| In the era of technology advancement, with the increase of communication speed and capacity and the development of communication equipments, conventional narrowband antennas can’t meet the demand of modern communication, which are being replaced by wideband antennas with wider bandwidth and smaller dimensions. Meanwhile, referring to modern radio monitoring, EMC testing and broadcasting testing, the working frequency ranges from hundreds of megahertz to tens of gigahertz. The utilization of conventional narrowband antennas both increase equipment cost and decrease the working efficiency greatly, since antennas of different frequencies are set up to meet the corresponding measurement demands. Therefore, wideband antennas are widely used in the fields mentioned above. To accommodate the development of wireless communication in the future, wide bandwidth, miniaturization, polarization and radiation pattern stability, gain stability, etc. should all be considered. Since the characteristics of antennas are determined by dimensions to a great extent, which is an objective fact, the compromise between electric performance indexes and dimensions should be made by following objective laws.Initially, antenna structures with the potential of ultra-wideband impedance bandwidth are analyzed theoretically and compared, which include gradient structures in the semi-open planar form, ridged waveguide structures in the closed form, and bi-conical structures in the open form. The mechanization of power transmission in the ultra-wide bandwidth is focused on, providing the analytical results as the theoretical foundation of antenna analysis and design in the following chapters.Secondly, a novel planar loaded horn antenna is proposed with low profile, wide radiation pattern bandwidth, high gain and low sidelobe level. The simulation and experimental results show that the rectangular slot loaded on the conventional antipodal Vivaldi antenna along the axis direction decreases the lowest working frequency by9%, from4.4GHz to4GHz, while the high frequency end is not influenced; planar dielectric lens loaded at the aperture of the antipodal Vivaldi antenna with rectangular slot can widen the antenna’s radiation pattern bandwidth by decreasing the aperture phase error in the high frequency band which leads to serious radiation pattern distortion in conventional antipodal Vivaldi antennas; Choke slot loading is introduced at the aperture of the antipodal Vivaldi antenna with rectangular slot and lens loading to decrease the effect of the transverse current at the end of the antenna radiation flare angle on the radiation pattern to improve the radiation gain in the high frequency band and lower the sidelobe level further. The measurement of the novel planar horn antenna shows that the working frequency band is4-30GHz with gain ranging from5dBi to8dBi, the front-to-back ratio of13dB and no distortion.Thirdly, a novel miniaturized double-ridged horn antenna with wide radiation pattern bandwidth, high gain and low sidelobe level is proposed. The simulation and experimental results show that a pair of metal slots loaded on the ridges of the double-ridged horn antenna can improve the stability of the port impedance and widen the impedance bandwidth of the antenna effectively. Compared with conventional ridged antennas, the lowest working frequency is lowered by60%, from5GHz to2GHz; a dielectric lens with hyperbolic-cylindrical boundary calculated by Fermat Principle is loaded at the antenna aperture to improve the phase distribution at the aperture and widen the antenna’s radiation pattern bandwidth greatly. Compared with the antenna without lens loading, the highest working frequency is increased by82%, from22GHz to40GHz; a wedge metal block is loaded inside the feeding waveguide of the ridged horn to restrain the higher modes, maintain the stability of the dominant mode, and widen the gain bandwidth greatly. Compared with the antenna without wedge metal block loading, the highest frequency of the gain bandwidth is increased by82%, from22GHz to40GHz; modal analysis is carried out to explain the mechanism of the gain bandwidth enhancement initially. The measured results of the proposed double-ridged horn antenna show that the working frequency band is3.2-40GHz with gain ranging from5dBi to20dBi, the front-to-back ratio of higher than15dB and no distortion.Finally, a novel dielectric lens loaded bi-conical horn antenna with cones of different central axes is proposed. Based on a miniaturized dielectric loaded omni-directional bi-conical antenna, the novel antenna is realized by inclining the cones and cutting the back side of the cones. The simulation and experimental results show that the bi-conical antenna can realize ultra-wideband impedance bandwidth with fractional bandwidth of20:1(1-20GHz). The dimension of the omni-directional bi-conical antenna can be decreased greatly by dielectric loading. Maintaining the impedance bandwidth unchanged, the diameter of the cone underside can drop from150mm to96.6mm, by35.6%, which guarantees the bandwidth and miniaturization of the bi-conical horn antenna; the directivity of the antenna can be increased by inclining the cones and focusing the beam; the antenna gain can be enhanced further by loading a specially-designed dielectric lens with elliptic-cylindrical boundary. The measurement of the novel bi-conical horn antenna shows that the working frequency band is2-20GHz with gain ranging from3dBi to17dBi, front-to-back ratio of higher than15dB and no distortion. The dissertation mainly focuses on the characteristics of ultra-wideband horn antennas. Planar horns, ridged horns and bi-conical horns are analyzed intensively by theoretical analysis, simulation and experiments. The instability of the above-mentioned antennas’ parameters in the working frequency band is improved by kinds of loading methods such as dielectric loading, to provide more thorough and effective theoretical foundation, experimental demonstration and optimization design methods for ultra-wideband horn antennas. |
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