In the thesis, we designed a metamaterial radome to increase the antenna gain. Owing to the need of high-directivity radiation in fix-point communications, antenna array and reflective antenna had been used to increase the directivity of antenna traditionally. Complicated feed and huge antenna size are the disadvantages of these techniques. We proposed a simpler metamaterial radome to increase the antenna gain. We find the subwavelength-hole structure formed by four Jerusalem cross structures can collimate electromagnetic wave originally spreading out from the holes. With the same size, multiple subwavelength holes in metamaterial radome can further enhance the antenna gain. We showed that metamaterial radome with 9 subwavelength holes can improve the gain by about 3.5 dB. In addition, we applied the concept of Fabry-Perot Cavity (FPC) to find the suitable distance between the radome and the antenna. When the resulting electromagnetic waves are in-phase, the radome can increase the antenna gain effectively. Recently, high-directivity radiation in fix-point communications is required and in the meantime multi-mode communication systems have become more and more popular. For practical purposes, we designed a coplanar dual-band metamaterial radome to be operated at 2.5 GHz and 3.5 GHz for WiMAX. This structure allows adjustment of its characteristics independently at each band. This coplanar dual-band radome can enhance the antenna gain by about 1.74 dB and 2.08 dB at 2.5 GHz and 3.5 GHz, respectively.

In the thesis, we use the concept of single-negative metamaterials to reduce the antennas¡¦ coupling. Firstly, the multilayer insulator is proposed to enhance the isolation bandwidth. The isolation bandwidth is broadened by adjusting the individual layer of insulators with close but different operating frequencies. Then, the designed multilayer insulator is inserted in a planar antenna system. Isolation of the MIMO antenna system is below than -20dB. The measured and simulated isolation bandwidth is 8% and 6.9%, respectively. We then design dual-band insulators for dual-band MIMO antenna applications. The proposed dual-band insulator is implemented bystacking the insulators with different operating bands and the isolation of the dual-band MIMO antenna can be improved at both 2.6 and 3.5GHz bands. In the broadband insulator design, the T-shaped branch is proposed to broadenthe operating bandwidth. The measured isolation bandwidth is improved by 12.8% than that of the multilayer insulator. The bandwidth of the proposed broadband insulator can be used in other broadband communication standards.