A Metasurface-based Single-Layered Compact AMC-Backed Dual-Band Antenna for Off-Body IoT Devices

In this article, a compact printed monopole dual-band antenna using artificial magnetic conductor (AMC)-plane with improved gain and broader bandwidth, applicable for off-body internet of things (IoT) devices is presented. The monopole antenna consists of two C-shaped resonators connected through a U-shaped monopole, parasitic elements, discrete ground circular rings and a co-planar waveguide (CPW) feedline. Each artificial magnetic conductor (AMC) unit cell consists of a slotted circular and a square stubs, designed with two zero-crossing phases for improving the radiation characteristics and to achieve the high gain. The overall size of the proposed AMC-backed antenna is 44.4 mm × 44.4 mm × 1.6 mm with electrical dimensions of 0.75λg×0.75λg×0.027λg. This AMC-backed antenna featured measured bandwidths of 9.6% and 12.4% with improved measured gain values of 4.88 dB and 4.73 dB at 2.45 GHz and 5.8 GHz, respectively. The specific absorption rate (SAR) values are analysed and found to be 1.58 W/kg at 2.45 GHz and 0.9 W/kg at 5.8 GHz. Therefore, the proposed AMC-backed antenna is useful for off-body IoT devices operating at 2.45 and 5.8 GHz industrial, scientific, and medical (ISM) band applications.


I. INTRODUCTION
The main objective of a Wireless Body Area Networks (WBANs) is to provide a consistent interconnection between various body-centric devices for communication and sensing. An example of its application in remote patient health monitoring system is presented in Figure 1. These devices can either be used for (in, on, or off)-body communication devices. In off-body communication, networking among body-worn devices and their surroundings is established for effective transmission/reception. Antenna structures for the off-body application typically must be compatible with clothing with consistent performance under bending and human proximity effects and comply to specific absorption rate (SAR) restrictions [1][2][3][4]. Since the antenna is utilized for off-body IoT devices therefore it is necessary to discuss its application in brief details [5]. The antenna can be utilized for many off-body IoT devices like smart hand watch wearable applications, patient to doctor communications, and man to vehicle communications etc. The antenna integrated on an IoT device will exhibit a directional radiation pattern in order to send the information of the patient to the doctor's gadget [6]. Since the directional radiation pattern is an important feature of off-body antennas, therefore it is proposed that the antenna is suitable for off-body IoT devices [7][8][9][10].
A compact size low-profile antenna comprising of an L-shape slotted patch for biomedical and internet of things (IoT) was reported in [9]. The simple microstrip patch antenna for the applications in WBAN was explained in [11]. The antenna was a single band operated at 2.45 GHz. The antenna consisted of three metallic fabric layers of substrate separated by the fourth layer of felt substrate. The complete volume of the antenna reported is 80 × 80 × 8.6 mm 3 (0.71 × 0.71 × 0.076 ). The antenna covered the bandwidth 2.4-2.52 GHz (4.9%) at 2.45 GHz. A microstrip simple patch antenna for wearable applications operated in ISM band was presented in [12]. The total area of the patch antenna was found to be 60 × 60 mm 2 (0.73 × 0.73 ) The reported frequency range of the antenna was 2.4-2.5 GHz (4.1%) at 2.45 GHz. The ovalshaped radiating patch antenna was presented in [13]. It was comprised of an extended CPW semi-circular ground plane based on electromagnetic bandgap (EBG) resonating at 2.45 GHz for WBAN applications. The overall volume of the design with EBG was 75.7 × 75.7 × 0.1 mm 3 (1.05 × 1.05 × 0.0014 ) with a bandwidth from 2.4 to 2.56 GHz (6.53%). Despite the satisfactory performance, its size was relatively large. Another EBG-based single band antenna with discrete ground structure was proposed for medical wearable applications [14]. The patch antenna consisted of an E-shaped radiator with half ground plane to operate at 2.4 GHz with a bandwidth of about 32.08% and a size of 60 × 60 × 2.4 mm 3 (0.64 × 0.64 × 0.026 ) on a fabric substrate.
A low-profile dual-layered substrate antenna operating in dual-frequency mode was designed [8]. Both substrate layers are made up of FR-4 (lossy), each with 0.6 mm thickness. The radiating patch contained a thin copper line with two arms and two rectangular patches. Both arms were connected with the second layer of the substrate's ground via two shorting pins. The 50 × 50 × 0.6 mm 3 (0.856 × 0.856 × 0.01 ) sized antenna operated with bandwidths of about 4.2% at 2.45 GHz and 10.2% at 5.8 GHz. The antenna featured a peak gain of 1.2 dB at 2.45 GHz, and 7.9 dB at 5.8 GHz. The spherical patch antenna was designed to implemented on a FR-4 substrate (1.57 mm-thick) for on-, and off-body links [15]. The radiating patch consisted of two circular shape radiators connected to the feedline with the  In another study, a dual-band circular patch antenna printed on the F4B substrate with a thickness of 3.2 mm was presented in [19]. The operating bandwidth of the antenna was 2.57% at lower band and 5.22% at upper band. The overall size of the antenna was 100 × 100 × 3.2 mm 3 (1.33 × 1.33 × 0.042 ). In [20], a dual-band circular patch textile-based antenna with the size of 100 × 100 mm 2 (0.9 × 0.9 ) for on-, and off-body links was explained. The circular patch had eight slots on its edge. The reported antenna resonated at 2.45 GHz and 5.8 GHz from 2.398 to 2.517 GHz (4.9%) at 2.45 GHz and from 5.697 to 5.915 GHz (3.8%) at 5.8 GHz.
A diamond-shaped dual-band AMC-backed patch antenna printed on Roger 3003C for indoor and outdoor wearable applications was proposed in [21], This antenna resonated at 1.575 GHz and 2.45 GHz with 1.84% and 0.736% of bandwidths, respectively. The thickness of the semi-flexible substrate was 3.04 mm, with an overall antenna size of 130.8 × 130.8 mm 2 (1.85 × 1.85 ). The proposed antenna had comparatively large size with narrow impedance bandwidths. Another AMC-backed dual-band antenna working at 0.86 GHz and 2.4 GHz for wearable applications was reported in [22]. This meandered shaped antenna was fully rolled onto a watch strap and was simulated on a human arm phantom (skin, fat, and muscle) but the antenna did not show a good performance on human arm model. However, the area of the reported antenna was 195 × 30 mm 2 (1.67 × 0.4 ) and its SAR values were not calculated. An AMCbacked antenna for health monitoring applications was fabricated on a polyimide substrate of 0.15 mm-thickness in [23]. It had size of 59.1 mm × 59.1 mm (0.9 × 0.9 ) with operational bandwidths of 4.08% at 2.45 GHz and 2.24% at 5.8 GHz. The proposed antenna showed wide bandwidth but the antenna size was relatively large. This paper describes a compact metamaterial-based With AMC-plane of 3 × 3 array, the antenna has achieved broadsided directional radiation pattern, higher bandwidth, high gain, and reduced SAR values. In this paper, the antenna design procedure is explained in Section II, followed by the AMC unit cell design in Section III. The performance of the antenna with AMCbacking in free space and in proximity of human body is illustrated in Section IV and V, correspondingly. Finally, our concluding remarks are discussed in Section VI.

II. ANTENNA DESIGN A. DESIGN PROCEDURE OF THE DUAL-BAND ANTENNA
A simple dual-band monopole antenna with two C-shaped resonators connected with a U-shaped patch is proposed, fed by a co-planar waveguide (CPW) technique. The presented antenna contains three distinct layers: a ground plane, a lowcost 1.6 mm-thick FR-4 substrate (εr = 4.3 and tanδ = 0.025) and a radiating patch (from bottom to top), as shown in  The design procedure of the dual-band antenna is explained as follows: The basic antenna design (ANT I in Figure 3(a)) consists of a 50-Ω CPW feedline, discrete ground circular rings, and a rectangular patch. The patch's width and length are calculated using (1) and (2) [19], as follows: and  The design steps for the presented antenna are shown in Figure 3. ANT I is the basic rectangular patch with -10 dB reflection coefficient but in this instance, the antenna is resonating only at 2.6 GHz and 6.8 GHz frequencies. 'Lp' and 'Wp' are the radiating patch's length and width (8.8 × 9.8 mm 2 ). Then, to shift the frequency towards the lower band, another small patch is created at the bottom side of the first rectangular patch, as illustrated in ANT II (Figure 3(b)). This shifts the resonating frequency to the lower frequency band and keep its S11 value below -10 dB at 2.55 GHz and 6.2 GHz. Now in the third case (ANT III), the rectangular patch is truncated to create two monopoles. This results in an operation at 2.5 GHz and 5.82 GHz. In order to move the lower band towards 2.45 GHz while keeping the size compact, the discrete ground circular rings are added behind the substrate and the parasitic elements are introduced on the left and right side of the substrate just below the radiators as presented in ANT IV. Simulation results of all design steps are compared in Figure 4 indicating the ease of impedance tuning in the upper and lower bands using the parasitic elements and the discrete ground circular rings.

B. PARAMETRIC ANALLYSIS OF THE DUAL-BAND ANTENNA
The analysis of the parameters of the dual-band antenna is fully explained in this section. The reflection coefficient in the lower and upper bands can be adjusted by varying the values of the pole's length 'L3', width of the poles 'W3' and length of the pole 'W2' as presented in Figure 5. varying the length of the parasitic elements 'Le' from 10 mm to 15 mm, the lower frequency band shifted from 2.2 GHz to 2.7 GHz (500 MHz) while the upper band changed a bit as presented in Figure 5 (e). When the width of the parasitic elements 'We' are varied from 2 mm to 6 mm then the upper frequency band displaced from 5.8 GHz and shifted to the higher frequency bands as shown in Figure 5(f).

C. FABRICATED PROTOTYPE OF THE PROPOSED ANTENNA
The proposed dual-band radiating patch is fabricated on a low-cost FR-4 (lossy) substrate, the front view and the back view of the fabricated model are presented in Figure 6. In   The current density of the dual-band antenna is presented in Figure 9. Since, the current density helps to find out the resonating elements inside the patch antenna and tells the direction of the current flow. At the lower band (at 2.45 GHz), the current mostly flows around the U-shaped patch, CPW-feedline, and some number of current flows through the parasitic elements and the discrete ground circular rings, while in the case of upper frequency band (at 5.8 GHz), the surface current circulates in C-shaped resonators and most of the current flow through the discrete ground circular rings that have proved the highest surface current at 5.8 GHz.  To obtain operation in the lower band i.e., 2.45 GHz, two capacitors are connected in parallel with two inductors and one resistor. In the same way, another parallel RLC circuit is designed to achieve the upper-frequency band i.e., 5.8 GHz. In this case, a capacitor is connected in parallel with an inductor and a resistor. The S11 of the circuit model can be improved by adjusting the values of the resistors. The comparison among the simulation results of the circuit model and the full-wave optimization of the presented antenna are presented in Figure 11.

E. PARAMETRIC ANALYSIS OF THE EQUIVALENT CIRCUIT MODEL FOR THE DUAL-BAND ANTENNA
To change the operation in the lower band, the values of 'L1', 'C1', 'L2', and 'C2' can be varied, as presented in Figure 12 (a

III. AMC UNIT CELL DESIGN A. DESIGN PROCEDURE OF THE UNIT CELL
Artificial magnetic conductor (AMC) can be used to control the propagation of electromagnetic waves, which makes them suitable to improve the gain and to ensure the directional radiation pattern [25]. A simple and miniaturized AMC unit cell is printed on an FR-4 substrate (hsub=1.6mm and tanδ = 0.025) in this work. Its overall size is 14.8 × 14.8 × 1.6 mm 3 (0.25 × 0.25 × 0.027 ), as presented in Figure 13, and for a reflection phase of 0° at 2.45 GHz and 5.8 GHz the optimized values are given in TABLE 3. A parametric study of the unit cell is also presented to understand its operation and optimized design procedure, as illustrated in Figure 14.  As in the first step, a simple square patch (ANT I) is designed. It is observed that the unit cell has provided a zero-reflection phase at 3.5 GHz as shown in Figure 15 (a). Next, a circular ring slot with outer stub slots (ANT II) are integrated into the patch to introduce a dual reflection phase. It can be realized in Figure 15 (b) that the design has produced zero-reflection phases at 2.5 GHz and 6.6 GHz. Then in the third step, an inner circular stub (ANT III) is introduced into ANT I, this is also aimed at enabling a dual reflection phase behavior. This step results into a zeroreflection phase at 2.6 GHz and 6 GHz, as illustrated in Figure 15(c). Finally, both ANT II and ANT III are combined to produce the final proposed unit cell (ANT IV). This unit cell exhibits a reflection phase of 0° at 2.45 GHz and 5.8 GHz in a miniaturized form to operate as a dualband reflector for the proposed antenna. Simulated reflection phases for the different design steps are summarized in Figure 15, with a parametric study presented in the next section. The current density of the dual-band AMC unit cell is shown in Figure 17. Both operating bands are correlated with the two metallic stubs i.e., circular, and square. In the case of lower band (at 2.45 GHz), the current mainly flows around the outermost square stub and small amount of current through the circular stub. Since, in operating the lower frequency band, the outer square stub has major contribution. While in the case of upper frequency band (at 5.8 GHz), the current circulates the inner circular stub and some amount of current flows through the outer square stub.  The dual reflection phase behavior of this equivalent circuit can be modeled using the equivalent impedances of the circular and square patch in the unit cell, 'Zs1' and 'Zs2'. Their impedance is calculated as follows:

C. EQUIVALENT CIRCUIT MODEL FOR THE UNIT CELL
when 'Zs1 and Zs2' approaches to infinity, the dominators of (7 and 8) approaches to zero. The working frequencies and the total surface impedances of the AMC depend on the values of the lumped components which can be estimated as follows: = ℎ .
where, ' ' are the permittivity and the permeability in free space, 'hsub' is the height of the substrate, ' ' is the relative permittivity of the substrate's material, and ' ' is the effective length of the metal. The optimized parameters of the capacitors, inductors and resistors are summarized in TABLE 4.  Figure 19 shows the comparison of the reflection phases obtained from the circuit model, simulated in ADS software (in Figure 18), indicates the reasonable agreements with results of the unit cell simulated in CST software. It can be noticed that both have good agreement results. The reflection phase has narrow bandwidth when it is drawn in CST while in the case of ADS the phase of the reflection coefficient has wider bandwidth as compared to CST. But both have zero-reflection phases at lower and upper bands. This validates the circuit model for the dual reflection phase AMC design.

D. PARAMETRIC ANALYSIS OF THE EQUIVALENT CIRCUIT MODEL FOR THE UNIT CELL
A parametric study of the circuit model for the unit cell is also performed to examine the S-parameters using the time domain solver in CST. According to (7 and 8)

IV. AMC-BACKED ANTENNA IN FREE SPACE
To reduce the coupling towards human body, a 3×3 AMCplane is enrolled as the back plane of the antenna as presented in Figure 21. The antenna radiator which contains a CPW feed, a ground plane and with a radiating patch to be located on top of the AMC-plane at a distance 'D' (of 10 mm) to function as the electro-magnetic reflector. The front view and the side views are presented in Figure 21.

A. ANALYSIS OF THE AMC-BACKED ANTENNA IN FREE SPACE
When the AMC is placed at an angle of 0°, the antenna did not show any optimal performance. Therefore, it is necessary to examine the effects of the AMC arrays on the antenna's performance with different angles. The reflection coefficient [dB] of the reported AMC-backed design is studied when the AMC array is rotated at various angles i.e., 0°, 15°, 30°, 45° and 60°. Their results are summarized in Figure 22, indicating that the reflection coefficient of the AMC-backed antenna provided a better response except for zero-degree rotation. While in the case of other angles, the reflection coefficient is stable at both frequency bands. But it is preferred to use the angle of 45° because, in the case of the prototype, it is easy to place the AMC behind the antenna with 45° rotation. The optimal performance of the AMC placement is also found at an angle of 45°. Different arrays configuration of the AMC-plane is investigated in Figure 23, and the resulting antenna peak gains are given in TABLE 5. It is realized that there is not much difference in terms of reflection coefficient when different array configurations are used. However, the antenna realized gain improved when the elements in the array are increased in number. Noted that there is also a minimal increase in performance when a 4 × 4 array is used in comparison with a 3 × 3 configuration.   Figure 24 illustrates that there is a minimal change in the S11 of the AMC-backed antenna with the variation in 'D' (at 4, 6, 8, and 10 mm) in free space. From the graph, the antenna did not operate well at 2.45 GHz and 5.8 GHz, when the AMC is kept at 0 mm distance. However, as 'D' is increased, the reflection coefficient started improving. The antenna featured a good S11 in both operating bands i.e., 2.45 GHz and 5.8 GHz when placed at a distance of 10 mm. |S11| is improved by increasing the distance of 'D' as can be seen in Figure 24.

B. MEASUREMENT OF FABRICATED PROTOTYPE AMC-BACKED ANTENNA IN FREE SPACE
The fabricated prototype of the 3 × 3 AMC-backed antenna is depicted in Figure 25. To ensure consistency in the distance 'D' between the dual-band antenna and the AMCplane during measurements, a 10 mm thick foam layer is used. A vector network analyzer (VNA) is utilized to check the S11 of the antenna with AMC-backing. The radiation pattern is evaluated in an anechoic chamber and its evaluation setup is presented in Figure 25 (b).

(a) (b) FIGURE 25. (a) Fabricated prototype of the AMC-backed antenna in Free space, (b) its evaluation setup in an anechoic chamber.
The comparison of S11 of the antenna with AMC-backing and without AMC-backing is simulated and measured in Figure 26, indicating the consistency with each other. Simulated reflection coefficients showed operation in the frequency ranges of 2.36 to 2.6 (9.8%) in the lower band, and from 5.66 to 5.90 (4.13%) in the upper band. In the measurements, the antenna with AMC-backing in free space operated within the bandwidths from 2.365 to 2.595 GHz (9.38%) at 2.45 GHz, and from 5.45 to 5.94 GHz (8.44%) at 5.8 GHz.

V. AMC-BACKED ANTENNA WITH HUMAN PROXIMITY
To explore the appropriateness of the antenna with AMCbacking for off-body IoT applications, the proposed design is simulated on human body phantom model in this section. Consequently, to design an antenna for the applications in BCC that had high precision and the presence of the human body phantoms must take into consideration while calculating SAR. For that the antenna performance in proximity of human phantoms and the SAR values at 2.45 GHz and 5.8 GHz are evaluated.

A. HUMAN PROXIMITY EFFECTS
In an ideal case, off-body IoT-based antennas should be planned to avoid significant coupling effects from the human tissues. To estimate these effects, the reported design with AMC and without AMC is kept at 1 mm spacing over a 100 mm × 100 mm various layers of tissues. It consists of  The S11 of the antenna with AMC-backing in free space as well as on human phantoms are simulated and measured in Figure 31. Simulations    . While the total radiation measured efficiency is found to be more than 53% when the AMC-backed antenna is placed over human tissues (HT). The simulation and measurement graphs for the gain and the efficiency are presented in Figure 33.

B. SPECIFIC ABSORPTION RATE (SAR) ANALYSIS
The radiations of electromagnetic waves may cause health risks to human body and such risks are calculated in terms of SAR. The relationship between the input power and the SAR is as follows [23]: where ' ' and ' ' denotes the thermal conductivity (S/m) and the mass density (kg/m 3 ), while 'E' is the electric field intensity (V/m). The electric power intensity is related to the signal power as below:  Figure 34. The simulated SAR values are less than the limit of 2 W/kg over 10 grams of the human tissues, indicating the capability of the reported AMC-plane in decreasing the electromagnetic absorptions by the human arm. Figure 34 presents the distributions of the SAR when the antenna is kept at 1 mm away from human arm tissues. SAR will be reduced more when we increase the gap among the human tissues and the antenna with AMC-backing.

VI. CONCLUSION
A novel IoT-based antenna with AMC-backing for off-body devices working at 2.45 GHz and 5.8 GHz bands is presented monopole radiating patch, which is then integrated with a 3 × 3 AMC-plane to decrease the backward radiation and to in this research. The antenna consists of a CPW-fed planar reduce the coupling effects on human body in IoT applications. The unit cells of the AMC are designed based on rectangular and circular stubs. The rectangular stubs generate a zero-reflection phase at the lower frequency band (2.45 GHz), whereas the circular stubs are used to generate another zero-reflection phase at the upper-frequency band (5.8 GHz). Both antenna and AMC-plane layers are designed using a single 1.6 mmthick FR-4 substrate with a compact size of 44.4 mm × 44.4 mm × 1.6 mm (0.75 × 0.75 × 0.027 ). Satisfactorily measured bandwidths of 9.6% and 12.4% are achieved in the lower and the upper bands, correspondingly, with measured peak gains of 4.88 dB (at 2.45 GHz) and 4.73 dB (at 5.8 GHz). Calculated SAR on a three-layered human phantom is observed to be 1.58 W/kg at 2.45 GHz and 0.91 W/kg at 5.8 GHz. Experimental evaluations also indicate that the performance of the proposed antenna with compact size is suitable for off-body IoT devices.