Research On Applications Of Subwavelength Structures In Electromagnetic Radiation And Absorption

Subwavelength structures are artificial structures,composed with period array,non-period array,or just non-array units,and possess anomalous electromagnetic(EM)properties.Due to their flexible EM controlling abilities,subwavelength structures are widely applied in military stealth,radar antenna,microwave and millimeter wave communication,and many other fields,resulting in the emergence of various high-performance functional devices based on sub-wavelength structures utilizations.As the EM radiation devices,the design of antenna with high performance is always the focus of RF front-end research.And the explosive growth of various portable electronic devices and compact application platforms makes the miniaturization of antennas become an inevitable development trend.Correspondingly,the EM radiation complicates the EM environment,that makes an urgent application requirement to develop the high-efficiency absorbing materials with light,thin,wideband and wide-angle absorbing merits to meet military confrontations and civil protection needs.Hence,this thesis would focus on the advanced applications of subwavelength structures in microwave EM radiation devices and absorbing metamaterials(MMs).Considering that,antennas suffer the trade-off between efficiency,gain and bandwidth during their miniaturization processes,as well as low-profile broadband absorbers sustain some difficulties to satisfy large-angle absorption and polarization insensitivity,this work carries out the research on applications of subwavelength structures in electrically small antennas(ESAs)and MM absorbers.The main content and innovation are:(1)Limited by its electric size,the ESA has its sub-wavelength radiator operate at dipole or monopole mode with weak radiation directivity.To satisfy the requirement of ESA with high directivity,two 28 GHz Huygens source ESAs are proposed in this thesis by employing forward-radiating Huygens principle.One of ESAs is linearly polarized,and the other one is circularly polarized.Based on the design method of near-field resonance parasitic(NFRP),the two ESAs successfully realize the miniaturized size,high efficiency,high front-to-back ratio and wide beamwidth by exciting the near-field Huygens radiator with dipole feeding.The two ESAs are planar multi-layer and suitable for system integration.Besides,their low backward radiation characteristics reduce the human body's absorption of radiational energy,so that both the ESAs have a low specific absorption rate,which make them are suitable for human portable device applications.(2)According to the Wheeler-Chu Limit,the physical bandwidth of ESA is limited by its electric size and has a theoretical upper limit.To satisfy the requirement of antenna with miniaturization and bandwidth expansion,Huygens dipole ESA and monopole ESA realize the bandwidth expansion by inserting non-Foster circuits into their subwavelength radiators.Finally,the non-Foster monopole ESA is experimentally verified and analyzed with return loss,radiation pattern,gain,and signal-to-noise performances.Experimental data shows that:under the condition of ka=0.39,a 5-times enhancement of its-10 d B impedance bandwidth is achieved,significantly surpassing its passive Wheeler-Chu Limit.Within the-10 d B impedance bandwidth,the proposed non-Foster monopole ESA achieves a relative high transducer gain with the maximum total efficiency of 44.5%and average 5.0 d B signal-to-noise improvement.(3)Up to date,the realization of microwave MM absorber with low profile and wide bandwidth is effective and fruitful.However,it is still challenging to meet the wide-angle absorption and wide-angle polarization insensitivity of the absorber under the broadband and low-profile conditions.To satisfy the requirement of broadband MM absorbers with wide-angle absorption and polarization insensitivity,two MM absorbers are proposed in this thesis.One is a wideband,wide-angle,and polarization-independent MM absorber based on the uniaxial homogeneous model.The electric size of absorber unit cell is 0.14?₀×0.14?₀×0.09?₀.With the tailoring of dispersion parameters,for arbitrary incident plane wave with incident angle of less than50°,the absorber can achieve over 90%absorption within a relative bandwidth of48.6%.The absorber has its designed thickness value close to the calculation according to Rozanov limit,and its wide-angle absorption functions are close to the results obtained from the theoretical calculations based on the perfect uniaxial homogeneous model.The second absorber is designed based on the nonlocal homogenization model.From the perspective of field analysis,this design method first uses the characteristic mode theory to synthesize the scattering pattern of the metal unit,and then reshape the metal unit with impedance sheets,to transform the scattering characteristic into the absorption performance.This design method can directly avoid the complex spatial dispersion tailoring and the equivalent circuit parameter optimization,which means simple and effective design advantages.Under the condition of low profile and wide bandwidth,the proposed absorber achieves the near omnidirectional absorption of TM wave,as well as effective absorption for TE plane wave and surface TEM wave.Experimental verifications demonstrate the absorption within incident angle of 70°agrees well with the simulation results.