| In recent years,the breakthroughs in wireless communication technology have prompted the emergence of a variety of wireless communication devices,and their working frequency bands are also different.With the advantages of being compatible with multiple operating frequency bands,broadband antennas and multi-band antennas have become the focus of many antenna researchers.Compared with linearly polarized antennas,circularly polarized(CP)antennas can effectively reduce the interference from the natural environment and electromagnetic signals,and are compatible with electromagnetic waves in various polarization directions,which have great practical application value.Multiple-Input Multiple-Output(MIMO)antenna system can achieve higher signal-to-noise ratio and channel capacity,and occupy an important position in modern wireless communication systems.Therefore,this thesis studies the broadband CP microstrip antenna,the broadband CP MIMO antenna and the multi-band CP microstrip antenna.The main research contents of this thesis include:(1)The advantages of broadband and multi-band CP antennas are analyzed,the design methods of broadband and multi-band CP antennas from both china and overseas are summarized.The performance parameters of the microstrip antenna,the basic theory of CP and the decoupling method of the MIMO antenna are also described.(2)According to the application requirements of broadband and CP,two broadband CP microstrip slot antennas fed by Coplanar Waveguide(CPW)are designed.One of the antennas expands the CP bandwidth through the opening on the left side of the wide slot,adds a perturbation patch to obtain a new CP mode,and optimizes the microstrip line and wide-slot structure to combine multiple CP modes.Taking the first antenna as a MIMO antenna unit,a dual-unit MIMO antenna with high isolation is designed using polarization diversity technology.Different from the first antenna,the second antenna uses a gun-shaped radiating patch as the main radiating element,which excites two circularly polarized modes in the higher frequency band,and further optimizes the radiation performance of the antenna by adjusting the wide slot structure.An measured impedance bandwidth of 134.6% is obtained,and the simulated axial ratio bandwidth reachs 55.8%.(3)A dual-band CP microstrip antenna is proposed.The antenna adopts the monopole structure as the basic model and can obtain two wide impedance bandwidths.Modification of the upper monopole can excite the basic CP mode at 5GHz.By adding L shape long branches to the ground plane,the 2.5GHz CP mode is realized,and the L shape slot etched on the right side of the ground plane excited new CP modes and resonance bandwidths at 3.5GHz.Finally,two impedance bandwidths and three CP bandwidth are obtained,which are suitable for WLAN/Wi MAX/Bluetooth wireless communication devices.(4)Finally,a compact broadband CP MIMO antenna is proposed to solve the problem of too large size of MIMO antenna,and the antenna structure is symmetrically distributed along the diagonal.The antenna reduces port coupling through two orthogonal radiating patches and one decoupling microstrip strip,and the isolation reaches 15 d B.The introduction of the current perturbation structure on both sides of the port excites the basic CP mode,and the circular polarization bandwidth is expanded under the action of the microstrip. |
