Wireless local area networks (WLANs) provide the means of Internet access for millions of users worldwide. The technology has undergone a great deal of change since its inception, requiring antennas that are versatile and can provide high performance in a compact size. By following a simple design strategy, it is possible to increase the gain of a printed dipole antenna so that it can support a variety of different WLAN standards.
The WLAN standards have evolved into versions with different data rates and frequency bands, and well established among consumers. A growing trend in WLAN technology is for its use in portable products1,2, requiring WLAN antennas capable of operating in multiband applications. In these applications, a half-wave dipole antenna offers low gain of about 2.15 dBi. A simple method to enhance antenna gain is to use a 1.25? dipole antenna. To better understand the potential for this design approach, a conventional 1.25? dipole antenna with high gain is at 2.45 GHz was studied. Although the dipole has reasonable performance in terms of resonant frequency, its bandwidth at the required WLAN frequencies is limited. In order to increase its operating bandwidth at higher frequencies, the dipole antenna was modified from a symmetric structure to an asymmetric configuration3,4.
In this modification, four slits were embedded on the ground plane and one slit was embedded on the feeding plane. The proposed dipole antenna has a low band of 2.38 to 2.6 GHz and a high band of 4.67 to 7.37 GHz in support of IEEE 802.11a/b/g WLAN systems. The maximum gain of the proposed antenna is 5.37 dBi, which is higher than that of a conventional dipole antenna.
The proposed antenna is fabricated on 0.4-mm-thick FR substrate with relative dielectric constant, er, of 4.4 and dissipation factor, tan d, of 0.025. A 50-Ohm mini coaxial line was used as the feedline for the antenna. Figure 1 shows schematic diagrams for a conventional dipole antenna and the proposed, modified dipole antenna. The length of the conventional dipole antenna is 153 mm, which is a wavelength of 1.25? at 2.45 GHz. But the bandwidth of a conventional dipole antenna is too limited to cover all WLAN applications, especially at the lower frequencies (Fig. 2). Therefore, four slits (BCDE) were embedded in the antenna, as shown in Fig. 1(b), with spacing of about 14 to 15 mm (about one-quarter wavelength at 5.2 GHz) between the slits, into the left arm of the antenna to increase the high-frequency bandwidth. The width of the slits was set at 2 mm.
On the right arm of the proposed dipole antenna, the slit A was embedded to compensate for the effect of frequency shift at the lower frequencies and to improve the impedance match at higher frequencies Fig. 1(b)>. The dimensions of the antennas are listed as follows: Lsub Wsub = 155 12 mm2; L1 = 76 mm; W = 9 mm; G = 1 mm; silts A, E, and D are L2 W2 = 2 7 mm2; slits B and C are L2 W1 = 2 8 mm2; and L3 = 47 mm.
In order to optimize the modified antenna, it is necessary to understand the influences of the slits embedded in the antenna and complete the design for use from 2.4 to 2.5 GHz in the low band and from 5.15 to 5.83 GHz in the high band. The first factor to study is whether the four slits (BCDE) increase the frequency bandwidth. In Fig. 3, the impedance matching of both the high-frequency and low-frequency regions approach their lower-frequency limits after the four slits are embedded. Table 1 shows that the frequency bandwidth increased from 30 to 70 MHz for the low band and from 340 to 610 MHz for the high-frequency band after the four slits are embedded. The dipole design with four slits has a wider frequency bandwidth than the dipole with no slits. The lower-frequency band also approaches closer to the desired 2.4-GHz WLAN lower-frequency band.
The next step in the design is to add slit A in the feed plane. In order to examine its influence on the performance, the slit is placed in different positions for various dipole designs. The results of this positioning are shown in Fig. 4 and Table 2. In the low-frequency band, the bandwidth increases with the addition of the feed slit. When L3 for the slit equals 47 or 52 mm, the frequency bandwidth reaches the requirement of WLAN systems. In the high-frequency band, impedance matching is poor at 5.8 GHz for L3 = 42 mm and at 5.15 GHz for L3 = 52 mm. When L3 equals 47 mm, the dipole's bandwidth meets suits multiple WLAN bands.
Figure 5 shows that the measured return loss meets the desired frequency bandwidth at both 2.4 to 2.5 GHz and 5.15 to 5.83 GHz when L3 equals 47 mm. The current distributions of the proposed antenna at 2.45, 4.20, 5.10, and 5.60 GHz are shown in Figs. 6(a), 6(b), 6(c), and 6(d), respectively. Figure 6(a) shows that the current distribution of the proposed antenna is triple at 2.45 GHz and Fig. 6(b) shows the current distribution of the proposed antenna is quintuple at 4.2 GHz because the lowest resonant frequency of the proposed dipole antenna is 800 MHz. The current congregates at the edge of the four slits, affecting the impedance matching at 5.1 GHz Fig. 6(c)>. The current distribution is reduced on the surface of the left arm and maintained constant on the surface of the right arm at 5.6 GHz as shown in Fig. 6(d). The current distribution was found to be affected by the slit on the right arm of the modified dipole antenna.
Figures 7(a), 7(b), and 7(c) show the radiation patterns of the x-y and y-z planes for the modified dipole antenna at 2.45, 5.15, and 5.8 GHz, respectively. The three frequency points in the x-y plane show omnidirectional behavior; however, the field patterns are twisted and shifted due to the symmetric structure of the antenna. In the y-z plane, the radiation patterns have the shape of six lobes because the length of the antenna is 1.25?. At 5.15 GHz, the radiation pattern has an asymmetric shape due to the four slits having a quarter-wavelength spacing at 5.2 GHz in the ground plane.
Figures 8 and 9 show that the measured gains of the modified dipole are better than 5 dBi from 2.4 to 2.5 GHz and more than 4 dBi from 5.15 to 5.83 GHz. These measurements validate that the addition of the slits provides high gain over the bands of interest for multiple WLAN standards.
This work was supported by Southern Taiwan University and the Solution Of Connector Antenna Co. (SOCAA), Ltd. Program (the development program for the dipole antenna and array). The authors gratefully acknowledge the support by Mrs. Kelly Lo and Dr. Dennis Tsai from SOCAA.
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