Research And Application Of Multi-dipole Antennas

As the Fourth-Generation mobile communication has emerged into the industry and the Fifth-Generation will further propagate rapidly, multi-dipole antennas will play a more and more important pole in communication systems. By controlling relationship among amplitude, phase and positions of multiple dipoles, multi-dipole antennas can be widely used in base-station antennas, circularly polarized antennas, multi-input multi-output(MIMO) antennas and Yagi-Uda antennas. The main contributions of the dissertation can be summarized as follows.(I) A novel method to design base-station antennas with stable radiation patterns in a wide frequency band is presented. Through rational design positions of dipoles and a reflector, stable radiation patterns can be achieved. Two single polarized multi-dipole antennas were designed, fabricated and measured to verify the proposed method. Stable radiation patterns, wideband impedance matching and low cross polarization were achieved of the two antennas.(II) To reduce the effect of multipath fading, increase channel capacity of communication system and be compatible with fully integrated radio frequency(RF) front-end products, a differential-fed ±45° dual-polarized multi-dipole antenna was investigated. The half power beamwidth of the proposed antenna in about 65° at the horizontal plane, which is desirable in a three-sector base-station antenna.(III) Two novel methods to design ±45° dual-polarized base-station antennas with enhanced cross polarization ratio(XPD) are proposed. The first method to improve XPD is by addition of four horizontal parasitic elements around a simple ±45° dual-polarized base-station antenna. The second method is by adding four vertical parasitic elements between a simple ±45° dual-polarized base-station antenna and a reflective ground. Two ±45° dual-polarized base-station antennas were designed, fabricated and measured to verify the two proposed methods.(IV) Two new methods to design circularly polarized antennas with wide axial ratio beamwidth are displayed. The first method is designing circularly polarized antennas via two pairs of parallel dipoles in a square contour. After the spacing between two parallel dipoles is studied and set a proper value, wide axial ratio beamwidth can be achieved by placing the two pairs of parallel dipoles vertically and horizontally while setting a 90o phase difference between them. To reduce the whole size of circularly polarized antennas and improve their axial ratio beamwidth and power beamwidth, folded dipoles were employed instead of linear dipoles. The second method to enhance AR beamwidth is by addition of four vertical parasitic elements between a cross dipole circularly polarized antenna and a reflective ground. Three circularly polarized antennas were designed, designed, fabricated and measured to verify the two proposed methods.(V) A compact multiple-input multiple-output(MIMO) antenna with planar H-shaped directive antenna elements for WLAN/Wi MAX application is demonstrated. A printed H-shaped antenna without any reflectors and directors is applied as antenna element of the proposed MIMO antenna, but its radiation pattern exhibits a high front-to-back ratio in the azimuthal plane. Four those H-shaped antennas with compact size and directive radiation pattern are placed in a square loop array to obtain nearly orthogonal patterns. Therefore, a compact MIMO antenna with high isolation among those four elements is achieved. A laboratory model has been characterized experimentally, and the effectiveness of the proposed design in terms of theoretical achievable capacity is demonstrated.(VI) Yagi-Uda antennas with stepped-width reflectors are stated. The proposed reflectors were shorter than the driven element as a result of the stepped-width structure. An equivalent circuit to a dipole with a parasitic element was employed to explain the shortened length of the stepped-width reflector. Then the proposed stepped-width reflectors, shorter than the driven elements, were applied to design, fabrication and measurement of a single-ended Yagi-Uda antenna and a differential-fed Yagi-Uda antenna.