Miniaturized Novel UWB Band-Notch Textile Antenna for Body Area Networks

This paper presents the design and analysis of a miniaturized and novel wearable ultra-wideband (UWB) band-notch textile antenna for Body Area Networks (BANs). The major goal of building the antenna for wearable applications with band notch in X-band is to reject the downlink band (7.25 to 7.75 GHz) of satellite communication in the UWB frequency ranges of 3.1–10.6 GHz to keep away from interference. Computer Simulation Technology (CST) TM Microwave Studio, which is user-friendly and reliable, was used to model and simulate the antenna. The radiating element of the antenna is designed on Jeans’ textile substrate, which has a relative permittivity of 1.7. The thickness of the jeans’ fabric substrate has been considered to be 1 mm. Return loss, gain, bandwidth, impedance, radiation, and total efficiency, and radiation patterns are presented and investigated. The antenna is simulated placed on the three layers of the human body model, and the on-body results are summarized in comparison with free space. Results and analysis indicate that this antenna has good band-notch characteristics in the frequency range of 7.25 GHz to 7.75 GHz. The parametric study varying the relative permittivity of Jeans’ fabric substrate of this antenna is also evaluated. In addition, effects on the antenna parameters of variation of ground plane size have been reported. The antenna is 25 mm × 16 mm × 1.07 mm in total volume. Results reveal that this antenna achieves the design goal and performs well both in free space and on the body.


Introduction
Recently, Wireless Body Area Networks (WBANs) systems are growing to be a prevalent study area due to their medical, real-time health monitoring, and entertainment applications. WBAN consists of numerous wireless sensor nodes dispersed throughout the human body that communicate with an on-body base unit to 2 Jean's UWB Band Notch Antenna Design Figs. 1a-1c demonstrates the proposed miniaturized UWB band notch antenna showing the antenna's detail sizes of various components. In addition, it shows a detailed design of the front side and backside of the antenna. The antenna is modeled with (CST) TM software, which is user-friendly and reliable. Figs. 1d and 1e show the front and back views of the CST design antenna. The simulated results of the CST design are comparable with measurements [1,4,6,7].
Jean's textile substrate has a relative permittivity of 1.7, where the antenna's radiating element is designed. The height of the Jeans' textile has been considered as 1 mm in this study. The UWB antenna needs to be compact for wearable cases due to its longer battery life and user-friendly, textile-based design, and band rejection capability in the UWB frequency ranges to avoid interference from narrowband signals. By considering these facts, this antenna has been designed to be very compact. It has a total length and width of 25 and 16 mm, respectively. The ground plane's thickness and radiating element have been considered similar, and that is 0.035 mm. Perfect Electric Conductor (PEC)/Copper has been used as the proposed antenna's radiating element and ground plane. The antenna's overall height is 1.07 mm, which includes the ground plane and the radiating element. As can be seen from Figs. 1c and 1e, there is the partial ground plane on the backside of the antenna. Research shows that an antenna with a full ground plane is better and less sensitive to the human body [1]. But to get the ultra-wideband bandwidth and impedance matching, the full ground plane is not possible at the backside of the antenna. To acquire band-notch, a slit is inserted to the top of the antenna radiating element. At 7 GHz, the wavelength of the antenna is 42 mm. The antenna is considered to bean electrically small antenna because the size (length) of the antenna is smaller compared to the wavelength. The electrical size for the length of this antenna is 3:95 .

Results and Discussions
This section has shown and discussed the return loss, bandwidth, gain, radiation efficiency, total efficiency, and radiation patterns. Then parametric study has been performed varying the antenna ground plane size and relative permittivity values of the antenna substrate. Finally, the antenna was simulated on three layers of the human body, and the results were shown and studied. Fig. 2 depicts the antenna's return loss response. The antenna has an excellent band notch in the frequency range of 7.25 to 7.75 GHz, according to the graph in Fig. 2. The antenna operates in the ultrawideband entire frequency region (3.1 to 10.6 GHz). To reject X-band downlink satellite communication systems, it demonstrates band notch behaviors in the range of frequencies of 7.25 to 7.75 GHz. This was one of the goals of this paper, and the return loss result meets the goal. The return loss levels throughout the frequency spectrum of 7.25 to 7.75 GHz are well above −2 dB, allowing for interference-free communications in the UWB system for low-power BANs. Except for the band-notch frequency region, the antenna shows good return loss values in the range of operating frequencies. Tab. 1 lists the return loss values of the antenna at various frequencies.

Gain and Efficiency
The antenna's radiation efficiency and gain values are derived from the simulation results. Tab. 2 lists the antenna's gain, radiation efficiency, and total efficiency for six diverse frequencies. The gain and efficiency values are lower in the band-notch frequencies, and higher gain and efficiency are observed at the operational frequencies. For operating frequencies, the antenna gain is good enough for wearable applications for good power transmission. At 9 GHz, the highest gain (4.49 dB) is observed as compared to lower frequency bands of operations. It is noticed that at 9 and 10 GHz, the highest radiation efficiency is noticed.

Radiation Patterns in Free Space
From the simulation 3D radiation pattern, this antenna's polar plot radiation patterns at six frequencies were extracted. The polar plot radiation patterns of the antennas at 3.5, 5.8, 7.25, 7.75, and 10 GHz are shown in Figs. 3a-3l. The H and E planes (azimuth and elevation planes, respectively) radiation patterns are displayed. From Fig. 3 indicates that the radiation patterns at 3.5 GHz of the H plane look omnidirectional, but in the E plane radiation patterns of the same frequency band, there is a null near the angle of 120 degrees. The polar plot radiation pattern at 5.8 GHz in the H plane seems to be like 3.5 GHz, but in the case of the E plane, the radiation pattern is slightly distorted between the angles of 150 to 240. The polar plot radiation patterns for 7.25 GHz look close to 5.8 GHz in both the H and E planes. The radiation patterns at 7.75 GHz at H and E planes look distorted. The radiation patterns look omnidirectional at higher frequencies like 9 and 10 GHz, but slight distortion is noticed for E plane radiation patterns.

Antenna Ground Plane Size Variation
The antenna's ground plane size was changed to see what effect it had on the return loss results. Fig. 4 illustrates the variation of the size of the ground plane. Fig. 4a is the original size of the antenna named as the proposed ground plane; see Fig. 1c. Fig. 4b is the reduced ground plane from the original design (proposed ground plane size). In Fig. 4c the ground plane has been increased by 1 mm from the original design. Fig. 5 demonstrates the return loss curves for the variation of the antenna ground plane size. To return loss responses for different ground plane sizes are demonstrated in different colors, see Fig. 5. The red curve represents the return loss for the initially proposed ground plane size, the blue curve represents the return loss when the ground plane size is reduced by 1 mm from the original design, and the green curve represents the return loss when the antenna ground plane is increased by one mm for the original design.  It can be seen in Fig. 5 that decreasing the ground plane dimension has an effect on the return loss responses. The lower frequency at −10 dB bandwidth shifts lower than the original design for increasing the dimension of the ground plane. The lower frequency band at −10 dB impedance shifted to the right when the dimension of the ground plane was reduced by 1 mm more than the proposed ground plane size. Changing the size of the ground plane, on the other hand, has no effect on the frequency of the band-notch regions, according to the research (see Fig. 5). It has been discovered that changing the ground plane size has no influence on the frequency range of band rejection or the band notch. In all cases, the band notch is nearly the same. The antenna's impedance varies by changing the ground plane size at higher frequency, but the return loss remains below −10 very good dB bandwidth.

Variation of Relative Permittivity
In this section, the antenna's performance characteristics are investigated, varying the relative permittivity of the Jeans' fabric to study the effects on the results. In this study, the Jeans' textile's height was fixed to 1 mm, the same as the original design, but the relative permittivity of the jeans substrate has been varied. Results expressed that verifying the permittivity values has many effects on the antenna return loss. An interesting result is noticed for the 4.4 value. For a relative permittivity value of 4.4, the band notch of the antenna shifted to 5 to 6 GHz regions. When the permittivity value is 1.7, the antenna has a band notch in the X band; on the other hand, when the permittivity value is 4.4, the band notch shifts to the WLAN band. When the relative permittivity value of the Jeans is set to 2.2, the band notch shifted to the lower band. It can be seen from Fig. 6. that varying the relative permittivity of the jeans substrate also affects the antenna's impedance. The results concluded that varying the permittivity of Jeans' textile affects the antenna return loss.

4) of Jeans' Textile
The gain of the antenna is investigated by varying the relative permittivity values of Jean's fabric. In this case,Jeans' textile's relative permittivity is considered as 1.7, 2.2, and 4.4. Tab. 3 shows a comparison of gain for different relative permittivity values of Jeans' textile.
Results and analysis show that changing the relative permittivity value of jeans textile affects the antenna gain. This happens due to the shift or antenna operating frequencies. At 9 GHz, the change of the relative permittivity value of the jeans textile has a very slight effect on the gain of 9 GHz because the gain values are very close to each other for different permittivity values. Not much effect of gain is that the variation is noticed for the change of relative permittivity value of jeans on a lower frequency band of 3.5 GHz. The antenna has a band notch from 7.25 to 7.75 GHz when the dielectric constant of the jeans is 1.7, but when the dielectric constant value is changed to 4.4, the band notch is noticed at the WLAN band; therefore the gain changes more. However, variation of permittivity value affects the gain of 5.8 and 7.25 GHz that is happened due to frequency shifting and impedance matching reasons.

Radiation Efficiency for Variation of Permittivity Value of Jeans' Fabric
Tab. 4 compares radiation efficiency for different relative permittivity values of Jeans' textile at different frequencies. Results show that variation of relative permittivity value affects the radiation efficiency of the antenna. It is noted that the effect is higher for 5.8, 7.25, and 7.75 GHz and less for 3.5, 9, and 10 GHz. In this study, the highest radiation efficiency (96.46%) is noticed at 9 GHz when the dielectric value of the jeans fabric is used as 4.4. At 5.8 GHz, the poor radiation efficiency (61.63%) is noticed when the jeans' fabric's permittivity value is set to 4.4. This is due to changing the permittivity value of jeans that shift the band notch to 5.8 GHz.
Tab. 5 compares total efficiency for different relative permittivity values of Jeans' textile at different frequencies. It is observed from Tab. 5 that variation of relative permittivity value affects the radiation efficiency of the antenna. The effect is higher for 5.8, 7.25, and 7.75 GHz and lower for 3.5, 9, and  10 GHz when the dielectric permittivity of jeans changes the frequency of operation and band notch shift, which causes the variation of total efficiency of the antenna.

RadiationPatterns for Variation of Permittivity Values of Jeans' Fabric
The effects of the variations of the dielectric constant of Jeans' fabric on the radiation patterns of the antenna are also investigated. Figs. 7a-7l compares radiation patterns for various relative permittivity values of the antenna at 3.5, 5.8, 7.25, 7.75, 9 and 10 GHz. Radiation patterns are plotted based on elevation and azimuth planes. In Fig. 7, the radiation patterns for different dielectric permittivity are plotted, showing different colors. In Fig. 7, the red color shows the radiation pattern for the dielectric value of 1.7. The blue color shows the radiation pattern for the dielectric value of 4.4. The green color shows the radiation pattern for the dielectric value of 2.2. At 3.5 and 5.8 GHz in the H plane due to variation of relative permittivity does not affect the radiation patterns. However, in the E plane, a slight variation of power level near the angle of 120 degrees is noticed. Changing the relative permittivity value of jeans textiles has a higher effect on band notch frequencies

Study of on-Body Performance
To test the antenna's on-body performance, the on-body antenna parameters are examined and assessed in this part. Using the CST software, a three-layer human body model was created.

Three Layers Human Body Model
The three layers of the human body model are depicted in Figs. 8a and 8b, with the antenna on top of the skin layer. The skin (top layer), fate (middle layer), and muscle (bottom layer) are the three layers. The thickness of the skin layer, fat layer, and muscle layer is 2, 10, and 10 mm, respectively. Tab. 6 lists the height, width, and length of three human body tissues: skin, fat, a nd muscle. The dielectric properties of the human frequency dispersive body model are calculated from [26]. During CST simulations, the relative permittivity and loss tangent of the human body model layers where skin, fat & muscle formation are determined. The dielectric characteristics and tangent loss of skin, fat, and muscle are shown in Tab. 7. For this study, the gap between the antenna and the three-layer human body model was set to 4 mm; see Fig. 8.

Return Loss Responses (Free Space and On-body)
Comparison of free space and on-body return loss responses of the antenna are provided in Fig. 9. From  Fig. 9, it is observed that when the antenna is put on the human body model, slight frequency detuning is noticed at the lower band; however, the band notch still remains in the same range, 7 to 7.85 GHz. A slight change of impedance is observed while placing the antenna on the human body outside the band of both frequencies. Free space and on-body return loss responses have nearly similar results.

Free Space and on-body Gain, and Efficiency
Tab. 8 shows a comparison of free space gain and radiation efficiency for the antenna at six frequencies. From Tab. 5, it is observed that once the antenna is put at a 4 mm gap on the human tissue model, a reduction of gain is noticed for 3.5, 5.8, 7.25, and 7.75 GHz; an increase of gain is noticed for 9 and 10 GHz. The lowest gain of 1.32 dB for 7.25 GHz and 1.06 dB is noticed for 7.85 GHz on the human body. This is due to the band notch at these frequencies. This antenna shows a maximum of 5.20 and 5.80 dB gain at 9 and 10 GHz on the body, respectively. This antenna shows very goodon the body gain, which is good for power-efficient on body communication with other body-mounted nodes in body area networks. Placing the antenna on the human body model shows that the antenna still shows a very good gain. This could be owing to the ground plane on the backside of the antenna, which acts as a barrier between the antenna and the human body, reducing the lossy tissue effects of the human body on the antenna parameters. Previous research has shown that as the distance between the antenna and the human body widens, the antenna's performance approaches that of free space [1]. Therefore, if the antenna placement gap between the antenna and the body increases to 6 or 8 mm, the on-body results may improve further. The radiation efficiency of the antenna is also studied, placing the antenna on the human body model. In this case, the same gap (4 mm) was considered between the antenna and the human body model. Tab. 8 provides a comparison of free space and on-body performance. When the antenna is positioned on the body, the radiation efficiencies decrease from free space for all frequencies. This is due to the absorption of the human lossy tissues. However, on the body, this antenna shows well on body radiation efficiencies. The greatest radiation efficiencies, 57.30% and 65.33% are noticed for the 9 and 10 GHz on the body, respectively. In the band, both frequencies, the radiation efficiencies are very low, which is expected. As compared to a higher frequency, the lower frequency bands have higher radiation efficiency reduction when the antenna is put on the body.
The total efficiency of this antenna is also investigated. The free space and on-body total efficiency are also provided in Tab. 8. The total efficiency of this antenna is also reduced when the antenna is put on the body. However, this antenna illustrates very good body total efficiency, although it is very compact in size. The highest on-body total efficiencies (46.48% and 55.15%) are noticed at 9 and 10 GHz, respectively. Like radiation efficiency on the human body model, this antenna shows the lowest total efficiencies in the band of both frequencies. The lowest total efficiency of 7.25% is noticed at 7.25 GHz.

On-Body Radiation Patterns
The on-body radiation patterns were also shown plane-wise as azimuth and elevation, corresponding to the H and E planes. Figs. 10a-10l provides free space and on-body polar plot radiation patterns of the antenna for various frequencies of H and E planes. Figs. 10a, 10c, 10e, 10g, 10i and 10k shows H plane while Figs. 10b, 10d, 10f, 10h, 10j and 10l E plane radiation patterns. Mostly for all frequencies, the on-body radiation patterns and free space in the H plane and E plane are nearly similar, but reducing power level compared with free space radiation patterns is noticed. Both free-space and on-body radiation patterns for band-notch frequencies look distorted in companion with the radiation patterns of other operating frequencies. It can be seen in Fig. 10 that the on-body radiation patterns have higher power levels than the free space radiation patterns in some cases. This may be due to the human body model. The radiation patterns, in some cases, more directive in direction than others. The human body is lossy tissues putting the antenna on the human body, the power level of the radiation patterns are reduced. It is noticed in most of the cases here in this study. On the other hand, the radiation pattern is less affected by human lossy tissues because of the ground plane on the backside of the antenna.

Conclusions
A new miniaturized wearable ultra-wideband band notch jeans' textile-based antenna for Body Area Networks (BANs) is designed and presented. To avoid interference, the antenna operates in the ultrawideband frequency bands of 3.1 to 10.6 GHz with band-notch capabilities in X-band to exclude the downlink band (7.25-7.75 GHz) of satellite communication. Antenna performance parameters are presented and investigated. Parametric study results of the variation of antenna ground plane size are demonstrated and analyzed. Results and analysis show that variation of ground plane size does not affect the band rejection frequency range or band notch; however, slight frequency shifting at a lower frequency range is observed.
The performance parameters of the antenna for different relative permittivity values of Jeans' fabric are presented and investigated. Results show that variation of relative permittivity values of Jeans' fabric has huge effects on the antenna performance parameters. Finally, the on-body performance parameters of the antenna are presented and analyzed. Results show that when the antenna is put on the body, the band notch remains in the same frequency range (7.25 to 7.75 GHz). The antenna performs very well even when it is put very close to the human body model at operating frequencies such as 3.5, 5.8, 9, and 10 GHz. The antenna shows very poor results at 7.25 and 7.85 GHz both in free space and on the human body model due to band notch characteristics, and that is expected. The antenna performs very well at other frequency bands of operation. This antenna shows a maximum of nearly 65.33% radiation efficiency, 55.17 total efficiencies, and 5.80 dB gain at 10 GHz when placed even very close (4 mm away) to the human body model. The antenna's length, width, and height are 25, 16, and 1.07 mm, respectively. The antenna is electrically small at the center frequency of operation. The antenna was Figure 10: H and E planes free space and on-body radiation patterns for different frequencies printed on Jeans' fabric, and the total height of the antenna is considered to be 1.07 mm. Due to textile-based, this antenna will be a good choice for wearable applications. This UWB antenna will be appropriate for wearable applications in Body Area Networks (BANs) by considering the above-mentioned facts.

Conflicts of Interest:
The authors declare that they have no conflicts of interest to report regarding the present study.