Reflection coefficient characteristics

The reflection coefficient characteristics is represented in dB as return loss (S₁₁). The simulated and measured return losses of the proposed antenna are shown in Fig. 5.

Figure 5.

Return loss characteristics of the proposed antenna.

From the figure, it is seen that the simulated and measured results are in good agreement. In simulated result, the first lower order resonance occurs at 2.45 GHz which is used for ISM band applications. The middle order resonance was created at 3.45 GHz 5 G applications. The higher order resonances occur at 5.8 GHz (ISM) and 9.5 GHz, which operates in the UWB range of frequencies.

An antenna can be called as UWB antenna only if the bandwidth of the antenna is 500 MHz in the UWB range [Reference Mahmood, Ishak, Ismail, Zakaria and Alani34, 40]. To check this, the bandwidth of the proposed antenna was derived from the return loss graph with the reference point as −10 dB. As mentioned earlier, the ground plane dimension played a major role in enhancing the bandwidth of the proposed design. Hence, a parametric analysis was done for the width of the ground plane with various dimensions. This is shown in Fig. 6.

Figure 6.

Return loss of the proposed antenna at various ground plane dimensions.

Although the dimension was calculated using mathematical relations, this parametric analysis shall be helpful in finalizing the values. The value taken from the expressions (4) and (5) is 6.3 mm.

A value slightly lower than 6.3 which is 6 mm and slightly higher than 6.3 which is 6.6 mm were taken for analysis. From the analysis, it was found that the value obtained from the mathematical expressions was the most fitting one for high bandwidth.

On observing the return loss characteristics of the proposed antenna, it is found that the antenna covers the entire UWB band from 2 to 10 GHz, thereby providing 106% of bandwidth.

VSWR

The measured VSWR of the proposed antenna is shown in Fig. 7.

Figure 7.

VSWR of the proposed antenna.

In general, a good VSWR is a value less than 2, which represents only 10% of standing waves. On observing the graph, it is found that the simulated VSWR is less than 2 from 1.7 to 9.9 GHz. This confirms the proper operation of the antenna in the UWB band.

Efficiency

The overall radiation performance of the antenna is predicted by the parameter efficiency of the antenna, which can be viewed at every particular frequency of interest. The efficiency of the proposed antenna design is depicted in Fig. 8.

Figure 8.

Efficiency of the proposed antenna.

The efficiencies of the antenna are represented in dB obtained from CST microwave studio. Converting those values into percentage using basic mathematical calculations, it is found that efficiency of the proposed design is 91%, 95%, 96%, 96% at 2.4, 3.45, 5.8, and 9.5 GHz, respectively.

Radiation pattern

The directionality of the antenna’s radiation is given by the parameter radiation pattern. Figure 9a and b shows the radiation pattern (e-plane and h-plane) of the antenna at 2.4 and 5.8 GHz on free space. It can be inferred that the antenna radiates directionally at 2.45 GHz which makes it suitable for off-body communication [Reference Zhou, Fang and Jia10, Reference Mahmood, Ishak, Ismail, Zakaria and Alani34, Reference Farahat and Hussein41]. At 5.8 GHz, it radiates omnidirectionally which makes it suitable for on-body communication [Reference Tak, Lee and Choi4, Reference Mahmood, Ishak, Ismail, Zakaria and Alani34, Reference Farahat and Hussein41, Reference Singh, Dhupkariya and Bangari42]. Thus, the proposed design can act as a dual-mode antenna for WBAN applications.

Figure 9.

Radiation pattern of the proposed antenna. (a) 2.45 GHz and (b) 5.8 GHz.

Gain

The simulated gain of the proposed antenna is shown in Fig. 10. The simulated gain at 2.45, 3.45, 5.8. and 9.5 GHz is 1.95, 3.23, 3.46, and 5.95 dB, respectively. As WBAN devices will be communicating with other devices or networks which are probably closer to them, the obtained gain values of the proposed design are highly appreciable.

Figure 10.

Gain of the proposed antenna.

Within the desired frequency range, the peak gain of the antenna occurs at 7.5 GHz with 6.61 dB.

The comparison of return loss characteristics, gain, and bandwidth of the proposed design for three substrates is shown in Table 1.

Table 1.

Comparison

From the table, it is seen that the proposed antenna is smaller in dimensions than the antennas in papers [Reference Yeboah-Akowuah, Kosmas and Chen3, Reference Gupta and Kumar5–Reference Shakib, Moghavvemi and Binti Wan Mahadi7, Reference Zhao, Chao, Hong, Wang and Zheng9, Reference Zhou, Fang and Jia10, Reference Zhao and Geyi18, Reference Singh, Bangari, Ali, Vyas, Verma and Saxena29] and lesser in thickness than in papers [Reference Yeboah-Akowuah, Kosmas and Chen3, Reference Gupta and Kumar5–Reference Shakib, Moghavvemi and Binti Wan Mahadi7, Reference Zhao, Chao, Hong, Wang and Zheng9, Reference Zhou, Fang and Jia10, Reference Zhao and Geyi18, Reference Govindan, Palaniswamy, Kanagasabai, Kumar, Rao and Alsath26, Reference Singh, Bangari, Ali, Vyas, Verma and Saxena29, Reference Kumar, Kumar, Gupta, Sengar, Sharma, Gupta, Kumar, Gupta, Albreem, Ha and Sharma30, Reference Yadav, Singh, Verma and Singh32, Reference Chen, Wang, Liu, Wang and Lin33]. This lesser thickness of the proposed work makes the overall volume lesser. Also, the antenna provides better bandwidth than the antennas reported in papers [Reference Shakib, Moghavvemi and Binti Wan Mahadi7, Reference Veeraselvam, Mohammed, Savarimuthu, Marimuthu and Balasubramanian8, Reference Singh, Bangari, Ali, Vyas, Verma and Saxena29, Reference Kumar, Kumar, Gupta, Sengar, Sharma, Gupta, Kumar, Gupta, Albreem, Ha and Sharma30, Reference Yadav, Singh, Verma and Singh32, Reference Chen, Wang, Liu, Wang and Lin33] and dual-mode operation with good bandwidth than in papers [Reference Gupta and Kumar5, Reference Zhou, Fang and Jia10, Reference Zhao and Geyi18, Reference Saeidi and Karamzadeh31]. The proposed antenna also provides better gain than the antennas reported in papers [Reference Yeboah-Akowuah, Kosmas and Chen3, Reference Kerketta and Ghosh6, Reference Shakib, Moghavvemi and Binti Wan Mahadi7, Reference Zhou, Fang and Jia10, Reference Lai, Wang, Sun, Zhao and Zeng19, Reference Govindan, Palaniswamy, Kanagasabai, Kumar, Rao and Alsath26, Reference Singh, Bangari, Ali, Vyas, Verma and Saxena29, Reference Kumar, Kumar, Gupta, Sengar, Sharma, Gupta, Kumar, Gupta, Albreem, Ha and Sharma30, Reference Yadav, Singh, Verma and Singh32, Reference Chen, Wang, Liu, Wang and Lin33].

On observing the results, it is found that the FR-4 and rubber substrates provide better return loss values. On observing the bandwidth, ROGERS 4003C provides higher bandwidth with good gain, and hence it is chosen for further on-body analysis.

S₁₁ performance analysis

The S₁₁ performance analysis of the antenna in free space and on body and also with a human subject is done, and the results are presented below. An on-body tissue model initially with three layers, namely skin, fat, and muscle, was used for simulation. This means that the antenna is directly placed on the body without any air gap. Then, various air gaps such as 3 and 6 mm were used between the antenna and human multilayer model and the results were analyzed. Figure 11 shows the simulation setup of the antenna on body.

Figure 11.

On-body antenna simulation setup. (a) Front view and (b) side view.

The dimension of the body phantom model was chosen to be 100 × 100 mm², which is a bit larger than the dimensions suggested in papers [Reference Gupta and Kumar5, Reference Shakib, Moghavvemi and Binti Wan Mahadi7]. The thickness and electrical properties of the phantom model were considered as done in paper [Reference Shakib, Moghavvemi and Binti Wan Mahadi7] and presented in Table 3.

Table 3.

Electrical properties of on-body tissue model

From the graph (Fig. 12), it is found that when the antenna is directly placed on the human body with no air gap, there is detuning of the lower resonance at 3.45 GHz and the resonance at 2.45 and 5.8 GHz undergo change in loss values. The resonance at 9.5 GHz also slightly shifts to a lower frequency. The shift in resonance occurs toward lower frequency side.

Figure 12.

On-body S₁₁ performance analysis.

Later, air gaps were introduced between the antenna and the body phantom considering the fact that a cloth separation (dress) shall be there between the body and the antenna sensor.

When the gap is 3 mm, the antenna exhibits a slight shift in its resonances at 2.45 and 3.45 GHz to a lower value, but the resonance at 5.8 and 9.5 GHz remain almost the same without much effect. Later, a gap of 6 mm was introduced and it was observed that the behavior of the antenna almost remains the same at all the frequencies as compared to free space. Hence, to extract the same performance as it provides in free space, an air gap of 6 mm was used for on-body simulation and analysis.

SAR analysis

For any antenna used for WBAN applications, it is mandatory to analyze SAR. SAR gives information about what quantity of radiated power is entering the body when the antenna in brought to proximity of human body. According to the IEEE C95.3, the value of SAR should be less than 2 W/kg for 10 g of tissue and less than 1.6 W/kg for 1 g of tissue [43].

SAR can be calculated using the following expression:

(9)\begin{equation}{\textrm{SAR}} = {{\sigma {{\left| E \right|}^2}} \over {2\rho }}\end{equation}

where $E$—electric field intensity in V/m, $\sigma $—conductivity of the tissue in S/m, and $\rho $—mass density of the tissue in kg/m³.

The SAR analysis has been done for two categories of power such as a high value and a low value of power. Initially, the designed antenna was simulated for SAR analysis with an input power of 1 W for 10 g of tissue. For this power, the SAR was found to be almost closer to the health hazard standard given in papers [Reference Movassaghi, Abolhasan, Lipman, Smith and Jamalipour2, 43]. Hence, the input power was then reduced and set to 0.8 W for SAR analysis [Reference Sakurai, Kiyokawa, Narita, Suzuki, Taki and Miyakoshi44]. For 0.8 W, the SAR value was found to be 0.46 W/kg at 7.5 GHz and 0.57 W/kg at 5.5 GHz, which satisfies the health norms of electromagnetic exposure.

The SAR simulation is shown in Figs. 13 and 14. The percentage reduction in SAR is 77% at 7.5 GHz and 71.5% at 5.5 GHz. Then, to reduce the SAR values of the proposed design even at some higher values of input power, the antenna is backed with a reflector made of ROGERS 4003C as per the suggestions in papers [Reference Veeraselvam, Mohammed, Savarimuthu, Marimuthu and Balasubramanian8, Reference Targonski, Pozar and Waterhouse45, Reference Rowe and Waterhouse46], which is shown in Fig. 15.

Figure 13.

SAR performance without reflector (7.5 GHz).

Figure 14.

SAR performance without reflector (5.5 GHz).

Figure 15.

Antenna structure with reflector.

The dimension of the reflector is chosen to be 50 × 50 mm². A spacing of 10 mm is chosen between the antenna and the reflector. This 10 mm value was chosen based on the parametric results obtained from CST microwave studio, below which the bandwidth performance of the antenna slightly deviates. However, the other performance of the proposed antenna shall not get degraded due to the placement of the reflector directly behind the antenna. To ensure this first, the S₁₁, bandwidth and gain performance of the proposed antenna with and without reflector has been studied after which the design has been made. Figure 16 shows the S₁₁ performance.

Figure 16.

S₁₁ performance with and without reflector.

From Fig. 16, it can be seen that there is no much variation of S₁₁ and bandwidth values of the proposed design in the presence and absence of reflector.

In order to further validate the performance maintenance of the antenna with reflector, SAR simulation and analysis for the antenna backed with a reflector was done for two kinds of body phantom models. One is with Cartesian model whose sample picture is shown in Fig. 17. Other is cylindrical human arm model whose sample picture is shown in Fig. 18.

Figure 17.

SAR at 5.5 GHz Cartesian model.

Figure 18.

SAR at 5.5 GHz human arm model.

The SAR results for the antenna with reflector thus obtained are compared with tabulation in Table 4.

Table 4.

SAR values

From Table 4, it is evident that the proposed antenna exhibits limited SAR values at 5.5 and 7.5 GHz according to the IEEE health specifications and hence it shall be well suited for WBAN applications [Reference Yeboah-Akowuah, Kosmas and Chen3, 43]. The achieved reduction of SAR in percentage are 75.5% for Cartesian model at 5.5 GHz, 79.95% for cylindrical human arm model at 5.5 GHz, 79% for Cartesian model at 7.5 GHz, and 80.1% for cylindrical human arm model.

Thus, it can be said that, for slightly less values of input power whose values are nearly same as the power values used in papers [Reference Yeboah-Akowuah, Kosmas and Chen3–Reference Zheng, Zhang, Chen and Yan15], the proposed design without reflector which is shown in Fig. 11 can be used as such without making any further modifications. If the power is increased, a suitable reflector can be backed as shown in Fig. 15 to keep the SAR values within the specified levels.

Table 5 shows the comparison of efficiency, bandwidth, and gain of the proposed antenna with and without reflector.

Table 5.

Performance comparison (with and without reflector)

From Table 5, the efficiency of the antenna with reflector remains almost same as that of the antenna without reflector (taken at 7.5 GHz). With some improvement in peak gain (at 7.5 GHz), the bandwidth of the antenna with reflector undergoes a change of only 2% which is negligible. This ensures that placing the reflector behind the antenna does not affect the performance of the antenna.

On comparing the technique of placing reflector behind the antenna in papers [Reference Veeraselvam, Mohammed, Savarimuthu, Marimuthu and Balasubramanian8, Reference Targonski, Pozar and Waterhouse45, Reference Rowe and Waterhouse46], the distance of the reflector from the antenna and the dimensions of the reflector used are both quite lesser which ensures the betterment of the proposed design as compared to papers [Reference Veeraselvam, Mohammed, Savarimuthu, Marimuthu and Balasubramanian8, Reference Targonski, Pozar and Waterhouse45, Reference Rowe and Waterhouse46].

Radiation pattern (on body)

The radiation performance of the antenna under on-body condition was studied. Figure 19 shows the measured radiation pattern on free space and simulated radiation pattern under on-body condition at 2.4 and 5.8 GHz. Figure 19 shows that on comparing, the on-body radiation performance exhibits slight glitches with minimum variations. This is due to the additional permittivity effect of human body. However, the device preserves the shape of the pattern without any change under on-body conditions.

Figure 19.

Radiation pattern of the proposed antenna (on body) at (a) 2.45 GHz and (b) 5.8 GHz.

Bending analysis

For WBAN applications, as the devices shall be placed exactly on the body or very close to the body, possibilities exist for the device to be bent due to the shape of the human body.

Flexible and bending nature of antennas offers a favorable solution for body-centric communications [Reference Kirtania, Elger, Hasan, Wiśniewska, Sekhar, Karacolak and Sekhar47, Reference Kumar, Ali and Sharma48]. Hence, the analysis of the antenna performance has been extended to place the antenna on a bending surface and thus the results are evaluated. Figures 20 and 21 show the antenna setup with bending.

Figure 20.

Antenna bent in y orientation.

Figure 21.

Antenna bent in x orientation.

While bending the antenna, it can be taken up to some fixed radius. In this work, the bending radius of the antenna is taken to be 50, 75. and 100 mm which nearly resembles the bending radius of children and adults [Reference Veeraselvam, Mohammed, Savarimuthu, Marimuthu and Balasubramanian8].

Also, the antenna can be bent in both x orientation and y orientation. Keeping this scenario in mind, the S₁₁ performance of the proposed antenna was studied under aforementioned radii in both x orientation and y orientation whose results are presented in Figs. 22 and 23.

Figure 22.

S₁₁ performance under bending conditions (y orientation).

Figure 23.

S₁₁ performance under bending conditions (x orientation).

On taking a look into the S₁₁ performance of the proposed antenna under different radii (Figs. 22 and 23), it is observed that the antenna undergoes only slight detuning due to bending effects which ensures the flexible nature of the device proposed.

On bending the antenna in y orientation, changes in S₁₁ of the antenna are observed for the bending radii 75 and 100 mm. For 50 mm radius, the S₁₁ performance is same as that of the no-bending scenario. For 75 mm, the S₁₁ values changed from −30 to −27 dB at 3.45 GHz and remains the same without any noticeable change at other frequencies. For 100 mm radius, the S₁₁ value has changed from −30 to −25 dB at 3.45 GHz and from −21 to −19 dB at 5.8 GHz.

However, the antenna still clearly maintains the S₁₁ requirement (S₁₁ < −10 dB) without any degradation. This minor change in S₁₁ values happen because when the antenna is bent in y orientation, the width of the antenna is disturbed which in turn affects the impedance matching. This results in change in S₁₁ values.

On bending the antenna in x orientation, a slight shift in resonant frequencies of the antenna is witnessed. For the bending radius of 50 mm, the S₁₁ values and resonant frequencies remain the same as in unbent scenario. For 75 mm radius, a slight shift in resonant frequency happens from 3.45 to 3.5 GHz while other resonant frequencies remain the same. For 100 mm radius, a shift in resonant frequencies from 3.45 to 3.53 GHz and from 5.8 to 5.81 GHz is witnessed. This slight shift in resonant frequency does not affect the antenna’s functionality as it is within the prescribed ranges as realized in papers [Reference Mahmood, Ishak, Ismail, Zakaria and Alani34, Reference Kirtania, Elger, Hasan, Wiśniewska, Sekhar, Karacolak and Sekhar47].

Despite some shift in resonant frequencies, the antenna still provides good bandwidth. A slight change in bandwidth from 8 to 7.85 GHz is observed for the bending radius of 100 mm alone. This change in bandwidth is less than 2%, and the proposed antenna still provides the required bandwidth for UWB communication. This shows that the proposed antenna is quite less vulnerable to bending effects.

Analysis at various body locations

Also, the device was checked for its performance at various locations of the body. The locations chosen were an adult human arm, thighs, chest, and on ear phones. The sample images taken during measurements are shown in Fig. 24.

Figure 24.

Real-time on-body measurement setup.

To increase the usability of the antenna, the device was also checked under wet condition tied with a water bottle. The bottle was completely filled with water to ensure good humidity. Water droplets were poured at the exterior side also. Figure 24 shows the measurement setup of the antenna at headphone, arms, thighs, chest, and wet conditions respectively in the image.

The S₁₁ performance of the antenna under all the above mentioned conditions is presented in Fig. 25.

Figure 25.

S₁₁ performance under various on-body locations.