Abstract
A novel low-profile all-textile microstrip antenna for ultra-wideband (UWB) applications in wireless body area networks (WBANs) is presented. The antenna incorporates flexible materials such as felt and conductive fabrics which provide optimal wearing comfort and durability. The use of a single dielectric substrate layer facilitates the integration process. The antenna also features a full background plane that minimizes the back radiation towards the human body. Multiple branches are designed in a compact area to generate adjacent resonances, and their combination achieves broadband characteristics across the 4.83–9.57 GHz frequency band. The antenna has a miniaturized size of 60 mm × 60 mm, which is 1.6 × 1.6 (where represents the guided wavelength at the center frequency), and it has a high realized gain of up to 10 dBi. The fully grounded structure also ensures good isolation between the antenna and the human body, thereby alleviating concerns regarding safety and radiation degradation in WBAN context. Simulation results indicate that the antenna maintains high performance levels during various bending tests. Given its favourable properties like ultra-wide bandwidth, compact size, low profile, high flexibility, and low specific absorption rate (SAR), the proposed design could find broad application prospects in high-speed WBAN systems.
1. Introduction
With the rapid development of wireless communication, wearable materials, and sensor technologies, the wireless body area network (WBAN) has gained popularity in diverse fields including military activities, sports, healthcare, emergency rescue, and personal communications [1–5]. The antenna, as a crucial component enabling wireless transmission, requires thorough design to address challenges such as wearing comfort, high-speed transmission, safety, and cost-effectiveness. On the one hand, all-textile antennas offer a promising solution for better conformability, breathability, and aesthetics compared to their counterparts made from semirigid printed circuit board (PCB) or flexible polyester rubbers. The manufacturing techniques for textile antennas are also becoming increasingly mature and diversified, such as wet corrosion, manual cutting, laser cutting, screen printing, ink-jet printing, and embroidery [6, 7]. However, effectively integrating multiple fabric layers while avoiding detachment and dislocation between them still poses challenges. Reducing the number of dielectric and conductive layers can alleviate these difficulties. On the other hand, ultra-wideband (UWB) technology is desirable for high-speed data transfer, low power consumption, and strong anti-interference robustness. In addition, since human tissue is rich in water and consequently can serve as a good absorber, reflector, or scatterer of electromagnetic (EM) waves, effective insulation or shielding measures become necessary to minimize the interference between the human body and the antenna and prevent potential negative even harmful radiation impacts [8]. Wearable antennas should also keep steady radiation performance in complex conditions such as bending and humidity environment [9, 10]. In short, for short-range and high-speed WBAN applications, there is an increasing demand for low-profile all-textile UWB planar antennas with good back-shielding characteristic.
According to the shielding ability they have, UWB antennas reported from the existing literature can be categorized into three types. These include UWB antennas with partial ground plane, UWB antennas with electric band gap (EBG) structure or artificial magnetic conductor (AMC) structure, and UWB antennas with full ground plane.
Partially grounded antennas can achieve UWB characteristic with simple topologies [11–14]; however, their shielding performance is quite limited. When implemented near the human body, they often exhibit severe radiation distortion and can lead to high SAR values. To ensure personal safety, people tend to limit the radiation power of the antenna and maintain a thick gap between the antenna and the human body, which significantly restricts their application. To mitigate the interference between the antenna and the human body, a plane layer with periodic EBG or AMC structure can be added beneath the radiating antenna [15–18]. These structures also contribute to improved gain and radiation efficiency due to the in-phase reflection effects. The main deficiencies are the high overall thickness and the large surface area, since EBG or AMC structures must contain a sufficient number of periodic elements. Furthermore, the integration of multiple layers is also challenging, and small misalignments between layers may cause significant degradation in radiation when placed on the human body. In contrast, a fully grounded antenna does not introduce additional layers and has good shielding properties. However, the EM waves are inherently confined between the radiator and the ground plane, and only those around the resonance can effectively radiate, hence resulting in generally very narrow-band characteristics, such as the design in [19]. Extending the bandwidth of a fully grounded design is a challenge that has not been fully studied.
Several recent developments in fully grounded UWB antennas can be found in [20–25]. In [20], Poffelie et al. proposed an all-textile UWB antenna with two stacked substrate layers. The main radiating element is a partially grounded coplanar waveguide- (CPW-) fed patch, with a second substrate layer attached by sewing that is fully grounded to ensure good shielding. A parasitic patch is placed on the upper side of the second substrate layer just beneath the radiating patch to further expand the bandwidth. It covers the entire UWB 3.1–10.6 GHz band, but the only drawback is the high thickness and risks to introduce air gap between substrate layers under bending or crumpling conditions. In [21, 22], single-substrate-layer UWB designs are proposed using the multibranch/slot loading techniques and multiple TM mode excitation techniques, respectively. However, both designs adopt polydimethylsiloxane (PDMS) polymer as the dielectric layer, which hinders the breathability and wearing comfort. In [23], an extremely compact textile antenna is realized using the slot loading technique, while there is a large stop-band (4.2–6.5 GHz) in the middle. In [24], a multiresonance overlapped ultra-wideband width of 3.6–10.3 GHz is achieved thanks to three independent radiating branches and a surrounding patch that enables capacitive coupling, but its overall size is as large as 2.7 × 2.45. In [25], a multiband antenna operating at both the 4.5–10 GHz UWB and the 2.45 GHz WLAN is realized using capacitive coupling techniques. The design measures as large as 2 × 1.75 and is moderately breathable since foam instead of textile material was used as the substrate.
In this paper, we propose a highly compact all-textile ultra-wideband antenna suitable for high-speed WBAN off-body communication. Multiple radiating branches are combined to achieve adjacent resonance overlap, while a full ground plane ensures superior immunity to body proximity effects. The use of fully textile flexible materials results in good unobtrusiveness and wearing comfort.
2. Materials and Methods
Figures 1(a) and 1(b) show the topology, and Figure 1(c) shows the fabricated prototype of the proposed design. The substrate is made of 3.2 mm thick felt with a dielectric constant of 1.2 (measured by using the resonance method in [26]) and a loss tangent (tanδ) of 0.02 (measured by using the microstrip ring resonator method in [27]). The radiating patch and ground plane are made of flexible conductive fabric, which is a plain-weave polyester fiber layer with copper and nickel coated on the upper surface, and a very thin adhesive layer attached to the bottom to facilitate integration onto the substrate [28]. It has a thickness of 0.085 mm and a very low sheet resistance of less than 0.05 Ω/sq. The conductive and dielectric layers are processed and integrated using the manufacturing method proposed in [29]. A coaxial feeding is used, and the design parameters are as follows: L1 = 21.2, L2 = 20, L3 = 16.5, L4 = 22.5, L5 = 13.2, L6 = 10.5, L7 = 20, W1 = 47, W2 = 34, wf1 = 6, wf2 = 8, and h = 3.2, all in millimeters. The overall size is 60 × 60 × 3.37 mm3, which is 1.6 × 1.6 × 0.08 (where is the guided wavelength at the center frequency).
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Figures 2(a)–2(d) depict the evolutionary design process. Antenna 1, a half-wavelength rectangular patch antenna, serves as the starting point, with a resonance frequency around 6 GHz. Two L-shaped branches are added to the left and right sides to obtain antenna 2, introducing a new resonance point around 5 GHz. The middle branch’s higher-order mode is excited to generate another new resonance at 9.28 GHz, while the original resonance at 6 GHz is shifted to a higher frequency of 6.4 GHz. Antenna 3 is loaded with two L-shaped stubs above antenna 2, creating additional resonances at 5.76, 7.23, and 8.13 GHz. Two elliptical cuts and a circular slot are introduced separately on the upper and lower stubs to achieve better impedance matching. To expand the bandwidth further, another rectangular stub is added to the middle of the upper half of the patch in antenna 4, creating a new resonance at 9.1 GHz. Finally, the overlapped frequency band, in which the reflection coefficient |S11| ≤ −10 dB, covers 4.89–9.65 GHz, as shown in Figure 3. Figures 4(a)–4(f) show the surface current of the final design at six resonant frequencies of 5.06, 5.76, 6.49, 7.24, 8.46, and 9.1 GHz. We can identify their corresponding current path. The length of the current paths is approximately half the waveguide wavelength at their corresponding resonant frequency. Superposition or cancellation of far-field radiation occurs when there are multiple current paths at the same frequency. This results in enhancement or splitting of the radiation pattern.
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3. Results and Discussion
3.1. Bandwidth
To validate the proposed design, a proof-of-concept all-textile prototype was fabricated. The reflection coefficient |S11| of the antenna was measured using a vector network analyzer and compared with the values obtained from the full-wave simulation software CST 2019. Figure 5 depicts the reflection coefficients of the antenna when placed in free space or on the human body. In free space, the simulated and measured bandwidths cover the 4.89–9.65 GHz and 4.83–9.57 GHz bands, respectively. The measured resonances are slightly shifted towards lower frequencies, which may be due to slight inaccuracies in permittivity measure. Another possible explanation could be that the textile substrate, when exposed to open space, absorbs moisture from the air, thus slightly increasing its permittivity. When the antenna was placed on the human arm, its reflection coefficient remained unaltered visibly, indicating that the full ground plane adequately shields the human body.
Wearable antennas are typically integrated onto clothing surfaces. However, human body movements can cause the antenna to bend, crumple, or stretch to varying degrees. For illustrative purposes, bending tests around the x-axis and y-axis were performed. These tests simulate realistic operating condition on an adult arm by wrapping the antenna around a foam cylinder with a 45 mm radius (equivalent to a bending angle of 76.4°). Figure 6 shows the |S11| curves in both the bent and original planar states of the antenna. In the case of bending around y-axis (the green line), the bandwidth remains unchanged, and in the case of bending around x-axis (the red line), the bandwidth decreases slightly. In practice, one can choose the direction of placement of the patch.
The performance of the antenna operating at different humidity levels, i.e., dry condition with a relative humidity (RH) of 50%, moist condition with a RH of 70%, and very damp condition with a RH of 90%, was studied experimentally. The antenna was enclosed in a transparent box to keep the humidity level stable during the measurements. Figure 7 shows the changes in reflection coefficient. As the air humidity increases, the upper bound of the UWB range shifts slightly to lower frequency. It is because the dielectric constant of the substrate increased after absorbing moisture. |S11| increases in higher frequencies in the case of very damp condition but remains under −10 dB, which justifies its robustness in moist conditions.
3.2. Far-Field Characteristics
The far-field radiation of the proposed UWB design was both numerically calculated and measured in an anechoic chamber, as shown in Figures 8(a)–8(c). The simulated and measured radiation patterns at the resonant frequencies of 5.06, 6.49, and 8.46 GHz are in good agreement with each other. Additionally, the antenna exhibits smooth broadside radiation characteristics at all these resonant frequencies, indicating its suitability for off-body communications in WBANs. The realized gains of the antenna are 9.3, 7.1, and 5.9 dBi by simulation and 9.4, 7.1, and 6.0 dBi by measurement, respectively. The realized gain of the antenna ranges from 5.87 to 10 dBi in the entire passband, with a maximum gain as high as 10 dBi measured at 5.76 GHz. The radiation efficiency of the antenna falls within the range of 78.7% and 84% throughout the operating band.
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3.3. Wearing Safety Assessment
The specific absorption ration (SAR) values of the proposed antenna at the resonance frequencies of 5.06, 6.49, and 8.46 GHz were evaluated using CST. The European Union (EU) standard imposes a SAR limit of no more than 2 Watt/kg, averaged over the 10 g of tissue absorbing most of the signal. We used a simplified human model consisting of a 2 mm-thick skin layer, a 5 mm-thick fat layer, and a 20 mm-thick muscle layer, with an overall size of 100 × 100 × 27 mm3. The human tissues’ characteristic parameters at different frequencies can be found in [30] and are shown in Table 1. The antenna, with an input power of 0.5 Watt, was placed 5 mm above the human model, as shown in Figure 9. The SAR values calculated according to the EU standard are 0.38 W/kg, 0.23 W/kg, and 0.10 W/kg at 5.06, 6.49, and 8.46 GHz, respectively. These values are far below the EU standard’s upper limit and indicate that the antenna design is reliable in terms of safety.
3.4. Time-Domain Performance of the UWB Antenna
An antenna with frequency-domain UWB characteristics may not fully ensure short pulse transmission as it could radiate different frequency components through its different radiating parts or in different directions. This could result in distorted transmitted pulses. The system fidelity factor (SFF) is a measure of the antenna’s capability to transmit/receive short pulse signals in the time domain. We conducted SFF tests using CST for both free space and on-body environments. Two identical UWB antennas were positioned facing each other with a distance of 1 m between them. The excitation signal was set as Gaussian form pulse whose frequency spectrum covers the antenna’s operating band, as shown in Figure 10. The transmission coefficient (|S21|) was numerically calculated, from which we deduced the SFF value. In free space, the SFF value is 0.73, while on the human body, the SFF value is 0.69. The SFF values are higher than the generally acceptable SFF threshold for UWB signal transmission (SFF > 0.5) [31].
4. Discussion and Comparison
Table 2 presents a comprehensive comparison between our proposed wearable UWB antenna and several representative designs in recent literature. Antennas proposed in [13–15] adopt partial ground plane; as a result, they would have inferior antenna-body isolation performance. Compared to UWB antenna designs in [20–25], our proposed antenna covers the continuous broadband of 4.83–9.57 GHz and has a much higher realized gain, reaching 5.87–10 dBi. Its overall size (in terms of the guided wavelength ) is the smallest. Additionally, the all-textile materials and the single substrate layer jointly ensure low profile, easy integration process, high flexibility, mechanical robustness, and good breathability.
5. Conclusions
A novel single-dielectric-layer all-textile UWB patch antenna with a full ground plane is presented in this paper. The antenna is shaped like the Chinese character “出,” which means “Out” or “Send,” and exhibits good UWB radiation characteristics. The proposed antenna has low profile, compact size, high flexibility, good breathability, simple structure, and low SAR value. It has the potential for wide range applications in short-range, high-speed WBAN communications.
Data Availability
The data used in this study are available from the corresponding author on reasonable request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported partially by the Natural Science Foundation of China (grant nos. 61901253 and 61771300) and partially by the Aeronautical Science Foundation of China (grant no. 202000460S6001).