Ultra ‐ thin electromagnetic bandgap backed fractal geometry ‐ based antenna for 24 GHz ISM band WBAN

A compact, ultra ‐ thin electromagnetic bandgap (EBG) backed antenna is presented for the 24 GHz ISM band for WBAN applications. The proposed antenna has Koch fractal geometry ‐ based bow ‐ tie slots, designed with an overall dimension of 0.91 λ 0 � 0.84 λ 0 � 0.01 λ 0 and backed by a 5 � 5 element 0.01 λ 0 thick EBG structure; it is fabricated on a flexible Rogers 5880 substrate (thickness = 0.127 mm and dielectric constant ε r = 2.2, tan δ = 0.0009). In comparison to the previously published K band prototype antennas, our presented fractal antenna has a more compact and ultra ‐ thin form factor. The low profile, via ‐ less EBG unit cell structure with dimensions of 0.254 λ 0 � 0.254 λ 0 , possesses both Artificial Magnetic Conductor (AMC) and EBG characteristics. It is straightforward to fabricate at a millimeter ‐ scale. The performance parameters of the design are investigated in terms of on ‐ body reflection coefficient and free ‐ space radiation patterns with and without structural bending. The EBG structure enhances the antenna's front ‐ lobe gain by 2.3 dB, decreases back ‐ lobe radiation by 12.6 dB and decreases the specific absorption rate (SAR [1 g]) from > 50.9 W/kg to < 6.14 W/kg, significantly reducing the potential harm to the human body. Experimental investigations revealed high insensitivity of the proposed antenna to body proximity, and the performance is preserved with structural deformation.


| INTRODUCTION
Nowadays, wireless networks for wearables have gained substantial attention within both academic and industrial communities as they offer potential to further enhance the usage in military application, navigation, remote monitoring of healthcare systems and sporting performance [1,2]. The millimeterwave (mm-wave) spectrum offers many benefits such as large data rates, larger channel multiplexes, reduction in interference because of uncongested spectrum, miniaturisation of devices and unlicensed frequency bands [3]. Choosing 24 GHz (ISM) band over 60 GHz enables lower path-loss environment, less shadow fading and use of less directive antennas. However, wearable antennas for the body centric application have been mostly studied in the microwave spectrum rather than at mmwave frequencies [4,5]. Recently, two different inkjet-printed multilayer, multidirector mm-wave Yagi-Uda antennas have been developed on a thin flexible material for 24.5 GHz (ISM) band applications [6]. In Ref. [7], a wearable end-fire antenna has been proposed for 60 GHz on-body communication. However, these mm-wave body-worn antennas pose a potential radiation hazard to the human body because of high backward radiation (>5 dBi). The penetration depth of mm-waves to the human body is small due to the very short wavelength, and this restricts the risk to the skin. The value of the power transmission coefficient is extremely high in human tissues at mm-waves as compared to sub-6 GHz frequencies. Consequently, much of the electromagnetic energy is absorbed into the human skin. This issue can drastically decrease the efficiency of the antenna as well as cause a risk to the wearer's safety. Therefore, adequate isolation is necessary between the antenna and the human tissues [8]. An electromagnetic bandgap (EBG) structure is considered an effective approach to reduce harmful radiation by suppressing the back lobe and surface waves [9]. An EBG array in a phone case has been developed to minimise human exposure at microwave frequency in terms of SAR by 24%. This further reduces the effects of proximity of the human body and thereby enhances the gain of the antenna by 19% [10]. A textile-based square EBG has been developed at 26 GHz that improved the gain by 2.52 dB [11].
In this study, an ultra-thin Koch fractal geometry-based antenna integrated with a compact EBG/AMC structure having a total thickness of 0:02λ 0 is presented. The proposed antenna geometry consists of Koch fractal-based bow-tie slots on a radiating monopole, backed by a compact 5 � 5 cell EBG structure demonstrating both the bandgap and the required phase reflection at 24 GHz ISM band, which significantly reduces surface waves and the back-lobe of the antenna, respectively. The proposed structure uses extremely thin (127 μm) substrate of Rogers 5880 which made it flexible enough to fit on any shape or part of the human body. This structure is compact and electrically thinner than the comparable structures in literature. It also possesses a good bandwidth when attached directly to the skin, a minimum shift in S 11 while bending, a comparable front-to-back ratio (FBR) with a 30% decrease in thickness and the largest decrease in SAR percentage when compared with the existing wearable EBG backed antennas in K band. The paper is organised as follows: Section 2 presents the design and characterisation of the Koch fractal bow-tie antenna structure and EBG/AMC structure. Section 3 presents the results of the prototyped EBG backed antenna in terms of the reflection coefficient S 11 , radiations pattern, on-body operations, structural bending and SAR.

| EBG/AMC AND ANTENNA DESIGN
This section provides the geometry and structure of an EBG backed antenna. A split square ring resonator-based EBG geometry and Koch fractal bow-tie slot antenna are designed and investigated.

| EBG design
A unit cell of W � W = 3.18 � 3.18 mm 2 on a Rogers 5880 dielectric substrate (ε r = 2.2) and thickness of 0.127 mm over a ground plane was simulated in CST Microwave Studio using periodic boundary conditions (PBCs), Figure 1a. The proposed EBG unit cell consists of an approximately one wavelength perimeter split square ring resonator with coupling gaps on two corners. The EBG structure is symmetric along both the X and Y-axis as shown in Figure 1a, and the equivalent LC behaviour acts as a two-dimensional electric filter within the 24 GHz ISM band, where the surface exhibits high impedance. The change in the dimension of trace width and coupling gap varies the operating region, that is, a decrease in the trace width and slot gap shifts the band gap region to lower frequencies.
The unit cell was optimised to obtain the required phase reflection response at 24.125 GHz. This configuration provides 0 ο phase shift from the reflected wave and behaves as an artificial magnetic conductor (AMC). Figure 1b presents the simulated reflection phase diagram of a normally incident plane wave on the unit cell surface. The resonant frequency is 24.125 GHz, and the bandwidth is approximately 250 MHz. The AMC is a single layer and does not require vias, which simplifies the fabrication and reduces cost.
The characteristics of periodic EBG structures can be investigated by employing two different techniques such as the dispersion diagram and the suspended transmission line method. We analyse the bandgap region using both approaches. To calculate the dispersion diagram from the unit cell, periodic boundary conditions and an appropriate phase shift are applied to the sides. The unit cell repeats infinitely with periodicity P x , P y and lattice angle α. The CST Eigenmode Solver is used to generate the dispersion response in Figure 1c. The vertical axis shows frequency, while the horizontal represents transverse wavenumbers, specifically (Gamma), X and M as indicated in the inset. Hexahedral mesh type is used and the Jacobi-Davidson Method Eigenmode solver is chosen, which is considered to be faster for the given example. An EBG bandgap region from 23.9 to 24.48 GHz is observed, and surface waves will be suppressed in this band. Therefore, the interaction between the fields and the skin at the structure edge is reduced. The AMC characteristic improves the forward radiation, while the EBG effect improves the decoupling of the structure from the skin.
To further verify the bandgap region, the suspended transmission line approach was modelled using a 0.5 mm wide microstrip line placed on 0:02λ 0 above a 5 � 5 EBG array. The simulated transmission coefficient is presented in Figure 2. The S 21 is below −20 dB between 23.962 and 24.353 GHz.

| Antenna design and integration with EBG surface
In this subsection, the Koch fractal geometry-based slotted bow-tie antenna is presented in Figure 3a. To achieve flexibility, the antenna height is kept low, which reduces the impedance bandwidth. Then, the bandwidth of the proposed antenna is further enhanced by adding fractal repetition [12] along the resonant length of the patch. This fractal bow-tie slot antenna has single metallisation and dielectric layers, such that ungrounded coplanar waveguide feeding is a suitable option in terms of fabrication. After the integration of the antenna with the EBG array, grounded CPW feeding was chosen as the EBG array has a metal ground plane. The female type 50Ω SMA edge mount connector (HRM(G)-300-467B-1 specified to 28 GHz) was simulated with the full structure to improve the comparison with measurement. The fractal bow-tie slot antenna simulations were performed using the CST time domain solver, and a prototype was fabricated using chemical etching. A 5 � 5 EBG array was placed underneath the slot antenna. Figure 3b shows the dielectric and metal layer stack of the antenna-EBG structure. This prototype has an edge-mounted connector and no air gap between the layers. The EBG backed antenna was shown by measurement in Figure 3c to be well isolated from the skin, meaning the design process was valid without accounting for body loading. This configuration is suitable for wearing on the surface of the body. The resultant antenna is ultra-thin with overall dimensions of 16:19 � 16:19 � 0:254 mm 3 .

| RESULTS AND DISCUSSIONS
This section presents the simulated and measured S 11 and radiation patterns together with a consideration of body loading and a deformation study.

| Reflection coefficient evaluation
The prototype was tested in-house, and the measured reflection parameters taken with a 65 GHz VNA (Anritsu 37397c). Figure 3c shows that the S 11 values are less than −10 dB from 22.6 GHz to 25. 35 GHz (bandwidth of 2.75 GHz). The shaded bar in Figure 3c shows that the simulated and measured reflection coefficient of the EBGbacked antenna is below −15 dB in the ISM band (24-24.250 GHz). This investigation considers the antenna for the application in the 24 GHz ISM band. So although the structure is a wider band, this additional performance is not required here.

| Far-field radiation properties
The far-field radiation measurements were taken in an anechoic chamber. Figure 4 shows the H-and E-plane polar plots with and without the EBG/AMC array structure at 24 GHz. Placing the EBG/AMC structure underneath the antenna suppresses the back lobe by 12.6 dB at 24 GHz, whereas the realised gain increases by 2.3 dB to 5.1 dBi at 24 GHz. The results indicate the effectiveness of the EBG/ AMC structure in suppressing surface waves and reflecting radiation away from the human body.
Simulated radiation patterns on and off skin are included in Figure 4. It can be seen that the EBG reduces the skin loading effect, such that the difference at a given angle in the rear hemisphere is within about 10 dB.

| On-body performance
As the proposed EBG suppresses backward radiation, the prototype antenna should be tolerant to human tissue loading. On-body measurements were performed on the chest, forehead, thigh and wrist of a male volunteer who weighed 61 kg and was 178 cm in height. The S 11 of the human body loaded antenna is presented in Figure 5a. The curves differ due to the varying capacitive and dielectric constant values of human tissues at different mounting sites. However, apart from the thigh, all the measured S 11 were at <−10 dB over the desired 24 GHz ISM band. Figure 5a illustrates that without an EBG/ AMC, the structure is highly lossy across the entire measured band, and the antenna is detuned in all cases. When the EBG/ AMC is added, Figure 5b, the structure is tuned for the ISM band for all mounting sites, and the out-of-band loss decreases by around 5 dB. The prototype not only covers the 24 GHz ISM band but has 2.7 GHz bandwidth. Hence, this optimised EBG-backed Koch fractal bow-tie slot antenna with stable S 11 response when attached to the body is a suitable candidate for 24 GHz ISM band wearable applications.

| Structural conformability
The chosen antenna placement positions in Figure 5 can be considered as relatively flat. However, the capability to bear some structural deformation while retaining radiation performance with close conformance to the curved surface of the body is important for on-skin mounting. Therefore, the structure was further investigated under bending conditions. Figure 6 shows the reflection coefficient measurements when the antenna is bent around a polyethylene foam cylinder separately along the X and the Y-axis. The dielectric constant of the available polyethylene is between 1.3 and 1.6. A tissue phantom was not used, as it has been shown that the antenna is not strongly loaded by the body and therefore the curvature effect would dominate. Due to the reduction in the resonant length while bending, a slight shift in centre frequency can be seen from the plot when bent along the X-axis. A severe detuning and a shift of 400 MHz is observed when bent along the Y-axis. However, even with a 14 mm radius of curvature, the EBG backed bow-tie slot antenna still achieves reasonable reflection coefficient values (<−10 dB) at 24 GHz ISM band.
Furthermore, Figure 7 presents the radiation patterns under bending for radii R x and R y = 14 mm along the Yand Xaxis. The directivity is seen to be around −2.1 dB for both axes of curvature.

| Specific absorption rate (SAR) evaluation
To investigate the performance of the antenna in terms of the specific absorption rate (SAR), a rectangular block three-layer skin-fat-muscle model (Figure 8c) is used in CST MWS. Table 1 presents the dielectric properties of dry skin, fat and muscle layers, which were obtained from the online website of the Italian National Research Council [13]. The total size of the three-layer tissue model was 50 � 30 � 6 mm 3 . For the SAR evaluation there was a 2 mm gap between the skin and the EBG ground plane due to the height of the connector. Figure 8 presents the SAR distribution using the IEEE/IEC 62704-1 averaging technique for 1 g of tissue volume. The maximum SAR values were 6.14 and 50.93 W/kg at 200 mW input power, respectively. Thus, in the EBG case, the maximum SAR was reduced by 90% compared to the direct exposure from the antenna. To further analyse the effectiveness of the EBG structure, the rear ground plane without an EBG element array is placed underneath the antenna. The proposed antenna becomes 3.57 mm thick to get the required reflection coefficient performance at 24 GHz, and a significant decrease in the front-to-back ratio has also been observed. Therefore, the proposed EBG element is responsible for the ultra-thin structure of the antenna.

| CONCLUSION
In this study, a novel, ultra-thin and compact EBG-backed Koch fractal antenna structure is presented. Its periodic structure shows both AMC and EBG characteristics in the 24 GHz ISM band. The 5 � 5 element EBG array is evaluated beneath a Koch fractal bow-tie slot antenna and to the authors' knowledge is the thinnest (0.02λ 0 ) EBG-backed antenna reported in the K band. Table 2 presents the comparisons of different features of the proposed antenna with previously published K band EBG integrated antennas. The proposed antenna is the thinnest and smallest in size. This antenna also possesses a large bandwidth of 11.25% when attached directly to the skin; a minimum shift in S 11 while bending, a high increase in FBR and the largest decrease in SAR percentage. Anechoic chamber measurements of the EBG-backed antenna show a significant improvement in gain by 2.3 dB and in the FBR by 14.9 dB. The reflection coefficient performance of the antenna with EBG while attached to the human skin shows good insensitivity to body proximity.  -221 Furthermore, EBG-backed antenna structural deformation performed over a curve of 14 mm radius results in only slight detuning of the centre frequency and insignificant variation in bandwidth. The radiation pattern of the curved EBG antenna shows reasonable directivity and a gain of −2.1 dB under bending. Finally, the SAR simulation of the antenna over a threelayer skin-fat-muscle model under standard 200 mW input power shows a 90% reduction in SAR due to the EBG screen, which demonstrates suitability for 24 GHz ISM band conformally mounted WBAN applications.