Design of an Internal Multi-band Loop Antenna for

Multiple Mobile Handset Operations Chuandi Dai1, Duolong Wu1, and Yanjie Wu1

1 Guangdong University of Technology, Guangzhou, Guangdong, 510006, P. R China Email: ecddai21cn@gmail.com, edlwu@ymail.com, wuyanjie@gdut.edu.cn

Abstract — A compact internal multi-band folded loop

antenna for GSM850/900/DCS/PCS/UMTS/LTE2300/2500/ WLAN2.4GHz/WiMAX2.5GHz multiple mobile operations is proposed. The whole antenna structure proposed in this paper consists of a single folded meander loop track and a T-shape back-coupling element. The size of the total structure is 120 × 60 × 6.5mm3, meanwhile the area left for the loop track is only 60 × 15 × 6.5mm3. The compact structure makes it very suitable for small mobile phone applications. The design and optimizing of the performance of the proposed antenna are performed by using the simulator software HFSS. An actual model has been prototyped for testing. Measurement results and electromagnetic simulation ones are in good agreement, thus indicating that the proposed antenna can meet the actual demands.

Index Terms — Handset antenna, folded loop antenna, mobile phone antenna, multi-band antenna, LTE, WLAN, WiMAX.

I. INTRODUCTION

With the rapid development of wireless communication technologies, especially smart phones’ ever-changing with each passing day, mobile terminals have been increasingly growing towards miniaturized, ultra-thin, highly integrat- ed, multi-function and high-performance. Therefore the demands of designing novel compact multi-band antennas to meet the growing requirements of integrating more wireless services in the narrow space of the mobile terminals are increased continuously. In order to meet the actual needs, in recent years, a variety of novel mobile phone antennas have been developed and applied constantly.

Loop antenna which is a kind of self-balance antenna can operate an unbalanced 0.5λ mode and 1.5λ mode as well as a balanced 1λ mode [1]. Therefore up to three resonance modes can be generated in a single continuous loop track due to these unique multimode features [1], [11]. In addition, with the aid of appropriate tuning technology, loop antenna can even excite an extra 2λ wavelength work mode [1]. These advantages of multi-mode and multi-resonance make loop antenna very facilitated to cover the multi-band mobile communication operations. Meanwhile, the surface current paths excited by the loop pattern are in a closed form, which makes the coupling between the antenna track and the system ground much smaller than the conventional types of antennas,

such as PIFA and folded monopole [8]. Hence loop antenna is very suitable for the complex interference and dissipation environment, just as the mobile phones’ working environment [12]. These all make loop antenna very attractive for the research and application of new mobile terminal antenna designs.

To overcome the shortcomings of the loop antenna with a high resonant impedance so as to make it suitable for mobile phones’ 50Ω feeding scheme, recently, varieties of different tuning techniques have been continuously studied and developed. A reconfigurable loop antenna is introduced in [2], [3]. By using the matching bridge or P-I-N diode to control the antenna’s current flowing, thereby the resonance characteristic of the loop antenna is controlled. That the loop antenna is coupling motivated by the feeding monopole and an extra parasitic element is added to form two broadband resonances is mentioned in [4], [5]. The use of back-coupling element connected to the microstrip line to feed the loop path above the PCB is proposed in [6], [7], [8]. Most often, adding a central tuning pad or a passive matching network to improve the resonance performance of the loop pattern can yet be regarded as an effective approach, just as [1], [2], [3], [9], and [10] proposed.

In this paper, the design of a compact internal folded loop antenna for multiple mobile communication systems has been presented. The whole structure consists of a continuous meander line shape folded loop radiating path and a T-shape coupling element on the back plane of the PCB. The coupling unit is connected to the 50Ω feeding microstrip line, exciting the loop track above the positive plane of the PCB by electromagnetic coupling. The proposed antenna structure generates three broad bandwidth (797-1243, 1650-2165, and 2230-2847MHz) for a VSWR=3:1, thus being able to cover GSM850/900 (Low Band: 824-960MHz), DCS/PCS/UMTS (Middle Band: 1710-2170MHz) and LTE2300/2500/WLAN2.4- GHz/WiMAX2.5GHz (High Band: 2305-2690MHz) ope- rations, respectively.

The occupied space of the whole antenna structure measures 120 × 60 × 6.5mm3, whereas the space for loop radiating structure is only about 60 × 15 × 6.5mm3. Compact structure and low height make the design very easy to meet the narrow space constraints of the mobile

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terminals. The proposed antenna has been simulated, prototyped and tested. Obtained good agreement of the simulated and measured results indicates that the proposed loop antenna can work well as predicated.

II. ANTENNA DESIGN

Fig. 1 shows the geometry of the proposed antenna for the multiple operations in the mobile handset. The entire system has a very symmetrical form, whereas the antenna structure comprises a loop radiating pattern and a T-shape coupling element. The loop radiating path which is a closed metal patch is mounted on a 6.5mm high foam dielectric substrate with a relative permittivity of 1.07 and connected to the system monolayer ground plane by two shorting points. The PCB with a thickness of 0.8mm and an area of 120 × 60mm2 is made up of FR-4, of which the relative permittivity is 4.4 and loss tangent is 0.02.

The loop pattern has a meander line structure and is folded into several planar regions so as to increase its electrical length and reduce the space it occupies at the same time. It has mainly a uniform width of 3 mm, whereas in the system front plane, the loop is added a 3.5 mm wide and 70mm long tuning pad to increase the coupling between the loop pattern and the T-shape element. At the same time the tuning pad can play a role of tuning the whole antenna’s impedance performance. The T-shaped coupling element which is a section of rectangular metallic patch with size of 20 × 12mm2 is connected to the 50Ω microstrip line feeding structure with a width of 1.5mm. Detailed dimensions of the various parts are shown in Fig. 1(b) and Fig. 1(c).

Fig. 1. Geometry of the proposed loop antenna: (a) 3D view, (b) Plan view of the loop radiating structure, and (c) Plan view of the T-shape coupling-feeding structure.

III. RESULTS AND DISCUSSION In order to illustrate the coupling feeding scheme

described herein, we compare the simulated reflection coefficient of the proposed antenna and the reference antenna which is fed directly at the feeding point A, as shown in Figure 2.

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Fig. 2. Geometry of the direct-feeding reference loop antenna and simulated results of the two feeding mechanism: (a) Plan view of the reference antenna structure, and (b) Simulated reflection coefficient of the proposed antenna and reference antenna.

Relative to the reference antenna, the proposed antenna described herein can generate three broad resonance bandwidth at the LB, MB and HB for a VSWR = 3:1 or reflection coefficient = -6dB, therefore being able to cover the required seven bands for GSM/UMTS/LTE/WLAN /WiMAX operations. By observing the simulated Smith chart of the two antennas, the input impedance of the proposed antenna is evenly distributed in the vicinity of 50Ω matching point in the LB, MB and HB frequency range, thereby overcoming the drawback of high input impedance of the loop antenna to achieve a good matching performance.

To gain further understanding of the way each resonance is excited, vector and magnitude surface current distribution of the proposed antenna at the four resonant frequencies of 0.880, 1.020, 1.980 and 2.508GHz are plotted in Fig .3. It can be clearly seen that the resonances at the frequencies of 0.880 and 1.020GHz are 0.5λ folded monopole modes, whereas resonances at 1.980 and 2.508GHz are 1.5λ folded monopole modes. In the case of excluding of the tuning pad, the loop antenna path perimeter L = 247mm, so the resonant frequencies theoretically it excited at the 0.5λ and 1.5λ wavelength modes are

thus the theoretical resonant frequencies f1 and f2 are lower than the actual ones, which is mainly because the coupling feeding excitation mechanism shortens the electrical length of the loop antenna path.

Fig. 3. Simulated vector and magnitude current distributions on the proposed loop pattern at the four resonant frequencies: (a) 0.880GHz, (b) 1.020GHz, (c) 1.980GHz, and (d) 2.508GHz.

88

1 3

3 10 6 10 0.62 2 247 10

cf H z G H zL −

×= = ≈ × =

× × ×

88

2 3

3 10 18.2 10 1.822 / 3 2 / 3 247 10

cf Hz GHzL −

×= = ≈ × =

× × ×

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The proposed antenna is constructed for testing purposed, and top view and bottom view of the testing model are shown in Fig. 4(a) and Fig. 4(b), respectively. Then the measurements are accomplished by using the Agilent E5515C RF vector network analyzer, and a comparison curve of the measured and simulated result are plotted, as shown in Fig. 4 (c) below. The results show that in the entire testing frequency range (0.6-3.0GHz), measured and simulated results are in good agreement, and three broad bandwidth obtained can meet the band requirements for GSM850/900, DCS/PCS/UMTS and LTE2300/2500/WLAN/WiMAX operations.

Fig. 4. Photos of the proposed multiband loop antenna and the measured results along with the simulated results. (a) Top view of the proposed antenna, (b) Bottom view of the proposed antenna, and (c) Measured and simulated reflection coefficient.

VII. CONCLUSION

An internal multiband folded loop antenna for mobile applications has been demonstrated in this paper. By using the T-shape back-coupling mechanism, the proposed loop antenna generates three broad impedance bandwidths (797-1243, 1650-2165, and 2230-2848MHz) which can

be used to cover the multiple GSM/UMTS/LTE/WLAN/ WiMAX mobile phone operations. Simple, compact and flexible antenna structure makes the design easy to manufacture and convenient to install. The results of the electromagnetic software simulation and physical measurement are in good agreement, confirming the validity of the design method of the proposed antenna.

REFERENCES

[1] M. Zheng, and H. Y. Wang, “Internal Hexa-band Folded Monopole/Dipole/Loop Antenna with Four Resonances for Mobile Device”, IEEE Trans. Antennas Propag., vol. 60, pp. 1–6, Apr. 2012.

[2] Y. Li, Z. J. Zhang, J. F. Zheng, Z. H. Feng, and M. F. Iskander, “A Compact Hepta-Band Loop Inverted-F Reconfigurable Antenna for Mobile Phone”, IEEE Trans. Antennas Propag. , vol. 60, pp. 389–392, Jan. 2012.

[3] Y. Li, Z. J. Zhang, J. F. Zheng, and Z. H. Feng, “Compact Heptaband Reconfigurable Loop Antenna for Mobile Handset”, IEEE Antenna Wireless Propag. Lett., vol. 10, pp. 1162–1165, 2011.

[4] S. H. Lee, K. J. Kim, J. H. Jung, Y. J. Yoon and B. N. Kim, “Meander Line Loop Antenna with coupled Feed for Multiband Mobile Phone”, 2011 International Workshop on Antenna Technology (iWAT 2011), Mar. 2011, pp. 194–197.

[5] S. H. Lee, K. J. Kim, B. N. Kim, J. K. Oh, and Y. J. Yoon, “Multi-band Coupled Feed Loop Antenna for Mobile Handset”, 2009 Asia Pacific Microwave Conference (APMC 2009), Dec. 2009, pp. 2703–2706.

[6] C. W. Chiu, C. H. Chang and Y. J. Chi, “A compact folded loop antenna for LTE-GSM band mobile phone applications”, 2010 International Conference on Electromagnetics in Advanced Applications (ICEAA 2010), no. 3, pp. 382–385, Sept. 2010.

[7] Y. W. Chi and K. L. Wong, “Compact multiband folded loop chip antenna for small-size mobile phone,” IEEE Trans. Antennas Propag., vol. 56, pp. 3797–3803, Dec. 2008.

[8] W. Y. Li and K. L. Wong, “Surface-Mount Loop Antenna for WWAN/WLAN/WiMAX Operation in the Mobile Phone”, 2008 Asia Pacific Microwave Conference (APMC 2008), Dec. 2008, pp. 1–4.

[9] K. L. Wong and C. H. Huang, “Printed loop antenna with a perpendicular feed for penta-band mobile phone application,” IEEE Trans. Antennas Propag. , vol. 56, pp.2138–2141, July 2008.

[10] S. Hayashida, H. Morishita and K. Fujimoto, “Self-balanced wideband folded loop antenna,” IEE Proc.-Microw. Antennas Propag. , Vol. 153, No. 1, pp. 7–12, Feb. 2006.

[11] Z. J. Zhang, (2011) Antenna Design for Mobile Devices, IEEE Press.

[12] C. A .Balanis, (2005) Antenna Theory: Analysis and Design, 3rd edn, Wiley-Interscience.

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