Compact Multiband Slot Antenna for WLAN / WiMAX Operations

A novel compact triple-band slot antenna is proposed for WLAN/WiMAX applications. With the use of three L-shaped slots and an open-ended slot on the ground plane, three resonant modes centered at 2.4/3.5/5.8 GHz are excited. These modes cover the standards of 3.5 GHz WiMAX and 2.4/5.8 GHz WLAN, three useful frequency bands for present wireless communication systems. The proposed antenna fed by L-shaped microstrip line has a small overall dimension of 16 × 30mm. The numerical and experimental results show that acceptable radiation characteristic is obtained over the operating bands.


Introduction
With the rapid development of the modern wireless communication technology, the wireless local area network (WLAN) which operates in the frequency band of 2.4-2.484GHz/5.15-5.35GHz/5.725-5.825GHz and worldwide interoperability for microwave access (WiMAX) system covering 3.4-3.69GHz have been widely used in mobile devices.Meanwhile, small communication terminals lead to a great demand in integrating various services to collaborate with each other.Accordingly, antenna design has turned to focus on multiband and small simple structures.Some monopole antennas for multiband applications, such as dual U-shape [1], T-shaped [2], double-T shape [3], fork-shaped [4,5], trident-shaped [6], and meandering microstrip lineloaded [7] applications, have been reported.Slot antennas are another popular design for multiband applications.However, the printed slot antenna used in multiband operation design is relatively less in the open literature.In [8][9][10], bow-tieslot, T-slot, and ring-slot antennas provide dual or triple resonant modes for different wireless communication systems.Although these antenna designs mentioned above can achieve dual or multiple band property, they are somewhat large in size, which are not perfectly practical for small communication terminals.
In this paper, a novel compact microstrip-fed slot antenna designed with triple-band operations is proposed for WLAN and WiMAX applications.In the proposed design, the antenna structure consists of two L-shaped slots and an openended slot on the ground plane that is excited by an L-shaped microstrip feed line.Meanwhile, another etching small Lshaped coupling slot makes a positive effect to enhance the bandwidth of 3.5 GHz and improve the impedance match condition of 5.8 GHz.These design skills are introduced to approach excitation of triple resonant modes accompanied with good impedance bandwidths at 2.4/3.5/5.8GHz.Furthermore, acceptable radiation characteristics can be obtained over the operating bands.In the antenna design, the middle L-shaped slot (named Slot1) with horizontal part  1 and the vertical part  1 controls the resonant mode at 2.4 GHz, of which the notch is used to improve the impedance match of the proposed antenna.The L-shaped slot has smaller dimension compared to the common open-ended slot for the same resonant frequency, which      is more conducive to the miniaturization of antenna design.

Antenna Design
The resonant frequency  1 can be roughly approximated by In order to be compact in size as operating, an openended slot is embedded near Slot1 to have minimum antenna dimension for compact operation.The open-ended slot (named Slot2) occupying  2 ×  2 acts as the excitation of the second resonant mode centered at 3.5 GHz.The resonant frequency  2 can be roughly approximated by To introduce the upper resonant mode at 5.8 GHz-WLAN, another L-shaped slot (named Slot3), comprised of the horizontal arm  3 and the vertical arm  3 , is etched on the right side of the ground, which also has good effect on 3.5 GHz mode bandwidth enhancement.The resonant International Journal of Antennas and Propagation    frequency  3 has the same exciting principle as the resonant frequency  1 ; that is, Finally, owning to another small coupling L-shaped slot (named Slot4) is introduced aiming to improve the impedance match of resonant mode centered at 5.8 GHz and further broaden the bandwidth of 3.5 GHz.On the other hand, the slots are electromagnetically fed by a 50 Ω L-shaped microstrip feed line on the other side of board.
An electromagnetic software package, Ansoft HFSS based on the finite element method, is used for required numerical analysis to examine the performance of the proposed antenna design.By iterative trials, the final optimal dimension values are obtained and listed in Table 1.

Results and Discussions
A prototype of the proposed antenna was experimentally fabricated and measured to support the EM simulation.Figure 2 presents the photograph of the fabricated antenna, and a 50 Ω-SMA connector is used to feed the antenna.The simulated and measured reflection coefficients (| 11 |) of the proposed triband slot antenna are illustrated in Figure 3. Reasonable agreements between the simulation and measurement results are attained.Some slight discrepancies between them may be attributed to measurement errors, inaccuracies in the fabrication process, and the impact of the SMA connector.As observed and measured, the proposed antenna achieves three resonant modes over the frequency ranges of 2.35-2.55GHz, 3.28-4.15GHz, and 5.5-6.1 GHz for | 11 | < −10dB, simultaneously covering the 2.4/5.8GHz-WLAN and 3.5 GHz-WiMAX operation bands.
To further examine the effect of each slot on the proposed antenna's triple-resonance situations, the frequency response of |S 11 | for proposed antenna with different slots embedment is analyzed and presented in Figure 4.For the case of the antenna only with Slot1 (Case1), the fundamental mode around 2.5 GHz is excited.As for the antenna with Slot1 and Slot2 in Case2, the resonant frequency around 2.5 GHz in Case1 is shifted to lower band centered at 2.4 GHz and the second resonant situation around 3.5 GHz is emerged, while the bandwidth cannot cover the bandwidth of WiMAX system.When Slot3 is inserted into the antenna of Case2, the bands of 3.5 GHz are broadened to satisfy the bandwidth requirements of WiMAX system and the upper resonant mode at 5.8 GHz-WLAN is excited though the matching condition is worse than the situation of the proposed antenna.Slot3 is not only as the exciting of the resonant mode at 5.8 GHz but also as a coupling slot which is equivalent to provide additional coupling current path to broaden the impedance bandwidth of 3.5 GHz.As for the antenna in Case4, an interesting phenomenon that the impedance bandwidth of resonant mode centered at 3.5 GHz is broadened to 3.2 GHz-4.38 GHz emerged, which shows that Slot4 has the same effect as Slot3 on broadening the bandwidth of resonance frequency 3.5 GHz.In the proposed antenna case, triple resonant frequencies are achieved; meanwhile, better impedance match situation at 3.5 GHz-WiMAX and 5.8 GHz-WLAN operation bands is obtained than the antenna in Case3.
For better understanding the excitation behavior of the proposed antenna, the surface current distributions at 2.44 GHz, 3.5 GHz, and 5.8 GHz, obtained from the HFSS simulation, are studied and displayed in Figure 5.For the lower band at 2.44 GHz, the large surface current density could be observed along Slot1.Figure 5(b) shows the simulated current distribution of Slot2 at the operating frequency of 3.5 GHz.It is observed that the current distribution is mainly along the side of Slot 2, maximum one is emerged at point A and the minimum one at point B. For the higher band at 5.8 GHz, the surface current distribution in Figure 5(c) is similar to that of Slot1 at 2.44 GHz band.Thus, both from | 11 | responses and surface current distributions, the function of the related geometrical mechanism on the impedance matching condition of the three resonant modes is clearly presented.
According to the observed phenomena in current distribution and frequency response of | 11 | in different cases, some vital parameters are analyzed on resonant frequency bands.Figure 6 presents the tuning effect of length  1 on the | 11 | curves of the resonant mode at 2.44 GHz.As the length  1 is increased, the center resonant frequency of 2.44 GHz band is shifted towards lower frequency, and as the length  1 is decreased, the center resonant frequency of 2.44 GHz band is shifted towards higher frequency.The reason for this is that the longer  1 is, the longer wavelength at relevant resonant frequency is, and vice versa, which conforms to theoretical formula (1).Similarly, the changing of  2 and  3 has the same impact on the resonant point of 3.5 GHz band and 5.8 GHz band, respectively, as shown in Figures 7 and 8.
The measured and simulated radiation patterns in main cut planes at 2.44 GHz, 3.5 GHz, and 5.8 GHz are shown in Figure 9. From an overall view, the antenna behaves in  nearly omnidirectional performance at -plane.Reasonable agreements between the simulation and measurement results for   are attained.The measured peak gain of the proposed antenna is depicted in Figure 10.As observed, acceptable radiation performance is achieved at triple operation bands.16 × 30 mm 2 and exhibits triple-frequency resonant performance at 2.4/3.5/5.8GHz.Furthermore, acceptable radiation characteristics are obtained across the operation bands.Consequently, the proposed slot antenna design could be promising and suitable for 3.5 GHz WiMAX and 2.4/5.8GHz WLAN applications.

Figure 1
Figure 1 depicts the configuration of the proposed triple-band slot antenna.The antenna is fabricated on a thin FR4 substrate of thickness 1.0 mm, with relative dielectric constant of 4.4.In the antenna design, the middle L-shaped slot (named Slot1) with horizontal part  1 and the vertical part  1 controls the resonant mode at 2.4 GHz, of which the notch is used to improve the impedance match of the proposed antenna.The L-shaped slot has smaller dimension compared to the common open-ended slot for the same resonant frequency, which

Figure 1 :
Figure 1: Geometry of the proposed triple-frequency slot antenna.

Figure 2 :
Figure 2: Photograph of the fabricated prototype.
(a) 2.44 GHz B A (b) 3.5 GHz J surf (A per m)

Figure 5 :
Figure 5: Surface current distribution at different frequencies.

Figure 9 :
Figure 9: Radiation patterns in main cut planes.