A compact dual-band semi-flexible antenna at 2.45 GHz and 5.8 GHz for wearable applications

S. M. Shah, A. A. Rosman, M. A. Z. A. Rashid, Z. Z. Abidin, F. C. Seman, H. A. Majid, S. H. Dahlan, S. A. Hamzah, N. Katiran, A. Ponniran, F. Hassan, F. Zubir Research Center for Applied Electromagnetics, Universiti Tun Hussein Onn Malaysia, 86400 Batu Pahat, Johor Faculty of Electrical and Electronic Engineering, Universiti Tun Hussein Onn Malaysia, 86400 Batu Pahat, Johor School of Electrical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor


INTRODUCTION
These days, microstrip antennas are prevalent because of their low profile, light weight, flexibility and compatibility with integrated circuits [1]- [3]. Multiband microstrip antennas are required in various applications such as the wireless local area network (WLAN), wireless interoperability microwave access (WiMAX) and long term evolution (LTE) [4], [5]. Generally, the term dual-band antenna refers to two significant frequencies that the antenna can operate. This can be achieved by combining two frequency bands in the antenna design. One popular method to generate a dual-band frequency of operation is by introducing a slot on the radiating patch so as to alter the current path which governs the formation of each resonant frequency [6], [7].
Wearable antennas are essentially any antennas that are specifically designed to function while being worn on-body [8]. This requires the antenna to be very compact with a certain degree of flexibility, so as not to cause discomfort to the user. Due to the flexibility and durability of wearable antennas, they have been used in a widespread range of applications known as objects observation, wireless medical purposes and  [11]. Specific absorption rate (SAR) is described as a degree of rate at which energy is absorbed by the body when exposed to an electromagnetic field from an antenna [12], [13]. The European Union (EU) SAR limit is 2 W/kg averaged over 10 g of tissue following the IEC standard for wireless device (IEC 62209-1) [14]. On the other hand, the Federal Communication Commission (FCC) requires that the SAR should be lower than 1.6 W/kg averaged over 1 g tissues in the United States [15]. Influence of body proximity on the performance of antenna and the amount of SAR absorbed by human body are the two significant on-body requirements for wearable antennas.
The industrial, scientific and medical (ISM) frequency bands are reserved for international purpose in the non-commercial applications of industrial, scientific and medical applications [16]. The ISM bands are license-free with low power consumption [17]. Therefore, multiple work have been performed for the integration of ISM bands in a single device [18], [19]. The 2.45 GHz ISM band is a widely accepted frequency band for worldwide operations [20].
Traditional antennas are known to be rigid and inflexible, which makes them not a popular choice for wearable applications [21], [22]. On the contrary, wearable antennas are required to be flexible as they need to follow the body curves and bent for the comfort of the users. In addition, wearable antennas must be small and light weight as they are designed to be incorporated into clothings [23]. Hence, a semi-flexible wearable antenna (due to the nature of Rogers Duroid RO3003™ substrate material used in this work) for wearable applications is presented in this work as it offers the solutions for both rigidity and inflexibility of traditional antennas [24]. The dimensions of the antenna is designed to be compact so as not to cause discomfort to the user when being worn on body while at the same time, offers good flexibility. The proposed antenna will operate dual-band at 2.45 GHz and 5.8 GHz of the ISM bands for WLAN application in both the lower and upper frequency bands. This advantage will serve as an added-value to the proposed antenna.

RESEARCH METHODOLOGY 2.1. Antenna design and configuration
The design and simulation of the antenna are performed by using CST MWS ® software. Optimization has been performed in obtaining the exact dimensions to achieve the required frequency at 2.45 GHz and 5.8 GHz of the ISM bands with the best linear characteristics. A modified ground plane with dimensions of Wpg × Lpg mm 2 has been introduced in this design to further improve the reflection coefficient and an L-shaped slot is used to generate the upper frequency band of the antenna. Figure 1 shows the geometry of the proposed dual-band semi-flexible wearable antenna at 2.45 GHz and 5.8 GHz while Table 1 lists the dimensions of the antenna.

Antenna simulation in bending condition
The bending investigation is performed and simulated in CST MWS® software along the y-axis over a vacuum cylinder with diameters, d are varied at 50 mm, 80 mm and 100 mm to approximate the average human arm's diameter.

Antenna simulation of specific absorption rate
Since the dual-band wearable antenna will be used on human body, a numerical human phantom model has been replicated in CST MWS ® software to investigate the performance of the antenna while operating within a close proximity to the human body in terms of the specific absorption rate (SAR). In order to develop the numerical human phantom design, the information on the each layer of human tissue such as the dielectric constant, density and conductivity at the operating frequency of 2.45 GHz and 5.8 GHz are necessary [25] and has been included in the simulations. The antenna is placed at 2 mm over the human phantom model to take into account the approximate thickness of the clothings worn by the user.

Antenna fabrication and measurement
The prototypes of the dual-band wearable antenna can be viewed in Figure 2. The antenna is fabricated on a semi-flexible substrate material, Rogers Duroid RO3003™ with the following properties: relative permittivity, εr of 3, loss tangent, tan δ of 0.0010 and thickness, h of 1.52 mm. The ZVB14 Rohde & Schwarz Vector Network Analyser is used to measure the antenna.  Figure 3 shows the comparison of simulated and measured reflection coefficients of the dual-band semi-flexible wearable antenna at 2.45 GHz and 5.8 GHz. From the figure, the proposed antenna operates at 2.45 GHz with a simulated reflection coefficient, S11 of -13.472 dB and 5.8 GHz with S11 of -19.516 dB. The resonant frequencies of the fabricated antenna are measured at 2.65 GHz and 5.83 GHz with S11 of -32.37 dB and -32.05 dB, respectively. The shift of the measured resonant frequencies with respect to the simulated can be attributed to the fabrication tolerances and cable losses. However, the measured S11 when the marker is placed at 2.45 GHz and 5.8 GHz are -11.741 dB and -18.587 dB, respectively which show that the antenna can still operate at the desired ISM frequency bands of 2.45 GHz and 5.8 GHz.

Radiation pattern
The radiation patterns of the dual-band wearable antenna have been simulated at 2.45 GHz and 5.8 GHz. Figure 4 shows the radiations patterns of the antenna in the E-plane and H-plane at 2.45 GHz while Figure 5 illustrates the radiations patterns of the antenna at 5.8 GHz in both planes. From the figure, it can be observed that the radiation patterns at the lower resonant frequency of 2.45 GHz is almost bi-directional in the E-plane and omnidirectional in the H-plane. The bi-directional radiation pattern in the E-plane deteriorates at 5.8 GHz while the omnidirectional pattern remains in the H-plane. Figure 6 shows the surface current distribution of the antenna at 2.45 GHz and 5.8 GHz. It can be seen from the figure that the maximum surface current is concentrated around the edges of the microstrip feedline at 2.45 GHz. On the other hand, the maximum surface current is seen to be dominant around the edges of the slot at 5.8 GHz. The observation is consistent with the theoretical explanation from the literature that the length of the current path travels on the radiating patch does affect the resonant frequency [26]. The longer the current path travels across the radiating patch, the lower the resonant frequency or vice versa. Thus, as can be seen in the figure, the current path represented by the maximum current distribution is longer at the lower frequency which contributes to the lower resonant frequency at 2.45 GHz. On the other hand, the opposite occurs which is responsible to the formation of higher resonant frequency at 5.8 GHz.  Figure 7 shows the simulated reflection coefficients, S11 of the antenna in bending condition with varying approximate diameter, d of human arms with d=50 mm, 80 mm and 100 mm along the y-axis in CST MWS® software. From the figure, it can be observed that the reflection coeffitients for all diameters are very similar with a very slight difference to be seen in the upper frequency band for d=50 mm. In this case, the upper resonant frequency is shifted to 5.75 GHz with S11 of -25 dB. Nevertheless, the S11 when the marker is placed at 5.8 GHz is still acceptable at -13.23 dB which implies that the antenna can still operate at the upper resonant frequency of 5.8 GHz. The results show that the wearable antenna is working well under bending condition regardless of the approximate diameter of human arms. Figure 8 shows the measured S11 of the antenna over varying diameter of PVC pipes to emulate the human arms. From the figure, a shift in the resonant frequency is observed for each diameter. The differences between the simulation and measurement results might be attributed to antenna's misalignment. Table 2 lists the comparison between the simulated and measured S11 in bending condition. As can be viewed from the table, the simulation and measurement results of the antenna fairly agree with each other in bending condition.   Figures 9 and 10 show the simulated SAR values of the antenna at 2 mm away from the numerical human phantom model for 1 mW input power. The 2 mm distance is chosen as the approximate thickness of fabric materials worn by human as the antenna will not be placed directly on the bareskin but instead on the clothing of the user. From the figure, it is observed that at 2.45 GHz, the SAR limits of the antenna are simulated at 0.271 W/kg for 1 g and 0.0551 W/kg for 10 g. On the other hand, the SAR limits at 5.8 GHz are computed at 0.202 W/kg for 1 g and 0.0532 W/kg for 10 g. Therefore, 1 mW of input power is used for this work because it obeys the FCC and ICNIRP guidelines.

CONCLUSION
A dual-band semi-flexible microstrip antenna with Rogers Duroid RO3003™ substrate at 2.45 GHz and 5.8 GHz of the license-free ISM bands is presented in this work. The antenna is designed to be compact with dimensions of 30 × 38 mm 2 to avoid discomfort to the user when the antenna is worn on body and offers good flexibility, at the same time. The antenna bending performance on the approximate human arm diameters is also investigated to evaluate the performance of the antenna for wearable applications. The simulation and measurement results show that the antenna is working well under bending conditions at both 2.45 GHz and 5.8 GHz. The specific absorption rate (SAR) on 1 g and 10 g of human tissue have also been simulated for the SAR limits to observe the required guidelines by the FCC and ICNIRP. It is also shown from the simulated SAR that the input power of 1 mW obeys the guidelines. In a nutshell, the proposed dualband semi-flexible antenna is suitable for wearable applications due to its compact size, flexibility in bending conditions regardless of the diameter of human arms and the acceptable SAR limits that obey the FCC and ICNIRP guidelines.