analysis and Design, 3rd Edition, John Wiley & Sons, 1997.

[12] J.D. Kraus, Antennas for All Applications, 3rd edition, New York, McGraw-Hill, 2002.

[13] Z. N. Chen, Antennas for Portable Devices, John Wiley & Sons Ltd, 2007.

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Bend Profiling of Textile Antenna for Body Centric Wireless Communication

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