Research On Filtering Antennas Based On Multiple Resonant Modes

In recent years,wide frequency bands,size-miniaturization and system-integration have become the major trend for the development of the wireless communication system as the increasingly rapid development of the wireless communication technology.As one of key components of the modern wireless system,antenna affacts the quality of the whole wireless communication system.Along with the shortage of space resources,more and more antennas are integrated in a narrow section,which leads to the deterioration of the electromagnetic environment between antennas.Therefore,it is necessary to study filtering antennas with high out-of-band suppression levels.In this thesis,the design method of novel filtering antennas is studied based on the research topics and the design method of antenna system.The main contents of this dissertation are summarized as below:1.The design method of multi-frequency microstrip filtering antenna is studied.Based on the cavity mode theory of traditional microwave cavity,the new cavity mode with special structure is proposed,and the dual/triple frequency filter antenna is designed accordingly.In the new cavity resonance mode,when the microstrip patch antenna works under TM₁₀/TM₂₀/TM₃₀ mode,the current distribution that affects the radiation is about half wavelength(less than half wavelength under TM₁₀ mode and half wavelength in high-order mode)due to the symmetrical cancellation of the current in some areas.Therefore,a stable unidirectional radiation pattern is formed.In addition,radiation nulls are obtained between different resonant modes.By further introducing open-circuit resonant stubs,better out-of-band suppression characteristics can be achieved.Finally,a dual/triple frequency filtering antenna is obtained.Based on this principle,a low profile dual-band filtering antenna and a MIMO antenna array are designed.And the feasibility of this scheme is verified by simulation optimization and test.2.A dual-band and dual-polarized filtering antenna with low profile and compact size is studied.By etching four slots at each side of a square radiating patch,dual operation bands are observed as well as a natural radiation null between the two bands.Another two radiation nulls are generated by introducing a novel feeding structure,which is composed of a microstrip line with open-circuit stepped-impedance resonators.Thus,a dual-band bandpass filtering response of the realized gain is obtained.Finally,the proposed antenna has good dual-band filtering characteristics with good radiating performance.In addition,the proposed antenna meets the frequency band requirements of 3.3-3.6 GHz and 4.8-5.0GHz in the fifth generation(5G)communication,and achieves stable unidirectional radiation characteristics in both of the two operation bands.Besides,a differentially-fed,dual-band microstrip patch antenna with filtering response is also studied.By etching an H-shaped slot at a rectangle radiating patch,two resonant modes of the patch antenna are generated,as well as an additional radiation null between the two operation bands.Besides,two symmetric open-stub coupling feedlines are introduced to differentially excite the antenna,which offer two extra radiation nulls outside the two bands.Thus,a dual-band bandpass filtering response for the proposed antenna is obtained.Measurement results of the prototype agreed well with the simulated ones.Stable radiation patterns with low cross-polarization levels of better than-28 d B are observed.Besides,a deep roll-off of better than 22 d Bi for the realized gain is also obtained.3.A wideband dual polarized filtering antenna is studied.A novel E-shaped feeding structure with one end shorted to the ground is proposed to excite a square patch antenna,and an extra resonant mode is induced,which leads to a wider operation band.Besides,symmetric open/short-circuited stepped impedance resonators are proposed to generate the third resonant mode.And the bandwidth is further improved.Moreover,the gain-suppression level outside the operation bands is greatly enhanced as the introduction of open/short-circuited stepped impedance resonators.Finally,the proposed antenna meets the frequency band requirements of 3.3-3.6 GHz in 5G communication,and has stable directional radiation pattern and good port-isolation and cross-polarization performance,as well as excellent out-of-band gain suppression performance.4.A differentially-fed filtering antenna loaded with two resonant rings is studied.A square radiation patch is fed by inverted L-shaped structure.Two out-of-band radiation nulls are obtained by printing two ring resonant structures on the lower layer of the dielectric substrate of the patch antenna.Among them,the inner ring generates the high frequency radiation null,and the outer ring generates the low frequency radiation null.At the frequency corresponding to the radiation null,the antenna is in a weak resonance state.Moreover,the current on the radiation patch and the ring structure is symmetrically distributed and cancel out each other.Thus,a very low out-of-band gain is formed in the far field.Simulation results show that the antenna achieves 18.8%(3.27-3.95 GHz)impedance bandwidth and very low(better than-33.7 d B)cross polarization characteristics,and a good filtering performance for the realized gain is also obtained.Besides,a multifunctional dual-band dual-polarized filtering antenna is also studied.Firstly,a dual-polarized low profile patch antenna with coaxial feed is designed.Then,by introducing symmetrical open circuit branches into the feedline,a new resonant frequency is generated at the lower frequency band.At the original resonant frequency,the antenna obtains good directional radiation characteristics.At the same time,at the low frequency band,the transmission characteristics between the two ports of the antenna are excellent.At the same time,the antenna shows poor radiating performance.Thus,the antenna can be equivalent to a filter.Through parameter analysis,the working frequency of the filter and the radiation null between the two working bands can be adjusted by tuning parameters of the feeding structure.At last,the antenna structure has the advantages of multi-function,miniaturization and low profile,which provides a design guidline for the hybrid use of both antenna and filter.