Miniaturized Triple Wideband CPW-Fed Patch Antenna With a Defected Ground Structure for WLAN / WiMAX Applications

A coplanar waveguide (CPW)-fed patch antenna with triple wideband is presented for simultaneously satisfying Wireless Local Area Network (WLAN) and Worldwide Interoperability for Microwave Access (WiMAX) applications. The proposed antenna mainly consists by three radiating elements: inverted L-shaped Stub1, inverted L-shaped Stub2, and a rectangle Stub3 with a defected ground structure for band broadening. By adjusting the lengths of the three Stubs, three resonant frequencies can be achieved and adjusted separately. The proposed antenna with overall size of only 20×37mm 2 explores good triple wideband operation with −10 dB impedance bandwidths of 25.70%, 45.63% and 50.10% at 2.47, 3.20, and 4.92 GHz, respectively, covering the 2.4/5.2/5.8 GHz WLAN and 2.5/3.5/5.5 GHz WiMAX operation bandwidths. This design gives suitable results with a reduction in size and weight and allows integration in handheld devices. Furthermore, nearly omnidirectional radiation patterns over the operating bands have been obtained.


INTRODUCTION
In modern wireless communication systems, multiband antenna plays a very important role for wireless service requirements [1,2].Wireless Local Area Network (WLAN) standards of 2.400-2.484GHz (IEEE 802.11b / g) / 5.15-5.35GHz / 5.725-5.825GHz (IEEE 802.11a) and Worldwide Interoperability for Microwave Access (WiMAX) standards of 2.5-2.69GHz / 3.4-3.69GHz / 5.25-5.85GHz are extensively applied in mobile devices such as handheld computers and intelligent phones (4G smartphones).Many researchers pay lots of attention to designing antennas covering all frequency bands with characteristics of simple multiband, low-profile, compact size and omnidirectional pattern for multi-service systems, especially for WLAN and WiMAX applications in wireless communication systems [3].Furthermore, it is desirable to be able to control the antenna bandwidth over different frequency bands individually [4,5].
There are various reported antenna designs for wireless systems, but the most are single-band or dual-band [6][7][8][9][10][11][12][13][14][15][16][17][18].A microstrip fed monopole patch antenna with three stubs for dual-band WLAN Miniaturized Triple Wideband CPW-Fed Patch Antenna With a Defected Ground Structure for WLAN/WiMAX Applications Ahmed Zakaria Manouare 1 , Saida Ibnyaich 2 , Abdelaziz EL Idrissi 1 , Abdelilah Ghammaz 1 applications is depicted in [15].In [19], a simple miniaturized triple-band antenna is realized by using a defected ground plane for WLAN/WiMAX applications.Double L-slot microstrip patch antenna array for WiMAX and WLAN applications is proposed in [20].A coplanar waveguide (CPW)-fed planar monopole antenna with Y-shaped for PCS / WLAN applications and a compact dual-band planar antenna for DCS-1900 / PCS / PHS, WCDMA / IMT-2000 and WLAN applications are presented in [21,22].In [23], a compact antenna with a wide square slot in the center, a rectangular feeding strip and two pairs of planar inverted L strips connecting with the slotted ground for WLAN / WiMAX applications.
In [24], a triple band monopole antenna for WLAN/WiMAX applications is obtained; the radiator of the antenna is designed on a 40(W) × 40(L) mm 2 substrate and the radiator is composed of three elements, two branches and a short stub.In [25], a dual-wideband symmetrical G-shaped slotted monopole antenna is designed for WLAN / WiMAX applications.In [26], a dual wideband coplanar waveguide-fed notched antenna with two asymmetrical ground planes for multi-band wireless application has been reported.Further, in [27], a dual wideband printed monopole antenna has been designed to cover WLAN and WiMAX frequency bands.In [28], the rectangular patch is the main radiating element of the triple-band antenna combined with split-ring slot enclosed inside of it for WLAN / WiMAX applications is presented.However, the aforementioned techniques supporting triple/multi-band operations still suffer from large overall size or a complicated structure [29,30].In [31], a square-slot antenna with symmetrical L-strips is presented for WLAN and WiMAX applications, but the three resonant frequencies cannot be tuned independently.
In this paper, a simple miniaturized triple wideband patch antenna with a defected ground structure for WLAN and WiMAX applications is proposed, which is fed by a coplanar waveguide (CPW).The antenna is created by three Stubs and three resonant frequencies can be obtained and tuned individually.Simulated results show that the three operation bandwidths of the proposed antenna are 635 MHz, 1460 MHz and 2465 MHz, respectively, which satisfy the required bandwidth of the 2.4 / 5.2 / 5.8 GHz (WLAN) and 2.5 / 3.5 / 5.5 GHz (WiMAX) with an S 11 less than -10 dB.Details of the antenna design and the effects of the key structure parameters on the antenna performances are neatly examined and discussed.

II. ANTENNA DESIGN
The geometry of the proposed triple-band antenna is illustrated in Fig. 1.The antenna substrate is a Rogers 4350 (thickness H = 0.508 mm, relative permittivity ε r = 3.66 and dielectric loss tangent of 0.004).The dimension of the substrate is 20×37 mm 2 .
In the geometry, the resonant path length L 11 (L 1 + W 1 + G + Ls 1 ), L 22 (L 1 + W 1 + G + Ls 2 ) and L 33 (L 1 + G + Ls 3 ) of the three Stubs are set close to quarter-wavelength at their fundamental resonant frequencies and can be calculated from the following equations [32]: The effective permittivity is given approximately by [33]: where Where c is the speed of light, W ƒ is the conductor width, ε r is the relative permittivity, H is the substrate thickness, ε eff is the effective permittivity; ƒ 1 , ƒ 2 and ƒ 3 denote the fundamental resonant frequencies of the Stub1, Stub2 and Stub3 respectively.The planar antenna is fed by a 50Ω impedance CPW that has a central strip having as width W ƒ = 1.55 mm and the gap between the central feed line and the ground is fixed at g = 0.3 mm.The proposed antenna was excited by a Wave Port with a width W CPW = 10 mm and a height h CPW = 5 mm.For detailed design, all parameters of the proposed antenna are optimized using the software package High Frequency Structure Simulator (HFSS  ) which is based on the Finite Element Method (FEM) and are shown in Table I.
The simulated reflection coefficient is presented for the optimized set of antenna parameters in Fig. 2.
From the simulated result, it is apparent that a single antenna having three multiple resonant frequencies with a wide bandwidth is obtained.The simulated impedance bandwidths for S   Table II shows the values of resonant frequencies and bandwidths at 10 dB of the three resonant modes.For the first resonant mode, the resonant frequency is about 2.47 GHz (S 11 = 13.25 dB), the second resonant frequency for the second mode is about 3.20 GHz (S 11 = 41.85dB), and the third resonant frequency is about 4.92 GHz (S 11 = 48.11dB).
It is worth mentioning that the configuration of the ground plane also affects the characteristics of the antenna.In this design, the two symmetrical ground planes are defected for ameliorating the impedance performance and broadening the bandwidth especially for the second and the third operating bands.In fact, ground plane has been useful to design multiband handset antennas [34][35][36].

A. The Ls 1 Variation Effects
The effects of the length of the first stub (Stub1) on the antenna performance are plotted in Fig. 3.
This figure shows the simulated reflection coefficients when the length of Ls 1 changes.By adjusting the length of Ls 1 , the total length of Stub1 varies.It is seen that the increase in Ls 1 decreases the resonant frequency of the first band and vice versa, while other resonant frequency bands are slightly affected.

B. The Ls 2 Variation Effects
Fig. 4 shows the simulation of the reflection coefficients with variation of Ls 2 (Stub2).By tuning the length of Ls 2 from 9.5 mm to 12.5 mm, it is clear that the raise in Ls 2 decreases the resonant frequency of the second band.The third band shifts to higher frequencies as Ls 2 is increasing, which is possibly due to the changing coupling between the two inverted L-shaped.The first band is not affected.

C. The Ls 3 Variation Effects
Varying the length Ls 3 of Stub3 to be 8.5, 9.5, 10.5 and 11.5 mm, it can be seen from Fig. 5, Tables III and IV that with the increasing of length Ls 3 , the third band shifts towards the lower frequency with the bandwidth increases slightly, while other resonant frequency bands have not been changed.

D. The Effects of the Defected Ground Structure (DGS) on the Antenna Performance
To demonstrate the effect of the square and rectangular truncation on the ground plane, Fig. 6(a

E.
The Gap Variation Effects on the Impedance Matching Performance of the Antenna An important feature of the proposed antenna is the capability of impedance matching at three operating frequencies using a single CPW feed line.For this, the coupling effect between the feed line and the two ground planes is investigated.Fig. 7 shows the simulated reflection coefficient for the proposed antenna with different gap width (g) of 0.1, 0.2, 0.3 and 0.35 mm.As seen in figures 7 and 8, the gap width has a significant effect on the impedance matching performance of the proposed antenna for the three resonant frequencies.
The matching condition of the three bands is sensitive to the variation of g.The impedance matching condition for the three operating bands can be optimized with g equal to 0.3 mm.

F.
The L 1 Variation Effects As shown in Fig. 9, L 1 (the gap between the radiating patch and the two ground planes) could disturb the performance of the second and the third band antenna but the first band is not affected.It is evident that the reflection coefficient of the proposed antenna with L 1 = 3.8 mm is better than that L 1 = 4.3 mm and L 1 = 4.8 mm.

IV. CURRENT DISTRIBUTIONS AND RADIATION PATTERNS
In order to better understand the antenna behavior, the current distributions of the three-band antenna at frequencies of 2.47 GHz, 3.  The simulated antenna gain against frequency for the proposed antenna across the three bands is shown in Fig.

Fig. 2 .
Fig. 2. Simulated result of the reflection coefficient against frequency for the proposed antenna.
)and Fig.6(b)  show the different evolution of the geometry of the two ground planes with and without truncation and its corresponding simulated reflection coefficients.We remark from Table V and Fig.6(b) that the first truncation (P 1 , P 2 ) influences on the impedance matching of the second band (WiMAX) and the reflection coefficient is enhanced from -21.5 dB to -37.24 dB.Furthermore, the second truncation (W 2 , L 2 ) affects the upper band; the reflection coefficient is enhanced from -30.68 dB to -39.69 dB.For the proposed antenna, more amelioration of the reflection coefficient and the bandwidth were registered for second and third resonant band.The change of ground structure has small effects on the first band.

Fig. 11 .Fig. 12 .
Fig.12.Simulated peak antenna gain across the three operating bands for the proposed antenna.

TABLE II .
SIMULATED IMPEDANCE BANDWIDTHS OF THE PROPOSED ANTENNA

TABLE IV .
THE VALUES OF BANDWIDTH OF THE THIRD BAND FOR DIFFERENT VALUES OF L S3

TABLE V .
THE VALUES OF REFLECTION COEFFICIENTS FOR DIFFERENT GEOMETRY OF THE TWO GROUND PLANES 20GHz and 4.92 GHz are simulated and shown respectively in 12. First over the 2.4 GHz operation band (2.085 -2.72 GHz); the simulated gain is varied from -2.4 dBi to -0.08 dBi.Then for the medium band (2.89 -4.35 GHz), the simulated antenna gain is about -1.27 dBi to 2.62 dBi.Finally at the higher band (4.59 -7.055 GHz), the simulated gain of the antenna changes from 1.05 dBi to 4.39 dBi.