1 Introduction

The growing popularity of commercial wearable technology has aroused the interest of various research improvements in the field of wearable sensing technologies as a result of recent developments in the field of contemporary wireless communication systems. Future Internet of Things (IoT) technology will change how gadgets communicate with wearable electronics, such as those with flexible antennas [1]. Hence, flexible antennas have consequently drawn a lot of interest. As a result, recent research has focused on various flexible antennas for biomedical and other applications [2, 3]. As a part of wireless body area network (WBAN) systems, wearable textile antennas have recently piqued the interest of researchers from both academia and industry due to the variety of applications they can be used for, including in security, health, and military settings [4]. The use of wearable textile antennas in WBAN applications is favored because they have significant benefits, including being lightweight, having great flexibility, being cheap, having low production complexity, and being simple to integrate into the garment [5]. There have been a number of flexible and textile-wearable antenna layouts proposed recently. [6] describes a flexible, wearable antenna for WLAN applications that is constructed on a denim substrate and has notch characteristics. [7] describes a flexible textile antenna for body-centric wireless communications running in the 2.45 GHz industrial, scientific, and medical spectrum and employing FR4, denim cotton, and Teflon substrates. A multiband Fractal Koch dipole textile antenna for wearable applications is described in [8]. An innovative, small, dual-band, all-textile PIFA for wireless body area network applications is presented in [9]. Two textile-based antennas are suggested for IoT and body-centric communication applications. The radiating structure, comprised of conductive copper threads, was created using the sewing embroidery process, and the antennas were constructed on textile cotton material. Most of these antennas have poor gain or a narrow bandwidth. Moreover, the Federal Communications Commission (FCC) selected the ultra-wideband (UWB) frequency spectrum for WBAN systems in 2002 [10]. This UWB frequency range is typically preferred for WBAN systems to operate on. On the other hand, as a result of recent advancements in wireless technology, the next generation of communication systems requires an antenna that is small, inexpensive, light, efficient, low-power, portable, and must also be capable of working with high data rates and high-quality wireless connectivity. The necessity for high-data-rate wireless communication over short distances can be satisfied by the employment of ultra wideband (UWB) radio technology [11, 12]. Yet, because of its general applicability through both short- and long-distance data communication, super wideband (SWB) technology has received a significant amount of attention in recent years [13].

SWB technology includes all the sophisticated features of UWB and offers better channel capacity with a higher data rate and increased resolution [14]. Due to the recent developments in wireless technology, WBAN requires wearable wideband antennas that can exchange digital information with high data rates and high-quality wireless connectivity that can easily maintain reliable and high-performance communication for both short- and long-distance [15] by using the human body as a data network. Lately, various [16] SWB antenna configurations using different flexible materials have been investigated. [15] describes a novel conformal super wideband (SWB) wearable textile antenna. A flexible antenna is employed on a material named Dacron fabric whose substrate with a bandwidth of almost 35 GHz. The designed antenna has been investigated with conformal analysis in terms of return loss and radiation characteristics. [17] Present a Super Wide Band (SWB) antenna design and its performance evaluation. This antenna is made of textile material for body area network applications. In [18] a low-cost PET paper has been used to design SWB antenna for wearable medical applications, vehicular navigation systems, and any other applications that require flexibility. The designed antenna operated from 1.6 to 56.1 GHz. In [19], a compact SWB antenna is designed on a thin and flexible ultralam 3850 laminates with a physical dimension of 60 × 40 × 0.1 mm3, and it uses a circular disc monopole structure. The designed antenna is thoroughly analyzed in terms of its reflection coefficient, radiation pattern, gain, radiation efficiency, and surface current for 5G application.

It has been observed that most of the UWB or SWB antennas created for body-worn applications have been found to function worse when placed in close proximity to the body in terms of gain, directionality, and efficiency. Moreover, placing an antenna on a body causes electromagnetic waves to enter the body more deeply, increasing the system-specific absorption rate (SAR) value and rendering the intended antenna unsuitable for body-worn applications. Also, a variety of flexible materials are used to design an antenna for on-body applications. However, to ensure the comfort of users, such on-body antennas need to be designed on flexible textile or plastic substrates. Moreover, textiles are excellent substrates for wearable antenna designs because of their low dielectric permittivity, which widens the antenna impedance bandwidth [20].

Due to the rising popularity of wearable technology, particularly health monitoring systems, the market for flexible electronics is expanding. Hence, the antenna must offer improved channel capacity and a higher data rate in order to be designed for application in health monitoring systems. Furthermore, the antenna for on-body application needs to be flexible and conformal, taking into account that different areas of the human body have varying dimensions and curvatures [15]. Also, most importantly, the antenna's gain must be high, as wearable antennas have been shown to have poor gain, and the antenna's SAR must be low to avoid interactions with the body, which is the final and most crucial requirement.

To address all the above-aforementioned issues, we proposed a compact, flexible Super Wide Band (SWB) antenna designed on a jeans substrate. The suggested antenna is compact and extremely flexible since it uses a Jeans substrate. The impedance bandwidth of the proposed antenna is 28 GHz (3.2–31.2 GHz) with a peak gain of 5.61 dBi. Since the antenna is intended for body wearable applications, a single-layer frequency selective surface (FSS) reflector is employed to reduce the specific absorption rate (SAR) and improve the gain of the proposed antenna for wireless body area network (WBAN) applications. A novel FSS reflector, consisting of a 2 × 2 array of elements, is located at a distance of 1.138 λ (13 mm) below the proposed antenna. However, in order to observe the SAR reduction, the same FSS structure is placed between the antenna and the human’s forearm. This placement reduced the specific absorption rate (SAR) by more than 95%. There is a 3–4 dBi improvement in antenna gain after the application of the FSS. The proposed antenna has great potential to be applied for WBAN applications not only for short range but also for long range, due to its wide bandwidth, high gain, compact size, low SAR value, and also for providing comfort to users with the use of polyethylene foam.

2 Antenna configuration

The proposed arrowhead slot Super Wide Band (SWB) antenna is illustrated in figure 1. The antenna is designed on a Jeans substrate with a relative permittivity (εr) of 1.6, a loss tangent (tan δ) of 0.002, and a thickness of 0.3 mm. The overall dimensions of the substrate are Ws × Ls mm2. To boost antenna performance and achieve 50-ohm impedance matching, a tapered connection is placed between the feed line and the main radiating patch. The optimized design parameters of the proposed SWB antenna have been depicted in table 1. The radius of a circular patch antenna ‘R’ is calculated using the Equation (1) [21, 22].

Figure 1
figure 1

Arrow head slot super wide band (SWB) antenna.

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Table 1 Parameters of the proposed SWB antenna.
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$${{\text{f}}}_{{\text{L}}}=\frac{7.2}{\left({{\text{L}}}_{{\text{g}}}+{\text{R}}+{\text{p}}\right)}\mathrm{ GHz}$$
(1)

The design of the SWB antenna starts with a simple circular monopole antenna, as it’s a basic technique to get wide and ultra-wide bandwidth, and the antenna is fed with a tapered feed line, as it provides good impedance matching. The rectangular DGS structure is inserted in the ground plane with the added inductive effect, thereby neutralizing the capacitive effect, and the antenna becomes more resistive, increasing the impedance bandwidth. Finally, the arrowhead structure is removed from the circular patch antenna as it increases the current density in the patch and enhances the bandwidth. The bandwidth of the designed antenna with the jeans substrate was achieved from 3.2 GHz to 31.2 GHz, as shown in figure 2. The gain varies from 3.87 to 5.61 dBi. The antenna achieves good bandwidth but has variable gain and continuously decreasing efficiency with frequency, i.e., from 0.65 to 0.78, as shown in figure 3.

Figure 2
figure 2

Simulated return losses of antenna with respect to stage-wise modification.

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Figure 3
figure 3

Antenna Gain and Efficiency plot with respect to frequency.

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3 SWB antenna impact on human body

Because the antenna is intended for body wearable applications, its performance should be evaluated by implanting the antenna near a human body phantom, as the body absorbs and reflects some electromagnetic energy. Figure 4 shows the phantom model for a hand structure. The designed body phantom has five layers. The last three layers are muscle, fat, and skin, and the inner two layers consist of bone layers such as cortical (80% of the total skeleton dimension) and cancellous (20% of the total skeleton dimension) [23,24,25].

Figure 4
figure 4

Proposed antenna on the phantom model for a hand structure.

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For the layers of a body phantom that represent the muscle, fat, skin, bone (cancellous), and bone (cortical), the dielectric and thermal characteristics with regard to frequencies are taken into account. Table 2 below [26], where ‘ε’ is permittivity and ‘σ’ is the electrical conductivity in S/m, gives the specific dielectric parameters for each layer with respect to frequency.

Table 2 Dielectric properties of body phantom layers.
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The phantom model has been designed by considering the dielectric properties of each phantom layer for different frequencies, as shown in table 2. Further, the SAR study has been carried out by implanting the antenna 14 mm away from the human body phantom [27], in order to examine the effect of the proposed antenna on the human body, as illustrated in figure 5. The International Commission on Non-Ionizing Radiation Protection imposes regulatory limitations to safeguard the human body from the effects of hazardous radiation. The maximum SAR value for 1 gm of tissue, is 1.6 W/Kg, and for 10 gm of tissue, it is 2 W/Kg according to regulatory criteria.

Figure 5
figure 5

Proposed antenna on a human body phantom for SAR calculation for 5.5 GHz, 8 GHz, 12 GHz, and 20 GHz for 1 gm (a), (b), (c), (d) and 10 gm (e), (f), (g), (h). respectively.

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Table 3 displays the SAR values for 1 gm and 10 gm tissues determined for various frequencies within the proposed SWB antenna bandwidth range. As shown in table 3 below, it has been found that placing an antenna on a person results in electromagnetic waves penetrating the body more deeply, raising the system-specific absorption rate (SAR) value and decreasing the gain.

Table 3 SAR values of patch antenna at different stages (14 mm from the body).
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The system-specific absorption rate (SAR) value rises when an antenna is placed near or directly over a person's body. The interface between the antenna and body should be avoided in order to lower the SAR value. One strategy is to integrate Frequency Selective Surface (FSS) structures at the back of the antenna to prevent the antenna from coming into direct contact with the body. As a result, the FSS structure is used as a reflector, as an extra effort to lower the SAR values to safe levels and also to enhance the gain [24, 25], as detailed in the section below.

4 SWB antenna with FSS structure

Figure 6 depicts the proposed FSS unit cell's geometry, and table 4 lists the optimum design parameters for the proposed FSS unit cell. The FSS structure is printed on the RT Duroid substrate with a thickness of 0.254, a permittivity ‘\({\upvarepsilon }_{{\text{r}}}\)’ of 2.2, and a loss tangent of 0.0009. It consists of three metallic square loop structures and four dipole elements. The unit cell (L1) dimension of FSS can be calculated approximately from Equation (2) as given below [27].

Figure 6
figure 6

Configuration of the proposed FSS.

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Table 4 Parameters of proposed FSS.
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$${\text{L}}1=\frac{{\text{c}}}{4{{\text{f}}}_{{\text{L}}}\sqrt{\left({\upvarepsilon }_{{\text{r}}}+1\right)/2}}$$
(2)

The FSS is designed in three steps, as shown in figure 7, in order to achieve a wider bandwidth in the SWB range. Initially, a square loop along with four slots at each corner provides the bandwidth from 4.75 GHz to 23 GHz. Further to enhance the bandwidth, a square with a phase shift of 45 degrees has been added to the design in step 2, and the FSS starts resonating from 4.2 GHz to 25 GHz. At last, another square is inserted in order to improve surface current, which shifts the lower frequency to 3.05 GHz and increases the transmission characteristic bandwidth in the frequency range of 3.05 GHz to 28 GHz. The comparison of the transmission coefficients of FSS structures is shown in figure 8.

Figure 7
figure 7

Step-wise modification of the proposed FSS.

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Figure 8
figure 8

Transmission characteristics of square loop FSS and proposed FSS.

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The 2 × 2 FSS array of dimensions 38 × 38 × 0.254 mm3 is shown in figure 9. The proposed FSS array arrangement sets it behind the antenna in order to keep it away from the body, as seen in figure 10. The FSS is situated behind the antenna and optimised so that the reflected wave from the FSS and the transmitted wave from the antenna are in phase [28, 29] in order to reduce the SAR value and achieve the gain increase.

Figure 9
figure 9

Array of unit cell FSS.

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Figure 10
figure 10

Front view of antenna integrated with FSS.

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The FSS, which is placed behind the antenna, impairs electromagnetic performance. The simulated reflection coefficient with and without FSS are contrasted in figure 11. The impedance bandwidth is unaffected by the application of FSS, but placing FSS with an antenna causes a negligible variation.

Figure 11
figure 11

Front view of antenna integrated with FSS.

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5 Antenna with FSS analysis for on-body scenario

It has been noted that the suggested antenna's impedance bandwidth is unaffected by the use of FSS. Further, the performance analysis of the suggested antenna with FSS has been carried out in terms of placement of the antenna with FSS and conformal analysis in this section to make the antenna appropriate for on-body use.

5.1 Placement of antenna with FSS

As depicted in figure 10, the FSS is positioned 0.138 λ beneath the antenna. Merely air fills the space between the antenna and the FSS construction. To test the antenna performance with FSS in an on-body situation, this positioning is ineffective. To maintain this set distance between the antenna and the FSS, a metallic enclosure must be used. Unfortunately, the performance of the antenna may be negatively impacted by this metallic housing, and users may find it uncomfortable. In order to make the proposed antenna fully suitable for body-worn applications with user comfort, the gap between the antenna and FSS is filled by polyethylene foam, with a dielectric constant of 2.26, a loss tangent of 0.00031, and a density of 2.2 lb [30] as shown in figure 12(a). The antenna is located on top of the polyethylene foam, and at the bottom of the foam, the FSS structure is placed. Further, the belt has also been attached to the FSS structure to wear a complete antenna directly on hand as shown in figure 12(b).

Figure 12
figure 12

(a) Front view of the antenna with FSS filled by polyethylene foam. (b) A complete wearable structure of the proposed antenna.

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Figure 13 compares the simulation of an antenna's reflection coefficient with FSS when the gap is filled with polyethylene foam and when it is filled with air. It has been observed that the use of polyethylene foam between the antenna and FSS has no impact on the impedance bandwidth.

Figure 13
figure 13

Comparative analysis of Return loss for gap filled with polyethylene foam and with air.

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5.2 Conformal analysis of antenna with FSS

Considering that different parts of the human body have distinct dimensions and curves, the antenna for on-body use needs to be flexible and conformal [20]. Due to its design on a denim substrate, the suggested antenna is completely flexible. To make the proposed antenna appropriate for mounting on a human body with different dimensions and curvatures, a conformal analysis of the designed antenna is also required. Several bending possibilities have been considered in the conformal analysis. As shown in figure 14, the performance of the suggested antenna has been tested at various bending angles of 20°, 40°, and 60°.

Figure 14
figure 14

The proposed antenna with (a) 20°, (b) 40° and (c) 60° bending angles.

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Figure 15 illustrates the characteristics of an antenna's reflection coefficient with and without bending. It has a very slight effect on higher frequencies because it has been observed that as the bending angle increases, the higher frequency shifts slightly to the lower side. The bending analysis has been done for a complete structure, i.e., an antenna with FSS along with foam in between to ensure the complete structure's placement on the human body.

Figure 15
figure 15

Proposed antenna reflection coefficient characteristics for bending angles.

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6 SAR and gain analysis of antenna using phantom model

Because of the direct interface between the antenna and the body, part of the electromagnetic energy radiated from the antenna is absorbed and part of it is reflected by the body [31], hence the SAR value increases above acceptable levels and the antenna gain also decreases, as shown in table 3. By integrating the suggested FSS structure between the body and antenna, as shown in figure 16, the direct interface between the antenna and body has been eliminated. Investigating the performance modification of the suggested antenna with FSS and its impact on the human body is crucial for on-body applications, hence further simulation was carried out by placing the antenna with an FSS array on the phantom model for a hand structure.

Figure 16
figure 16

Antenna with FSS on (a) Hand structure, (b) On phantom model and (c) Side view.

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The 2x2 array FSS is placed on the phantom model directly and the antenna is placed above 13 mm from the FSS. This placement is supported by filling the gap between the antenna and the FSS with polyethylene foam. SAR analysis has been carried out in order to examine the effect of the proposed antenna on the human body, as illustrated in figure 17. It has been observed that whenever the wave radiated towards the FSS is reflected back and added to the wave radiated from the antenna, If the two wave components are added in phase, then the back radiation is suppressed by integrating the FSS. The significant reduction in back radiation helps to achieve a lower SAR level [31, 32].

Figure 17
figure 17

Proposed antenna with FSS on a human body phantom for SAR calculation for 5.5 GHz, 8 GHz, 15 GHz, 20 GHz for 1 gm (a), (b), (c), (d) and 10 gm (e), (f), (g) and (h) respectively.

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According to regulatory criteria, the maximum SAR value for 1 gm of tissue is 2 W/kg, and for 10 gm of tissue, it is 1.6 W/kg. The SAR values of the SWB antenna with FSS of various stages for 1 gm and 10 gm tissues were calculated for various frequencies within the intended SWB antenna bandwidth range and are shown in table 5 for 1 gm and 10 gm tissues. The specific absorption rate (SAR) value has been found to decrease when an FSS is integrated between an antenna and a human body. The SAR obtained is below the FCC's recommended level of 1.6 W/kg. The SAR is reduced by 95.45% compared to the antenna without FSS.

Table 5 SAR Values of SWB antenna with FSS of different stages.
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Table 5 also shows that the FSS structure implemented with the antenna serves as a reflector [31, 33] and is responsible for increasing the antenna's gain for on-body applications. As the radiated waves impinge on the FSS reflector, they reflect back, which in turn affects the antenna characteristics. The gain has been enhanced as the wave radiated by the antenna and the wave reflected from the FSS are in phase to enhance the antenna gain [29, 33].

Figure 18 shows a comparative analysis of gain and efficiency for the proposed SWB antenna without FSS, the antenna with FSS, and the proposed antenna with foam in between over the complete SWB range. The gain is increased by 48.12% as compared to an antenna without FSS. The gain for foam placed between the proposed antenna and FSS is near the same as for air filled between the antenna and FSS, and there is a slight enhancement in efficiency shown in figure 18(a). Efficiency is nearby constant for all the three cases shown in figure 18(b).

Figure 18
figure 18

(a) Gain of the proposed antenna with and without FSS over the complete SWB range. (b) Efficiency of the proposed antenna with and without FSS over the complete SWB range.

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The suggested flexible SWB antenna with FSS is also contrasted with a few other flexible and semi-flexible antennas found in the literature in order to compare the proposed work with the literature already in existence. Table 6 depicts the comparison.

Table 6 Comparative analysis of the proposed SWB antenna with a few flexible antenna’s available literature.
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The proposed flexible SWB antenna has been compared mostly in terms of the SAR reduction technique used on flexible antennas for body wearable applications based on the literature. Further, the proposed antenna has also been compared in terms of size, percentage of bandwidth, conformal analysis, and peak gain for wearable applications, with most of the options available for body applications, as shown in table 6.

In [32], SAR reduction using FSS has been achieved. However, the size of the antenna is very large, and the bandwidth is very low. [34] shows SAR reduction using metamaterial; the bandwidth is in the UWB range, whereas the gain is less, and conformal analysis has not been performed. In [35], a dual-band antenna designed on polyamide substrates and metamaterials has been used for SAR reduction. However, the antenna can be used only for short-distance communication, and conformal analysis has not been performed. [36] used polyethylene foam to design a dual-band antenna with a very large size. In this work, electromagnetic band gap (EBG) structures are used for SAR reduction, and antennas can be used for short-distance communication. In [37], a metal rim TP substrate was used to design a tri-band antenna and TCM of composite PEC-lossy dielectric structures used for SAR reduction. This antenna can be used only for short-distance communication, and conformal analysis has not been performed. [38] shows a UWB antenna using SAR reduction using a reflector. A backed UWB antenna has been performed. This antenna is not flexible, and hence conformal analysis has not been performed, and gain is also on the lower side.

Finally, the proposed antenna designed on a flexible material shows a very compact size for a SWB range, and the FSS reflector is used to reduce the SAR and enhance the gain. The conformal analysis has also been performed, which makes the proposed antenna most suitable for wearable applications. According to a review of previously published work available in the open literature, no such work has yet been reported in which an SWB antenna was designed on a jeans substrate and used FSS for SAR reduction and gain enhancement.

The suggested flexible SWB antenna's compactness and superior performance characteristics in terms of size, bandwidth, flexibility, and SAR make it the most ideal antenna for a variety of WBAN and medical applications.

7 Measured results and discussion

7.1 Fabricated antenna and FSS with its measurement setup

The proposed SWB antenna is built on a Jeans substrate with a thickness of 0.3 mm and a thickness of 0.05 mm of copper tape, as shown in figures 19(a, b) below. The FSS structure is designed on an RT duroid substrate with a thickness of 0.254 mm, as shown in figure 19(c). The measurement setup for the fabricated antenna is shown in figure 20. The MS46122B VNA up to 20 GHz is used to measure the reflection coefficient with Shockwave software on a laptop, as shown in figure 20(a). The results are measured using VNA for the various stages, i.e., only for SWB, SWB antenna with FSS, where air is in between FFS and antenna, and SWB antenna with FSS, where foam is in between FFS and antenna.

Figure 19
figure 19

Fabricated Patch antenna (a) Front side, (b) back side and (c) FSS front side.

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Figure 20
figure 20

Measurement set up for (a) Antenna with VNA for S11 and (b) Dielectric constant of Jeans Material.

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Figure 20(b) shows the setup measurement of the dielectric properties of the jeans material using the Keysight N1501A Dielectric Probe Measurement kit. Figure 21 shows the assembly of the designed antenna, which consists of a SWB antenna, FSS, foam, and a supporting belt for wearable purposes. The belt and foam are 1 mm and 13 mm thick, respectively. Foam is placed between the SWB antenna and the FSS structure. The whole assembly is mounted on a belt, as shown in figure 21.

Figure 21
figure 21

Fabricated patch antenna assembly.

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Figure 22 shows the comparison result of the simulated and measured reflection coefficient graph of the antenna with the FSS array structure. Figure 23 shows the comparison graph of the reflection coefficient of patch antenna assembly for on-body applications for both simulated and fabricated cases.

Figure 22
figure 22

Return loss versus frequency graph for simulated and measured antenna with FSS.

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Figure 23
figure 23

The return loss versus frequency graph for simulated and measured antenna.

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The suggested SWB antenna with FSS-simulated reflection coefficients (S11) is compared with the measured reflection coefficients in the body scenario in figure 23. The antenna resonates between 3.2 and 31.2 GHz, and the fabricated antenna has only been tested using the Vector Network Analyzer up to 20 GHz. The results from simulation and measurement have been found to be very close to each other. However, the tiny variation is brought on by a fabrication tolerance fault.

7.2 Radiation pattern

The comparative analysis of simulated and measured radiation patterns of the SWB antenna with and without FSS at 5 GHz, 10 GHz, 15 GHz, and 20 GHz are shown in figure 24(a), (b), (c), and (d) respectively. It can be seen that the antenna without FSS has almost stable radiation patterns that are nearly bidirectional in the E (Y-Z)-plane (ϕ = 900) over the entire ultra wide band bandwidth. However, for the antenna with FSS nearly unidirectional radiation pattern in the E (Y-Z)-plane (ϕ = 900). At higher frequencies, undesirable side lobes are generated with the main lobe due to the multi-directional current distribution and higher-order modes of excitation. The simulated and measured radiation patterns are found to be in close agreement. There is a slight difference due to measurement testing and alignment errors.

Figure 24
figure 24

Radiation pattern of the proposed antenna with and without FSS (a) 5 GHz, (b) 10 GHz, (c) 15 GHz and (d) 20 GHz.

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7.3 SAR measurement

The SAR measurement is possible by the following three methods [39].

  1. 1.

    Electric Field Distribution Measurement Method

  2. 2.

    Temperature Distribution Measurement Method

  3. 3.

    Magnetic Field Measurement Method

In this work, the Temperature Distribution Measurement Method has been used to calculate the SAR value practically. The thermal analysis method is used for the practical measurement of SAR values, where the antenna is placed on the human body and powered by VNA as a signal generator, as shown in figure 25.

Figure 25
figure 25

Thermal analysis arrangement for SAR measurement.

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In this method, thermal screening is used to calculate the change in temperature. The change in temperature is calculated after twenty minutes by considering the initial time, i.e., starting from zero for different frequencies as shown in table 7. Depending upon temperature change, SAR can be calculated using Equation (3) [32, 39].

$$\mathrm{SAR }=\mathrm{ c }\frac{\Delta {\text{T}}}{\Delta {\text{t}}}$$
(3)

Where “c” is the heat capacity of the skin layer of the body phantom. The heat capacity of skin layers is 3391, for SAR analysis [26]. The thermal analysis is carried out to analyze the impact of radiation exposure of signal on the body, and consequently, changes in SAR values are observed. Table 7 shows the comparison between simulated and measured SAR values to judge the realistic performance of the proposed antenna.

Table 7 Practical Measurement of SAR values using Thermal analysis.
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Figure 26 shows the comparative analysis of measured and simulated SAR. It has been noted that there are slight changes in the measured and simulated SAR values. However, the measured SAR obtained is also below the FCC's recommended level of 1.6 W/kg.

Figure 26
figure 26

A comparative analysis of measured and simulated SAR.

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8 Conclusion

This paper designs and analyses a compact, fully flexible, arrowhead slot SWB antenna with FSS structure for body wearable applications. The impedance bandwidth of GHz (3.2–31.2 GHz) with a peak gain of 8.31 dBi has been achieved for a very compact physical dimension. In order to prove the relevance of the proposed antenna for wearable applications, SAR analysis has been performed. In order to reduce SAR, a compact FSS structure of 38 × 38 × 0.254 mm3 was used, which reduced the SAR by 95.45% and enhanced the gain by 3–4 dBi compared to an antenna without FSS. The proposed SWB antenna is suitable for both short- and long-distance data communication with a fractional bandwidth of 162%. In addition to this, the flexibility and bending capabilities of the proposed antenna were also tested under different bending conditions and found suitable considering the distinct dimensions and curves of the human body. The use of foam material between the antenna and FSS makes the proposed antenna highly comfortable for wearing applications without any adverse effect on antenna performance. Furthermore, with the complete setup, the antenna's efficiency has been increased. The proposed SWB flexible antenna with FSS maintaining low SAR, high gain, and efficiency with bending capabilities makes it highly suitable for body wearable applications with user comfort for both short- and long-distance data communication.