Introduction
Rapid technological advancement in the field of wireless body area networks (WBAN) paved the way for multiple real time applications including health care industry, emergency rescue, defense, location tracking, sports, and physical fitness etc. WBAN enables either on-body or off-body communication in which information can be communicated between devices within on the body or to remotely connected devices and servers like in hospital for health care monitoring. WBAN application requires wearable antennas with special features like, comfort, compactness, durability, flexibility, robustness, and higher gain. Body tissues are highly vulnerable to electromagnetic signals, thus wearable or any on-body antenna should have low specific absorption rate (SAR) which can be easily achieved with low back radiation. Extensive research on wearable on-body antennas is performed. Wearable antennas are designed as printed, embroidery or button types in rigid (FR-4) and flexible (Jean, polydimethylsiloxane [PDMS]) substrates to suit the application [Reference Paradiso, Loriga and Taccini1–Reference Sayem, Simorangkir, Esselle, Hashmi and Liu9]. Antenna performance is evaluated in the presence of water and moisture and shows that textile material has a larger impact on performance degradation on longer life. Rigid materials are long lasting with better performance but not comfortable for wearing. Thus flexible and comfortable substrate like PDMS is always a better equivalent providing good performance. Most applications comply with dual band antenna compared to single band antennas for wireless applications [Reference Le and Yun10–Reference Sreelakshmy and Vairavel17]. Most dual band wearable antennas operate in 2.4 and 5.8 GHz frequency bands falling in industry, scientific, and medical application. In order to accommodate high data rate and multiple operation frequency Multiple Input Multiple Output (MIMO)and reconfigurable antennas are preferred [Reference Kumar Biswas, Pattanayak and Chakraborty18–Reference Dang, Chen, Ranasinghe and Fumeaux20]. Miniaturization is another important factor on antenna design, miniaturized antennas with stubs, slots and fractals can improve the number of operating bands with wider bandwidths [Reference Sambandam, Kanagasabai, Natarajan, Alsath and Palaniswamy21–Reference Dash, Saha, Ghoshal and Palai27]. Pattern and polarization diversity button antennas are suitable for on/off-body applications [Reference Zhou, Fang and Jia28–Reference Hu, Yan and Vandenbosch32]. In wearable antennas button antennas are prominently used as they are easily designed with metallic structures or snap buttons themselves act as radiators with different arrangements to make multiple bands with better performance. Their ease of integration to cloths and comfort of wearing make it as a best choice of wearable antenna for WBAN application.
This paper presents a miniaturized novel circular button antenna with multiple bands using PDMS and jean as the substrate. Full ground plane with an arc minimizes the SAR value with an additional benefit of extra band. Thus the proposed button antenna is compact, flexible, and comfortable to integrate in the blazer or jacket.
Submission
The proposed circular blazer cuff button antenna is designed with 10 mm radius on dual dielectric substrate consisting of 0.5 mm jean at the bottom and a 1 mm PDMS elastomer at the top. The dielectric permittivity ε of Jean and PDMS substrate are 1.6 and 2.7 with loss tangent tan δ = 0.02 and 0.0314, respectively. The unique geometry consists of a circular conductor and four interconnected arcs with radius r1, r2, r3, and r4 from the center which are interconnected to form a single slotted arc geometry. The radiating element of multiple arc section with a circular center attached with a narrow strip is printed on the PDMS substrate. Top, bottom, and side view of the proposed antenna is shown in as shown in Fig. 1. The circular ground conductor with an arc slot acting as the ring resonator is printed on the bottom of the jean substrate. Adhesive is used to connect both copper and substrate materials. The inner and outer radius of the ground slot arc are r5 and r6. Dimensions of the antenna are tabulated in Table 1. The antenna is fed by a 50 Ω coaxial probe at the center for better impedance match.
Figure 1. Proposed button antenna (a) Top view (b) Bottom view (c) Side view.
Table 1. Proposed antenna dimensions
Antenna evolution stages
Evolution of antenna geometry in different stages are shown in Fig. 2. Initial geometry has a circular patch with an arc connected through a stub. It resonates at 8.7 GHz with a return loss of 20.29 dB. Additional arc attached to stage 1 contributed with multiple resonances as shown in Fig. 2. Stage 2 resonates at 8.85 GHz frequency and 2.85 GHz frequency with S11 respectively as −14.89 and −13.19 dB. Antenna design started with basic expression of patch with radius a as shown in equation (1). Stage 3 also has three resonances at 6.25, 8.75, and 10.1 GHz which are not the intended frequencies. Addition of 4th strip as in stage 4 achieved resonance at 2.5, 5.8, and 10 GHz with better S11 values, −27.03, −22.94, and −21.18 dB. In stage 5 an arc slot is introduced in the ground plane to enable resonance at 3.6 GHz which can be utilized for 5G application.
\begin{equation}f = \frac{{1.8412\,{v_0}}}{{2\pi a\sqrt \varepsilon }}\end{equation}
Figure 2. Evolution of cuff button antenna.
Thus the multiband antenna of stage 5 resonates at four frequencies respectively at 2.5, 3.6, 5.75, and 9.8 GHz suitable for WLAN, WBAN, 5G, and X band applications with return loss of 15.39/18.43/22.19/18.18 dB. All four arc strips have an angle of 260°. The four arcs are interconnected with smaller angular strips. Optimization was performed to for arc lengths to achieve the required resonance. Inner interconnection arc has an angle of 80˚ and the outer two interconnecting arcs have an angular gap of 100˚ each. Return loss plots of various stages are in Fig. 3. The antenna with slotted ground had better reflection characteristics with resonance at four frequencies including sub 6, 5G band of 3.6 GHz, but the gain at 2.4 GHz was low showing −5.07 dB. In order to improve further gain at lower frequency an additional strip is attached to the top of 2nd arc. Gain plots of the antenna for last two stages are compared in Fig. 4. The additional stub in the proposed antenna improved the gain at 2.45 GHz from −5.07 to 1.85 dB. All simulations were carried out using HFSS. Flexibility of proposed cuff button antenna is confirmed by bending analysis along X and Y direction as shown in Figs. 5 and 6. The S11 plots show stable performance on bending at all four frequency bands. Proposed antenna has lower bandwidth 60 MHz (2.47–2.53 GHz), 30 MHz (3.76–3.73 GHz), 140 MHz (5.83–5.69 GHz), and 220 MHz (9.95–9.73 GHz), respectively due to smaller strip widths. Simulated efficiency of the antenna at operating frequencies of 2.5, 3.75, 5.75, and 9.8 GHz are 84.52%, 86.21%, 77.77%, and 86.18%, respectively. The antenna is tilted in different angles along X and Y direction during simulation and Fig. 7 shows the tilted antenna return loss at 20° in front & back along X axis and left & right along Y axis. The performance of the antenna remains good on tilting.
Figure 3. Reflection performance of antenna during evolution.
Figure 4. Gain performance of last two antenna stages.
Figure 5. Effect of antenna bending along x axis.
Figure 6. Effect of antenna bending along y axis.
Figure 7. Effects of antenna tilting along X and Y axis at 20° indicating front, back, right, and left.
Current distribution
The surface current distribution of the proposed multilayer multiband cuff button antenna is shown in Fig. 8. Maximum current is for 2.5 GHz and minimum for 9.8 GHz. Coverage of strips is more in case of 5.75 GHz. The current distribution indicates the strip position responsible for maximum radiation at different frequencies.
Figure 8. Surface current distribution of the button antenna at resonating frequencies.
Results and discussion
Off-body antenna performance
The prototype of fabricated antenna is shown in Fig. 9. The antenna reflection performance is measured with the help of R&S’s Network analyzer ZNLE14 as in Fig. 10 and the results of simulation and measurement shows a close agreement with each other. The measured results indicate resonance at 2.48, 3.69, 5.84, and 9.96 GHz with corresponding S11 readings as −22.11, −12.28, −16.08, and −18.40 dB, while simulated return loss was found to be 21.38/12.2/22.41/18.87 dB at frequencies of 2.5, 3.6, 5.75, and 9.87 GHz. Measurements are also carried out placing the antenna at different position on body. Figure 11 depicts the real time return loss performance of the antenna placing on hand and chest in comparison with simulated result. In order to analyze the water absorption property of jean substrate (wet and moisture condition), the antenna is dipped in water and partially dried and the return loss is measured. Later, when it is fully dried the measurement is repeated to check for any deformity. The results are plotted in Fig. 12. Measured return loss results are tabulated in Table 2. The Voltage Standing Wave Ratio (VSWR)and gain plots of button antennas are depicted in Figs. 13 and 14. VSWR is below 2 for the resonant frequencies. Measured peak gains at resonance are 3.08, 2.09, 1.79, and 3.03 dB while the simulated gains are 1.85, 2.03, 1.47, and 3.27dB for radiating frequencies.
Figure 9. Fabricated cuff button antenna.
Figure 10. Comparative S11 of simulation and measurement with measured photograph.
Figure 11. Measured S11 performance at different on-body positions.
Figure 12. Measured S11 performance with water and moisture content.
Figure 13. Comparative VSWR of simulation and measurement.
Figure 14. Comparative gain of simulation and measurement.
Table 2. Return loss performance of antenna in different measurement scenario
The radiation patterns in azimuth and elevation plane are measured against 18 GHz rigid horn antenna in an anechoic chamber of size 5 m × 3 m × 2.6 m and the measurements are taken at operating frequencies of 2.5, 3.6, 5.75, and 9.8 GHz which is shown in Fig. 15. Normalized radiation patterns on simulation and measurement in E and H plane are plotted in Fig. 16.
Figure 15. Radiation pattern measurement setup in anechoic chamber.
Figure 16. Radiation patterns of cuff button antenna.
On-body antenna performance
The antenna performance on the body was analyzed by placing it on male right arm with spacing with 2, 5, and 10 mm. Figure 17 show the return loss performance of the antenna at different spacing and resonance frequency. When the antenna is placed at 2 mm distance the lowest two resonances have poor S11 magnitude of −9.25 and −6.94 dB at 2.6 and 3.65 GHz. At 5 and 10 mm distance all four bands show S11 readings below −10 dB reference. Resonance frequencies and corresponding S11 magnitude at 5 mm distance are 2.55/3.75/5.8/9.9 GHz & 10.94/11.28/17.34/19.68 dB and at 10 mm distance the S11 values are 11.34/21.42/15.07/17.66 dB at 2.6/3.75/5.8/9.9 GHz. This indicates that the on-body dielectric variation has a tendency of smaller frequency shift with reduced S11 value when placed close to the body.
Figure 17. On-body reflection performance of antenna.
Antenna SAR analysis
SAR is the amount of radiation absorbed by the body tissues on sensing of electromagnetic radiation. As per the regulatory standards by Federal Communication Commission (FCC), SAR is limited to have a protective environment from radiation hazards with a limit of 1.6 W/Kg for 1 g of tissue or 2 W/Kg for 10 gm of tissue. In this study, human arm phantom is considered for SAR analysis as the antenna is designed to place on the sleeves. The simulation is carried out with 1 g of tissue with an input signal power of 100 mW and 5 mm gap from the body. The evaluated SAR distribution at different frequency are plotted in Fig. 18. SAR value of designed antenna is minimum (0.0113 W/Kg) at 2.5 GHz and maximum (0.6347 W/Kg) at 3.6 GHz. Simulated SAR values at 5.75 and 9.8 GHz frequency are found to be 0.155 and 0.175 W/Kg. Figure 18 clearly shows that the proposed antenna provides a SAR of acceptable FCC limit suitable for safe on-body application. Table 3 show comparison results of the proposed antenna with few existing literatures.
Figure 18. SAR analysis of proposed antenna.
Table 3. Comparison with existing literature
Conclusion
In this article, a miniaturized cuff button antenna is presented for off-body communication for medical application. The multiband antenna covers application frequency bands of 2.5, 3.6, 5.75, and 9.8 GHz with corresponding bandwidth of 60/30/140/220 MHz. The radiation patterns are directional with better gain suitable for real time on-body and off-body communication. The designed miniaturized cuff button antenna has low SAR values and make comfortable for wearable biomedical application.
Acknowledgements
This work was supported by All India Council for Technical Education, under research promotion scheme. File No.8-122/FDC/RPS/POLICY-1/2021-2022.
Competing interests
The authors declare that they have no conflict of interest.
Sapna B A obtained her B.E. in electronics and communication engineering from Madras University, India in 2000, an M.E. in communication systems in 2007, and Ph.D. in Electronics and Communication Engineering in 2022 from Anna University, India. She is working as an assistant professor at KIT-Kalaignar Karunanidhi Institute of Technology, Coimbatore, India. She is a life member of ISTE. Her research interests include planar antennas, wearable antennas, implantable antennas frequency-selective surface (FSS), and metamaterials.
K. Ramasamy obtained B.E. ECE from Madurai Kamaraj University, M.E. applied electronics from Bharathiar University, India, and Ph.D. communication engineering from Multimedia University, Malaysia, in 1988, 1993, and 2006, respectively. He is a fellow in IETE, and member in ISTE. He has more than 35years of experience in academics and administration in different engineering colleges in India and abroad. Currently, he is professor and dean Academics and Research at Kalaignar Karunanidhi Institute of Technology, Coimbatore. He published around 150 papers and his research interests include design of microstrip antennas and wireless communication. He guided three Ph.D. scholars. He is a recipient of National Merit Scholarship Award by Government of India, and EMC2 Academic Leader Award 2015, also awarded the silver medal in KERIE 2006, conducted by Faculty of Engineering, International Islamic University Malaysia, for best Ph.D. work Asymmetric Turbo Code for Enhanced Performance of JPEG Coded Image Transmission over 3G Systems.
