Abstract
A low-cost, easy-to-fabricate, wideband and high-gain dual-polarized array antenna employing an innovative microstrip slot patch antenna element is designed and fabricated. The design parameters of the antenna are optimized using commercial softwares (Microwave Office and Zeland IE3D) to get the suitable -parameters and radiation patterns. Finally, the simulation results are compared to the experimental ones and a good agreement is demonstrated. The antenna has an approximately bandwidth of 14% (5.15–5.9 GHz) which covers Worldwide Interoperability Microwave Access (WiMAX)/5.8. It also has the peak gain of 26 dBi for both polarizations and high isolation between two ports over a wide bandwidth.
1. Introduction
Recently, microstrip patch antennas are one of the most commonly used antenna types due to many advantages such as light weight, low fabrication costs, planar configuration, and capability to integrate with microwave integrated circuits. Thus, the patch antennas are very suitable for various applications such as wireless communication systems, cellular phones, satellite communication systems, and radar systems [1–6]. Due to their inherent features they are found attractive for applications in broadband networks. WiMAX is a standard-based wireless technology for broadband networks providing high data rate communication by using low-cost equipment. The access points in this network are usually built with large physical spacing. Therefore, the high-gain antenna is necessary to execute the long distance transmission with a lower power. WiMAX has three allocated frequency bands called low band (2.5 GHz to 2.8 GHz), middle band (3.2 GHz to 3.8 GHz), and high band (5.2 GHz to 5.8 GHz). In this work, the low-cost microstrip slot array antenna () is designed, simulated, and fabricated for operation in the frequency band of 5.15 GHz to 5.9 GHz. In each antenna element, two rectangular slots are used for coupling the microstrip feed lines to the radiating patch. This antenna has high isolation between the two ports over a wide bandwidth more than 14%. Furthermore, this high-gain (25.5 dBi) array antenna has dual polarization with a minimum half-power beamwidth (HPBW) (vertical: 7°; horizontal: 6°). The impedance characteristics, radiation pattern, return loss, and isolation between two ports for the designed array are analyzed, simulated, and optimized using Microwave Office and Zeland IE3D softwares. Also, , , and radiation pattern are measured and compared to the simulated ones.
2. Configuration of Element Antenna
Microstrip patch antennas can be excited by different types of feeds. In order to achieve the desired performances of WiMAX antenna, an aperture coupled feed is used because of its good characteristics such as wide operational bandwidth and shielding of the radiation patches. Moreover, an aperture coupled feed yields better gain and radiation pattern for a dual-polarized antenna aimed for wireless applications [7–12]. An exploded view of the dual-polarized microstrip antenna and a simplified equivalent circuit model for an aperture coupled microstrip antenna are shown in Figure 1. The antenna consists of only one substrate (Rogers TMM 4 with dielectric constant ), an air layer for enhancing the bandwidth, and a radome. The input impedance of the antenna at the center of the slot is given by [13, 14] where is the patch admittance and is the aperture admittance (Figure 1(b)). , , and are the microstrip line parameters in this equation. Also the coupling of the patch to the microstrip line is described by a transformer [14]. The dimensions of the element antenna such as slots, feed lines, circular patch, and spaces between them are optimized with the use of IE3D to achieve best radiation characteristics, wide impedance bandwidth, and high isolation between two ports. The optimized element antenna has a circular patch with 11.89 mm radius positioned at the bottom side of Rohacell. Furthermore, two 50 ohms microstrip feed lines ( mm, mm, and mm) at the bottom side of the substrate (Rogers TMM 4 with mm, ) are electromagnetically coupled to circular patch through two rectangular slot apertures in the common ground plane. As shown in Figure 2, in order to reduce the antenna back lobes, a metallic plate is located at the back of antenna, for example, 22 mm from the bottom of the antenna structure. Additionally, Figure 3(a) shows the simulated return loss for two ports ( and ) versus frequency for one element antenna and Figure 3(b) shows simulated gain against frequency for one element. As depicted in Figure 3, in the desired bandwidth (5.1–5.9) return loss for both polarizations is more than 15 dB and the isolation between two ports () is better than 35 dB.
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3. Array Antenna
To obtain the desired radiation pattern characteristics, an planar microstrip slot array antenna is designed (Figure 2(a)). The bottom side of substrate consists of the feeding network which is designed to give equal amplitude and phase to each element (Figure 2(b)). Additionally, by using T-junction design and a quarter-wave matching transformer (Figure 2(c)), the feeds are matched to 50 ohms feed line [15, 16]. To provide a match, the transformer characteristic impedance should be , where is the characteristic impedance of the input transmission line and is the input impedance of the antenna. The transformer is usually another transmission line with the desired characteristic impedance (Figure 2(d)). The spaces between elements are set at 50 mm for better radiation characteristics. The simulated and measured return loss () and isolation () of dual-polarized microstrip patch slot array antenna are illustrated in Figures 3(c) and 3(d). Furthermore, the metal plate at the back of array antenna reduces the front-to-back ratio about −20 dB, as can be seen in Figure 4. Likewise, the gain of the array antenna in different frequencies is demonstrated in Figure 5. Moreover, the simulated and measured and plane far-field radiation patterns of the array antenna at center frequency are shown in Figures 6 and 7. Finally, all vital parameters such as antenna size, its gain, beamwidth, side lobe level, and front-to-back ratio are summarized in Table 1.
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4. Conclusions
This paper has reported the design of a low-cost high-gain dual-polarized patch array antenna for WiMAX applications in the 5.15–5.9 GHz frequency band. The antenna has an approximately bandwidth of 14% and the peak gain of 26 dBi for both polarizations. The design has been achieved with the use of commercial software packages AWR Microwave Office and Zeland IE3D. The design process aimed at best return losses and fine quality radiation characteristics over the assumed frequency band. The designed antenna has an impedance bandwidth of approximately 14% and the peak gain of approximately 26 dBi for both polarizations. This performance has been confirmed experimentally.