Research On Broadband Multi-frequency And Circularly Polarized Microstrip Filtering Antenna

With the innovation of wireless technology,the communication methods become more and more convenient,denser of the division of radio spectrum is,more complicated the communication environment will be.The realization of“lightweight,multifunctional and high performance”of the wireless communication system becomes an urgent problem,and the biggest obstacle of the RF front-end miniaturization is the size of the filter and antenna.Although the miniaturization technology of the filter has made significant progress in recent years,it's difficult to achieve the miniaturization of the antenna due to the limitation of its working principle.During the traditional design of RF front-end,the filter and antenna are designed independently,and they are connected directly through a section of coaxial line,waveguide or microstrip transmission line.This direct cascading method may cause the mismatch between the ports of filter and antenna,and even affect the radiation efficiency and directionality of the antenna.Meanwhile,due to the introduction of the matching circuit,the circuit system becomes more complicated,the size becomes larger,and the efficiency is lower.Even if the impedances of the filter and antenna match well after direct cascading,the structure of them may have large differences,which is not conducive to the system integration.Therefore,the filtering antenna with radiation,filtering and balance conversion functions attracts the attention of scholars at home and abroad because of its compact fusion structure and higher efficiency.In this thesis,the importance of developing filtering antennas is presented,and the development history of filtering antennas is reviewed.The structure and performance of several existing filtering antennas are introduced briefly.The superiority and emphasis of each design methods of filtering antennas are summarized.In this thesis,three kinds of microstrip filtering antennas with compact structure,good selectivity and stable radiation performance are proposed to realized broadband,multi-frequency or circular polarization characteristic,respectively.First,a miniaturized low-profile broadband filtering antenna for LTE communications is designed in this thesis,which is based on the method of forming a notch on the radiation patch of the MPA?microstrip patch antenna?.Two radiation nulls are introduced at the upper and lower band edges of the boresight gain curve to make the edge steep,respectively.The multimode resonance is realized by loading the short-circuited pins and etching the gaps,and the relative impedance bandwidth of the antenna is broadened to25.3%.Then the miniaturization of the antenna is realized base on the method of extending the current path by etching the slits,which reduces the size by more than 65%,and the cross polarization and the sidelobe level are suppressed.The matching,filtering and radiation performances of the original antenna and the improved antennas are compared,and the influence of structure change on the performance is analyzed.The antenna has a wide bandwidth,a stable radiation pattern and a flat gain in the band,and the sharp edges,three radiated nulls out of the band and high out-band suppression on the boresight gain curve show good bandpass filtering characteristic of the antenna.Second,a low-profile tri-band microstrip filtering antenna for WLAN and WiMAX communications is designed in this thesis.The antenna is achieved through displacing the ports of the tri-band coupled band filter with three sets of radiating patches.Five radiated nulls on the antenna's boresight gain curve are introduced to improve the sideband selectivity of the antenna by using open-circuit branch resonators and electromagnetic hybrid coupling.The dual-mode resonance is realized in all three passbands to broaden each band of the antenna through combining the resonance mode of the patches and the filter's resonator.The three sets of radiating patches of the antenna adopt a common aperture design to achieve compact structure.The only one-layer structure has a very low profile?0.02?₀?and make it machined easily.The antenna has such a flat boresight gain in the low and medium frequency bands and five radiation nulls outside the band that performs good band-pass filtering characteristics.Finally,a high-performance circular polarization microstrip filtering antenna is proposed in this thesis.Some gaps are etched on an MPA's radiator to realize a linearly polarized filtering antenna unit which has radiation nulls on both sides of the passband of boresight gain.Then a circular polarization microstrip filtering antenna array is realized by using sequential rotation feeding.Finally,the filtering performance of the antenna array is further improved by loading a coupling bandpass filter on the feeding networks.A compact low-profile circular polarization filtering antenna array is obtained,which performs high rectangularity filtering characteristic due to the very flat boresight gain in the pass band and the high rejection outside the pass band.