Research Article Design of a Dual-Band On-Body Antenna for a Wireless Body Area Network Repeater System

Transcription

1 Antennas and Propagation Volume 212, Article ID 3797, pages doi:.11/212/3797 Research Article Design of a Dual-Band On-Bod Antenna for a Wireless Bod Area Network Repeater Sstem Keol Kwon, Jaegeun Ha, Soonong Lee, and Jaehoon Choi Department of Electronics and Counications Engineering, Hanang Universit, 17 Haengdang-dong, Seongdong-gu, Seoul , Republic of Korea Correspondence should be addressed to Jaehoon Choi, choijh@hanang.ac.kr Received 17 Jul 212; Accepted 28 September 212 Academic Editor: Z. N. Chen Copright 212 Keol Kwon et al. This is an open access article distributed under the Creative Coons Attribution License, which permits unrestricted use, distribution, and reproduction in an medium, provided the original work is properl cited. A dual-band on-bod antenna for a wireless bod area network repeater sstem is proposed. The designed dual-band antenna has the maimum radiation directed toward the inside of the human bod in the medical implantable counication service (MICS) band in order to collect vital information from the human bod and directed toward the outside in the industrial, scientific, and medical (ISM) band to transmit that information to a monitoring sstem. In addition, the return loss propert of the antenna is insensitive to human bod effects b utiliing the epsilon negative eroth-order resonance propert. 1. Introduction Recentl, there has been increasing interest in wireless bod area network (WBAN) sstems for a variet of applications such as biomedical, militar, and coercial services. Especiall in biomedical applications, in order to monitor a patient s health status, an implanted device needs to collect various phsiological data and wirelessl transmit the information to eternal medical devices in real time [1]. However, the have a short transmission range due to the low radiation efficienc and the effective radiated power (ERP) regulation of 2μW. This limitation demonstrates the necessit of a dual-band on-bod repeater antenna to deliver weak signals from implanted devices to eternal devices. In addition, antenna performance is significantl affected b bod tissues due to the high dielectric constant and conductivit at the microwave frequenc band. Also, the input impedance and resonance frequenc cannot be changed, but the gain and radiation efficienc of an antenna can also be deteriorated when an antenna is operated on or in a bod. In order to be insensitive to the human bod effect, compact eroth-order resonance (ZOR) antennas for implantable and wearable WBAN sstems were proposed in [2, 3]. Additionall, to protect the human bod from radio wave eposure, the structure of the antennas for WBAN must have a low specific absorption rate (SAR) [4]. In this paper, we present a novel on-bod antenna for a WBAN repeater sstem. The proposed antenna has a dual-band propert that covers the industrial, scientific, and medical (ISM) bands and the medical implantable counication service (MICS) band. Also, the return loss of the antenna is insensitive to human bod effects, and the radiation pattern of the antenna is suitable for a repeater sstem. The antenna structure is designed and analed using FEM-based coercial software ANSYS HFSS v.14.. []. A semisolid human bod phantom is used for verification of the performance of the antenna both numericall and eperimentall. 2. Antenna Design Figures 1 and 1 show the laout of the proposed antenna for a WBAN repeater sstem. The proposed antenna has the dimensions of 1.6, and a FR-4 dielectric with a relative permittivit of 4.4 is used as a substrate. The antenna is comprised of a top patch fed b a microstrip line and a bottom patch fed through via connecting top and bottom patches. The top patch is used to suppress the radiation towards the outside of the bod in the MICS band and transmit signals to eternal devices in the ISM band.

2 2 Antennas and Propagation L L nh Chip inductor 27. Via.2.2 C R 7 8 Via C R Feeding point Semisolid human bod phantom ε r = 6.7, σ =.94 S/m for 3. MH ε r = 2.7, σ = 1.9 S/m for 24 MH (c) Figure 1: Configuration of the proposed antenna: top view, bottom view, (c) calculation setup for the on-bod antenna on a semisolid human bod phantom. The bottom patch is designed to counicate with the implanted devices in the MICS band and to reduce the human bod effects of the ISM band. The bottom patch is fed b using a via at the center of the top patch, and a chip inductor with a value of nh is mounted between the bottom patch and the ground plane to realie epsilon negative (ENG) ZOR. The gap between the bottom patch and the ground plane can be modeled as the shunt capacitance (C R ), and the chip inductor can be modeled as a shunt inductor (L L ). The ZOR frequenc from the above circuit description can be found using: 1 ω = 2π, (1) C R L L where ω is the ZOR frequenc [6]. B adopting the large inductance of the chip component, the bottom patch can still have a compact sie (.19λ.94λ )thatisevensmaller than the top patch. This allows a reduction in the off-bod side radiation in the MICS band. For a numerical simulation of the antenna, a homogeneous semisolid human bod phantom ( ) is modeled as illustrated in Figure 1(c). 3. Results and Discussion Figure 2 shows the fabricated antenna and the measurement setup in an anechoic chamber. To measure the return loss and radiation pattern, the antenna was placed awa from the surface of the fabricated semisolid phantom, and a Strofoam slab (ε r = 1.) was inserted between the antenna, and the phantom for spacing. In Figures 3 and 3, the measured electrical properties of the fabricated phantom using an Agilent 87E Dielectric Probe Kit and 8719ES network analer together are shown. As shown in the figure, the measured relative dielectric constant and conductivit of the phantom (ε r = 6.1 andσ =.92 S/m at 3. MH, and ε r = 2. and σ = 1.98 S/m at 24 MH) closel agreed with the values

3 Antennas and Propagation 3 Strofoam slab Fabricated antenna Fabricated semisolid human bod phantom 1. 1 Measured εr of semisolid flat phantom Measured σ of semisolid flat phantom FCC εr ±% margin at 3 MH [7] FCC σ ±% margin at 3 MH [7] Frequenc (MH) Measured εr of semisolid flat phantom Measured σ of semisolid flat phantom FCC εr ±% margin at 2.4 GH [7] FCC σ ±% margin at 2.4 GH [7] Frequenc (GH) Conductivit (σ) (S/m) 2 Relative permitivit (εr ) Conductivit (σ) (S/m) Relative permitivit (εr ) Figure 2: Photographs of the manufactured antenna and measurement setup: manufactured antenna, Measurement setup. 1 3 Figure 3: Measured 3D radiation pattern of the antenna on a semisolid human bod phantom: at 3. MH, at 24 MH. MICS band ISM band Return loss (db) from the equivalent electrical properties (εr = 6.7 and σ =.94 S/m at 4 MH, and εr = 2.7 and σ = 1.9 S/m at 24 MH) of the whole human bod [7]. Figure 4 shows the return loss characteristics of the proposed antenna in free space and on the human bod phantom. All the setup of the measurements and the calculations had been taken for a -ohm load. The measured results agreed well with the calculated results. The measured db impedance bandwidths of the antenna were 3.7% ( MH) at the MICS band and 3.7% ( MH) at the ISM band. In addition, the return loss propert of the proposed antenna was insensitive to the eistence of the human bod phantom. This is because the ZOR frequenc at the MICS band is insensitive to the surrounding medium [2], and the bottom patch reduces the human bod effect in the ISM band. Figures and depict the simulated electric field distributions of the substrate at each resonance frequenc of the proposed antenna. In Figure, the electric field distribution is in phase b the virtue of the ENG ZOR resonance emanating from the bottom patch at 3. MH. Besides, tpical 18 out of phase propert, which is a general characteristic of a patch antenna, is observed in Figure since the resonance of the top patch occurs at 24 MH. Figures 6 and 6 depict the measured radiation patterns of the antenna on a semisolid human bod phantom. Even though the ZOR antenna has a dipolar radiation Frequenc (GH) Calculated RL w/o phantom Calculated RL w/ phantom Measured RL w/o phantom Measured RL w/ phantom Figure 4: Return loss characteristics of the proposed antenna. pattern, the maimum power was delivered toward the bod at the MICS band due to the suppression caused b the top patch; therefore, the proposed repeater antenna is advantageous for counication with other implanted devices in the MICS band. In the ISM band, on the other hand, the maimum power was delivered outward from

4 4 Antennas and Propagation E field (V/m) E field (V/m) 1e e e e e e e + 3 e e e e e e e + 2 e + 2e e e e e e e + 3 1e e e e e e e + 3 e + Figure : Simulated electric field distributions of the proposed antenna: for the MICS band (3. MH), for the ISM band (24 MH). (dbi) 2 (dbi) Ma.24 2 Ma.63 Ma Min Min Ma Figure 6: Measured 3D radiation patterns of the antenna on a semisolid human bod phantom: at 3. MH, at 24 MH. W/kg W/kg Figure 7: Measured SAR distribution: at 3. MH, at 24 MH. the bod, which improves the counication efficienc between the repeater and an eternal device. In addition, the peak gains were dbi and.24 dbi at 3. MH and 24 MH, respectivel. The measured SAR distributions are shown in Figure 7. The SAR was measured at the Radio Research Agenc of Korea using the ESSAY sstem [8]. The proposed antenna is ecited b the signal generator. The input power of 2 mw, which is usuall used for SAR measurements of mobile application devices, is used to measure SAR. Hot spots are observed underneath the center of the top patch where the via is located, and the maimum SAR values were.411 W/kg at 3. MH and.4 W/kg at 24 MH. Although a high input power of 2 mw was delivered to the antenna, the maimum values of the measured SAR were still well below the regulated SAR limitation (1.6 W/kg) of the American

5 Antennas and Propagation National Standards Institute (ANSI/IEEE) for short-distance biotelemetr [9]. 4. Conclusion We proposed a dual-band on-bod antenna for the WBAN repeater sstem. The bandwidths of the proposed antenna were wide enough to cover the MICS band (2 MH) and ISM band (2 248 MH). Also, the resonance frequencies are stationar whether the antenna is placed in the air or on a human bod phantom. In addition, the radiation pattern of the antenna is advantageous for counication with implant devices in the MICS band and eternal devices in the ISM band. The measured maimum SAR values were low enough to conform to the SAR limitation of the ANSI. Consequentl, the proposed antenna can be a good candidate for a WBAN repeater sstem owing to the dualband propert, the insensitivit to an human bod effect, and the desirable radiation pattern. Acknowledgments This work was supported b the National Research Foundation of Korea (NRF) Grant funded b the Korean government (MEST) (no ). References [1] P.S.HallandY.Hao,Antennas and Propagation for Bod-Centric Wireless Counications, Artech House, Norwood, Mass, USA, 26. [2] J. Ha, K. Kwon, and J. Choi, Compact eroth-order resonant antenna for implantable biomedical service applications, Electronics Letters, vol. 47, no. 23, pp , 211. [3]J.Lee,S.I.Kwak,andS.Lim, Wrist-wearableeroth-order resonant antenna for wireless bod area network applications, Electronics Letters, vol. 47, no. 7, pp , 211. [4] U. Kim and J. Choi, Design of a microstrip patch antenna with enhanced F/B for WBAN applications, IEICE Transactions on Counications, vol. E94-B, no., pp , 211. [] HFSS: High Frequenc Structure Simulator Based on the Finite Element Method, v.14.., ANSYS. [6] A. Lai, C. Calo, and T. Itoh, Composite right/left-handed transmission line metamaterials, IEEE Microwave Magaine, vol., no. 3, pp. 34, 24. [7]D.L.MeansandW.Kwok,Evaluating Compliance with FCC Guidelines for Human Eposure to Radiofrequenc Electromagnetic Fields, Supplement C (edition 1-1) to OET Bulletin 6 (Edition 97-1), Federal Counications Coission Office of Engineering & Technolog, June 21. [8] [9] IEEE Standard for Safet Levels with Respect to Human Eposure, IEEE Standard C , 1999.

6 Rotating Machiner Engineering The Scientific World Journal Distributed Sensor Networks Sensors Control Science and Engineering Advances in Civil Engineering Submit our manuscripts at Electrical and Computer Engineering Robotics VLSI Design Advances in OptoElectronics Navigation and Observation Chemical Engineering Active and Passive Electronic Components Antennas and Propagation Aerospace Engineering Volume 2 Modelling & Simulation in Engineering Shock and Vibration Advances in Acoustics and Vibration