Antenna evolution stages

Evolution of antenna geometry in different stages are shown in Fig. 2. Initial geometry has a circular patch with an arc connected through a stub. It resonates at 8.7 GHz with a return loss of 20.29 dB. Additional arc attached to stage 1 contributed with multiple resonances as shown in Fig. 2. Stage 2 resonates at 8.85 GHz frequency and 2.85 GHz frequency with S₁₁ respectively as −14.89 and −13.19 dB. Antenna design started with basic expression of patch with radius a as shown in equation (1). Stage 3 also has three resonances at 6.25, 8.75, and 10.1 GHz which are not the intended frequencies. Addition of 4^th strip as in stage 4 achieved resonance at 2.5, 5.8, and 10 GHz with better S₁₁ values, −27.03, −22.94, and −21.18 dB. In stage 5 an arc slot is introduced in the ground plane to enable resonance at 3.6 GHz which can be utilized for 5G application.

(1)\begin{equation}f = \frac{{1.8412\,{v_0}}}{{2\pi a\sqrt \varepsilon }}\end{equation}

Figure 2.

Evolution of cuff button antenna.

Thus the multiband antenna of stage 5 resonates at four frequencies respectively at 2.5, 3.6, 5.75, and 9.8 GHz suitable for WLAN, WBAN, 5G, and X band applications with return loss of 15.39/18.43/22.19/18.18 dB. All four arc strips have an angle of 260°. The four arcs are interconnected with smaller angular strips. Optimization was performed to for arc lengths to achieve the required resonance. Inner interconnection arc has an angle of 80˚ and the outer two interconnecting arcs have an angular gap of 100˚ each. Return loss plots of various stages are in Fig. 3. The antenna with slotted ground had better reflection characteristics with resonance at four frequencies including sub 6, 5G band of 3.6 GHz, but the gain at 2.4 GHz was low showing −5.07 dB. In order to improve further gain at lower frequency an additional strip is attached to the top of 2nd arc. Gain plots of the antenna for last two stages are compared in Fig. 4. The additional stub in the proposed antenna improved the gain at 2.45 GHz from −5.07 to 1.85 dB. All simulations were carried out using HFSS. Flexibility of proposed cuff button antenna is confirmed by bending analysis along X and Y direction as shown in Figs. 5 and 6. The S₁₁ plots show stable performance on bending at all four frequency bands. Proposed antenna has lower bandwidth 60 MHz (2.47–2.53 GHz), 30 MHz (3.76–3.73 GHz), 140 MHz (5.83–5.69 GHz), and 220 MHz (9.95–9.73 GHz), respectively due to smaller strip widths. Simulated efficiency of the antenna at operating frequencies of 2.5, 3.75, 5.75, and 9.8 GHz are 84.52%, 86.21%, 77.77%, and 86.18%, respectively. The antenna is tilted in different angles along X and Y direction during simulation and Fig. 7 shows the tilted antenna return loss at 20° in front & back along X axis and left & right along Y axis. The performance of the antenna remains good on tilting.

Figure 3.

Reflection performance of antenna during evolution.

Figure 4.

Gain performance of last two antenna stages.

Figure 5.

Effect of antenna bending along x axis.

Figure 6.

Effect of antenna bending along y axis.

Figure 7.

Effects of antenna tilting along X and Y axis at 20° indicating front, back, right, and left.

Current distribution

The surface current distribution of the proposed multilayer multiband cuff button antenna is shown in Fig. 8. Maximum current is for 2.5 GHz and minimum for 9.8 GHz. Coverage of strips is more in case of 5.75 GHz. The current distribution indicates the strip position responsible for maximum radiation at different frequencies.

Figure 8.

Surface current distribution of the button antenna at resonating frequencies.

Off-body antenna performance

The prototype of fabricated antenna is shown in Fig. 9. The antenna reflection performance is measured with the help of R&S’s Network analyzer ZNLE14 as in Fig. 10 and the results of simulation and measurement shows a close agreement with each other. The measured results indicate resonance at 2.48, 3.69, 5.84, and 9.96 GHz with corresponding S₁₁ readings as −22.11, −12.28, −16.08, and −18.40 dB, while simulated return loss was found to be 21.38/12.2/22.41/18.87 dB at frequencies of 2.5, 3.6, 5.75, and 9.87 GHz. Measurements are also carried out placing the antenna at different position on body. Figure 11 depicts the real time return loss performance of the antenna placing on hand and chest in comparison with simulated result. In order to analyze the water absorption property of jean substrate (wet and moisture condition), the antenna is dipped in water and partially dried and the return loss is measured. Later, when it is fully dried the measurement is repeated to check for any deformity. The results are plotted in Fig. 12. Measured return loss results are tabulated in Table 2. The Voltage Standing Wave Ratio (VSWR)and gain plots of button antennas are depicted in Figs. 13 and 14. VSWR is below 2 for the resonant frequencies. Measured peak gains at resonance are 3.08, 2.09, 1.79, and 3.03 dB while the simulated gains are 1.85, 2.03, 1.47, and 3.27dB for radiating frequencies.

Figure 9.

Fabricated cuff button antenna.

Figure 10.

Comparative S₁₁ of simulation and measurement with measured photograph.

Figure 11.

Measured S₁₁ performance at different on-body positions.

Figure 12.

Measured S₁₁ performance with water and moisture content.

Figure 13.

Comparative VSWR of simulation and measurement.

Figure 14.

Comparative gain of simulation and measurement.

Table 2.

Return loss performance of antenna in different measurement scenario

The radiation patterns in azimuth and elevation plane are measured against 18 GHz rigid horn antenna in an anechoic chamber of size 5 m × 3 m × 2.6 m and the measurements are taken at operating frequencies of 2.5, 3.6, 5.75, and 9.8 GHz which is shown in Fig. 15. Normalized radiation patterns on simulation and measurement in E and H plane are plotted in Fig. 16.

Figure 15.

Radiation pattern measurement setup in anechoic chamber.

Figure 16.

Radiation patterns of cuff button antenna.

On-body antenna performance

The antenna performance on the body was analyzed by placing it on male right arm with spacing with 2, 5, and 10 mm. Figure 17 show the return loss performance of the antenna at different spacing and resonance frequency. When the antenna is placed at 2 mm distance the lowest two resonances have poor S₁₁ magnitude of −9.25 and −6.94 dB at 2.6 and 3.65 GHz. At 5 and 10 mm distance all four bands show S₁₁ readings below −10 dB reference. Resonance frequencies and corresponding S₁₁ magnitude at 5 mm distance are 2.55/3.75/5.8/9.9 GHz & 10.94/11.28/17.34/19.68 dB and at 10 mm distance the S₁₁ values are 11.34/21.42/15.07/17.66 dB at 2.6/3.75/5.8/9.9 GHz. This indicates that the on-body dielectric variation has a tendency of smaller frequency shift with reduced S₁₁ value when placed close to the body.

Figure 17.

On-body reflection performance of antenna.

Antenna SAR analysis

SAR is the amount of radiation absorbed by the body tissues on sensing of electromagnetic radiation. As per the regulatory standards by Federal Communication Commission (FCC), SAR is limited to have a protective environment from radiation hazards with a limit of 1.6 W/Kg for 1 g of tissue or 2 W/Kg for 10 gm of tissue. In this study, human arm phantom is considered for SAR analysis as the antenna is designed to place on the sleeves. The simulation is carried out with 1 g of tissue with an input signal power of 100 mW and 5 mm gap from the body. The evaluated SAR distribution at different frequency are plotted in Fig. 18. SAR value of designed antenna is minimum (0.0113 W/Kg) at 2.5 GHz and maximum (0.6347 W/Kg) at 3.6 GHz. Simulated SAR values at 5.75 and 9.8 GHz frequency are found to be 0.155 and 0.175 W/Kg. Figure 18 clearly shows that the proposed antenna provides a SAR of acceptable FCC limit suitable for safe on-body application. Table 3 show comparison results of the proposed antenna with few existing literatures.

Figure 18.

SAR analysis of proposed antenna.

Table 3.

Comparison with existing literature