| With the explosive growth of smart home,smart medical,smart transportation,smart city,and other fields,as the basic technical support of the internet of things（Io T）,the wireless communication system has increasingly demanding requirements on the transmission rate and communication capacity.For different application scenarios,the structure（profile）,performance（bandwidth,gain）and function（multi-polarization,multi-beam）of omnidirectional coverage antennas are in higher demand.Therefore,how to design high performance and multi-functional omnidirectional coverage antenna in a limited space is valuable work.Facing the application requirements of larger coverage capability,large communication capacity,and higher communication reliability,this thesis focuses on omnidirectional coverage antennas with systematic research work on how to improve the gain and communication reliability.The main innovations are as follows:1.Compact and high gain planar cavity omnidirectional antenna arrays with composite resonance structure.A compact and high gain planar cavity omnidirectional antenna array is proposed for the problems of oversize and frequency sweep of the omnidirectional antenna array.Since the series feed has the problem of frequency sweep and the parallel feed has a complex and asymmetric feed network structure,a medium feed was chosen.A slot-quasi-CPW composite structure is proposed,and the two-elements planar cavity omnidirectional antenna array is studied and designed.Parasitic dipoles are introduced to form a slot-dipole composite structure,introducing a new resonance to extend the bandwidth.A compact high gain ten-elements planar cavity omnidirectional antenna array with slot-dipole composite structure is proposed,achieving a high gain（>10d Bi）and compact（5.3λ0*0.4λ0*0.02λ0）structure,which has potential in wide-range wireless coverage,micro-base station and intelligent vehicle networking application scenarios.2.High gain millimeter-wave omnidirectional antennas based on lens-loaded technology.For the problems of narrow operating bandwidth and unstable beam of the high gain omnidirectional antenna array,a lens omnidirectional antenna design is proposed based on the biconical antenna（traveling wave antenna）and lens-loaded technology.Based on the above idea,two kinds of lens omnidirectional antennas are researched and designed:ordinary lens omnidirectional antenna and oversized lens omnidirectional antenna.For the performance comparison between the biconical antenna and ordinary lens omnidirectional antenna,the oversized lens omnidirectional antenna has a higher gain.It achieves high gain（>10 d Bi）,broadband（24-30 GHz）,low roundness（<±0.6 d Bi）,and stable radiation pattern in a whole operating band,providing a good method for the omnidirectional antenna applied to 5G millimeter-wave channel measurement and large-range wireless communication.3.Low-profile dual-polarized（DP）omnidirectional antenna with dual-mode resonance technology.Based on the annular open-cavity dipole array structure with dual-mode resonance,a design method is proposed to achieve a low profile,high omnidirectional gain,and stable DP beam using shared-aperture technology.Based on the above method,a center-fed circular patch antenna acts as both a vertically polarized（VP）driven element and a horizontally polarized（HP）reflector.The annular open-cavity dipole array is introduced evenly around the circular patch,and the annular open-cavity dipole array plays two roles in the proposed design,i.e.,the VP director and the HP radiator.Both orthogonal polarizations radiate energy from the same physical aperture and vertical space,contributing to a low profile.Meanwhile,the annular open-cavity dipole array is used as the VP director,improving the omnidirectional gain without changing the vertical profile,and introducing another resonance to improve the VP bandwidth.The whole antenna structure is symmetrical,laying the foundation for the DP beam stability in a whole operating band.It has a DP overlapped bandwidth of 20%,a profile height of 0.13λ0,and a DP horizontal omnidirectional gain greater than 1.8 d Bi.The method overcomes the difficulties of high profile,low gain in a horizontal plane,and unstable beam of DP omnidirectional antenna,which has great potential application in wireless communication scenarios with stable and large coverage.4.Broadband quasi-monopole antennas with multi-mode radiation patterns.A compact wideband omnidirectional/directional beam switchable antenna scheme is proposed based on multi-mode resonance technology.Firstly,the multi-mode resonance operating principle of the quasi-CPW feeding network and quasi-monopole radiator,and the orthogonal modified structure of the above structures are analyzed,thus two compact and wideband omnidirectional/directional beam switchable antennas are designed.Four and seven complementary beams with good radiation performance are realized by controlling the excitation port and phase,and both achieve more than 30%of the operating bandwidth.By controlling the excitation port and phase to control the current distribution of the radiation structure to achieve complementary multi-beam,the spatial diversity capability is effectively enhanced,providing a good design solution for indoor communication and other micro base station applications with specific mode coverage. |
