Slot Antenna Tames Three Wireless Bands

The antenna, which is printed on low-cost FR-4 substrate material with thickness of 0.8 mm and relative dielectric constant of 2.95, has overall dimensions of 34.5 × 40.0 mm. Simulation and measurement results show that the proposed antenna configuration can be applied to applications in wireless local area networks (WLANs), Fourth-Generation (4G) cellular equipment, and WiMAX devices with almost omnidirectional radiation patterns.

As wireless applications have spread, requirements have increased for sufficiently small printed antennas that can be produced at low cost. Increasing attention has been paid to antenna designs capable of handling two or more wireless frequency bands within one compact structure, although the frequencies of interest may vary depending on the geographic area and/or country.¹ For example, within the wireless communications bands, the 2.45-GHz frequency band is used by WLAN services and equipment, the 2.60-GHz frequency band is used by 4G cellular communications systems, and frequencies from 3.3 to 3.6 GHz are used by IEEE WiMAX systems.

Various types of traditional multiband antennas have been proposed and published for multiband wireless communication applications.^2,3 Many can be categorized as the elliptical slot antennas, compact slot antennas, and antennas with dual-band-notched characteristics.^4-6 To enhance or notch the impedance bandwidth of these multiple-frequency-band antennas, slots with different shapes are used.^7,8

In the current article, a novel tri-band slot antenna is proposed, featuring a simple architecture and performance that is well suited for wireless applications. Two U-shaped slots are used to produce dual-band resonant modes. The proposed antenna employs two L-shaped slots in the ground plane to broaden the impedance bandwidth, which can easily cover the WLAN, 4G cellular, and WiMAX bands.^9,10

This antenna offers promising features, including good impedance matching over the full operating frequency range and stable radiation patterns. In the analysis of the design, good agreement has been achieved between computer simulations and measurements of fabricated antennas.

Figure 1 shows the configuration of a triband slot antenna as well as the coordinate axis. The antenna is printed on FR-4 substrate with thickness of 0.8 mm, relative permittivity of 4.3, and loss tangent of 0.02. The antenna is fed by means of a 50-Ω signal strip transmission line. Two U-shaped slots in the rectangular radiating patch generate two resonant modes, while the other resonant frequency is produced by the Z-shaped strip. The L-shape slots help enhance the impedance bandwidth.

The antenna was simulated and optimized with the help of the High-Frequency Structure Simulator (HFSS) electromagnetic (EM) simulation software from Ansoft using the finite-element method (FEM). The table shows the antenna parameters and their optimized values (in mm).

Figures 2(a) and (b) illustrate the current distributions of the proposed triband slot antenna at different frequencies. As shown in Fig. 2, the U-shaped slots increase the current routes to achieve two resonant bands while the third band is achieved by the Z-shaped strip at the back of the substrate. Figure 3 plots the antenna’s measured and simulated S₁₁ responses as functions of frequency. As these responses show, the simulations agree closely with the measured results at the three frequencies of interest: 2.45, 2.64, and 3.44 GHz.  

Figure 4 shows radiation patterns for the prototype antenna measured at 2.45, 2.64, and 3.44 GHz. The radiation patterns in the xz-plane and yz-plane correspond to the antenna’s E-planes, while the radiation patterns in the xy-plane correspond to the antenna’s H-plane.

At low-band frequencies, a figure-eight radiation pattern is obtained in the E-plane, while a nearly omnidirectional radiation pattern is associated with the H-plane. Although the patterns obtained for this antenna are not as good as those for a simple conventional monopole antenna, they are close to monopole-like patterns for a multiband configuration.

In Figure 5, the measured gain varies across a range of about 2.24 to 4.89 dBi in the first frequency band, about 1.71 to 4.70 dBi in the second frequency band, and about 3.12 to 4.19 dBi in the third frequency band. However, the gain variations extend more than 1.0 dB across the operating bandwidths in the first and second frequency bands.

Because antenna gain is a function of its electrical dimensions relative to the wavelength of interest, current distribution, and radiation pattern, the proposed antenna has higher gain in the lowest-frequency band. Figure 6 presents a photograph of the prototype fabricated triband antenna.

In short, the antenna is a compact design that shows great promise for handling multiple frequency bands in wireless applications. By using two U-shaped slots and a Z-shaped slot, it achieves three resonant operating bands on low-cost circuit-board material.

Owing to the L-shaped slot in the ground plane, the bandwidths are enhanced further. Simulations and measurements have indicated that the proposed antenna can be effectively used for WLAN, 4G cellular communications, and WiMAX applications.