| With the rapid development of wireless communication systems,performance upgrade and structure miniaturization of the wireless devices have brought new challenges to the antenna design.Portable mobile devices bring great convenience to people’s travel,but also leave a smaller design space for the antenna.Therefore,the antenna needs to satisfy specified technical indices in the ever-shrinking space to achieve antenna miniaturization and low profile.At the same time,with the diversified development of modern wireless systems for adapting to their application background,new design requirements for different characteristics of the antennas,such as wide beamwidth,dual frequency,broadband,and dual polarization,have also been put forward.Especially in dealing with the Faraday rotation effect and the multipath effect,the circularly polarized（CP）antenna has become the preferred alternative,because it can ensure stable transmission performance.Therefore,studying low-profile circularly polarized antennas with different characteristics is of great significance and application value.Both the microstrip antenna and the planar magnetoelectric（ME）dipole antenna have the advantages such as low profile,easy processing and low manufacturing cost.Based on these two types of antennas,this dissertation conducts in-depth research on the design and implementation of low-profile circularly polarized antennas with the wide beamwidth,dual-band,polarization reconfigurable,and omnidirectional characteristics.Main contributions of this dissertation can be summarized as following.1.Design of a low-profile broadband CP microstrip antenna with wide beamwidth.Main radiator of the antenna is a circular microstrip antenna,and the air medium is sandwiched between the patch and the floor.At the same time,in order to eliminate the inductance introduced by the direct feeding of probe,four sequentially rotatingΓ-shaped probes are adopted in the feeding structure for the coupling feeding.Subsequently,in order to adjust the antenna matching and expand its bandwidth,four claw-shaped parasitic branches are loaded in four corners of the floor.Based on the analysis of active reflection coefficients for the multi-port antenna,amplitudes and phases of the antenna ports are adjusted by etching slots on the patch to achieve broadband matching characteristics.Meanwhile,another function of the claw-shaped parasitic branches is to expand half-power beamwidth of the antenna.By qualitatively analyzing the currents on the antenna and the claw-shaped parasitic branches,the relationship between the branches and the half-power beamwidths is discussed.Finally,a second-order Wilkinson power divider is used to feed the fourΓ-shaped feed branches with equal amplitude and 90°sequential phase difference,and make the antenna radiate the CP waves.After synthetically considering the bandwidth,the size and the beamwidth,a novel broadband circularly polarized microstrip antenna with wide beamwidth is designed and implemented.The measured results show that the antenna has a-10 dB reflection coefficient bandwidth of 72.5%,a 3-dB axial ratio bandwidth of 54%,and a circular polarization overlap bandwidth of 54%.The antenna beamwidths are greater than 100°within the operating frequency band.In addition,size of the antenna is only（0.35×0.35×0.06）λ03.Therefore,the proposed antenna will be a good candidate for the navigation applications.2.Design of a novel dual-band CP planar endfire EM dipole antenna with adjustable frequency ratio and enhanced front to back ratio.First,an equivalent magnetic dipole is constructed by using a half-mode SIW cavity to provide the vertical polarization.Current and electric field distributions of the magnetic dipole with different modes are analyzed,and appropriate radiation modes are selected accordingly.The frequency ratio is adjusted by etching two slots on the overlap positions of two selected modes with the maximum and minimum currents,and dual-band vertical polarizations at appropriate frequencies are realized.The corresponding dual-band horizontal polarizations are provided by two pairs of electric dipoles.The magnetic dipole and the electric dipole are connected by the parallel double wires.Amplitudes and phase difference of these two orthogonal polarizations can be effectively controlled by lengths and widths of the parallel double wires,so that the dual-band CP radiation is realized.A second-order reflector designed for the magnetic dipole is subsequently introduced to significantly improve front-to-back ratios of the antenna.Based on the above design steps and ideas,a dual-band CP planar endfire EM dipole antenna working at 2.4/5 GHz is finally designed and fabricated.The measured results show that front-to-back ratios of the proposed antenna at the low frequency and the high frequency reach 15 dB and 14.2 dB,respectively.3.Design of a dual-CP planar endfire ME antenna.First,CP radiation principle of the planar endfire ME dipole antenna is analyzed,and the idea of changing the CP sense of rotation is discussed.That is,without changing the phase of magnetic dipole,the polarization of electric dipole is reversed.Based on this idea,two CP planar endfire antennas are designed respectively.The first one is a CP reconfigurable planar endfire ME dipole antenna,in which a half-mode SIW cavity is still used as a magnetic dipole to provide the vertical polarization and a printed electric dipole is still used to provide the horizontal polarization.The PIN diodes inserted between the parallel double wires and the electric dipole arms control connection modes of the magnetic dipole and the electrical dipole,so that two endfire CP senses of rotation are adjusted accordingly.In order to simplify the DC bias circuit,the electric dipole arms on both sides of the substrate are connected through metallized via holes,reducing half of the DC bias lines on the electric dipole.Designed center frequency of the antenna is 2.45 GHz.The measured results show that the-10 dB reflection coefficient bandwidth and the 3-dB axial ratio bandwidth are 2.1%and 10.7%,respectively.The second antenna is a dual-port dual-CP planar endfire ME dipole antenna.By using the same form of orthogonal linear polarizations,the antenna has no direct connection between the magnetic dipole and the electric dipole.Instead,the magnetic dipole and the electric dipole are connected to the feeding network respectively,and two endfire CP senses of rotation are reversed by selecting different feeding ports to change the phase difference between the ME dipoles.At the same time,a strip-shaped director is added in front of the antenna to improve its front-to-back ratios.The simulation results show that the antenna can radiate the dual CP waves at 2.45GHz,and its front-to-back ratios are improved more than 7 dB,compared with the antenna without loading the director.4.Design of a low-profile omnidirectional CP ME dipole antenna.First,a half-mode SIW cavity equivalent magnetic dipole and a printed electric dipole are used to provide omnidirectional orthogonal linear polarizations,respectively.At the same time,omnidirectional CP radiation can be achieved by virtue of their inherent 90°phase difference.However,the omnidirectional CP characteristics after directly combining the magnetic dipole and the electric dipole are not so good,which is caused by the shielding of electric dipole by the magnetic dipole in the-z-axis direction.In order to solve this problem,a V-shaped element model is introduced to assist the design of electric dipole,which improves the roundness of omnidirectional radiation pattern.Moreover,the relationship between the opening angle of V-shaped dipole and its phase center are adopted to improve axial ratio characteristics of the proposed antenna.The measured results show that the gain fluctuations in the omnidirectional LHCP radiation pattern of the antenna in the azimuthal plane are less than 1.5 dB at 2.45GHz,and the omnidirectional axial ratios are less than 1.8 dB.Compared with the published similar omnidirectional ME dipole antennas,profile of the proposed antenna is reduced by 55.7%. |
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