ISSN: 2319-9873

Reach Us +447723598358
All submissions of the EM system will be redirected to Online Manuscript Submission System. Authors are requested to submit articles directly to Online Manuscript Submission System of respective journal.

Wearable Small, Narrow Band Highly Efficient, Compact Conformal, Low-Profile Metasurface Antenna For Medical Devices

Ashraf Sayed Abdel Halim*

Department of Communication, Canadian International College (CIC) Cairo, Egypt

*Corresponding Author:
Ashraf Sayed Abdel Halim
Department of Communication
Faculty of Engineering, Canadian International College (CIC) Cairo, Egypt
E-mail: Ashraf_sayed@cic-cairo.com

Received date: 25/05/2019; Accepted date: 13/06/2019; Published date: 19/06/2019

Visit for more related articles at Research & Reviews: Journal of Engineering and Technology

Abstract

In this research, I suggest a compact orthomorphic wearable antenna that works in the 2.36-2.4 GHz medical body-area network band. The antenna is implemented by installing a highly truncated metasurface, composed of only a 2 2 array of Ishaped ingredients, beneath a planar monopole. In contradiction to formerly described artificial magnetic conducting ground plane backed antenna plans, in this study, the metasurface acts as a ground plane for isolation, as well as the principal radiator. An antenna prototype was manufactured and examined, illustrating a powerful compromise between simulation and measurement. Comparing to formerly suggested wearable antennas, the validated antenna has a compact form factor of 0.5λ0 , 0.3λ0 , 0.28λ0 all while achieving a 5.5% impedance bandwidth, a gain of 6.2 dBi, and a front-to-back ratio more than 23 dB. The more numerical and experimental analysis demonstrates that the act of the antenna is extremely potent to both structural distortion and human body loading, much better to both planar monopoles and microstrip patch antennas. Furthermore, the introduced metal backed metasurface let a 95.3% decrease in the specific absorption rate, producing such an antenna a prime candidate for establishment into many wearable devices.

Keywords

Wearable sensor antenna, Body communication, Medical monitoring systems

Introduction

The prompt progress in Body Area Network (BAN) consideration [1,2] has generated major research prosperity over the past decade because of their encouraging applications in multiple fields, including health monitoring, patient tracking, wearable computing, battlefield survival, and so on.

As wearable antennas work close to the human body, the loading effect due to lossy tissue makes the plan of a high radiation efficacy antenna challenging when additionally. It is needed for it to have low weight and low-profile natures [1,3]. Meanwhile, the influence of such wearable antennas on human tissue also requires to be forwarded, such as the peak provided Specific Absorption Rate (SAR). Up till now, a lot of forms have been examined for their appropriateness as wearable antennas, including perpendicular monopoles [3-5], planar micro-strip monopoles [6,7], different inverted-F antennas [8-10], microstrip patch antennas [11], cavity-backed slot antennas [12,13], reflector patch radiators [14], and Artificial Magnetic Conducting (AMC) surface backed antennas [15,16]. The monopoles [3-5] and inverted-F antenna [4] are valuable for inside-body communications nevertheless, they bulge from the body and are thus not orthomorphic. The planar monopole antennas [6,7] and some of the planar inverted-F antennas [8-10] have a small footprint; but, a considerable amount of energy is forwarded into the human body because of their near-omnidirectional radiation properties. Micro-strip patch antennas [11] and cavity-backed slot antennas [12] are susceptible for outside-body communications because of their broadside radiation patterns, but they express a narrow bandwidth and ground plane size dependent front-to-back (FB) ratios. Substrate-integrated waveguides can decrease the configuration of slot antennas [13,14], however, they still have a large lateral size. In [15,16], isotropic AMC ground planes were used to provide a marked degree of isolation among the antenna and human tissue while keeping a reasonably small overall shape. But, they still experience almost a large footprint, frequency shifts due to bending, and/or low FB ratios.

This paper focuses on narrowband highly efficient, low-profile, and small form- factor, In addition, I will report, the effects of structural tissue make the plan of a high radiation efficacy antenna challenging when additionally. It is needed for it to have low weight and low-profile natures [1,3]. Meanwhile, the influence of such wearable antennas on human tissue also requires to be forwarded, such as the peak provided specific absorption rate (SAR). Up till now, a lot of forms have been examined for their appropriateness as wearable antennas, including perpendicular monopoles [3-5], planar micro-strip monopoles [6,7], different inverted-F antennas [8-10], microstrip patch antennas [11], cavity-backed slot antennas [12,13], reflector patch radiators [14], and artificial magnetic conducting (AMC) surface backed antennas [15,16]. The monopoles [3-5] and inverted-F antenna [4] are valuable for inside-body communications nevertheless, they bulge from the body and are thus not orthomorphic. The planar monopole antennas [6,7] and some of the planar inverted-F antennas [8-10] have a small footprint; but, a considerable amount of energy is forwarded into the human body because of their near-omnidirectional radiation properties. Micro-strip patch antennas [11] and cavity-backed slot antennas [12] are susceptible for outside-body communications because of their broadside radiation patterns, but they express a narrow bandwidth and ground plane size dependent front-to-back (FB) ratios. Substrate-integrated waveguides can decrease the configuration of slot antennas [13], however, they still have a large lateral size. In [15,16], isotropic AMC ground planes were used to provide a marked degree of isolation among the antenna and human tissue while keeping a reasonably small overall shape. But, they still experience deformation and human body loading on the act of the suggested antenna and it will be compared to conformal antenna designs primarily for off-body communications. I suggest and experimentally accomplish a light weight orthomorphic wearable antenna with a compact footprint and a low-profile in the MBAN band. First, I introduce the plan methodology and investigate the performance of the metasurface and that of the resulting integrated antenna. The radiation mechanism is realized through the calculation of the far-field pattern applying the uniform Geometrical Theory of Diffraction (GTD), reporting that the finite metasurface not only works as a reflecting ground plane but also as the primary radiator. This suggested antenna enables a more compact antenna footprint with better achievement. I will present the experimental results gained from the fabricated antenna prototype, which are in agreement with the simulation predictions. Conventional planar monopole and microstrip patch antennas. Both the numerical simulations and experimental characteristics revealed that the act of the suggested antenna is extremely potent with respect to both bending and human body loading. It is also demonstrated that the inclusion of the metasurface considerably reduces the peak SAR value.

Materials and Methods

Antenna Design

Antenna shape and Metasurface plan: The planar shape of the metasurface-permit orthomorphic antenna is illustrated in Figure 1a-1c.

engineering-technology-lower-views

Figure 1: (a): Shape of the integrated planar antenna; (b): Upper and lower views of the top monopole layer; (c): Upper and lower views of the definitely sized metasurface layer with a metal backing. The perfect dimensions are GX=62 mm, Gy=42 mm, AX=39 mm, AY=30 mm, dx=18.2 mm, dI=1.5 mm, d2=0.5 mm, d3=2 mm, Mw= 12 mm, M1=21.8 mm, tp=1.15 mm, msw=4 mm, ms1=11.3 mm, gnd1=10 mm, L1=16.86 mm, L2=5.1 mm, W1=9.75 mm, W2=16 mm, g1=2.15 mm, and G2=2.49 mm. The substrate material is Rogers RO3003 (Er=3, δtan=0.013).

It is composed of 2 ingredients separated by a thin foam spacer-a planar monopole on the superior and on the inferior a characteristic-planed highly truncated anisotropic metasurface with a metal sheet backing. The superior planar monopole is supplied by a micro-strip while the metasurface layer includes an array of 2 2 I-shaped components. The monopole is located parallel to the long axis of the I-shaped components for a valuable excitation. Over the 1ST step of the plan procedure, the geometrical measurements of the anisotropic metasurface were first tuned to give a zero reflection phase at around 2.5 GHz. Because a linear polarized monopole was utilized as the supplier, the metasurface layer was planned as it has a resonant response only in the direction parallel to the orientation of the monopole, i.e., in the x-direction. The I-shaped arrangement was selected because of its characteristic structural anisotropy, which gives a strong LC resonance through the x-direction while preserving a compact size in the orthogonal direction through the yaxis [17,18]. When the metasurface is illuminated by a plane wave at normal incidence with the electric field polarized in the x-direction, its efficient proportionate circuit model is illustrated in Figure 2a. The superior part of Figure 2b illustrated the reflection phase of the I-shaped array with a metallic sheet backing, demonstrated a near-zero reflection phase at around 2.49 GHz while an AMC band can be established. The reflection phase measured using the equivalent circuit agrees well with that obtained from the analogous full-wave simulation. The normalized maximum magnetic field of the Ishaped patch as a function of frequency is also illustrated in Figure 2b, expressing a peak at 2.48 GHz which analogous well to the zero reflection phase, frequency, i.e., the magnetic resonance band [19,20].

engineering-technology-single-unit

Figure 2: (a): Shape of a single unit cell composed of the I-shaped substance with a metal backing and its equivalent circuit under plane wave excitation; (b): Reflection phase of a definite array and the normalized magnetic field magnitude on the I-shaped patch element.

Work of the Integrated Antenna: As soon as the metal sheet backed metasurface was planned, a broadband monopole was joined and oriented at a distance d2 above the metasurface. The location of the monopole relative to the metasurface, which regulates the coupling between the antenna and the metasurface, and the geometrical measurements of the monopole was tuned to obtain the optimal impedance match, high gain and high FB ratio at the targeted band. The resulting integrated antenna has a total height of only 0.028λ0 and a footprint of 0.15λ02, which is markedly lesser than the prior reported wearable antennas at frequencies around 2.4 GHz. The simulated impedance performances of the monopole and the integrated antenna in free space are shown in Figure 3a and 3b, respectively.

engineering-technology-free-space

Figure 3: (a): Simulated and measured S11 of the monopole antenna alone in free space and the simulated S11 of the monopole antenna above a PEC ground plane; (b): Simulated and measured S11 of the integrated metasurface-enabled antenna and the reference patch antenna in free space. The inset shows the patch antenna with the same form factor as that of the integrated antenna. The measurements of the patch are PX=35 mm, PY=37 mm, D=12 m.

The monopole solely has a poor impedance match in the MBAN band, expressing S11 more than 6 dB. Additionally, the monopole located at a distance of 2.5 mm superior to a continuous metallic ground plane was simulated, illustrating an extremely poor impedance act throughout the whole band. In contradiction, when joined with the metal sheet backed anisotropic metasurface, a good impedance match is gained with an S11 lesser than -10 dB from 2.32 to 2.43 GHz, i.e., around 4.7% bandwidth. The majority of the prior illustrated integrated wearable antennas composed of a radiator backed by an AMC ground plane have an operating band around the zero reflection phase frequency of the AMC ground plane [16,21]. On the other hand, the operational band of the suggested antenna is around the frequency range where the reflection phase is about +90°, i.e., it acts as an inductive surface [22-24]. As the monopole solely is working at a frequency less its fundamental resonant mode, its input impedance is capacitive. Thus, when the monopole is loaded by the inductive impedance surface which stores more magnetic energy, the reactance of the integrated antenna can be inhibited, hence achieving a good impedance match. Additionally, this corroborates prior theoretical research on this topic [22]. These results reveal that a properly designed metallic sheet backed metasurface, even with a substantially truncated extent in the horizontal plane, can give rise to significant improvement in the impedance matching for a miniaturized antenna. Additionally, this concept can be applied to many other types of planar and wire monopole/dipole antennas. The monopole solely in free space has a dipole-like a pattern in its E-plane and an omnidirectional pattern in its H-plane (Figure 4a and 4b).

engineering-technology-monopole

Figure 4: Simulated and measured (a): E-plane and; (b): H-plane normalized radiant pattern of the monopole alone at 2.38 GHz. Simulated and measured; (c): E-plane and; (d): H-plane normalized radiation pattern of the integrated metasurface enabled antenna and the reference patch antenna at 2.38 GHz.

The integrated antenna, in contrary, has most of its energy radiated towards the +Z half-space with a Half Power Beam Width (HPBW) of about 81 and 102 in the E-plane and H-plane, respectively (Figure 4c and 4d). The insignificant back lobe demonstrated that too little energy would be radiated into the tissue when that antenna is located on the human body. This character helps to decrease the SAR value and keeps the antenna more potent to loading effects of the human body, both of which are wanted properties for wearable procedures. It should be considered that, even though the FB ratio and SAR are characteristics related to different “radiation zones” of the antenna, they are interrelated to a certain degree because the SAR and the back radiation of the integrated antenna are both results of the diffracted waves at the edges of the ground plane. The monopole solely has a simulated gain of around 2 dBi whereas the integrated antenna has a simulated peak gain at the broadside of around 6.2 dBi in the frequency band of interest, as shown in Figure 5a.

engineering-technology-peak-gain

Figure 5: (a): Simulated and measured peak gain of the monopole alone and the integrated antenna; (b): Simulated and measured FB ratio of the monopole alone, the integrated metasurface-enabled antenna and the reference patch antenna.

While the monopole has identical field strength in the anterior (+Z) and posterior (-Z) directions, the integrated metasurface-enabled antenna has a simulated FB ratio greater than 29 dB, as presented in Figure 5b. This benefit is much higher than the previously demonstrated antennas backed by an artificial ground plane, especially when realizing its compact footprint of 0.15λ02 . Especially, a FB ratio of around 16-18 dB was achieved with an antenna footprint of around 1λ02 [16,21,23,24] . In the end, it is of value to consider the favorable angular coverage of the resulting integrated antenna. Such radiation characteristics of the integrated antenna are important for outside-body communication between the wearable device and remote base stations. For a better comparison, a reference patch antenna, which has been widely used for wearable procedures, was additionally planned to resonate at 2.38 GHz using a micro-strip feed. It has an identical form factor as that of the metasurface-enabled antenna. As illustrated in Figures 3-5, the patch antenna reveals an S11<-10 dB band from 2.33 to 2.40 GHz, i.e., around 2.9% bandwidth, and a broadside gain of 5.9 dBi. Of vale, its FB ratio is only 11 dB, which is important, i.e.~ 15 dB, less than that of the suggested antenna.

Radiating Mechanism of the Integrated Antenna: To realize the radiating mechanism of this integrated metasurfaceenabled antenna, the fields at 2.38 GHz are plotted, as illustrated in Figure 6a.

engineering-technology-electric-field

Figure 6: (a): Simulated electric field distribution of the integrated metasurface enabled antenna at 2.38 GHz. An array model consisting of three magnetic current sources (M→1, M→2, M→3) was used to represent the radiating slots of the metasurface; (b): Normalized far-field patterns based on HFSS full-wave simulations of the integrated antenna compared to the three-element.

The electrical work is mainly concentrated on the edge of the planar monopole and in the capacitive gaps between the I-shaped patches along the x-direction. The monopole can be considered as an electric current source. Its radiation is greatly suppressed when it is placed close to the ground plane. However, the space between the ground plane and the ends of the I-shaped patches in the ± x-direction, act as slot antennas, which can be considered as magnetic current sources. They have the ability to radiate precisely even in close proximity to the ground plane, according to image theory [25]. Thus, contrary to prior plans where the metallic backed metasurface works only as a high-impedance AMC reflector [15,16,23,24], the definite-sized metasurface suggested in my design works as the primary radiator. More specifically, the antenna acts as a 3-element slot array with magnitude tapering, which leads to a very high FB ratio when taking into consideration the compactness of its overall footprint.

In order to substantiate this, we measured the radiation arrangement of an array of 3 uniforms identical magnetic current densities (M→1, (M2)→, M→3) placed in the y-direction on both an infinite and a definite sized ground plane as illustrated in Figure 6a. The central element has double the power as that of the magnetic current sources on every side. The GTD technique [26] was employed to account for the edge diffraction of the definite ground plane. In the E-plane (X- Z plane), the total far-field radiation arrangement as a result of the superposition of the direct Geometrical Optics (GO) fields generated by each of the 3 magnetic current sources[27]. To obtain higher precise outcomes, the twice diffracted fields were considered. In the H-plane (y-z- plane), the far-field contribution resulted in the direct GO fields is the same from each magnetic current source. Rather than the zero contribution of the 1st order diffraction as a result of the vanishing electric field at the peripheries, y= ± Gy/2, the slope diffraction was accounted [26]. In the backlobe region of the H-plane, the contribution from the E-plane edge diffraction was gained by utilizing the equivalent edge current technique as discussed [27].

To justify this design, I performed full-wave simulations of the suggested antenna on both an infinite ground plane and a definite ground plane with a size of 2λ0 by 2λ0 and compared the outcomes to those gained from the analytical formulas (Figure 6b). The geometrical measurements in the magnetic current source array model were settled from the actual geometrical dimensions of the suggested integrated metasurface-enabled antenna. It can be realized that in both the E-plane and H-plane, good acceptance is obtained for both ground sizes. Because this design does not take the radiated fields from the currents on the monopole into account, small deviations can be found between the simplified analytical model and the full-wave simulations of the actual integrated antenna, especially in the angular region near the first null in the E-plane. However, the overall good correspondence verifies that the radiation of this integrated antenna comes mainly from the artificial metallic sheet backed metasurface rather than the monopole, whose radiation is largely canceled by the imaging source. This research illustrates that, in addition to acting as a high-impedance reflector for antennas, the metasurface character can be exploited to act as the main radiator of the antenna system while simultaneously producing the isolation functionality when the antenna is placed in very close proximity to another object, e.g., the human body. This is significantly different from most of the prior reported AMC backed antenna plans that depend on large metasurfaces (>2λ0 by 2λ0) to work in-phase reflection for high gain and low FB ratio [15,22-24,28-30].

This study approves the preeminence of the suggested antenna over the patch antenna. The radiating slots for the current patch antenna are placed near the margins of the ground plane, which leads to significant diffraction of the fields. However, the suggested antenna has the most powerful radiating slot placed at the center of the ground plane, along with the tapered amplitude of the array shape, which leads to a greatly decreased back radiation. This work illustrated that further to acting as a high-impedance reflector for antennas, the metasurface characteristics can additionally be overworked such that it acts as the main radiator of the antenna system, meanwhile giving a high degree of isolation when located very near to another object, e.g., the human body.

Results and Discussion

The metasurface and the monopole were 1st manufactured separately utilizing standard PCB board etching. A foam spacer with the defined thickness was then supplemented in between the two components (Figure 7a and 7b).

engineering-technology-ground-plane

Figure 7: Photographs of (a): the manufactured copper ground plane backed metasurface and; (b): the assembled integrated antenna.

An Agilent E8364B network analyzer was used to describe the S11 of the monopole and the integrated metasurfaceenabled antenna in free space. As illustrated in Figure 3a and 3b, the acceptable agreement can result between simulations and measurements of the 10 dB bandwidth as well as respect to the resonance positions. The measured S11 of the metasurface-enabled antenna has a 5.5% -10 dB bandwidth varying from 2.30 to 2.43 GHz, which is slightly wider than the simulation prediction because of a too small decrease in the quality factor. The decreased quality factor is mostly due to the rise Ohmic loss resulting from the solder on the manufactured prototype.

The normalized radiation arrangement of the monopole and the integrated antenna were described in an anechoic chamber with an automated antenna movement platform. As illustrated in Figure 4, the calculated radiation arrangement in both the E-plane and H-plane concede well with the simulated outcomes, establishing the predicted attitude of the suggested antenna plan. The monopole alone in free space has a dipole-like arrangement, while the integrated antenna gives unidirectional radiation with wide angular coverage in the +Z hemisphere excluding the human body in a wearable configuration. In the E-plane, the calculated HPBW of the integrated antenna reaches 89°, slightly wider than the simulated value. A stronger back lobe is also noticed mainly because of manufacturing imperfection. In the H-plane, the calculated HPBW has good agreement with that developed from the simulation which is around 101°. In the 2 planes, the calculated cross-polarization level is below -20 dB in all directions. The calculated gain and FB ratio in the band of interest are illustrated plotted in Figure 5a and 5b. The calculated gain parameters for the 2 monopoles and the integrated antenna are slightly lesser than the simulated outcomes, however with a difference less than 0.4 dB. The integrated antenna achieves a gain more than 5.85 dBi in the band ranging from 2.30 to 2.43 GHz within which the S11 is below -10 dB. The calculated FB ratio illustrated a value of more than 23 dB in the same band. Although a FB ratio difference of a few decibels is noticed between the measurement and simulation, the absolute power difference is lesser than 1%. Keeping in mind that the integrated antenna has a strongly truncated ground plane, nearly the same size as that of the definite anisotropic metasurface, the high FB ratio achieved is critical to the efficient radiation of the antenna on human body surfaces.

Effects of Structural Deformation

In medical applications, the wearable antenna demands to be deformed thus becoming tailored to human body configurations. Before studying the human body loading effect, we 1st investigate the antenna act under various degrees of structural deformation in free space, where a parameter “Rα” is utilized to denote the bending radius of the antenna. As illustrated in Figure 8a and 8b, the integrated antenna with 4 different radii of curvature values have been investigated, including Rα=15, 20, 30, and 40 (mm).

engineering-technology-photograph

Figure 8: (a): Structurally deformed integrated metasurface-enabled antenna with various values of the radius of curvature ranging from Rα=15 mm to Rα=40 mm; (b): A photograph of the deformed antenna with a Rα=40 mm.

The reference patch antenna with the same bending radius values has also been simulated for comparison. The selected curvature values are proper representations for the radius of various sized human arms. Figure 9a-9c illustrate the simulated S11, gain, and FB ratio of the antennas with the 4 bending radius values.

engineering-technology-patch-antenna

Figure 9: Simulated (a): S11; (b): gain, and; (c): FB ratio of the deformed integrated metasurface-enabled antenna and the deformed reference patch antenna with various values for the radius of curvature. Measurements for the deformed integrated antenna with Rα=40 mm are included.

As we can realize, for the 2 integrated antennae as well as the patch, the resonance frequency, i.e., the lowest point in S11 is well preserved with a frequency shift less than 10 MHz, which is approximately nil. Furthermore, as the radius of the bending curvature decreases, the -10 dB impedance bandwidth becomes broader. Especially for the integrated antenna case, this band increases from 2.33-2.43 GHz when Rα=40 mm to 2.31-2.45 GHz when Rα=15 mm. It belongs to the reduction in the quality factor of the radiating TMZ100 modes of both the metasurface as well as the patch [31]. This also proposes that the radiated fields from the integrated antenna are resulted mainly by the slots among the metasurface and the ground plane. However, even at the near-extreme deformation where Rα=15 mm, the 2 antennas are capable to cover a reasonable bandwidth of about 70 MHz.

The radiation characteristics of the deformed integrated antenna undergo observable, yet still fairly insignificant, changes as the radius of curvature decreases. As Figure 9b shows, the gain is reduced by 0.8 dB due to energy leakage in the angular ranges off broadside. This further leads to a drop in the FB ratio of around 9 dB, as shown in Figure 9c. In contrast, the patch antenna exhibits an inferior radiation performance when it is bent, yielding a gain of around 4.7 dBi and a significantly degraded FB ratio of only 5.3 dB when Rα=15 mm. To demonstrate the simulation researches the integrated antenna prototype was bent around a cylindrical object with the curvature of about 40 mm and verified (as shown in Figure 8b). The overall acceptance between calculated outcomes and simulation predictions is agreed. The -10 dB impedance bandwidth is slightly narrower but without observable frequency shifting. The calculated gain and FB ratio are nearby 0.5 and 2 dB lesser than the simulated parameters, respectively, because of manufacturing inaccuracy and non-ideal uniformity of the bending radius through the structure in case of the antenna were deformed. In all, the act of the integrated antenna has verified to be very strong to structural deformation in comparison with many prior reported plans where considerable band shifting and gain drop were noticed [14,32].

Effects of Human Body Loading

I examine the influences of human body loading on the integrated metasurface-enabled antenna. I do full-wave simulations utilizing the planar monopole, the integrated antenna, and the reference patch antenna for comparison. A cylindrical multilayer human tissue model with an outer radius of 40 mm was chosen to mimic the human upper limb, which consists of four layers each representing skin, superficial fascia, muscle, and osseous tissue as Figure 10a-10c shows.

engineering-technology-human-upper

Figure 10: Structurally deformed (a): monopole antenna; (b): integrated metasurface-enabled antenna, and; (c): reference patch antenna when located on a cylindrical multilayer tissue model constituting human upper limbs.

For each layer, typical permittivity, conductivity, thickness, and mass density values reported in the literature were assigned, as listed in Table 1 [33].

  Skin Fat Muscle Bone
Er 37.95 5.27 52.67 18.49
σ (S/m) 1.49 0.11 1.77 0.82
Density (Kg/m3) 1001 900 1006 1008
Thickness (mm) 2 5 20 13

Table 1. Material characteristics of the multilayer human tissue model.

The length between the antenna and the tissue model was different in my full-wave numerical experiments to recognize how the proposed metallic sheet backed metasurface mitigates the loading effect of the human body. As Figure 11 illustrates, the integrated antenna has a powerful input impedance even in case of its placement in extremely close proximity (e.g., dα=1 mm) to the multilayer tissue model. Bandwidth broadening is noticed as compared to the Rα=40 mm case depicted in Figure 9a, showing a -10 dB bandwidth extending from 2.33-2.43 GHz to 2.31-2.47 GHz, because of the decreased quality factor of the radiator due to the lossy tissue model loading [1,16].

engineering-technology-human-body

Figure 11: Measured S11 of the structurally deformed integrated metasurface-enabled antenna placed on different parts of the human body. The inset shows a photograph of the integrated metasurface-enabled antenna tailored to a human arm.

Similar effects can be noticed in the impedance act of the reference patch in that it has a radiating mechanism similar to that of the integrated antenna. In contradiction, the impedance act of the monopole show sensitivity to the length between the antenna and the tissue model and illustrates a very broad impedance matched bandwidth. But, as can be illustrated by Figure 11, the outcome of the monopole decreases dramatically from about 2 dBi in free space to -8~ -4 dBi as it gets very near to the multilayer tissue model, proving that most of the input power is absorbed in the antenna close field by the skin and superficial fascia layers of the tissue and scattered as heat. In contradiction, the integrated antenna has very stable gain, only decreasing from 5.9 to 5.8 dBi over the band of interest, while the gain of the patch antenna is much lesser and underwent a slightly more severe decrease from 4.5 to 3.8 dBi. This indicates that with the presence of the metal-backed metasurface, the antenna is capable to maintain good impedance match and high efficiency when located at different distances and, especially, in too close proximity to the human tissue model. For the monopole and the suggested antenna, the FB ratios were almost fixed when changing antenna locations relative to the human tissue model, with differences lesser than 1.5 dB, while more difference in the FB ratio, i.e., around 2 dB, can be noticed for the patch antenna. To prove the power impedance character of the integrated meta Surface-enabled antenna, experiments were carried out with the antenna on various parts of a human body. As illustrated in Figure 12a, a very stable S11 is maintained in the case where the antenna is directly placed on an arm, chest, and leg.

engineering-technology-tissue-model

Figure 12: Simulated 1 g averaged SAR values for (a): the planar monopole; (b): the integrated metasurface-enabled antenna, and; (c): the reference patch antenna at 1 mm away from the multilayer tissue model.

Similar to that we have noticed in the numerical simulations, for the antenna on arm case as shown in Figure 12b and 12c, the -10 dB bandwidth of the integrated metasurface-enabled antenna is broadened from 2.30-2.43 GHz in free space to around 2.32-2.46 GHz, which corresponds well to the full-wave predictions.

Specific Absorption Rate (SAR) Analysis

SAR is a standard measure usually used to assess the absorption of electromagnetic power in human tissues. Agreeing to the designation given by the FCC, SAR values must be no more than 1.6 W/kg averaged over 1 g of tissue. The SAR value belongs to the adapted input power by

image is the electric field in V/m, and p is the mass density of the tissue in kg/m3. As a benchmark, a 100 mW power agreed by the antenna is selected to investigate and correlate the SAR act of the monopole, the suggested integrated antenna and the reference patch antenna. As Figure 12 illustrates, for the studied input power, the monopole provides a maximum 1 g averaged SAR value of around 16.8 W/kg because of its omnidirectional radiation property.

Even at a distance of 5 mm away from the tissue model, the monopole encounters a maximum 1 g averaged SAR value as high as 11.3 W/kg, as shown in Table 2.

da 1 mm 2 mm 3 mm 5 mm
Monopole 16.8 15.5 13.9 11.3
Integrated antenna 0.79 0.67 0.61 0.48
Patch antenna 3.98 3.47 3.12 2.53

Table 2. Maximum 1 g averaged SAR value for the monopole, the integrated metasurface-enabled antenna and, reference patch antenna at various distances away from the tissue model (units: W/KG).

For the reference patch antenna case, a considerable decrease in the maximum 1 g averaged SAR, which is around 3.98 W/kg, can be observed because of the presence of the solid ground plane. With the metallic sheet backed metasurface included, a further decrease is noticed in the maximum 1g averaged SAR value-dropping to 0.66 W/kg even in case of the integrated metasurface-enabled antenna is only 1 mm away from the tissue. In addition to the fact that the antenna characteristics are powerful to tissue loading, there is a 95.3% and 83.4% decrease in the 1 g averaged SAR in comparison to the planar monopole and the patch antenna, respectively, realizing the superiority of the integrated antenna for working very near to the human body. The maximum permissible input power levels for the 3 antennas with various values of dα are illustrated in Table 3.

da 1 mm 2 mm 3 mm 5 mm
Monopole 9.52 10.32 11.51 14.16
Integrated antenna 202.53 238.81 262.3 333.33
Patch antenna 40.2 46.11 51.28 63.24

Table 3. Maximum permissible input power levels for the monopole, the integrated metasurface-enabled antenna and, reference patch antenna at a various distance away from the tissue model (units: MW).

Effect of Change in Substrate Material Properties

With practical considerations, wearable antennas may be exposed to different types of environments for a long time. This may result in changes in the antenna substrate material characteristics, e.g., the permittivity, which may then affect the activity of the wearable antenna [1]. To test the power of the suggested plan, a ± 5% variation was introduced in the permittivity of the antenna substratum material, due to the possible influence of humidity and temperature changes. Figure 11 illustrates the S11, gain, and FB ratio of the antenna loaded with the multilayer human tissue model Figure 10b. Some frequency shift develops in the impedance band as the substrate permittivity predicts the resonance frequency however the S11 values of all three antennas are still below -10 dB within the MBAN band. The FB ratios are well maintained, illustrating a broadside gain more than 5.7 dBi and a FB ratio more than 17 dB in the MBAN band. This research reveals that the suggested antenna plan actually has a definite degree of tolerability of material character variation due to environmental change.

Conclusion

In conclusion, I have suggested and tested a compact conformal antenna for wearable processes, which was implemented by applying a definite sized metallic sheet backed anisotropic metasurface. In contradictions to prior validated antennas integrated with AMC ground planes, the metasurface in the suggested antenna integration acts as the main radiator as well as providing isolation. The accomplished antenna has performance metrics in high compliance with simulation predictions, achieving a 5.5% -10 dB bandwidth, a gain of about 6.2 dBi, and a FB ratio higher than 23 dB, far superior to traditional patch antennas. Numerical simulations and experimental calculations have additionally reported that the antenna is powerful to structural deformation and loading effects of human tissue, and the possible influence of environmental changes. Additionally, the metallic sheet backed metasurface greatly reduces the maximum 1 g averaged SAR value. These realized characteristics of the suggested antenna establish it an optimal candidate for prospective wearable devices for medical sensing, observations and other purposes.

References