Proposed antenna configuration and analysis

Antenna design

The schematic diagram of the CP antenna is shown in Fig. 1(a). The designed CPW fed CRLH-TL-based CP antenna is fabricated on Rogers RT duriod 5880 and it employs an asymmetric co-planar waveguide feeding strip scheme. The CRLH-based TL unit cell is realized on an L-shaped rectangular strip that serves as the antenna's feedline. The proposed antenna uses an asymmetric ground plane consisting of a rectangular-shaped slot. The equivalent circuit of the proposed structure based on CRLH-TL is shown in Figs 1(b) and 1(c). The L-shaped feed of the antenna gives the series inductance (L_r) of the CRLH-TL, as shown in Fig. 1(b), while a small rectangular ring slot will offer capacitance in series (C_l), forming the series arm (L_r and C_l) of the CRLH inspired TL. Further, rectangular strip attached to the feed line provides shunt inductance (L_l). The virtual ground for the shunt inductor is provided by the rectangular stub, which substitutes conventional CRLH-TL-based design concept of using shorted vias or metal posts for ground [Reference Ameen and Chaudhary29, Reference Ameen and Chaudhary30]. The coupling capacitor C_c is formed by the gap w ₅ between feed line and asymmetric ground plane. Further, coupling between the CRLH-TL and the ground plane supplies the shunt capacitor (C_r) that makes up the CRLH-TL shunt arm (L_l, C_s, C_c, and C_r). A rectangular annular ring slot with gap w ₁₀ is loaded in the ground plane to keep the antenna compact. A shunt tank circuit consisting of capacitor C ₁ with inductors L ₁ and L ₂ is provided by a rectangular ring slot and ground plane as shown in Figs 1(b) and 1(c). CST Microwave Studio is used for antenna modeling and analysis. Table 1 shows the dimension of the proposed antenna.

Fig. 1.

(a) Top view of the antenna. (b) L-C representation of the antenna. (c) Equivalent circuit diagram of the antenna.

Table 1.

Dimensions of the proposed antenna

Considering an open-ended TL condition, the input impedance calculated from one end of the resonator to the other is inversely proportional to the shunt admittance and number of unit cells. As a result, the resonant frequency of an open-ended ZOR antenna is identical to the shunt frequency, and it is only dependent on the shunt parameters [Reference Ameen and Chaudhary29, Reference Ameen and Chaudhary30].

(1)$$f_{zor} = f_{sh} = \displaystyle{1 \over {2\sqrt {L_l\left[{\displaystyle{{C_s( {C_r + C_c} ) } \over {C_r + C_c + C_s}}} \right]} }}.$$

Proposed AMC unit cell design and analysis

The configuration of AMC unit cell is shown in Fig. 2(a). The proposed AMC unit cell derived from EBG structure, and designed AMC consists of a circular patch with multiple rectangular slots. The AMC structure is designed on polyimide with a dielectric constant of 3.5, a loss tangent of 0.0027, and a thickness h ₃ of 0.254 mm. The ground plane of the AMC structure is separated by h ₄ = 2.7 mm. Figure 2(b) depicts reflection phase values of AMC unit cell. The AMC ±90° reflection phase bandwidth ranges from 2.1 to 2.8 GHz, with a 0° reflection phase at 2.5 GHz. To study the AMC for wearable applications, we have simulated reflection phase values at various degrees of bending and it is observed that the results of AMC unit cell is considerably good throughout a wide range of bending degrees. The AMC unit cell is durable and well-suited for wearable applications. The AMC metasurface shown in Fig. 3 is comprised of an array of 5 × 5 AMC unit cells.

Fig. 2.

(a) Top view of AMC unit cell. (b) Reflection phase of AMC unit cell under bent conditions.

Fig. 3.

Configuration of AMC-based metasurface.

Proposed design integration of CRLH-based antenna and AMC

Figure 4 depicts the side view of the proposed antenna, which is a combination of a CRLH-TL-based CP antenna and an AMC metasurface. A 5 × 5 AMC unit cell array is used as in-phase reflector to suppress the effect of the human body on the antenna and increase forward radiation. S parameter results of the antenna with and without AMC reflector are shown in Fig. 5(a). The impedance bandwidth of the antenna without AMC is 2.36–3.1 GHz, whereas the impedance bandwidth of the antenna with AMC is 2.3–3.1 GHz. It is observed that the resonance frequency of the AMC-loaded antenna is slightly shifted to the left side in comparison to the CRLH-based antenna. The simulated gain values of both antennas are shown in Fig. 5(b), with the AMC-loaded antenna exhibiting stable gain within the impedance bandwidth and a peak gain of 6.1 dBi at 2.6 GHz. It is also noticed that the AMC-loaded antenna (2.3–2.8 GHz) exhibits an improved 3 dB ARBW than the CRLH-TL-based antenna. As a result, we can conclude that combining AMC with the CRLH-TL-based CP antenna enhances overall radiation characteristics.

Fig. 4.

Side view of CRLH-based CP antenna integrated with AMC.

Fig. 5.

Simulated (a) S ₁₁. (b) Gain. (c) Axial ratio of antenna with and without AMC.

Evaluation of AMC antenna performance under bending conditions

For practical wearable applications, the designed antenna must be able to display robust performance owing to changes in shape caused by the curvature of the human body. Thus, in this section, we have evaluated the performance of proposed AMC antenna by varying its shape as per the requirement. The different AMC antenna combinations under conformal conditions placed on cylinders of varying diameter is designed. The simulated results show that the antenna's S-parameter has minute impact due to variation in sizes, and the antenna is suited for wearable devices. Figure 6(a) shows the simulated reflection coefficients of AMC antenna under different bending conditions and it is observed that impedance bandwidth of AMC antenna almost remain same for R = 30, 40, and 50 mm bending diameter. The simulated gain of the antenna for bending R = 30, 40, and 50 mm is shown in Fig. 6(b); it is observed that as the gain of the antenna reduces, bending diameter reduces. The axial ratio of the AMC antenna is shown in Fig. 6(c) and 3 dB ARBW reduces to 2.35–2.6 GHz for R = 30 mm. It is clear that as bending level increases, the performance of the antenna slightly deteriorates.

Fig. 6.

Simulated (a) S ₁₁. (b) Gain. (c) Axial ratio of AMC antenna under different conformal cylinder radius.

Experimental results and discussion

To investigate the bending performance, proposed antenna is positioned on a cylinder (radius = 50 mm) made up of foam material. The radius of foam cylinder is deliberately selected to mimic the antenna's conformation to a typical human arm. Figure 7 shows the photographs of measurement of AMC antenna using vector network analyzer. The measured and simulated S ₁₁ of the antenna are shown in Fig. 7(a). It is observed that the measured bandwidth of AMC antenna is 2.3–3.2 GHz with relative impedance bandwidth of 21%. However, the performance of antenna slightly changes in the conformal shape as the human arm of radius is 50 mm. It is seen that impedance bandwidth of AMC antenna with bending is 2.32–3.1 GHz. Therefore, it is clear that antenna has little effect of bending and is suitable for wearable application. Figure 7(b) shows the simulated and measured gain of the antenna, it is observed that the proposed antenna exhibits stable gain in both planar and conformal forms with peak gain of 5.89 and 5.72 dBi in planar and conformal form, respectively. The axial ratio of the proposed antenna is shown in Fig. 7(c), AMC antenna have 3 dB ARBW of 2.36–2.78 GHz, while 3 dB ARBW of bent AMC antenna is 2.38–2.67 GHz. Further, the normalized radiation patterns of antenna at 2.4 GHz under normal and bent conditions are plotted in Figs 8 and 9, respectively. It has been observed that the proposed antenna LHCP pattern is 15 dB below in comparison with RHCP pattern in the boresight direction. It is seen that HPBW of the AMC antenna and bent AMC antenna is 52° and 45° in phi = 0° plane while HPBW of the AMC antenna and bent AMC antenna is 42° and 41° in phi = 90° plane. Therefore, it is clear that AMC antenna shows stable results in bent form also.

Fig. 7.

Simulated and measured (a) S ₁₁. (b) Gain. (c) Axial ratio of AMC antenna under normal and bent condition.

Fig. 8.

Normalized LHCP and RHCP radiation pattern of AMC antenna under normal condition. (a) Phi = 0°. (b) Phi = 90° at 2.4 GHz.

Fig. 9.

Normalized LHCP and RHCP radiation pattern of AMC antenna under bent condition. (a) Phi = 0°. (b) Phi = 90° at 2.4 GHz.

Performance of the proposed antenna on the human body

We have executed full-wave simulations with the designed antennas close to a human body model to verify the on-body performance by using human phantom model named Hugo (from CST MWS voxel family). The proposed antenna is placed on the arm of human body phantom as shown in Fig. 10. Instead of modeling the antenna on the complete numerical body model, a reasonable region of Hugo's arm is chosen for simulation to shorten simulation time. In all the studied scenarios, antenna is assumed to be separated from the skin layer by an additional distance of 2 mm to account for clothing.

Fig. 10.

3-D CST human body model (Hugo).

The comparison of simulation results of reflection coefficients of the proposed antenna in free space and with human body model (Hugo) is shown in Fig. 11(a). It is observed that impedance of the proposed antenna is slightly shifted to right side and small bandwidth widening is seen with −10 dB impedance bandwidth of 2.32–3.23 GHz due to the loading of human phantom model and impedance matching of the antenna is also reduced. The reduction is due to high dielectric constant values of different tissue layers of human body model. The gain values of the proposed antenna are shown in Fig. 11(b), and it is seen that implementation of human body model has little impact on antenna gain and radiation efficiency performance. The simulated peak gain of the antenna is reduced to 5.82  as compared to 6 dBi in free space. The radiation efficiency of the proposed antenna is shown in Fig. 11(c) and we can say that radiation efficiency of the antenna is above 85% in the whole impedance bandwidth for both the cases. Figure 12 shows the simulated radiation patterns of the proposed antenna in free space and with human body condition at 2.4 GHz. The results illustrate that radiation characteristics of antenna change trivially between free space and on-body status because of the good isolation achieved by the AMC structure.

Fig. 11.

Simulated results of the proposed antenna in free space and with human body phantom. (a) Reflection coefficient. (b) Gain. (c) Radiation efficiency.

Fig. 12.

Simulated normalized radiation patterns of the proposed antenna in free space and with human body phantom. (a) Phi = 0°. (b) Phi = 90°.

SAR results of proposed antenna for wearable safety

To guarantee compliance with safety laws, the SAR level of a wearable antenna must be evaluated while designing the antenna. Furthermore, SAR is used to assess the risks that wearable antennas pose to human health. Thus, the RF energy received by human tissues must not go over the critical threshold of 1.6 W/kg for any 1 g tissue or 2 W/kg for 10 g tissue, according to the IEEE C95.1-1999 and IEEE C95.1-2005 standards. The AMC antenna is placed on the CST human body model (Hugo) to assess the SAR of the human body. Figure 10 depicts the numerical simulation of a 3-D body with approximate human tissue data. After loading the AMC structure, the EM wave radiation to the human body is effectively reduced. Figure 13 depicts the simulated SAR distribution on human hand. From Fig. 13(a) it is seen that at 2.4 GHz, the maximum value of SAR is 0.67 W/kg of 1 g/m³ under the condition of 1 W input power. Further, Fig. 13(b) depicts the SAR distribution for 10 g human body tissue and it is observed that peak SAR value is 0.568 W/kg of 10 g/m³.

Fig. 13.

SAR simulation. (a) 1 g human tissue. (b) 10 g human tissue at 2.4 GHz.

Table 2 depicts the SAR simulation at 2.4 GHz, as shown in the table below. The AMC antenna's assessed SAR values are all considerably below the regulated SAR criteria, but the monopole antenna does not. The reason for this is that antenna without AMC produces an omnidirectional pattern, but an AMC antenna produces a directing radiation pattern. The SAR study demonstrates the AMC antenna's advantage for operating close to the human body. A comparison of the proposed work with the work discussed in literature is presented in Table 3. It is seen that the proposed antenna has smaller foot print with stable gain as compared to the work reported. The proposed AMC antenna exhibits circular polarization which makes it a good candidate for ISM band wearable applications.

Table 2.

SAR analysis of antennas at 2.4 GHz (1 g tissue (w/kg))

Table 3.

Comparison of the proposed work with the previous work