Small Planar Monopole UWB Wearable Antenna with ... - IEEE Xplore

2014 IEEE Region 10 Symposium

Small Planar Monopole UWB Wearable Antenna with Low SAR Khairun Nidzam Ramli Wireless and Radio Science Centre Universiti Tun Hussein Onn Malaysia Batu Pahat, Johor, Malaysia [email protected]

Wadhah A. M. Al ashwal Wireless and Radio Science Centre Universiti Tun Hussein Onn Malaysia Batu Pahat, Johor, Malaysia [email protected]

(WPANs) and medical sensors. The introduction of body worn medical sensors has enabled specialists to monitor patients at a distance [4].

Abstract— This paper reports an overview of UWB jeans antenna and evaluates the safety limits by indicating the computed Specific Absorption Rate (SAR). Simple geometry of the design was aimed in order to fabricate the antenna with minimum errors. The proposed design is a rectangular patch placed on 32 × 34 mm2 jeans substrate with partial ground. Simulated and measured S11 parameter for the antenna at free space is reported in this paper. Simulated radiation patterns are also presented in this paper. The performance of the antenna has then been examined in close proximity to a developed model of human arm. Evaluation of SAR has included calculating 10-g SAR when the antenna placed at 5, 10, 15 and 20 mm far from the phantom.

The very recent development has aimed to utilize textile material in antenna substrates which offers more flexibility and entertainment possibilities. Textile antennas can be made fully textile as in [5], while fabric antenna refers to that the substrate is textile material such as fleece fabric. Textile materials have attractive properties such as very low dielectric constant, which helps reduce surface wave losses and enhances the overall bandwidth [6]. Textile and fabric UWB antennas have been studied in [7], [8] and antennas of a metallic radiator on a textile substrate was studied in [9].

Keywords-UWB, Specifice Absorption Rate (SAR), textile antenna, body tissue, phantom.

I.

A considerable attention has been given to the impact of the interaction between electromagnetic (EM) fields and the human body as in [10]–[13]. The interaction between human head and cellular phones has been studied in [14], [15]. These researches have been conducted in order to examine whether or not the antenna radiation does not exceed the limits set by the standards [16], [17].

INTRODUCTION

Ultra wideband (UWB) technology has received the interest of the new ongoing researches on data communication. This focus has made most of the people presume that UWB is a new born technology; that is because of its implementation in advanced systems like audio-visual applications, which require the very two attractive attributes: large and quick data processing. The fact is that UWB is more than a century of age. In 1800, Hertz used a simple experiment setup to generate an electromagnetic signal in a form of short pulse using spark-gap generator [1], [2].

Studies on the evaluation of the power absorbed by human body and the Specific Absorption Rate (SAR) have been conducted in different methods. Kuster [14] has evaluated the absorption mechanism for homogenous body model while Kivekas [18] has considered homogenous and layered body model. Klemm [19] has further studied the interactions of UWB antennas used in wearable applications on homogenous and layered human body models. SAR results of very near antennas to the body have been computed in order to investigate the influence of the body. The first evaluation of SAR was on a simple homogenous model composing one tissue (muscle of 50 mm thickness). The second model consisted of three-layers (skin: 0.5, 1 and 2 mm; fat: 1, 3 and 6 mm; muscle: 50 mm). The study assumed all models to be planar neglecting the curvature of the body.

The reason behind the gap between UWB now and then was political, which prevented it from commercialization. It is due to the fact that frequency regulators assign narrow frequency bands to specific services, while UWB violates those assignments by emitting a radiation in a large range. Proponents of UWB had then convinced the Federal Communication commission (FCC) that the emissions would not interfere with the narrow frequency bands devices. At then, the FCC issued the ruling agreement in 2002 allowing a range of 3.1GHz-10.6GHz for emission with certain restrictions for the radiated power spectrum [1].

This work studies a UWB antenna made on a textile substrate for wearable applications. Performance of the antenna at free space and close to the body is presented in this paper. SAR evaluation has been included in this study.

The recent development in communication systems has led to combining wearable applications and miniaturization of electronic devices which created small devices that can be attached to the body [3].

II.

Researchers have increased their interest in integrating wearable antennas with different types of area networks like body area networks (BANs), wireless personal area networks

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ANTENNA DESIGN

This section consists of two parts: antenna materials and geometry of the antenna.

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Optimization of the ground plane has shown that the bandwidth improves with the reduction of the ground plane as it reaches a certain point under the patch. Figure 3 shows how the ground plane affects the overall return loss. The illustration represents the impact of optimizing the ground plane while the antenna is fed at the center.

A. Antenna Materials The proposed antenna consists of a substrate made of jeans and a metallic radiator (adhesive copper tape). This study has required examining the properties of the sample used for substrate. An experiment was carried out to determine the permittivity and loss tangent of the sample. The average value of the dielectric constant and loss tangent found to be 1.76 and 0.078 respectively. B. Antenna Geometry Simplicity was the main aim during designing stage. Since the antenna is fabricated on a piece of cloth, bare hands and sharp tools are employed to cut the materials (jeans and copper tape). Usually, to improve the impedance bandwidth of a planar antenna, one needs to use techniques such as slots and steps [20], [21], beveling [22], slits [23] and many other techniques, which may lead to a complex design and create a difficult task to depict the details of design on the substrate. The proposed antenna has an overall size of L × W (32 × 34 mm2) and thickness of 1mm. The patch of the antenna is l × w (17 × 18 mm2) and has feed line of length lf = 13 mm and width wf = 3.64 mm. The ground plane is truncated to lg × W (10 × 34 mm2) to help match the patch with the feeding line; this is because of that the truncation can create a capacitive load that neutralizes the inductive nature of the patch to produce nearlypure resistive input impedance [24].

Figure 3: Ground plane optimization.

The feeding mechanism has also contributed to further improvement in the bandwidth impedance. Figure 4 shows how off-center feeding and shifting the patch increased the range of the operating frequency.

Geometry of the proposed antenna is illustrated in Figure 1, while Figure 2 shows the fabricated antenna.

(a) (b) Figure 1: Antenna geometry (a) front view (b) back view

Figure 4: Feeding mechanism optimization.

The return loss of the proposed antenna has been simulated within the range of 2 GHz- 12GHz. The results from simulation show that the antenna can operate within the range 2.8 GHz – 11.6 GHz, while the measured results show the range of 3.04 GHz 11.3 GHz. Figure 5 illustrates the simulated and measured return loss of the antenna operating at free space condition. The antenna is classified to be UWB if the fractional bandwidth satisfies a minimum of 20%. Fractional bandwidth is defined by equation 1 [25]. From the simulation result, the proposed antenna has a fractional bandwidth of 122%, while it has a minimum of 109 % according to measured results.

(a) (b) Figure 2: Prototype of the proposed antenna, a) front and b) back views

III.

RESULTS AND DISCUSSION

The performance of the antenna was predicted using CST MWS software. Before producing the final version of the geometry, necessary parameters such as ground plane and feeding mechanism have been studied in order to arrive to a design with an optimum performance.

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E-plane

FB

fh fl u 100%..................................(1) fc

fc

fh fl ...............................................(2) 2

The radiation pattern of the antenna at free space has been plotted and shown in Figure 6. The patterns of E-plane (phi = 90) appear as a ring-like shape especially for 3 GHz, while at 5 GHz the patterns show that most of the radiation is more intense at the back, the top left corner , right side and in front of the antenna (considering the patch is pointing outward). One can relate that to the shifting of the patch from the center to the left side of the antenna, which appears to have effect on 7 GHz as well. At 10 GHz, the radiation is more focused at the direction of top right corner of the antenna with also considerable patterns from front and top left corner.

H-plane

Figure 6: Simulated radiation pattern at free space

Dielectric properties for the body tissues have been obtained from FCC official website [27]. The performance of the antenna has been predicted at about 5 mm far from the arm. The resulting S11 parameter is illustrated in Figure 8. The return loss shows that the antenna has a good chance to operate well in the range of higher frequencies.

From H-plane plot (phi=0), the pattern could appear like omni-directional for frequencies 3 GHz, 5 GHz, and 7 GHz except that it deteriorates as it goes higher to 10 GHz. IV.

ANTENNA PERFORMANCE AT THE PRESENCE OF THE BODY

Radiation patterns are presented in Figure 9 for both of Eplane and H-plane. The simulated results indicate that the radiation is directed to the front side of the antenna (the ground plane faces the skin layer, while the patch faces the free space), this can be seen from the direction of the main lobe where it is found to be around the lower degrees with a gain of higher than 6 dB for 5 GHZ and 7 GHz, while it drops to around 2 dB at 3 GHz and 4 dB at 10 GHz. The graphs also tell that the antenna operates very poorly at 180 degree direction due to the blockage of the phantom at near field region.

Since the antenna is proposed for wearable applications, there was a need to predict how the antenna performs in close proximity to the human body. For that purpose a portion of the human arm was selected to be the phantom that represents the body. Many studies have considered homogeneous rectangular models with number of tissue layers for simulating the characteristics the proposed antennas, ignoring the curvature of the part of the body. The model presented in this paper is a four-layer phantom with a curvature approximated to a conical shape with top and bottom radiuses. The thickness of each layer is taken as skin = 2 mm, fat = 3 mm, muscle = 8 mm and bone = 10 mm. Although the thicknesses in [19] and [26] were represented as the height of the rectangle layer, there are taken here as a radius. Figure 7 illustrates the top and front view of the phantom.

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radiation pattern, the antenna still achieves a good efficiency that could reach to higher than 65% at the higher band of UWB. Figure 10 shows the plot of the total efficiency with respect to frequency.

(a) (b) Figure 7: Human arm model developed in CST MWS, a) top view, b) front view.

Figure 10: Total efficiency at the presence of the body.

V.

SPECIFIC ABSORPTION RATE (SAR)

The core merit of wearable antennas is whether or not they can be safely used. Specific Absorption Rate (SAR) indicates the quantity of the power absorbed in the human body. The SAR should agree with the available safety limits and standards [16], [17]. The 10-g SAR of the proposed antenna has been evaluated for selected frequencies at different distances. The graphic illustration in Figure 11 shows the total SAR for frequencies 3 GHz, 5 GHz, 7 GHz and 9 GHz at 5 mm, 10 mm, 15 mm and 20 mm placements from the phantom.

Figure 8: S11 parameter at 5 mm far from the arm model.

The computed values show that the antenna has very low total SAR as the antenna is placed farther from the phantom. This can be seen more clearly at the lower band frequencies such as 3 GHz and 5 GHz, where the total SAR for 3 GHz has reduced from more than 8 W/kg at 5 mm to lower than 2 W/kg at 20 mm. It can be understood that the phantom has a bigger chance to absorb more radiated and reflected power at near spacing than far one.

E-plane

The graph also shows how the total SAR decreases as the frequency increases for a fixed distance, where the total SAR at 5 mm reduces from more than 8 W/kg at 3 GHz to about 1 W/kg at 9 GHz.

H-plane

Figure 9: Radiation pattern

This can also be related to the fact that most of the radiated power towards the body is absorbed and reflected by the layers of the phantom. Despite of all of the deterioration in the

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Figure 11: Total SAR [W/kg] in the 4-layer body phantom.

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VI.

[16]

CONCLUSION

In conclusion, this paper has given an overview of the development of UWB technology and UWB wearable antennas. The proposed design is compact in size and flexible for attachment to the body. The performance of the proposed jeans antenna has been examined in free space and close to the body states. The proposed arm phantom has approximated the total SAR of the antenna for selected frequencies at different distances. The antenna could work with acceptable efficiency especially in the higher band region. Future work will focus on improving the design so that it can sustain an operating frequency for the full spectrum of UWB.

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