Studies On Low Scattering Techniques For Tightly Coupled Ultrawideband Planar And Conformal Arrays

Phased antenna arrays are widely used in modern wireless electronic systems due to their flexible beam forming and scanning abilities.Meanwhile,modern advanced electronic systems are gradually required to achieve multifunctionality and miniaturization.Therefore,phased antenna arrays are required to achieve ultra-wideband,wide-angle beam scanning,low profile,and low scattering simultaneously.In some applications,conformal design of phased arrays is also in great demand due to its ability to enlarge radiation aperture,broaden scanning angular ranges,reduce radar cross-section,and increase the air/hydrodynamics of the on-board platforms.Tightly coupled phased arrays achieve wideband impedance matching by utilizing tight coupling among elements and have a unique advantage over traditional ultra-wideband phased arrays in lower antenna profile.However,low-scattering studies on tightly coupled wideband antenna arrays are rarely seen,especially for low-scattering tightly coupled conformal arrays.To meet the requirements of advanced multifunctional electronic systems,this dissertation investigates the low scattering technology of ultra-wideband wide-angle scanning planar and conformal phased arrays based on tightly coupled antenna elements.The main contents and novelties of this dissertation are summarized as follows:1.A new approach is proposed for the efficient calculation of the radiation and scattering properties of finite phased arrays based on scattering-matrix theoryA novel method based on the combination of scattering-matrix approach and microwave network theory is proposed for the realization of in-band scattering and radiation tradeoff of broadband phased arrays.In particular,the proposed method can efficiently predict both radiation performances and scattering characteristics of finite phased arrays with element-independent matching networks/loads.The in-band scattering and radiation tradeoff of finite arrays can be achieved by the optimization of the loaded element-independent matching networks/loads.Moreover,the accuracy of this method is guaranteed by taking into account of some factors such as the EM coupling and the edge effects of finite arrays.To verify the effectiveness of the proposed method,a 1×16 linear array operating at 8.0 ～ 10.0 GHz for scanning up to 45° is designed as the reference prototype and the quasi-coaxial lines are served as the element-independent matching networks.The optimized results show that an obvious scattering reduction is achieved throughout the operating bandwidth in comparison with the reference array.Finally,the optimized linear array is fabricated and measured,and good agreement is obtained to further demonstrate the effectiveness of the proposed method.2.In-band scattering control technology of tightly coupled planar phased arraysWith the polarization selective metamaterial absorber(PSMA),a unique approach for reducing the in-band scattering while preserving the radiation performance of a tightly coupled dipole array(TCDA)is proposed.The proposed PSMA structure is comprised of resistive films,dielectric layers and polarization gates.In detail,the resistive films are loaded on the surface of a TCDA,and the polarization gates are utilized to broaden the absorption bandwidth of PSMA as well as to improve the polarization purity of the array.Moreover,the process of the co-design of the phased array with PSMA are presented.The designed infinite array is able to achieve a 3:1 bandwidth from 6 to 18 GHz with the active VSWR < 2.7 for the scan up to 60° in E-/H-plane.To verify the proposed approach,a 10×10 single-polarized prototype array is fabricated and measured.Measured results are in good agreement with simulated ones,thus demonstrating that the PSMA structure is able to reduce the scattering significantly throughout the operating band of array under normal and oblique incidence.3.Ultra-low scattering design of tightly coupled wideband conformal arrays based on optimally loaded resistors on meta-surfacesA novel approach based on the use of a resistor-loaded meta-surface(RL-MS)with optimized resistor values is proposed for scattering control in wideband conformal phased arrays.Specifically,the resistance of the loaded resistor in each RL-MS element is varied.As a result,the proposed RL-MS structure not only absorbs incident EM waves but also scatters the remainders away from threatening directions.To accelerate the design process,the scattering-matrix approach is used to calculate the scattering patterns efficiently.The proper resistor element distribution can be achieved by optimizing the scattering characteristics.The RL-MS structure is used as the cover layer of a wideband conformal array.By loading the optimized resistors,scattering in the proposed array is reduced to a relatively low level with little degradation of radiation performances.The final designed array achieves 3:1 impedance bandwidth with scanning up to ±60°/±45° in E-/H-plane.Remarkably,a monostatic scattering reduction of approximately 30 d B is observed throughout X band in the simulated results of the optimized array.To demonstrate the effectiveness of this design,prototype arrays are fabricated and measured.The measured results are in reasonable agreement with the simulated results.4.Conical conformal tightly coupled dipole arrays co-designed with low-scattering characteristicsConformal designs and scattering reduction have long been significant challenges to ultra-wideband phased arrays.In this work,a conical conformal tightly coupled dipole array with low-scattering characteristics is developed.The design procedure from a planar antenna element to a two-dimensional conformal finite array is presented in detail.In this design,a novel structure of conformal resistive strips is used as a cover layer for the radiating dipoles to control scattering,and its ability to control scattering is insensitive to variations in the dimensions of the designed conformal array.To achieve a wide scanning range and reduce scattering in a wide band,a metasurface composed of full-split rings is employed as the superstrate.The final designed array achieves a 3:1 impedance bandwidth for scanning up to 60° in two principal planes.Moreover,scattering is reduced by more than 10 d B throughout the entire operating band.Finally,an array prototype is fabricated and measured.Measured results are in good agreement with the simulated results,thus demonstrating the effectiveness of the proposed array.