Study Of Unidirectional Dipoles Antenna

Dipole antennas have excellent radiation characteristics,such as wide impedance bandwidths,high gains,and stable radiation patterns,and have been widely used in wireless communication.Although the dipole antenna has been proposed for over a century,the research on it is still enduring,and many new kinds of dipole antennas have been proposed.Recently,the research about dipole antenna is basically focused on ensuring stable radiation characteristics by utilizing the complementarity theory.There are three main methods: 1.Magnetoelectric dipole antenna,that combines electric dipole and magnetic dipole in an antenna element;2.Cross-dipole antenna,which is equivalent to place two electric dipoles vertically and crosswise to achieve the complementary radiation pattern in two polarization planes;3.Multi-dipole antenna,that combines electric dipole and an in-phase binary array to obtain the stable radiation pattern.In this thesis,the unidirectional dipole antennas are studied for application in the base stations of mobile communication.Based on the complementarity theory,seven unidirectional dipole antennas are proposed and designed in the thesis,and their working principles are elaborated in detail.All antennas are manufacutured and measured to verify the rationality of the design.The main work of this thesis is as follows.1.The properties of loop dipole are studied.The characteristics of the linear dipoles,the planar dipoles and the loop dipoles are similar.The difference between the linear dipoles and the planar dipoles is the transverse widths,and the loop dipoles are obtained by digging out the middle part of the planar dipoles.It is well known that the impedance bandwidth of dipole antenna is broadened with the transverse width of the dipoles increasing,thus the dipoles antenna with wide strip has wider the impedance bandwidth than linear dipoles antenna.However,a wider transverse width will lead to increasing of the dipole's transverse current,which worsens the cross-polarization of the antenna.In order to block the transverse current on the planar dipoles,the middle part of the wide strip dipoles is excavated to form the loop dipoles.Based on this method,three loop multi-dipole antennas are designed by combining the loop dipole with the in-phase binary array.Two of them are the dual-polarized antennas,one of which is fed by differential signals.All multi-dipole antennas have excellent radiation characteristics.Compared with the linear dipole antennas,the impedance bandwidth is greatly enhanced,and the good cross-polarization characteristics are also retained.2.A crossed multi-dipole antenna fed by capacitive coupling is proposed.This antenna is essentially a multi-pair cross-dipole antenna.A pair of large crossed loop dipole dominates the low frequency performance,while a pair of small crossed loop dipole controls the high frequency performance.The two pairs of large and small loop dipoles are fed by capacitance coupling simultaneously,and the feeding device is a pair of Y-shaped feeders and H-shaped couplers.The equivalent circuit model of the antenna is given and its working principle is described.The dipole antenna has four resonant frequencies,so the bandwidth is greatly broadened.In addition,the dipole antenna loaded with the complementary split-ring resonators is investigated,which can increase the bandwidth of the antenna and adds a resonant frequency.3.A folded-loop dipole antenna is proposed.The folded-loop dipoles have been seldom used in the design of the antenna due to the reverse current on them,which will greatly damage radiation performance of the dipoles.In the design of the antenna,a big octagonal loop patch is used as a parasitic element to suppress the reverse current on the folded-loop dipoles.The current on the parasitic unit and the reverse current on the loop folded-dipole form a loop circuit,and they cancel out.Thus the radiation characteristics of the folded-loop dipole are greatly improved.This antenna has a wide beamwidth and is suitable the base-station applications.4.A novel 45° dual-polarized cross-dipole antenna is proposed.Compared with the traditional cross-dipole antenna,the antenna has wider impedance bandwidth and four resonance frequencies,in contrast to two resonance frequencies achieved by the traditional cross-dipole antenna.In convention,the loop structure is used as the cross-dipole element.In the design of this antenna,a special structure is adopted as the dipole element,and a part of the dipole element forms coplanar striplines.The coplanar striplines are equivalent to impedance transformers,thus the impedance bandwidth of the antenna is greatly broadened.