| With the rapid development of mobile communication systems,the fourth generation mobile communication system(4G)has already been commercialized on a large scale.In order to take the previous 2G/3G communication systems into account,the base station antenna,as the frontend device of a mobile communication system,plays a significant role and should meet more requirements.However,the most basic requirement is that its impedance bandwidth needs to be able to cover 2G/3G/4G LTE band(i.e.1710-2690 MHz).At the same time,with the increasing number of mobile users,the number of base station antennas should be increased to ensure the quality and speed of communication services.Unfortunately,this will bring the trouble for installation.In addition to this,the public's fear of radiation of base station antennas also cause the difficulties to construct more base stations.As a result,those operators are in an increasing need of wideband and miniaturized base-station antennas so as to cover more frequency bands and reduce the cost of installation and operation,and to realize convenient construction,high concealment,as well as to overcome the psychological panic of the public.On the other hand,since the circularly polarized antenna can accept any linearly polarized electromagnetic waves,and different types of circular polarization(left-hand and right-hand)are isolated from each other,the circularly polarized antenna has been widely used in the GPS system,Beidou system,as well as WIFI and RFID systems.Specifically,in the satellite positioning and navigation system,in order to receive circularly polarized electromagnetic waves in any place of the earth,the circularly polarized antenna need to achieve a wide axialratio beamwidth of more than 120 o.The mail contributions of the dissertation can be summarized as following: 1.Three wideband dual-polarized base-station antennas is proposed.By loading small loops,or bent stubs,or straight metallic trace and notch into the traditional crossed loop dipoles,the impedance bandwidth of the antenna can be improved significantly to cover modern frequency bands of 2G/3G/4G LTE systems.2.Two miniaturized dual-polarized antenna elements with independently controllable resonant modes are presented,and both of which have been developed into antenna arrays to face the practical applications.Based on the traditional crossed loop dipole,by embedding a pair of straight dipole into the gap of them,the higher resonant mode of the antenna can be controlled independently.By changing the traditional loop dipole into a stepped-width loop dipole,the lower resonant mode can be controlled independently and a miniaturized antenna structure is achieved.By inserting meander lines into a traditional loop dipole,the lower resonant mode can also be controlled independently,and the lower resonant frequency can be shifted down without increasing the antenna size,resulting in a miniaturized antenna structure.3.A dual-polarized base-station antenna based on stub-loaded dipoles is proposed.By loading an inverted-L shaped stub to a straight dipole,a new resonant mode located at the higher frequency of the primary 0.5? resonant mode of the dipole is formed;thus,the impedance bandwidth of the straight dipole can be enhanced.Based on the stub-loaded dipole,a dualpolarized antenna with several good performances is presented for base-station applications.4.Three circularly polarized antennas with wide axial-ratio beamwidth are proposed.By employing straight dipoles with different lengths,or different types of stepped-width dipoles,two self-phased circularly polarized antennas are realized.At the same time,the property of wide axial-ratio beamwidth is achieved by the using of a cavity-backed reflector.On the other hand,by loading four parasitic elements to a simple circularly polarized crossed-dipole antenna,the axial-ratio beamwidth of this antenna can be enhanced significantly with properly adjusting the length and position of these parasitic elements. |
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