Design of a Multi-band Aperture Coupled Microstrip Antenna for Mobile Communication Base Station

Abstract

This dissertation is dedicated to the design and implementation of novel wide and multi-band aperture coupled antenna and a 1 x 4 linear microstrip antenna array for use in a Cellular (824-894MHz) / PCS (1750-1870MHz) / W-CDMA (1920-2170MHz) base station for mobile communication services. When a communication system need to operate at more than two frequency bands very far apart, a multi-band antenna structure can be implemented to avoid the use of separate antennas, which will bring numerous merits, such as, compact size, low cost and ease of installation. In this dissertation, to achieve wide and multi-band functionality, the aperture coupled method was selected and to obtain a small patch size (~ 0.25 lo, ?o is free space wavelength), two slits at patch were used along the radiating edge. The aperture operates not only as a feeding purpose but also as a resonator at the additional resonant frequency. By adding the reflector patch to the antenna, the FBR (Front-to-Back Ratio) was improved. A single microstrip antenna aperture coupled with a dual offset microstrip feed structure, which easily achieves impedance matching with a high level of coupling, is proposed in this dissertation. In order to apply this antenna to a commercial base station antenna, which requires increased gain, an array-type antenna must be considered. Firstly, a 1 x 2 microstrip antenna array was proposed. The optimal spacing between the radiating elements was investigated by employing a stacked approach to form the array. Second, a 1 x 4 microstrip antenna array was configured by four aperture coupled microstrip antennas arranged in a stacked method, too. To verify the unique array characteristics of the proposed multi-band microstrip antenna, the electrical characteristics of single, 1 x 2 and 1 x 4 microstrip antenna arrays were compared. Finally, 1 x 4 microstrip antenna array was placed in a UV (Ultra Violet) stabilized ABS (Acrylonitrile Butadiene Styrene) radome casing for a 3-sectorized base station antenna In this dissertation, the multi-band aperture coupled microstrip antenna and 1 x 4 microstrip antenna arrays for a Cellular/PCS/W-CDMA base station are studied by both simulation and fabrication methods. It was discovered that the impedance bandwidth of the microstrip antenna can be greatly increased by changing the aperture coupled characteristics. This proposed microstrip antenna provides two wide -10 dB impedance bandwidths (VSWR≤1.5) covering, about 9% (77MHz) of the Cellular band and 39% (760MHz) of the PCS/W-CDMA bands. Further, a peak antenna gain of 8.3 dBi in the Cellular band and a peak gain in the range of 6.3 to 7.7dBi in the PCS/W-CDMA band were observed. The aperture coupled microstrip antenna originally has the worst FBR characteristics due to its own wide aperture. To improve the FBR, a reflector patch was added near the bottom of the ground substrate without harm to performance. The FBR improved by 12 dB and 5 dB in PCS and W-CDMA band, respectively. In particular, the characteristics of the 1 x 4 microstrip antenna array improved significantly by the addition of a reflector patch and backside shields. The fabricated 1 x 4 microstrip antenna array showed increased peak gain, ranging from 12.4 to 14.6 dBi, and superior FBR over the 27dB on the whole band. The performance of this antenna was reviewed to find the possibility of using it in commercial applications form electrical, mechanical and economical viewpoints. The proposed microstrip antenna and array maintained wide and multi-band impedance bandwidths in the various configurations in this dissertation: therefore it would be feasible to serve this antenna as base station antenna to provide multi-band services in almost all Cellular / PCS / W-CDMA mobile communication systems.