Dual Ban MIMO DRA

Abstract

Multiple Input, Multiple Output (MIMO) antenna technology is being used extensively to meet the current wireless standards. It consists of multiple transmit and receive antennas to increase the signal capturing power and throughput. However, to achieve these goals, it is necessary to keep the coupling between antenna elements in an acceptable range so that multiple signals can effectively be resolved. In most of the existing MIMO antennas, separate antenna elements are used for occupying spatial diversity technique. In such a diversity scheme, isolation enhancement techniques may effectively be applied between these antenna elements. However, in spatial diversity, multiple antenna elements occupy more space to be accommodated in the devices with size constraints. Moreover, at higher frequencies, losses of metallic antennas become severe which significantly degrade the performance of the MIMO system. Recently, a possible alternative to these metallic antennas has been introduced in the form of dielectric resonator antennas (DRAs). DRAs offer potential advantage of high radiation efficiency and negligible conductor losses. DRA supports more than one resonant modes at different frequency bands, enable to meet the requirements of different applications with a unique device. Multimode excitation feature in the DRA makes it a suitable choice for use in multiband MIMO applications. However, more critical and challenging issue is coupling between modes excited in the single DR volume, especially when used for dual-band MIMO applications. This is the reason that dual-band MIMO DRAs are rarely found in literature. This thesis is composed of two single-band and four dual-band MIMO DRA designs. The first single-band MIMO DRA is excited by means of symmetric microstrip feeding and other DRA is excited by means of hybrid feeding mechanism. It is observed that hybrid feeding provides more isolation between antenna ports. This concept is extended to the dual-band MIMO DRA designs using similar and hybrid feeding mechanisms. The first dual-band MIMO design consists of symmetric microstrip slot feeding for pattern diversity at both WiMAX and wireless local area network (WLAN) bands. In the second design, compactness is achieved by stacking the DRA with high a permittivity material. Orthogonal modes at the two bands are

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excited in the DRA by coaxial probes and coupling between two ports is reduced by means of defected ground structure (DGS). Almost 80 % volume reduction has been achieved in this design by means of stacking approach. The last two designs presented in this work are based on hybrid feeding mechanism. Both of the dual-band MIMO DRAs have also been investigated for geometric scalability of the designs. As the resonant frequency of the DRA is inversely related to its dimensions, therefore, same design with different dimensions can be operated on other frequency bands as well. This property is termed as geometric or size scalability of the design. This re-sizing or scalability of the dual-band MIMO DRAs is a very interesting property and has not yet been investigated in literature. Frequency ratio is the ratio of higher band to the lower band which is an important parameter in the dual-band designs. Frequency ratio as a result of design scalability has also been presented in this work