Design of a modified circular-cut multiband fractal antenna

December 2016, 23(6): 68 75 www.sciencedirect.com/science/journal/10058885 The Journal of China Universities of Posts and Telecommunications http://jcupt.bupt.edu.cn Design of a modified circular-cut multiband fractal antenna Hu Zhangfang, Xin Wei (), Luo Yuan, Hu Yinping, Zhou Yongxin Institute of Photoelectronic Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065, China Abstract This paper presents a modified circular-cut multiband fractal antenna with good radiation patterns designed for digital cellular system (DCS), personal communication system (PCS), 2.4/5.2/5.8 GHz wireless local area networks (WLAN) and 2.5/3.5/5.5 GHz worldwide interoperability for microwave access (WiMAX) applications simultaneously. Originally, the modified circular monopole antenna is designed to resonate at around 2.1 GHz and 3.6 GHz. After subtracting the circular iterative tree fractal structure, it can produce three other resonances at around 5.6 GHz, 6.47 GHz and 7.89 GHz. Besides, as the number of iterations increases, not only do the new frequency bands appear (it demonstrates the good self-similarity property of the proposed antenna), but also the operating bands shift from high frequency to low frequency (it shows the well space filling property). Furthermore, the proposed antenna owns a compact structure, which can achieve the 5.28 dbi of relative high gain. And the measured results are basically accordant to simulated results, which proves the effectiveness of the proposed antenna. Keywords multiband, fractal monopole, self-similarity, space filling 1 Introduction With the development of wireless communication technology, the channel capacity is expanding, the speed of transmission is increasing, the different operating systems are becoming more and more compatible with each other, and the service modes are also turning more flexible, which make the operating bands of the wireless communication expand again and again. From the global system for mobile communication (GSM), DCS, PCS to the 4th generation communication system (4G) and from WLAN to WiMAX, more and more standards may be used simultaneously in various systems, so there is an urgent need for small size, multiband and integrated antennas. Fractal theory is a novel method for antenna design like using the self-similarity property of the fractal body to realize the multiband of the antenna [1], and using the space filling property to reduce the size [2 4]. Such as the proposed Hilbert structure in 1891 [5], the Koch structure Received date: 14-06-2016 Corresponding author: Xin Wei, E-mail: 512185894@qq.com DOI: 10.1016/S1005-8885(16)60072-9 in 1904 [6] and the Sierpinski structure in 1915 [7], etc, they are all some of the classic fractal structures. In Ref. [8], an H-fractal used for a multiband antenna is presented, the proposed antenna can excite multiple resonances with reasonable antenna directivity, which successfully covers the WLAN (2.4/5.2/5.8GHz) bands. In Ref. [9], a hybrid fractal planar monopole MIMO antenna is investigated, by incorporating the Minkowski island curve and Koch curve fractals to decrease the resonant frequency for covering several wireless communication bands (WiFi/WiMAX/ WLAN). In Ref. [10 12], some fractal printed slot antennas with the appropriate feeding ways are proposed, all of these are suitable for WLAN and WiMAX operations. In this paper, an improvement in the modified circular monopole antenna is presented, by subtracting a novel circular iterative tree fractal structure from it, which gets a multiband antenna for DCS, PCS, 2.4/5.2/5.8 GHz WLAN and 2.5/3.5/5.5 GHz WiMAX applications simultaneously. The resonant frequencies of the proposed antenna are 2.05 GHz (1.7 GHz~2.92 GHz), 3.65GHz (2.92 GHz~ 4.28 GHz), 5.6 GHz (5 GHz~5.98 GHz), 6.47 GHz

Issue 6 Hu Zhangfang, et al. / Design of a modified circular-cut multiband fractal antenna 69 (6.37 GHz~ 6.78 GHz) and 7.89 GHz (7.33 GHz~8 GHz). Finally, the proposed antenna with the size of 40 mm 58 mm 1.3 mm has omnidirectional radiation characteristics and relative high gain, which meets the requirements of portable multiband mobile devices. 2 Antenna design and analysis 2.1 Tree fractal structure The tree fractal structure proposed in this paper is shown in Fig. 1. The basic unit of tree fractal structure is a circle, which is denoted as S 0, the basic unit S 0 shrinks by a scaling factor (b=0.54) and the resultant structure is referred to as S 1, then S 1 shrinks by the same scaling factor to get S 2. The first circular iterative tree fractal structure is composed of S 0, the second is consist of S 0 and S 1, and the third is made up of S 0, S 1 and S 2. (a) Modified circular patch (b) First iterative circular-cut structure (a) First iteration (b) Second iteration (c) Third iteration Fig. 1 Tree fractal structure The third circular iterative tree fractal structure rotates around the center followed by 45, 90, 135, 180, 225, 270, 315 and the resultant structure is referred to as the third circular iterative tree fractal structure array. According to the above principle, the 4-array and 6-array antenna can be obtained. In order to increase the current flow path and improve the radiation characteristics of the antenna, based on the general circular patch has been improved to give a modified circular patch shown in Fig. 2(a). Then the first iterative, second iterative and third iterative circular-cut fractal structures can be obtained by the modified circular patch subtracting 8-array circular iterative tree fractal structure, as illustrated in Figs. 2(b) 2(d). (c) Second iterative circular-cut structure (d) Third iterative circular-cut structure Fig. 2 Patch geometry

70 The Journal of China Universities of Posts and Telecommunications 2016 2.2 Antenna design The proposed antenna is composed of the third iterative circle-cut fractal structure, microstrip feed-line and the defected ground structure. The initial radius R of the modified circular patch is derived from Eq. (1) [13]. R = F 1 (1) 2h π 2 F 1 + ln + 1.772 6 πεrf 2h 9 8.791 10 F = (2) f ε r r where f r is the resonant frequency of the antenna, ε r is the permittivity of the dielectric, h (cm) is the thickness of the dielectric layer. The radius of the modified circular patch is given by Eqs. (1) and (2), then optimized by high frequency structure simulator (HFSS) further. The overall structure of the antenna is shown in Fig. 3. R and R 1 are the outside and inside radius of the improved circular patch respectively, r 1 and r 2 are the outside and inside radius of the third circular iterative structure respectively, C 1, C 2 and C 3 are the distance between each center of tree fractal structure and the center of the improved circular patch respectively. In order to get a better impedance matching, the input impedance of the antenna is required to be designed in 50 Ω. The proposed antenna fed by the microstrip transmission line with a strip width (W) and length (L) is supported by FR4 substrate (A B) with relative dielectric constant of 4.4, thickness of h =1.3 mm, and loss tangent of δ =0.02. The detailed dimensions of the antenna are given in Table 1. (a) Front view (b) Rear view Fig. 3 Proposed circular-cut antenna Parameter Size/mm A 40.0 L 20.3 R 18.0 C 1 12.0 C 3 3.3 r 2 4.0 l 1 1.0 Table 1 Antenna details Parameter Size/mm B 58.0 W 2.2 R 1 16.5 C 2 6.4 r 1 4.5 L 1 19.7 l 2 2.0 2.3 Effect of the defected ground structure The rear view of the modified circular-cut fractal antenna with ordinary rectangular ground plane is shown in Fig. 4(c), the height L 2 is 19.7 mm. In order to get a better impedance matching and adjust the resonant frequency, defected ground structure (DGS) is introduced in the ground plane [14 15]. Fig. 4(b) shows the small periodic rectangular defects etched on the ground plane, whose width l 1 is 1 mm and length l 2 is 2 mm, the distance d between each other is 2mm. Fig. 4(a) shows the simulated reflection coefficients of the DGS and the rectangular ground plane antennas. The simulated result shows that the DGS antenna has lower reflection coefficient and wider bandwidth compared with the common rectangular ground plane antenna in operating bands. 4.00 GHz and 5.80 GHz resonant frequencies are shifted to 3.65 GHz and 5.60 GHz respectively, the reflection coefficients of the DGS antenna in 2.05 GHz, 3.65 GHz, 5.6 GHz, 6.47 GHz and 7.89 GHz are 40 db, 25 db, 28 db, 27 db and 37 db respectively, which are better than the reflection coefficients of the rectangular ground plane antenna. So we can draw the following conclusions: the frequency characteristics of the original rectangular ground antenna are improved due to the existence of the DGS without increasing the overall

Issue 6 Hu Zhangfang, et al. / Design of a modified circular-cut multiband fractal antenna 71 size of the antenna. (a) Reflection coefficient of different ground planes (a) Reflection coefficient (b) Modified circular patch (c) First iteration (b) DGS (c) Rectangular ground plane Fig. 4 Reflection coefficient of different ground planes 2.4 Effect of the different iterations Fig. 5 shows the simulated reflection coefficients of the antenna with different iteration stages. As expected, the third iterative circular-cut fractal antenna has five resonant bands, which are operating in 1.70 GHz~2.92 GHz, 2.92 GHz~4.28 GHz, 5.00 GHz~5.98 GHz, 6.37 GHz~ 6.78 GHz and 7.33 GHz~8.00 GHz. The second iterative circular-cut fractal antenna has four resonant bands, which cover 1.73 GHz~2.92 GHz, 2.92 GHz~4.16 GHz, 5.20 GHz~6.28 GHz and 6.97 GHz~7.46 GHz. The first iterative circular-cut fractal antenna has three resonant bands, which are working in 1.80 GHz~2.92 GHz, 2.92 GHz~4.30 GHz and 7.30 GHz~8.00 GHz. And the modified circular patch antenna has two resonant bands of 1.80 GHz~2.94 GHz and 2.94 GHz~5.19 GHz. As the number of iterations increases, not only do the new frequency bands appear (it demonstrates the good self-similarity property of the proposed antenna), but also the operating bands shift from high frequency to low frequency (it shows the well space filling property). (d) Second iteration (e) Third iteration Fig. 5 Reflection coefficient of different iterations 2.5 Effect of the different arrays Different arrays of the circular-cut fractal antenna have different self-similarity properties and different current characteristics. Therefore, it is necessary to discuss the influence of different arrays on the performance of the antenna. Figs. 6(b) 6(d) show the 4-array, 6-array and 8-array circular-cut fractal structures, the reflection coefficients of different arrays are shown in Fig. 6(a). 4-array circular-cut fractal antenna has three resonant frequencies of 2.06 GHz, 3.64 GHz and 7.78 GHz. Six-array circular-cut fractal antenna has four resonant frequencies of 2.06 GHz, 3.54 GHz, 5.61 GHz and

72 The Journal of China Universities of Posts and Telecommunications 2016 7.76 GHz. Eight-array circular-cut fractal antenna has five resonant frequencies of 2.05 GHz, 3.65 GHz, 5.6 GHz, 6.47 GHz, 7.89 GHz. Compared with the 4-array and 6-array antennas, 8-array circular-cut fractal antenna has the lowest reflection coefficient and the widest bandwidth, which demonstrates it has the better self-similarity property and the longer current flow path. (a) Reflection coefficient of different arrays (d) 8-array structure Fig. 6 Reflection coefficient of different arrays 3 Parameter optimization and experimental results 3.1 Effect of D Fig. 7(a) shows the simulated reflection coefficients of the antenna with different widths of the ring D= R R 1. (b) 4-array structure (a) Reflection coefficient of different D (c) 6-array structure (b) Reflection coefficient of different R

Issue 6 Hu Zhangfang, et al. / Design of a modified circular-cut multiband fractal antenna 73 When r 2 is 3.5 mm, five resonant frequencies (2.09 GHz, 3.66 GHz, 5.99 GHz, 7.21 GHz and 7.95 GHz) are acquired. When r 2 is 4.0 mm, five resonant frequencies (2.05 GHz, 3.65 GHz, 5.6 GHz, 6.47 GHz and 7.89 GHz) are achieved. The length of r 2 affects the position of band 1, 3, 4. 3.4 Measured results (c) Reflection coefficient of different r 2 Fig. 7 Reflection coefficient of different parameters With the increase of D, band 3 and 4 move to higher frequency and have a wider bandwidth, the resonant frequency of band 3 shifts from 5.24 GHz to 5.36 GHz, 5.50 GHz, 5.60 GHz respectively and band 4 shifts from 5.77 GHz to 6.06 GHz, 6.29 GHz and 6.47 GHz respectively, which show that the width of the ring D affects the resonant frequency and bandwidth of band 3 and 4, by adjusting the ring width D, the position of band 3 and 4 can be adjusted. Fig. 8 shows the photograph of the fabricated antenna. The proposed antenna is measured by Agilent E8363B. The measured bandwidths below 10 db are 1.56 GHz~ 2.90 GHz, 2.90 GHz~4.60 GHz, 5.35 GHz~5.98 GHz, 6.32 GHz~6.60 GHz and 7.54 GHz~8.00 GHz. Fig. 9 demonstrates the simulated and measured reflection coefficient of the proposed antenna, according to the figure, we can see that they are in good agreement. Some discrepancies between the measured and simulated results can be attributed to the fact that the welding of the antenna is not perfect. 3.2 Effect of R Fig. 7(b) shows the simulated reflection coefficients of the antenna with different outsider radiuses of the ring R. With the increase of R, the resonant frequencies of the antenna move to the lower frequency and the new frequency bands appear. When R is 14.0 mm, three resonant frequencies (2.36 GHz, 4.32 GHz, 7.28 GHz) are obtained. As R is 16.0 mm, four resonant frequencies (2.22 GHz, 3.91 GHz, 6.5 GHz, 7.9 GHz) are achieved. When R is 18.0 mm, five resonant frequencies (2.05 GHz, 3.65 GHz, 5.6 GHz, 6.47 GHz and 7.89 GHz) are acquired. The length of R affects the number of resonant frequency, by adjusting the length of R, the number of operating band can also be adjusted. (a) Front view (b) Rear view Fig. 8 Photograph of the fabricated antenna 3.3 Effect of r 2 Fig. 7(c) shows the simulated reflection coefficients of the antenna with different radius of the third circular iterative structure (r 2 ). With the increase of r 2, the resonant frequencies of the antenna shift from high frequency to low frequency, when r 2 is 3.0 mm, three resonant frequencies (2.16 GHz, 3.66 GHz, 7.80 GHz) are obtained. Fig. 9 Reflection coefficient results of simulated and measured Since a microstrip patch antenna radiates normally to its patch surface, the useful portion of radiation pattern will be elevation plane for ϕ = 0 and ϕ = 90. The simulated and measured radiation patterns of the proposed antenna in the XZ plane (E-plane) and YZ plane (H-plane)

74 The Journal of China Universities of Posts and Telecommunications 2016 are shown in Fig. 10. (d) 6.47 GHz (a) 2.05 GHz (e) 7.89 GHz Fig. 10 Radiation pattern of the antenna (b) 3.65 GHz (c) 5.6 GHz It is observed that the radiation patterns are stable, nearly omnidirectional in the YZ plane and bidirectional in the XZ plane, while the omnidirectional radiation characteristics at the upper frequency are less stable because the ground plane has more electromagnetic radiation at high frequencies so that it affects the radiation characteristics of the antenna. According to Fig. 10, we can get that for the 2.05 GHz pattern, the maximum directivity occurs at θ = 0, the peak gain is 2.16 dbi. For the 3.65 GHz pattern, the maximum directivity occurs at θ = 210 and the peak gain is 4.42 dbi. For the 5.6 GHz pattern, the maximum directivity occurs at θ = 225 and the peak gain is 1.15 dbi. For the 6.47 GHz pattern, the maximum directivity occurs at θ = 330 and the peak gain is 2.95 dbi. For the 7.89 GHz pattern, the maximum directivity occurs at θ = 270 and the peak gain is 5.28 dbi. Table 2 presents a comparison between the circular-cut fractal multiband antenna and other antennas presented in the literature. And it also accordingly indicates the operating standards and the dimensions of each antenna.

Issue 6 Hu Zhangfang, et al. / Design of a modified circular-cut multiband fractal antenna 75 4 Conclusions Table 2 Comparison of the proposed antenna against other antennas Structure DCS PCS WLAN (2.4 GHz) WiMAX (3.5 GHz) WLAN (5.0 GHz) Dimension/mm 3 In Ref. [16] In Ref. [17] In Ref. [18] In Ref. [19] In Ref. [20] In Ref. [21] Proposed 57 37.5 0.8 55 52 1.52 56 44 0.80 45 40 1.00 40 30 0.80 67 47 1.00 40 58 1.30 The design of the modified circular-cut multiband fractal antenna for DCS, PCS, WiMAX and WLAN has been presented. The proposed antenna is obtained by the modified circular patch minus 8-array circular iterative tree fractal structure. Simulation and measurement are applied to discuss the performance of radiation characteristics of the antenna. Results show that the antenna has five frequency bands at 2.05 GHz (1.70 GHz~2.92 GHz), 3.65 GHz (2.92 GHz~4.28 GHz), 5.60 GHz (5.00 GHz~ 5.98 GHz), 6.47 GHz (6.37 GHz~6.78 GHz) and 7.89 GHz (7.33 GHz~8.00 GHz). 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