| The circularly polarized(CP)antenna is a type of antenna that transmits and receives circular polarization waves.Due to the characteristics of the circular polarization wave,CP antennas have advantages of anti multipath fading,requiring no strict orientation between transmitting and receiving antennas and reducing the ‘faraday rotation' effect,which have been widely applied in mobile communications,satellite communications,global positioning systems(GPS),radio frequency identification(RFID),wireless local area networks(WLAN),direct broadcasting service(DBS)television reception systems,wireless sensors,wireless power transmission and so on[1].In these systems,broadband represents high data-rate,high capacity communication or systems with different functions interoperated in one module.Thus CP antennas as key components of these systems are required to be broadband.On the other hand,general broadband CP antennas only seek for the largest bandwidth,and they are unable to balance other important performances,which greatly limit the performances of communication systems.Therefore,the research on high-performance broadband CP antennas is significant and urgently needed to be solved.High-performance broadband CP antennas can be realized by two ways.One way is to realize a CP antenna developed from the high-performance broadband linearly polarized(LP)antenna.The other way is to improve the bandwidth and performance of a general CP antenna.The novel LP magneto-electric dipole(MED)antenna has an ultra-wideband characteristic and excellent performances,which can be designed for a high-performance broadband CP antenna.Microstrip CP antennas are an important branch of CP antennas,which have the advantages of low profile,light weight,easy to fabrication,easy to integration and so on.Nevertheless,general microstrip CP antennas have a narrow bandwidth of about 10%,and their performances are urgently needed to be improved.This thesis focuses on the novel MED antenna and microstrip antennas for high-performance broadband CP antennas,the major contributions of which are summarized as follows:1.An ultra-wideband dual CP MED antenna is presented.First,through introducing an another resonance frequency by improving the feed structure,a dual LP MED antenna with a bandwidth of 73.3% is developed from the previous design with a bandwidth of 52%.Second,through the multi-branch coupler synthesis,design of the microstrip line for high characteristic impedance,and design of folded main lines,a six-branch hybrid coupler is designed on the printed circuit board(PCB),which achieves an efficient bandwidth of 80.7% and a miniaturization ratio of 53%.Third,through sharing the ground,the dual LP MED antenna and six-branch hybrid coupler are combined for dual circular polarization.Furthermore,a perturbation is applied to the antenna structure for good circular polarization.As a result,an ultra-wideband dual CP MED antenna is realized,which achieves an impedance bandwidth of 84.5% from 0.6GHz to 1.53 GHz,an axial-ratio(AR)bandwidth of 81.8% from 0.63 GHz to 1.53 GHz,and a nearly symmetrical,stable unidirectional radiation pattern with an average gain of 8d Bic during the impedance bandwidth.2.Implementation of a small circular-arc shaped ground to improve the performances of a CP single-layer single-feed loop antenna is presented.First,the frequency-domain radiation model of a CP single-layer single-feed loop antenna is developed,which demonstrates that an unsymmetrical surface current distribution on the ground is caused by an unbalanced feed.As a result,the dimensions of the ground have an influence on the circular polarization of the loop antenna.Thus the ground should be designed as a minor radiation part of the loop antenna.Second,the CP single-layer single-feed loop antenna with a small circular-arc shaped ground is proposed.Through choosing different angles of the small circular-arc shaped ground for different design objectives,the bandwidth and tiled radiation pattern of the loop antenna can be improved respectively.Third,the small circular-arc shaped ground is applied to the CP single-layer single-feed exponentially tapered loop antenna,and the antennas with an improved bandwidth and radiation pattern are realized respectively,which verify the validity of the small circular-arc shaped ground.3.Broadband dual CP and single CP patch antennas with wide beamwidths are proposed.Based on the slot coupled patch antenna fed by a microstrip ring,a series of methods are introduced to improve the performances of the antenna.In order to broaden the AR bandwidth and AR beamwidth,irregular multi coupling slots are presented;in order to broaden the impedance bandwidth,a microstrip ring feed constructed by multi transform lines is presented;for good impedance matching without sacrificing gain,a small coupling slice replacing the resister load at the end of the microstrip ring feed is presented;for high isolation between adjacent elements in an array,a metal wall surrounding the antenna is applied;for high front-to-back ratio,a metal back-cavity under the multi coupling slots is applied;to improve the bandwidth further,assistance methods of the parasitic patch and thick air substrate are applied.By these schemes,the dual CP patch antenna obtains an impedance bandwidth of 24%,an AR bandwidth of 16%,a 3d B beamwidth of 60°,and a 3d B AR beamwidth of 100°.The single CP patch antenna achieves an impedance bandwidth of 29.7%,an AR bandwidth of 21.6%,a 3d B beamwidth of 70°,and a 3d B AR beamwidth of 90°.Moreover,they also have an excellent overall performance and a simple feed,which are very valuable for arrays.4.High-gain and shaped-beam broadband CP patch antenna arrays are proposed.Based on the 2 × 5 broadband wide-beamwidth single CP patch antenna array,different performances of the array with different arrangements are analyzed.Simulation results demonstrate that a sequentially rotated array of 0°,90° arrangement is the optimal choose while the element with wide bandwidth and wide AR beamwidth.For the long-distance communication network,a high-gain broadband patch antenna array is realized,which achieves a gain of 17.1?18.3d Bic from 3.4GHz to 4GHz.Its 3d B beamwidth can coverage 40° in the horizontal plane and 14 o in the vertical plane.Moreover,good circular polarization is also obtained in the coverage angle.The influences of the coupling in the array on the radiation patterns of elements are also analyzed.Simulation results reveal that the coupling has different influences on the radiation patterns of different elements in the array,thus it is necessary to make a difference between the radiation patterns of elements for high-precision beam-shaping while the array has few elements.Moreover,in reference to a broadband array,it is also necessary to make a difference between the radiation patterns of the element at different frequencies during the bandwidth.For the air-ground integration communication network,a shaped-beam broadband patch antenna array is realized by the multi-objective pattern synthesis,and a genetic algorithm with the assistance of HFSS simulation is employed.The array achieves a shaped-beam with the gain of 14?15.8d Bic from 3.4GHz to 4GHz,the shaped-beam and 3d B AR beamwidth of which can coverage-10°?+90° and-10°?+45° in the vertical plane respectively. |
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