G-Shaped Wearable Cuff Button Antenna for 2.45 GHZ ISM Band Applications

Laila K. Hady Salman, and Larbi Talbi

Department of Computer Science and Engineering Université du Québec en Outaouais

101 Saint-Jean Bosco, Gatineau, Québec, Canada J8X 3X7

I. INTRODUCTION Wireless communication technology has matured as a viable way to provide ubiquitous unrestrained and convenient infrastructure free communications everywhere. Wireless communication systems are diffusing around the world faster than any other communication technologies from global cellular telephone systems to local and even personal-area networks. Hence, wireless radio systems have increased in importance for both voice and high speed data transmission in military, commercial and personal communications. Recently, intensive research work has been devoted in the body-area networking where computing devices are allowed to have wireless communication with each others over the human-body surface. In fact, it becomes challenging to integrate the developed technologies on clothing to achieve what so-called “body-worn communication systems” [1-4]. Both miniaturization and compactness in microelectronics along with other technologies are the key parameters in enabling wearable computing devices to integrate their functionality in clothing and to be used for military, medical, commercial and personal applications [5-7]. Wearable communication systems are of interest for on-body applications such as supporting the user in the various hazardous environments or enhancing the soldier’s performance, awareness and survivability on the battle field or monitoring the human body’s functions for healthcare monitoring. Consequently, it is important to design optimized body-worn antennas with acceptable radiation performance in the proximity of human

body, compact, low cost, low-profile and ease of manufacture. Early wearable antennas were designed in textiles where printed planar antenna structures such as patch or microstrip antennas were attached to the human-body or mounted on flexible textile substrates as part of the cloths [8-10]. Also, textile coplanar patch antenna with electromagnetic band gap (EBG) substrates was introduced for dual ISM band applications in [11]. The reported designs were fed either by probe or microstrip feeding techniques to ensure wearing comfort. In addition, advanced antenna designs topologies and different polarizations were proved to be applicable in textile wearable antennas. On the other hand, button antennas were introduced as alternative type of wearable antennas such as jeans-button antennas [12-14]. Most of the reported wearable antennas were proved to have acceptable radiation performance over single frequency band for wireless communications around 2.45GHz while few were able to operate for dual frequency bands allowing mobile network connections around both 2.45GHz and 5.8GHz. In this paper, the study of the radiation performance of a single band wearable cuff button antenna is presented. The radiation element is designed as a G-shaped structure excited by a printed microstrip feeding line placed on a RT/duroid® microwave material to operate at 2.45 GHz for WLAN/Bluetooth applications with omni-directional radiation patterns that are required for data transmission with nearby located wearable devices or mounted on the human body.

2010 14th International Symposium on Antenna Technology and Applied Electromagnetics [ANTEM] and the American Electromagnetics Conference [AMEREM]

978-1-4244-5050-3/10/$26.00 ©2010 IEEE

II. PROPOSED ANTENNA DESIGN GEOMETRY The proposed design geometry is shown in Fig. 1 where the complete structure is illustrated to have a better understanding of the modeled configuration. The design to be presented here consists of two parts: the metallic button and the grounded microwave RT/duroid 5870 substrate with the microstrip feeding network printed for excitation purpose. Figure 2(a) explains the detailed modeling of the metallic G-shaped cuff button. It is designed of three main parts to ease the fabrication procedure later on. The upper part is taking a G-shape which is placed on top of a circular plate of R3 = 12.5 mm radius and H3= 0.825 mm height. A conducting post of radius 0.5 mm and H2 = 2.175 mm height is used to connect between the G-shaped and the solid cylinder plate. The spacing between the G-shaped and the circular plate is optimized to have the appropriate capacitive effect that helps in improving the impedance matching of the antenna. The base part is formed of a metallic solid cylinder of radius R4= 8.25 mm and height H5 = 1.8mm while connected with the upper layer through an intermediate solid metallic cylinder of radius R6= 1.2 mm and height H4= 5.37 mm. This structure is fed using printed microstrip line on a rectangular grounded microwave substrate of 2.33 dielectric constant and Hsub = 1.57 mm thickness as shown in Fig. 2(b). The feeding part consists of annular circular patch of R5= 6.6 mm outer radius and R6= 1.2 mm inner radius. Additional microstrip line stages are used for matching as illustrated in Fig. 2(b). Table I summarizes some of the used design parameters.

Figure 1. The proposed geometry of the dual-band G-shape cuff button antenna for ISM bands applications.

(a)

(b)

Figure 2. (a) The detailed modeling of the metallic G-shaped cuff button part, (b) Top view of the printed microstrip feeding network.

III. SIMULATED RESULTS

As mentioned earlier, the main idea behind the designed structure is to obtain a small, low cost and acceptable radiation performance wearable antenna that can be attached easily to clothing and not significantly affected by the proximity of the human body. The proposed design explained in the previous section and illustrated in Fig. 1 is still under investigation for the effect of the body-antenna coupling that exists and further may affect on the antenna radiation pattern, operating frequencies or antenna input

z y

x

z y

x

2010 14th International Symposium on Antenna Technology and Applied Electromagnetics [ANTEM] and the American Electromagnetics Conference [AMEREM]

impedance [15]. The designed structure was simulated using the commercial software Ansoft HFSS [16] and both reflection coefficient at the input port and input impedance on Smith chart were obtained as shown in Fig. 3. It can be noticed that the antenna has two resonances around 2.45 GHz with acceptable impedance matching.

(a)

(b)

Figure 3. Simulated reflection coefficient and input impedance on Smith chart at the input port.

TABLE I. Proposed Design Parameters Parameter

Name Parameter

Value (mm) Parameter

Name Parameter

Value (mm) L 40 W 40 L1 9.9 R1 8.745 L2 6.6 R2 5.445 L3 10.4 W1 6.6 L4 3.3 W2 13.2 L5 3.8 H1 1.65 L6 2.53 W3 6 L7 4 L8 7.75

Far field radiation patterns are also computed at 2.45 GHz where acceptable omni-directional radiation pattern can be observed in the XY plane as shown in Fig. 4. It is also noticed that the cross polar level is at least 20 dB below the copolar level. In addition, far field radiation patterns in both XZ- and YZ- planes are computed as shown in Fig. 5. An 8-shaped radiation pattern was observed around 2.45 GHz as illustrated in Fig. 5.

Figure 4. Simulated far field radiation pattern for the proposed G-shaped cuff button antenna in the XY-plane at 2.45 GHz

IV. CONCLUSION

The concept of using a small size metallic cuff button as a single band wearable antenna for ISM band applications has been discussed. The antenna is having a G-shaped metallic structure mounted on a small piece of grounded microwave substrate and excited using simple microstrip feeding network. The antenna has its resonance around 2.45 GHz. Promising results were obtained for the simulated design geometry and future experimental work will be done for verification. Both simulated antenna return loss and far field radiation patterns were obtained at the resonance frequency. Acceptable omni-directional pattern was obtained with at least 20 dB cross polar level below the copular level. The current development in the use of standard buttons in realizing realistic on-body type of sensors for different applications threw up immense challenges for the coming technology generations.

m1

2010 14th International Symposium on Antenna Technology and Applied Electromagnetics [ANTEM] and the American Electromagnetics Conference [AMEREM]

(a)

(b) Figure 5. Simulated far field radiation pattern for the proposed G-shaped cuff button antenna in the (a) XZ-, (b) YZ- planes at 2.45 GHz

References

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[16] HFSS software is distributed by the Ansoft Corp: http://www.ansoft.com

2010 14th International Symposium on Antenna Technology and Applied Electromagnetics [ANTEM] and the American Electromagnetics Conference [AMEREM]