| Phased antenna arrays,which are also known as electronically scanned arrays,have the advantages of fast beam scanning and precise beam pointing,and are also capable of beam shaping,multi-beam scanning and anti-jamming nulling.Thus,it has been years since phased arrays played important roles in multi-aim targeting and tracking and long range point-to-point communication.Recently,with the rapid development of electronic techniques and the explosive increasing of communication capacity,the studies on phased arrays are no longer limited to scanning capabilities,but more and more focused on bandwidth extension to adapt to the requirements of increasingly integrated multi-functional electronic systems.Moreover,to save the limited space and load of the platform,as well as to reduce aerodynamic drag and RCS(radar cross section),features of low profile and light weight are also demanded for UWB(ultra-wideband)phased arrays.UWB phased arrays based on inter-element tight coupling were proposed to meet all the aforementioned demands.The wideband performance of the tightly coupled dipole arrays briefly attributes to:(1)the close element-spacing,which results in approximately constant current distribution on the array aperture in free space;(2)the strong inter-element capacitance,which counteracts the inductance effect from the conducting ground plane;(3)the dielectric superstrates,which help with the impedance transition between the radiation aperture and the free space.Generally,tightly coupled dipole arrays achieve at least 4:1 bandwidth within a beam scanning range of ±45°,subject to Active VSWR < 3.0.The profile height of a tightly coupled dipole arrays is around 1/10 wavelength at the lowest operating frequency.As seen,the tightly coupled dipole arrays are superior in achieving ultrawide bandwidth with low profile when compared to conventional Vivaldi antenna arrays.However,as a relatively new technique,further studies need to be conducted on the tightly coupled dipole arrays to explore their potentials and performance improvements.The focuses of this dissertation are on UWB 1-D(one-dimensional)tightly coupled arrays and the low-profile and lightweight alternatives of dielectric superstrates,which were both rarely studied before.The foundation of the dissertation is the wideband principle of tightly coupled dipole arrays.The main contributions of this dissertation are as follows:1.Studies on wideband,wide-scan and lightweight dual-polarized linear array of tightly coupled elementsPrevious studies on tightly coupled dipole arrays are mainly focused on 2-D phased arrays.In this dissertation,a wideband and lightweight dual-polarized linear array based on tightly coupled dipoles is proposed to meet the demands of application platforms with long but narrowly confined space,such as the aircraft wings.Different from 2-D arrays,the wideband factors of continuous current and the capacitive coupling between the elements in the dual-polarized tightly coupled array are destroyed in the 1-D environment.To solve these issues,a series of compensation designs are proposed.Simulation results show that the tightly coupled linear array achieves 5:1 bandwidth(0.4~2 GHz)and 4:1 bandwidth(0.5~2 GHz)in the vertical polarization and horizontal polarization,respectively,for active VSWR < 3.0,while scanning up to ±60°.The width of the array is smaller than a wavelength at the highest operating frequency.The dielectric superstrate that is commonly used in tightly coupled dipole arrays for impedance matching is bulky at the operating band.Thus,two lightweight designs are proposed in the dissertation and the FSS(frequency selective surface)loading with lighter weight is finally applied to the linear array.To validate the proposed design,prototype for a 1 × 26 linear array was fabricated and measured.Good agreement is achieved between measured and simulated results.2.Studies on ultra-wideband and wide-scan phased arrays based on tightly coupled open folded dipolesThe thickness of the dielectric superstrate used in tightly coupled arrays is normally around a quarter wavelength at the highest operating frequency,which largely increases the height of the array profile.Nevertheless,the dielectric superstrate is bulky when applied to lower frequencies,such as the UHF band.To solve this issue,a technique for designing ultrawideband tightly coupled arrays excluding the use of dielectric superstrates is proposed.The proposed array is consisting of tightly coupled open folded dipoles,which provide additional reactive components as compared to traditional tightly coupled dipoles and form a multi-stage matching network together with their integrated baluns.Thus,ultra-wide bandwidth and wide scan range is achieved in the proposed array.Moreover,the array profile is reduced effectively.As a result,the proposed array achieves 7.33:1 bandwidth(0.3~2.2 GHz)while scanning up to ±70? in E-/D-plane and ±50? in H-plane,subject to active VSWR < 3.0.The total profile height of the propose array is only 0.08? with reference to the lowest operating frequency.A prototype for an 8 × 8 array was fabricated and measured.Good agreement is achieved between measured and simulation results,thus validating the good performance of the proposed array.3.Studies on low-profile and ultrawideband tightly coupled arrays loaded with split ringsTo further reduce the profile of the tightly coupled arrays,a novel split ring loading printed on planar single-layer printed circuit board and with wideband tuning performance is designed to help with the wideband impedance matching in the tightly coupled arrays.The array achieves 7.2:1 bandwidth(0.3~2.15 GHz)while scanning up to ±70° in E-plane,±45° in H-plane and ±65° in D-plane for active VSWR < 3.0.The total profile height of the propose array is only 0.067? with reference to the lowest operating frequency,which is the lowest among the published UWB tightly coupled arrays without using any lossy materials.Moreover,the polarization purity of the array is excellent,which approximates the theoretical limit while scanning in the D-plane.To validate the proposed design,an 8 × 8 prototype array was fabricated.Measured results are in good agreement with the simulated results.The proposed concept is further verified in a dual-polarized configuration with a larger element spacing,which reduces the number of T/R modules and thus the cost in practical applications. |
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