Design of a compact triple band fractal planar inverted F antennafor handheld application

In this paper, a novel Fractal PIFA antenna for triple-band is presented. The proposed antenna is based on the self affinity and lengthen shorting pin of PIFA structure. The combining design of the F-PIFA antenna is optimized and validated via simulation using CST-MW Studio as well as measurement.

DESIGN OF A COMPACT TRIPLE-BAND FRACTAL PLANAR INVERTED F ANTENNAFOR HANDHELD APPLICATION

Duong Thi Thanh Tu1,2*, Nguyen Ngoc Tu1, Vu Van Yem2

Abstract:In this paper, a novel Fractal PIFA antenna for triple-band is

presented The proposed antenna is based on the self affinity and lengthen shorting pin of PIFA structure The combining design of the F-PIFA antenna is optimized and validated via simulation using CST-MW Studio as well as measurement The measured result is well in agreement tothe simulated result Being on the FR4 substrate with the height of 1.6mm, the antenna getsa compact size with patch dimension of 9.36x9.68mm 2 that is quite suitable for handheld terminals.The proposed F-PIFA antenna operates at 2.6GHz, 5.0GHz and 7.3GHz with large bandwidth of 10.55%, 25% and 5% respectively All leads to the meet for mobilebroadband services: LTE, Wi-Fi and X-band of Satellite on each handle mobile equipment.

Keywords:PIFA,F-PIFA, Fractal, Triple band

1 INTRODUCTION

Recently, the wireless communication system has advanced incredibly, especially in mobile phone system The dimensions of end used equipment is not only more and more decrease but also the number of internal antennas in one terminal increase rapidly [1-2] These require that internal antennas must be compacted to used practical mobile handsets and have multiband for multi technologies In last three decades, Planar Inverted F Antenna (PIFA) has emerged as one of the most promising candidate for satisfying above demands [3-4] However, one of the limitations of PIFA antenna is narrow bandwidth which makes this antenna type unsuitable for wide-band commercial applications

To make multiband PIFA antenna, there are several methods that have been proposed However, using Fractal structure for making multiband in antenna design has attracted rather studies [5-9] just because this technique not only gets the multiband behaviour but also has small size and large bandwidth Recently, there have been a number of studies that combine Fractal and PIFA structure [10-12]

Because the resonant frequency of PIFA antenna is inversely proportional to the antenna height, most of previous F-PIFA designs are based traditional PIFA structure which has the antenna height are higher than thickness of substrate This is why the PIFA antenna is fabricated much more complex than other patch antennas Here, we propose a novel antenna design which combines fractal configuration and innovated PIFA structure with lengthen shorting pin on the ground plane By this way, our F-PIFA antenna’s height

is as equal as the substrate thickness Therefore, the hybrid Fractal PIFA antenna is not only easy to fabricate but also can operatein three bands with large bandwidth, which is suitable for wide-band commercial applications

2 ANTENNA DESIGN

Figure 1 shows arecursive procedure of forming fractal for making multiband.Firstly, the circumscribed radius of a hexagonal shape, a, is calculated for lower resonant frequency of 2.6 GHz by the following equations [13]:

F =8.791x10

Where:r is the dielectric constant, fr is the resonant frequency and h is the height of the substrate

Figure 1 Recursive procedure of forming fractal

For giving freedom to cut fractal on the antenna surface, a microstrip line feeding is used The feed line is matched with the antenna through a quarter wave transformer and a characteristic impedance of 50 is obtained by the following equations [13]:

ε =ε + 1

ε − 1

2 1 + 12

h

Where:eff is the effective dielectric constant and W is the width of the feeding line

After that, we cut the initial shape by the hexagonal shape on the boundary and the round shape at the center to make the 1st iteration fractal Then the 2nd and 3rd one are created with a self similar The purpose of applying fractal geometry to the patch antenna

is to have multiband behavior as well as enlarge bandwidth

Figure 2 The structure of the proposed Fractal PIFA antenna

Finally, to miniaturize the size of the antenna, fractal geometry is applied on a FIPA structure The resonance frequency of a typical rectangular PIFA antenna is approximated

by Equation (5) [14] which is related to the electrical lengths  is the relative permeability

of the medium in between the ground and the radiating patch Also, H is the height of the

patch in reference to the ground, L and W are the patch length and the patch width, respectively

In our antenna design, the shorting pin is lengthened on the ground to rise the distance

of H to get the much compact size of PIFA antenna while the height of PIFA is 1.6mm which is the height of substrate This is a significant result in reducing thickness of equipment as well as complexity of antenna fabrication.The design of hybrid Fractal PIFA (FPIFA) antenna is optimized by CST software and illustrated in Figure 2 with the detail dimensions of the proposed antenna shown on Table 1

Table 1 Detail dimension of proposed FPIFA antenna

Based on the FR4 substrate of 1.6mm thickness with relative permittivity of 4.4 and loss tangent of 0.02, the total size of proposed FPIFA is 33.6 x 28.4 mm2 and the patch size is 9.36 x 9.68 mm2 that is equivalent 10% theoreticalantenna’s size and more compact than previous studies

3 SIMULATION RESULTS

Performance of the proposed FPIFA antenna has simulated in CST software S parameter with different steps of antenna procedure is shown in Figure 3 From Fig 3(a) and (b), it is clearly seen that using fractal geometry, the antenna not only gets multiband but also decreases resonant frequency from 8.2GHz down to 7.3GHz This means the antenna size is more compact if operates at same resonant frequency Appling this fractal

on lengthened shorting pin PIFA structure which still uses the microstrip line feeding, significant reduction of resonant frequency is achieved The lower band of FPIFA has decreased from 7.3 GHz down to 2.6 GHz as shown in Fig 3(c) Hence the F-PIFA antenna size is reduced by 64% comparing to the 3rd iteration fractal geometry and gets much compact patch size of 9.36 x 9.68 mm2 while ensures the total height of PIFA antenna of 1.6mm

(a) Initial hexagonal shape

(b) 3 rd iteration fractal geometry

(c) Hybrid Fractal PIFA antenna

Figure 3 S parameters of progressive Fractal PIFA antenna

It can be illustrated more clearly by analyzing the current distribution as shown in Figure 5 Electric current goes from the microstrip line feeding to distribute on the patch as well as on the lengthened shorting pin Thus the electrical length of antenna is increased This means that the antenna size is reducedwithout frequency variation

Fig 4 Surface current distribution on antenna at resonant frequency

Besides, from the Fig 4(c), it can be seen that the proposed antenna gets three bands which are 2.6 GHz, 5.0GHz and 7.3 GHz with large bandwidth of 274 MHz (10.55%),

1251 MHz (25%) and 369 MHz (5%), the reflection coefficient of 25dB, 32dB and -13dB respectively This result not only has solved the narrow bandwidth character which

is the huge challenge of PIFA antenna but also quite be good for compact handheld equipment with LTE Wi-Fi and X band satellite applications The 2D and 3D radiation pattern for the three bands of proposed antenna are illustrated by Figure 5 It can be seen that all 2D radiation pattern have dipolar and smooth radiation In addition, low radiation efficiency is one of drawbacks of patch antenna, especially using fractal geometry It is

reason why this parameter has not presented in almost previous fractal researches In our FPIFA design, as shown in 3D radiation, the radiation efficiency of antenna gets high value of 88% at 2.6 GHz This parameter gradually decreases but is rather high value of 80% at 5 GHz and acceptable one of 64% at 7.3 GHz Thus, from the simulation results, it can be concluded that the proposed FPIFA has solved both challenges of Fractal geometry and PIFA structure in antenna design which are low radiation efficiency and narrow bandwidth for wideband commercial applications

(a) At 2.6 GHz resonant frequency

(b) At 5 GHz resonant frequency

(c) At 7.3 GHz resonant frequency

Figure 5.The 2D, 3D radiation patterns of final designed antenna

In addition, Table 2 shows comparison between our proposed antenna and previous fractal designs It is clearly seen that the novel FPIFA antenna gets compact size than

Tot effic.: - 0.99 Tot effic.: - 0.58

Tot effic.: -2.24

communications.Going into detail, the considerably comparison in [6], our antenna design

is equivalent 4% patch size while the bandwidth makes twice The remarkable reduction of antenna size is decreased by 23.64% and 28.76% in [8] and [10] respectively but the bandwidth of our design still makes twice and even more in [10] Comparison with [7], the bandwidth of our FPIPA is smaller slightly, however, the patch size is still much more compact that is equivalent nearly 27% Finally, comparison with [9], the patch size of our FPIPA is smaller slightly and the bandwidth of our FPIPA is larger little but the results of [9] are not much significant as lack of measurement results

Table 2 Comparison between present design and previous fractal geometry

Ref [6] 2.6 0.416 x 0.416 12.3%

Ref [7] 2.65 0.199 x 0.131 31.3%

Ref [8] 2.5 0.215 x 0.316 8.1%

Ref [9] 1.2364 0.061 x 0.114 22.89%

Ref [10] 2.4 0.168 x 0.12 12.5%

4 MEASUREMENT RESULTS

The proposed F-PIFA antenna is fabricated on an FR4 substrate with the dielectric constant, r =4.4 and the thickness of 1.6mm.As shown in Fig.6 (a) and (b), the proposed antenna gets compact of total size is 28.4 x 33.6 x 1.6 mm3 With the height of antenna being the same of the height of substrate and the shorting pin be placed inner antenna structure, the proposed FPIFA is easy to fabricate comparing conventional PIFA antennas

(a) top view

(b) bottom view (c) Comparison between measured result and simulated one

Figure6 Fabricated proposed Fractal PIFA antenna

Moreover, using the microstrip line feeding, the measured result agrees well with the simulated one as shown in Figure 6(c) It is clearly seen that the FPIFA antenna operates at

2.6GHz, 5GHz and 7.3GHz with the bandwidth approximating 10.6%, 20% and 5%, the reflection coefficient of -23dB, -22dB and -13dB, respectively

5 CONCLUSION

In this paper, the novel hybrid antenna design based on innovated PIFA structure and fractal geometry is proposed The FPIFA antenna operates at three bands which are 2.6GHz, 5 GHz and 7.3GHz for LTE, Wi-Fi and X-band Satellite applications Thanks to fractal design as well as lengthened shorting pin of PIFA structure, the proposed FPIFA gets wide band of 10.6%, 24.5% and 5% and good radiation efficiency of 88%, 80% and 64% at 2.6GHz, 5 GHz and 7.3GHz respectively Thus, the proposed FPIFA has solved both challenges of Fractal geometry and PIFA structure in antenna design which are low radiation efficiency and narrow bandwidth for wideband commercial applications In addition, the total size of FPIFA is 28.4x33.6x1.6mm3 which is much more compact than previous Fractal antennas

REFERENCES

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TÓM TẮT

THIẾT KẾ ANTEN BA BĂNG TẦN CHO CÁC ỨNG DỤNG CẦM TAY DI ĐỘNG

DỰA TRÊN CẤU TRÚC PIFA VÀ HÌNH HỌC FRACTAL

Nội dung bài báo đề xuất một mẫu anten PIFA kết hợp kiến trúc Fractal (anten FPIFA) cho các ứng dụng đa băng trong thiết bị đầu cuối di động Anten đề xuất sử dụng đặc tính tự đồng dạng của hình học Fractal kết hợp với cấu trúc PIFA cải tiến chiều dài shorting pin trên mặt phẳng đất Toàn bộ cấu trúc hình học Fractal cũng như kiến trúc PIFA của anten được tối ưu trên phần mềm CST-MW Chế tạo trên vật liệu FR4, anten đề xuất cho kết quả thực nghiệm và mô phỏng tương đồng nhau Với chiều cao 1.6mm, anten FPIFA có kích thước vô cùng nhỏ gọn so với các thiết kế Fractal trước đó với bề mặt bức xạ anten đạt 9.36 x 9.68mm 2 , hoàn toàn phù hợp cho các thiết bị cầm tay di động Cộng hưởng tại ba tần số 2.6GHz, 5GHz

và 7.3GHz, anten đề xuất đạt được băng thông siêu rộng 10.55%, 25%, 5%, có thể đồng thời ứng dụng cho ba công nghệ băng rộng đang được phổ biến hiện nay LTE, WIFI và vệ tinh băng X

Từ khóa: PIFA, F-PIFA, Fractal, Ba băng tần

Author affiliations:

1

Posts and Telecommunications Institute of Technology;

2

Hanoi University of Science and Technology;

*Corresponding author: tudtt@ptit.edu.vn