Abstract
In this paper, a wearable dual-band T-shaped antenna using a coplanar waveguide (CPW) fed operating at 2.4 GHz and 5.2 GHz bands is proposed for onbody wireless communications applications. Without the metamaterial-based electromagnetic bandgap (EBG) layer, the original antenna covers two bands from 1.96 GHz to 2.77 GHz and from 5.07 GHz to 5.35 GHz. The antenna efficiency decreases when it is placed on the human arm due to the interference from the human body to the antenna, as shown by a high specific absorption rate (SAR) value. These SAR values are reduced to 77.1% at 2.4 GHz and 91.7% at 5.2 GHz by the proposed EBG. The antenna gain is therefore improved to 1.4 dBi at 2.4 GHz and 6.25 dBi at 5.2 GHz. The antenna prototype is evaluated using a Wi-Fi wearable device, resulting in an improved signal-to-noise ratio (SNR) of 6-12 dB.
1. Introduction
Nowadays, with the increase in average age in developed and developing countries, a telehealth monitoring system is a promising healthcare solution that allows elderly people to live alone. In this kind of system, wearable wireless devices are required to measure and collect human health parameters. Among them, the wearable antenna is an essential part of deciding the wireless communication quality [1–5]. The desired wearable antenna operates well when placed on the human body. In addition, it is compact and integrated into sensing circuits with a stable performance. To satisfy the IEEE 802.15 or IEEE 802.11 wireless local area network physical layer standards for most telemonitoring systems, a dual-band antenna at 2.4 GHz and 5.2 GHz bands design must be developed. However, the antenna performance is strongly affected by interference from the human body. To maintain its performance at both operation bands, several broadband and multiband antennas have been proposed based on the dipole, monopole, and slot antennas in [6–8]. It is indeed worth noting that while some flexible antennas were introduced in [9–11], they were not specifically designed for wearable devices. Regarding the size limitations of wearable devices, we have taken into consideration previous studies such as [12–14]. In [12–14], planar inverted-F antennas were utilized; however, their dimensions were relatively large (0.68 ×0.52 in [13] and in [14]), rendering them incompatible with wearable devices. Additionally, a dual-band monopole antenna was presented in [15], but it exhibited a narrow bandwidth of only 5.7% and 3.78% at 2.45 GHz and 5.85 GHz, respectively. As we mentioned before, unlike traditional antennas, which are generally positioned in free space, wearable antennas are located near human tissues. These tissues have a high dielectric constant, affecting the antenna performance such as reflection coefficient (), bandwidth, gain, and radiation characteristics [15]. Furthermore, electromagnetic wave radiated from a wearable antenna is regulated not to be unsafe for human tissue health [16]. It is qualified by a specific absorption rate (SAR) level. In general, a maximum SAR value of 2 W/kg averaged over every 10 grams of tissue is required and set by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) [17]. We have also examined several wearable antennas discussed in [18–20]. It is worth noting that the antenna presented in [20] had a relatively large size of . Furthermore, although the SAR (specific absorption rate) value was not mentioned in [18], it was found to be suboptimal in [19] at 0.536 W/kg, 0.593 W/kg, and 0.692 W/kg for 3 GHz, 6 GHz, and 10 GHz, respectively.
To overcome these limitations, metamaterial-based EBG layers were used to isolate the body and the antenna in [21], which reduces the SAR values from 2.57 W/kg to 0.68 W/kg. Different structures of the EBG elements were studied such as mushroom structure in [22] and some complex structures in [23], proving the potential function of EBG [24–28] to reduce the interference between the body and the antenna. Unfortunately, the proposed EBG antennas have complex designs and strongly sensitive electromagnetic behavior, which are not suitable for wearable devices.
In this paper, a small-size dual-band T-shaped antenna is proposed using coplanar waveguide (CPW) feed. The application of the rectangular ring-shaped dual-band EBG structure is also investigated, showing the improved wearable antenna SAR. Details of the design principle and the experiment results are shown in the following parts.
2. Antenna and EBG Designs
2.1. Antenna Design
The four-stacked T-shaped antenna fed by CPW is shown in Figure 1. It has four arms with unequal lengths and widths. The antenna parameters are listed as in Table 1. The evaluation process of the design using the T-shaped structure is shown in Figure 1. The first T-arm (first step) in Figure 1(a) responds to the resonance at 2.4 GHz. After adding the second arm as in Figure 1(b), an upper-frequency band appears at 5.2 GHz. Integrating the third and fourth arms corresponding to the third and final steps, respectively, as shown in Figures 1(c) and 1(d), leads to a bandwidth enhancement at the low-frequency resonance at 2.4 GHz (see Figure 2). As in Figure 3, the lower resonant band is determined by the first arm, and its bandwidth (defined at -10 dB of ) is extended by the third and fourth arms of the antenna. This enhancement is probably due to the coexcitation of resonances at adjacent frequencies, and thus forms a wider operating band. The other resonant band (5.2 GHz) is determined solely by the first and second arms of the antenna. The proposed antenna uses a RT/duroid 5880 substrate with a thickness of 0.8 mm, a relative permittivity of 2.2, and a loss tangent of 0.0009, with a 0.035 mm of the copper layer. This substrate is flexible and resilient, which is suitable for wearable devices. The antenna dimension is listed in Table 1.
(a)
(b)
(c)
(d)
(e)
To verify the applicability of wearable devices, a simple model (phantom) of a human arm is proposed in this paper. The human arm is modeled using CST and consists of four layers: skin, fat, muscle, and bone, as shown in Figure 4. The materials and design parameters of the human arm are the same as listed in [29, 30], which have been demonstrated to be suitable for describing the human arm. The intrinsic properties of each layer are summarized in Table 2.
As shown in Figures 5 and 6 when the antenna is placed in free space, it gives two bands, from 1.96 GHz to 2.77 GHz (33.75% bandwidth) and from 5.07 GHz to 5.35 GHz (4.8% bandwidth) for . The antenna peak gain at 2.4 GHz is 1.45 dBi, while that at 5.2 GHz is 1.56 dBi. When the antenna is placed on the body, the operating frequencies were redshifted and split (for the resonance frequency at 2.4 GHz), and their gains decrease dramatically due to the effect of the human arm. Furthermore, the SAR value was extremely high for an input power of 250 mW, as shown in Figure 7. The radiation efficiency also reduces from 84% to 9.2% at 2.4 GHz and from 58.8% to 13.1% at 5.2 GHz.
2.2. EBG Design
To improve the radiation efficiency and reduce the SAR value, the metamaterial-based EBG structure was designed and integrated into the antenna. The proposed EBG structure consists of double-ring rectangles, as shown in Figure 8. The EBG geometrical parameters are shown in Table 3. This EBG operates at both 2.4 GHz and 5.2 GHz. The size of the outer ring, which responds to the single-band resonance at 2.4 GHz, can be adjusted by tuning the parameters , , and . After adding the inner ring, the EBG operates at another frequency, as shown in Figure 9 and the size of this inner ring (, ) is tuned to operate at 5.2 GHz. Figure 9 shows that adding the inner ring makes an insignificant change in the 2.4 GHz band. It means tuning either one of two rings has almost no effect on the other one. So, the operating frequencies can be adjusted by tuning the size of each ring independently.
The EBG in free space and on the body can be simplified by an equivalent circuit shown in Figures 8(b) and 8(c). The inner ring is represented by the inductance . The outer loop is featured by the inductance . The central gap gives rise to the capacitance , while the gap between the outer and inner loops responds to the capacitance . The interelement capacitance is denoted by capacitance . The body is characterized by inductance . The gap between the EBG and the body gives rise to . The resonant frequency of each structure can be determined from the total capacitance and inductance of the equivalent circuits.
The EBG has two band gap frequencies, as shown in Figure 9. One of the bands covers the 2.4 GHz peak which has a band gap frequency between 2.06 GHz and 2.73 GHz. Another one covers the 5.2 GHz peak between 4.66 GHz and 5.96 GHz. Figure 9 also shows that the value is nearly 0 dB in the range of the band gap frequency, indicating none of the waves in the range of the band gap can be transmitted. The phase of the EBG goes from -90° to +90°, covering a band from 2.08 GHz to 5.24 GHz, as depicted in Figure 10.
3. EBG Antenna Design
When the antenna was placed on the human arm above the EBG, the operating frequencies were shifted away, and the parameters must be reoptimized. The EBG antenna parameters are shown in Table 4.
The wearable antenna using EBG unit cells was placed behind the antenna. There is an air gap between the antenna and the EBG structure to avoid impedance mismatching and short circuits. The overall size is , as shown in Figure 11.
(a)
(b)
(c)
(d)
(e)
To examine the performance of the EBG antenna on the body, its electromagnetic response on the human arm was computed, as illustrated in Figure 11(a). The EBG wearable antenna prototype measurement configuration is presented in Figures 11(b)–11(d). The simulated and measured reflection coefficient results are shown in Figure 12. It was found that the bandwidth of the EBG antenna is smaller than the bare one, from 1.81 GHz to 2.52 GHz (30% bandwidth) and from 5.14 to 5.3 GHz (3.1% bandwidth), respectively. But the SAR values with the same input power (100 mW) decrease considerably (77.1% at 2.4 GHz and 91.7% at 5.2 GHz), as shown in Figure 13. It is worth mentioning that the gain is impressively improved from 1.56 to 6.25 dBi at 5.2 GHz, as shown in Figure 14.
As shown in Figure 12, the measured bandwidth of the antenna is much smaller than the simulated one, from 2.15 GHz to 2.5 GHz (16.67% bandwidth) and from 5.17 GHz to 5.28 GHz (2.1% bandwidth). Figure 14(c) shows the simulated and measured radiation patterns of the EBG antenna at two frequencies of 2.4 GHz and 5.2 GHz. In general, the results show that the measured radiation patterns coincide with the simulated radiation patterns. There are still some distortions. This might have been caused by the radiation patterns measured at 10°. Besides that, since the antenna was fixed with the EBG by foam, the relative position between the antenna and the EBG may not be as in the simulation. The SMA connector and welding errors also cause some distortions. After adding the EBG structure behind the antenna, the maximum input power increased significantly, as shown in Table 5. The proposed EBG antenna is then connected to the Wi-Fi module to determine the signal-to-noise ratio (SNR) at the frequency band of 2.4 GHz, as shown in Figure 15. The measured SNR of the proposed EBG antenna is 6 dB more than that of the commercial antenna.
Table 6 provides a comparison between the proposed EBG wearable antenna and some related works. As shown in Table 6, the antenna in [14, 24] is smaller than this work, and the antenna in [31] has the largest bandwidth. However, antenna [14] has a very low gain, antenna [24] has a very small bandwidth, and antenna [31] is extremely large. Compared with other reported antennas in Table 6, the proposed antenna has a wide bandwidth, high gain, acceptable size, and low SAR. In addition, the proposed antenna is validated and tested in a Wi-Fi wearable device as shown in Figure 11(e) showing the improving signal-to-noise (SNR) ratio. The proposed EBG antenna is connected to the Wi-Fi module (ESP32 module) SNR measurement experimentation. We also use an ESP32 module to receive the signal. We determine the Received Signal Strength Indicator (RSSI) and calculate the SNR from the RSSI value and the noise floor. Normally, the noise floor is the sensitivity of the module (about -97 dBm with the ESP32).
4. Conclusion
In this paper, a wearable T-shaped antenna using EBG unit cells is designed for onbody wireless communications applications. Without the EBG layer, the basic antenna covers two bands from 1.96 GHz to 2.77 GHz and from 5.07 GHz to 5.35 GHz with a gain of 1.42 dBi at 2.4 GHz and 1.52 dBi at 5.2 GHz in free pace. The measured results show that using an EBG structure reduced the SAR value by 77.1% at 2.4 GHz and 91.7% at 5.2 GHz, and the antenna has a wide bandwidth, a high gain, and an acceptable size. Based on this result, the proposed T-shape antenna over a EBG array is a strong potential candidate for wearable applications.
Data Availability
Data is included in our manuscript.
Disclosure
Minh Thuy Le present address is the School of Electrical and Electronic Engineering, Hanoi University of Science and Technology, Hanoi, Vietnam.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This research is funded by the Asahi Glass Foundation 2023.