High-Performance Omnidirectional Antennas For Wireless Communication Systems

Omnidirectional antennas are able to receive and transmit signals in all directions in the azimuth plane to ensure a full coverage of the environment.Omnidirectional antennas can be widely used in radio broadcasting and mobile wireless communications systems.In wireless communication systems,antenna numbers are decreased and the system volume is miniaturized by using wideband antennas.Vertically-polarized(VP)omnidirectional antennas are more popular used than horizontally-polarized(HP)antennas in mobile wireless communication system.Base stations always transmit single polarized signals,such as VP signals.The multipath effect will cause polarization rotation during transfer process,and HP signals would be generated.In order to get good received signals,VP and HP antennas are always simultaneously installed on mobile devices.As an essential part of wireless communication systems,indoor distributed antenna systems(DASs)develop faster and faster and their data traffic increases significantly.In DASs,the communication operators have replaced the single polarized antennas by using dual polarized antennas and the upgraded the uplinks from 1×1 SISO to 1×2 SISO.In order to meet the high performance requirements of antennas in wireless communication systems and DASs,a series of omnidirectional antennas are investigated in this dissertation.The major works and contributions of this dissertation are summarized as follows.1.A new type of top shorted loading vertically-polarized omnidirectional antenna is proposed in this dissertation.These antennas are composed of monopoles and top loads.The loads are connected to ground plane with different numbers of shorted branches.The antennas with two,three and four shorted branches are designed in this dissertation.Effects of the shorted branch numbers on radiation patterns and gains are discussed.With the increase of shorted branch numbers,antenna patterns become more uniform and the gains decrease.The antenna with two shorted branches covers the frequency band from 0.8 to 3GHz.And the antennas with three and four shorted branches operate in the frequency range of 0.65-6GHz and 0.65-4.5GHz,respectively.It can be see that the antenna with three or four shorted has relatively wide bandwidth.In this dissertation,a low passive inter-modulation(PIM)antenna with three shorted branches is also designed.2.Based on the arc-shaped printed dipole and Y-shaped balun,six different horizontally-polarized omnidirectional antennas are designed.These antennas composed of 3,4 and 5 printed dipoles with corresponding wideband 3,4 and 5-way feeding networks and baluns.The printed dipoles are along a circle.In this dissertation,effects of the printed dipole numbers on omnidirectional characteristics are discussed.These antennas cover the frequency band from 1.7 to 2.7GHz with return losses larger than 15dB.Then,by adding arc-shaped loads along each printed dipoles,the return losses can reach at 20dB in the whole frequency band.Finally,straight strips and crossed strips are added between each adjacent dipole and the cross polarization performance of HP antennas by analyzing the antenna surface current.3.Two types vertical/horizontal dual-polarized omnidirectional ceiling mounted antennas are designed.Stable and good radiation patterns are obtained by soldering the HP element feed coaxial cable on VP antenna.First,a low profile dual-polarized omnidirectional antenna with high port isolation is designed.The VP element is a modified low-profile cone-shaped antenna shorted to the ground plane by four branches with a height of 45mm.The HP element is composed of three printed dipoles and three straight strips between adjacent dipoles.The distance between VP and HP elements is 40mm,and the total height of the antenna is 85mm.Its VP element covers the frequency band from 0.8 to 2.8GHz and HP element operates in the frequency range of 1.7?2.7GHz.The isolations between these two elements are larger than 38 dB in the whole overlapping frequency band.Then,a multiband high-performance dual-polarized omnidirectional antenna is designed.Its VP element is a modified paw-shaped discone antenna with three shorted branches.The HP element is a four-element printed dipole circular array matched by a feed network.A sleeve balun is added to reduce the common mode current along the HP feed coaxial cable,which improves the radiation patterns and enhances the isolation between the VP and HP elements.Its VP element covers the lower frequency band from 806 to 960MHz and the higher frequency band from 1.7GHz to 2.7GHz with return loss larger than 17dB.Its HP element works in the overlapping frequency band from 1.7GH to 2.7GH with |Shh|<-17dB.The isolation of the dual polarized antenna is larger than 32dB.Compared with the low profile design,the antenna has more stable radiation patterns.Both VP and HP elements remain stable radiation patterns.The horizontal plane ripples for VP and HP elements are less than 3dB and 2.6dB,and the cross-polarization levels in ?=60°conical cut are lower than-12dB for the VP element and-16dB for the HP antenna,respectively.4.Two HP omnidirectional arrays are designed in this dissertation.First,a ten elements HP omnidirectional array with low side lobe is designed.The array is composed of five pairs of symmetrical horizontal HP dipoles and shorted positions of the adjacent element baluns are opposite.The adjacent elements are in-phase after two times 180°phase inversion.Finally,all elements in the array are in-phase.Four shorted via holes are designed between each adjacent printed dipole of every element for through the feed coaxial cables and the cables have little effect on horizontal plane radiation patterns.The designed array operates in the frequency band from 1.7 to 2.7GHz with return loss larger than 15dB.The array gains are larger than 8dBi and the simulated horizontal plane ripples are less than 1.2dB in the whole frequency band.In this dissertation,the HP antenna of the low profile dual polarized omnidirectional antenna is replaced by a four elements in-phase array with uniform amplitude.The HP array gains are larger than 6dBi from 1.7 to 2.7GHz and it can provide a larger coverage range.