A Compact Dual-Band CPW-Fed Planar Monopole Antenna for 2 . 62-2 . 73 GHz Frequency Band , WiMAX and WLAN Applications

In this paper, we present a compact and low-profile monopole antenna with a simple structure for the 2.6-2.73 GHz frequency band, the Worldwide Interoperability for Microwave Access (WiMAX) and the Wireless Local Area Network (WLAN) applications. The first configuration of our antenna mainly consists by three radiating elements: inverted L-shaped Stub1, L-shaped Stub2 and a rectangle Stub3. By adjusting the lengths of the three Stubs, three resonant frequencies can be achieved and adjusted separately. Then, the assembled between Stub2 and Stub3 gives the final design of our proposed antenna with a small overall size of 20 mm × 37 mm × 1.56 mm. From the experimental results it is observed that, the antenna prototype has achieved two operating bandwidths (S11≤ -10 dB): the first band from 2.62 to 2.73 GHz (110 MHz) and a second broadband from 3.02 to 7.30 GHz (4280 MHz) which combines WiMAX and WLAN applications. The antenna also exhibits an almost omnidirectional radiation patterns over the operating bands. The parameters which affect the performance of the antenna in terms of its frequency domain characteristics are studied in this paper. The details of the monopole antenna design along with simulated and experimental results are presented and discussed.


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WLAN standards in the 5.2 GHz (5150-5350 MHz)/5.8GHz (5725-5825 MHz) operating bands and WiMAX 2.6/3.5/5.5 GHz (2500-2690/3400-3690/5250-5850 MHz) bands, multi-band antennas with low cost, compact size, easy fabrication and higher performance are required.Several dual-band antennas for WLAN applications were presented [1][2][3][4][5].To enhance the bandwidth, a dual wideband monopole antenna was realized with a parasitic patch using electromagnetic coupling mechanism to cover the whole WLAN bands and WiMAX bands.However, the overall size of the antenna is somewhat large (48 × 58 mm 2 ), occupying much of the device space [6].A compact wideband LIshaped monopole antenna with enough bandwidth to cover the whole WLAN and WiMAX operating bands was obtained in [7].A compact dual-wideband antenna with assembled monopoles is proposed in [8].A coplanar waveguide (CPW)-fed printed monopole antenna with an n-shaped slot for dualband operation is presented in [9].In [10], a dual-band slotted patch antenna with defective ground has been designed to cover WLAN and WiMAX applications.In reference [11], a CPW-fed compact meandered patch antenna for dual-band operation is presented.In [12], a CPW-fed dual-frequency monopole antenna has been reported.In [13], a square-slot antenna with symmetrical L-strips is presented for WLAN and WiMAX applications, but the three resonant frequencies cannot be tuned independently.Many promising UWB antennas have been discussed in the literature [14][15][16][17][18].
In this paper, we present a compact dual-band CPW-fed planar monopole antenna for 2.   I.   Table II shows a comprehensive comparison antenna size among our proposed antenna and other compact multi-band antennas.As for our proposed antenna with an excellent dual-band characteristic and a smaller size than those of the previously proposed dual-band antennas.III.

PARAMETRIC STUDY
The parametric study is important for a design because it provides some understanding of the antenna characteristics to the antenna designer.Therefore, the effects of the design parameters for L 1 , L 2 , L 3 , (L 2 and L 3 ), W 1 and L g on the initial antenna and the proposed monopole antenna characteristics are investigated here.The study is based on the antennas structures shown in Fig. 1.

A. The inverted L-shaped Stub1 (L 1 )
The effects of the length L 1 of the initial antenna are plotted in Fig. 4(a).This figure shows the simulated reflection coefficient when the length of L 1 changes (L 2 = 10.5 mm and L 3 = 8.5 mm).By adjusting the length of L 1 , the total length of Stub1 varies.It is seen that the increase in L 1 decreases the resonant frequency of the first band and vice versa.The resonant frequency of the third band is also affected.B. The L-shaped Stub2 (L 2 ) of the initial antenna Fig. 5(a) shows the simulation of the reflection coefficient with variation of L 2 (L 1 = 16.5 mm and L 3 = 8.5 mm) of the antenna presented in Fig. 1(a).By tuning the length of L 2 from 9.5 mm to 12.5 mm, it is clear that the raise in L 2 decreases the resonant frequency of the second band.The resonant frequency of the first band is slightly affected.

C. The rectangle Stub3 (L 3 ) of the initial antenna
Varying the length L 3 of the initial antenna to be 7.5, 8.5, 9.5 and 10.5 mm, it can be seen from Fig. III that with the increasing of length L 3 , the third resonant frequency shifts towards the lower frequency with an increase in the third band slightly, while the other resonant frequencies bands have not been changed.D. The simultaneous variation of L 2 and L 3 of the proposed antenna Fig. 6(a) shows the simulated reflection coefficient as a function of L 2 , the length of Stub2 and L 3 , the length of Stub3 of the proposed monopole antenna presented in Fig. 1(b).Small effects on the antenna's first and third resonant frequencies and large effects on the second resonant frequency are seen.The second band is shifted to lower frequencies with an increase in L 2 and the parameter S 11 of the third resonant frequency is ameliorated when raising the length L 3 .

E. The width W 1 of the proposed antenna
Fig. 6(b) shows the simulated reflection coefficient as a function of W 1 .It is seen from the plot that the 5-GHz operating band is strongly affected by the variations in W 1 , and the resonant frequency is shifted to lower frequencies when W 1 is increased from 6.225 mm to 7.725 mm and the level of S 11parameter is enhanced from -27.70 dB to -58.57dB.Small effects on the antenna's first and middle bands are also seen from the plot.

F. The effect of ground plane length L g of the proposed antenna
The finite ground CPW feeding mechanism of the proposed antenna is the capability of impedance matching at the operating frequencies.For this, the effect of the ground plane length L g on the antenna characteristics has been illustrated.The effect of ground plane length L g on the impedance matching is investigated and the reflection coefficient for the proposed antenna is shown in Fig. 6(c).It is observed that higher value of L g gives a good impedance matching in the 5-GHz band.IV.

A. Reflection coefficient results
The dual-band planar monopole antenna is simulated using the CST Microwave Studio V13.A prototype structure of the proposed antenna has been constructed and experimentally studied.The SMA female connector is used for feeding with characteristic impedance of 50 Ω, as shown in Fig. 7.
The reflection coefficient is measured with Rohde and Schwarz ZVB 20 vector network analyzer, which its frequency range is, limited to 20 GHz.Fig. 8 shows the simulated and measured results of the reflection coefficient of the proposed antenna.The measured impedance bandwidths for S 11 ≤ -10 dB are about 110 MHz (2.62 to 2.73 GHz, ƒ r1 = 2.69 GHz) and 4280 MHz (3.02 to 7.30 GHz, ƒ r2 = 4.28 GHz and ƒ r3 = 6.86 GHz), which makes it easy to cover the required bandwidths for WiMAX bands (3.40-3.69GHz and 5.25-5.85GHz), WLAN bands (5.15-5.35GHz and 5.725-5.825GHz) and a part of 2.60 GHz band from 2.50 to 2.69 GHz.Seen from Fig. 8, the simulated −10 dB impedance bandwidths for the first band is ranged from 2.519 to 2.729 GHz (210 MHz) and for the second band is ranged from 3.093 to 6.684 GHz (3591 MHz).We note a good agreement between the simulated and measured results with a good impedance matching in the operating bands.The small difference between the measured and simulated results is due to the effect of SMA connector soldering and fabrication tolerance.

B. Current distributions of the initial antenna
From the simulated reflection coefficient characteristics of the initial antenna presented in Fig. 1(a   GHz.In addition, the proposed antenna provides good radiation patterns in the working bands, which makes it suitable for integrating into portable devices.This CPW-fed planar monopole antenna is a good candidate for wireless communication systems. 6-2.73 GHz frequency band, WiMAX and WLAN applications.The first stage of our design is a simple antenna with three Stubs (Stub1, Stub2 and Stub3) which provides three resonant frequencies at 2.603 GHz, 3.429 GHz and 4.584 GHz.These frequencies can be tuned individually according to the parameters L 1 , L 2 and L 3 , as shown in Fig. 1 (a).In the second stage, by assembling the Stub2 and the Stub3, as shown in Fig. 1 (b), the proposed antenna satisfies a part of the 2.6 GHz band (first band) from 2.62 to 2.73 GHz and a second wide band from 3.02 to 7.30 GHz is formed to cover all the 5.2/5.8GHz WLAN operating bands and the 3.5/5.5GHz WiMAX operating bands.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 geometries of the initial and proposed CPW-fed monopole antennas are shown in Fig. 1.Both the antennas are designed on a 1.56 mm thick FR4 substrate having relative permittivity of 4.3, a loss tangent of 0.025 and having overall dimensions of 20 (W) × 37 (L) mm 2 and a coppering thickness of the radiator t= 0.035 mm.The electromagnetic simulation software CST Microwave Studio based on Finite Integration Technique (FIT) is used for the design.In both antennas, the CPW has a feed width of W f = 2.8 mm and a gap distance of g= 0.4 mm between the feed and the coplanar ground plane, which corresponds to 50 Ω characteristic impedance.

Fig. 1 .
Fig. 1.(a) Geometry of the initial antenna, (b) Geometry of the proposed antenna.The radiating element is composed by three Stubs: inverted L-shaped Stub1 with L 1 = 16.5 mm, Lshaped Stub2 with L 2 = 10.5 mm and a rectangle Stub3 with L 3 = 8.5 mm, as shown in Fig. 1 (a).By adjusting the lengths (L 1 , L 2 and L 3 ) of these Stubs, three resonant frequencies can be generated and adjusted independently.The optimized geometrical parameters describing the proposed antenna are tabulated in TableI.

Fig. 2 ( 2 .
Fig.2shows the different shapes in the evolution of the proposed antenna and the simulated result of the reflection coefficient of the proposed antenna is presented in Fig.3.The structure illustrated in Shape 1 of Fig.2is the basic CPW-fed planar antenna which consists by an inverted L-shaped Stub1 acting as the monopole.When an additional L-shaped Stub2 is embedded to the monopole of Shape 1 (Shape 2), a second resonant mode at 3.50 GHz band is generated and two operating bandwidths are

Fig. 3 .
Fig. 3. Simulated result of the reflection coefficient against frequency for the proposed antenna.

Fig. 4 (
Fig. 4(b) shows the variation of the reflection coefficient of the proposed antenna when the length of the first Stub L 1 changed from 15.5 mm to 18.5 mm.

Fig. 4 .
Fig. 4. Simulated reflection coefficients for different values of: (a) L 1 of the initial antenna and (b) L 1 of the proposed antenna.

Fig. 5 .
Fig. 5. Simulated reflection coefficients for different values of: (a) L 2 of the initial antenna and (b) L 3 of the initial antenna.

Fig. 6 .
Fig. 6.Simulated reflection coefficients for different values of: (a) L 2 and L 3 of the proposed antenna (b) W 1 of the proposed antenna and (c) L g of the proposed antenna.

Fig. 8 .
Fig. 8.The simulated and the measured reflection coefficient for the proposed antenna.

Fig. 10 .
Fig. 10.Simulated current distribution of the proposed antenna at frequencies (a) 2.65 GHz, (b) 3.50 GHz and (c) 4.60 GHz.D. Radiation patterns, gain and efficiencyThe Simulated E-plane (YOZ) and H-plane (XOZ) radiation patterns at 2.65, 3.50, 5.20 and 5.80 GHz are normalized and shown in Fig. 11.It is observed that the proposed antenna has almost an omni-directional radiation patterns in the H-plane and nearly bi-directional radiation patterns in the Eplane over the desired operating bands.

Fig. 12 (
Fig. 12(a) and Fig. 12(b) show simulated peak gain and radiation efficiency across the operating bands.The maximum simulated peak antenna gains and radiation efficiencies are 1.45/1.55/3.31dBiand 81.1/77.5/75%at the first band and the second band, respectively.

TABLE II .
COMPARISONS OF ANTENNA SIZE AMONG PROPOSED ANTENNA AND OTHER COMPACT ANTENNAS

TABLE III .
THE VALUES OF BANDWIDTH OF THE THIRD BAND FOR DIFFERENT VALUES OF L 3 OF THE INITIAL ANTENNA