1 Introduction

Wearable devices and wireless body area networks (WBAN) are envisioned to change the landscapes of body-worn sensors, medicine, health monitoring, and emergency rescue [1–9]. Enabling seamless accessibility between the 2.4 GHz WBAN frequency and the widely used Industrial, Scientific, and Medical (ISM; 5.2 GHz) band [2] requires WBAN devices to be capable of operating in both frequencies using a single hardware. Besides that, a wearable system is envisioned to be conformal, lightweight, miniature in size, low profile, inexpensive, and easy to fabricate to ensure its attractiveness. In recent years, textile antenna has been widely investigated as the main enabling technology for wearable devices. The choice of such material is mainly due to its ease of integration on clothing. Any fabric such as felt [1–5], silk, tween, panama, moleskin, fleece, PTFE, Perspex [5], denim jeans [6–8], nylon [9], etc., are potentially suitable as its substrate. Meanwhile, e-textiles are generally used to form its conducting elements and are required to be low in resistivity and flexible. They should preferably not be elastic due to the proneness of such antennas to easily degrade when deformed (stretched or bent). Meanwhile, metasurfaces such as artificial magnetic conductor (AMC), high impedance surface (HIS), and electromagnetic bandgap (EBG) structures are widely used due to its compatibility with low profile antennas. They can also be located close to the radiating elements and operate as their reflectors to reduce backward radiation of the overall structure toward the human body [1–5, 10–15]. One of the main advantages of AMC is its simple unit cell basic structure, which are typically square or rectangular-shaped patches. Besides that, such AMC is capable of operating across the 5 GHz wireless local area network (WLAN) band due to its wide bandwidth when properly optimized. In this paper, a dual-band planar textile slotted dipole integrated with an AMC plane operating in the WBAN and WLAN bands (2.45 and 5.8 GHz) is presented. Besides in planar condition, its operation has been studied under two different bending directions (x-axis and y-axis) around the human upper arm of different radii. Besides bending, specific absorption rate (SAR) is also assessed to evaluate the safety level of antenna operation in the vicinity of the human body based on [16].

2 Antenna and material specification

2.1 Characterization of the metasurface

The AMC is designed on a felt substrate with a thickness of 3 mm, relative permittivity (ε r) of 1.44, and loss tangent (tan-δ) of 0.044. Its conductive elements are formed using a 0.17-mm-thick ShieldIt Super with an estimated conductivity of 1.18 × 105 S/m. The AMC topology is illustrated in Fig. 1. Its unit cell structure is formed using five layers consisting of two 3-mm-thick substrate layers and three layers of ShieldIt. A full ground plane occupies the bottom-most layer, while the proposed AMC plane is located in between the two substrate layers. The dipole described in the following sections is planned to be placed on the top layer. The AMC plane is formed using a 3 × 3 array of rectangular patch, each integrated with a diamond-shaped slot as illustrated in Fig. 1b, c. The size of the inner and outer diamond-shaped slot determines the reflection phase points of the dual-band AMC. The shorter current path provided by the diamond-shaped slot resulted in a reflection phase bandwidth of 831 MHz (from 5.26 to 6.1 GHz) in the upper band. Meanwhile, the rectangular patch with a longer electrical length created a reflection phase bandwidth of 227 MHz (from 2.38 to 2.61 GHz) in the lower band. Its performance in both bands is illustrated in Fig. 1d.

Fig. 1
figure 1

a Unit cell of the dual-band AMC; b the 3 × 3 AMC plane; c fabricated AMC plane; and d reflection phase of proposed AMC. The dimensions of AMC unit cell (in mm) are L = 30, W = 30, L1 = 27.75, W1 = 27.75, inner = 10.3, outer = 11.3

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2.2 Antenna design

The proposed dual-band planar slotted dipole antenna is shown in Fig. 2. It is designed based on the conventional dipole topology [1] for operation in the 2.45 GHz (lower) and 5.8 GHz (upper) bands. It combines a slot antenna (planar magnetic dipole) with an electric dipole. Next, four slots in the x-direction are introduced onto rectangular patch to enable its dual-band characteristics. Careful selection of their sizes will enable proper resonance at the desired lower and upper bands. The overall dimension of the proposed antenna is 90 mm × 90 mm. It is built using the same felt and ShieldIt textiles and is placed over the AMC plane.

Fig. 2
figure 2

Details of the proposed antenna: a 3D view, b cross section; c topology; and d fabricated antenna. The dimensions of the proposed antenna (in mm) are L = 90, W = 90, L1 = 55, W1 = 50, a = 2, b = 5, c = 5, d = 2, e = 37, f = 10, g = 1, h = 3

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3 Results and discussions

Besides studying its performance in free space, the antenna performance is analyzed in bent conditions as it is expected that the on-body placement will result in some degree of deformation. In this work, the antenna is studied in the planar and two other bending conditions using CST Microwave Studio (MWS). Their performance is compared in the following sub-sections.

3.1 Planar antenna

The antenna in the planar state initially provided an impedance bandwidth of 162 and 592 MHz in the 2.45 and 5.8 GHz bands, respectively, see Fig. 3.

Fig. 3
figure 3

Simulated and measured reflection coefficient of the proposed antenna (in planar condition) in free space

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The reflection coefficient (S 11) is −12.97 dB at 2.45 GHz and −24.95 dB at 5.8 GHz. The planar antenna produced a directional radiation pattern and reduced back-lobe radiation. This is important as omnidirectional antenna usually results in a relatively higher SAR in the human body, potentially affecting the health of body tissues [3]. The simulated front-to-back ratio (FBR) is higher than 20 dB in the lower band and 32 dB in the upper band (Fig. 4). The realized gain of the antenna is about 3.01 dB at 2.45 GHz and 7.04 dB at 5.8 GHz.

Fig. 4
figure 4

Radiation patterns of the proposed antenna (in planar condition) in free space at; a 2.45 GHz (xz-plane); b 2.45 GHz (yz-plane); c 5.8 GHz (xz-plane); and d 5.8 GHz (yz-plane). Legend: Simulated (black solid line) and measured (red dashed line)

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3.2 Bending evaluations

To evaluate bending, the antenna is bent over cylindrical radii of r = 40 mm and r = 60 mm in the x-axis and y-axis, as illustrated in Fig. 5. The resulting S 11 is shown in Fig. 6. It can be observed when the bending radius is reduced; the bandwidth in the lower band increases, while the upper bandwidth narrows with bending at both x- and y-axes. The antenna S 11 improved by about 3 dB at 2.45 GHz when bent in the x-axis with r = 40 mm. On the contrary, similar S 11 values with small degradations are observed at both frequencies when bent in the y-axis. Nonetheless, the operation of the antenna in the desired bands is maintained with S 11 < −10 dB.

Fig. 5
figure 5

Bending configuration of the proposed antenna: a at x-axis with r = 40 mm; b at x-axis with r = 60 mm; c at y-axis with r = 40 mm; and d at y-axis with r = 60 mm

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Fig. 6
figure 6

Simulated reflection coefficient comparison of flat and bent antennas

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Figure 7 shows the radiation pattern comparison of the proposed antenna at 2.45 and 5.8 GHz in the planar form (in the xz- and yz-planes) and when bent (at the x- and y-axes). It can be seen that the bending did not severely affect the directional antenna pattern and therefore can be efficiently used in wearable applications. It also maintained a directional pattern with high FBR, which implies reduced backward radiation.

Fig. 7
figure 7

Simulated radiation patterns for the planar and bent antennas: a bent at the x-axis at 2.45 GHz (xz-plane); b bent at the x-axis at 2.45 GHz (yz-plane); c bent at the x-axis at 5.8 GHz (xz-plane); d bent at the x-axis at 5.8 GHz (yz-plane); e bent at the y-axis at 2.45 GHz (xz-plane); f bent at the y-axis at 2.45 GHz (yz-plane); g bent at the y-axis at 5.8 GHz (xz-plane); and h bent at the y-axis at 5.8 GHz (yz-plane). Legend: solid (black) line: planar antenna; (red) long dash line: bent with r = 40 mm; (blue) short dash line: bent with r = 60 mm

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Contrary to expectations, it is also seen that the bent antenna (for r = 40 and 60 mm) at x-axis provided the most significant improvement in terms of impedance bandwidth. An impedance bandwidth of 560 MHz (when bent with r = 40 mm) and 198 MHz (with r = 60 mm) is featured in the lower and upper band, respectively. In contrast, only 164 MHz (in the planar condition) of bandwidth is featured in the lower and upper bands, respectively. In general, the performance of the proposed antenna is satisfactory in terms of reflection coefficient, realized gain, directivity, bandwidth, and FBR in the flat and both bent conditions at 2.45 and 5.8 GHz. The performance of the proposed antenna is summarized in Table 1.

Table 1 Performance comparison of the proposed antenna in the planar and different bending configurations
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3.3 Specific absorption rate (SAR) analysis

To analyze the contribution of the proposed metasurface on the antenna, a series of SAR simulation is performed when it is bent around the upper arm of a Hugo human body model. The antenna is located 10 mm from this model considering both bending in the x-axis and y-axis. Figure 8 compares the simulated reflection coefficient of the proposed antenna with a varying degree of bending in the azimuth and elevation planes (with different radii). The calculated SAR values are summarized in Table 2. It is observed that the SAR value decreased with increasing frequency at both bending directions. However, when bending 60 mm at the x-axis, an increasing SAR value is evident with the increase in frequency. Nonetheless, all assessed SAR are acceptable since they do not exceed 2 W/kg averaged over 10 g of tissue, based on the European regulatory requirements (Figs. 9, 10).

Fig. 8
figure 8

Simulated reflection coefficient of the proposed antenna with varying bending conditions

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Table 2 Simulated SAR of the proposed antenna under different bending configurations
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Fig. 9
figure 9

SAR distribution of antenna bent at the x-axis with: a r = 40 mm at 2.45 GHz; b r = 40 mm at 5.8 GHz; c r = 60 mm at 2.45 GHz; and d r = 60 mm at 5.8 GHz

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Fig. 10
figure 10

SAR distribution of antenna bent at the y-axis with: a r = 40 mm at 2.45 GHz; b r = 40 mm at 5.8 GHz; c r = 60 mm at 2.45 GHz; and d r = 60 mm at 5.8 GHz

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4 Conclusions

A wearable dual-band antenna with dual-band AMC is proposed for operation in the 2.4 and 5.8 GHz frequency bands for WBAN/WLAN applications. The antenna features a directional radiation and high FBR which potentially reduces SAR. The AMC plane is integrated with the antenna to reduce back radiation toward the human body. It consists of a 3 × 3 array of rectangular patches slotted with diamond-shaped slots to enable dual-band operation. This AMC structure is then combined with a patch antenna with a slotted dipole to enable its dual-band property. The performance of proposed antenna is satisfactory in terms of reflection coefficient, gain, directivity, radiation pattern, and bandwidth.