| In recent years,wideband antennas are widely used in areas such as radar,communication and remote sensing systems.Generally,ultra-wideband low-profile antennas are recognized as one of the most significant components in the airborne radar and electronic warfare system.In particular,some special applications such as airfoil conformal applications raised higher requirements for wideband wide angle scanning phased arrays with low profiles.The tightly coupled phased arrays expand the impedance bandwidths by increasing the capacitive coupling between the dipole elements to offset the inductance of the ground plane.Due to this characteristic and as compared to the traditional phased arrays,the tightly coupled phased arrays have many advantages in reducing the antenna profile and wide-angle scanning.On the other hand,due to the ability to produce an in-phase reflection effect on the incident electromagnetic waves,the artificial magnetic conductor(AMC)can be used to substitute the metal grounds,thus reducing the antenna profile more effectively.However,AMC always has rather narrow bandwidth and poor applicability to broadband antennas.This thesis focuses on the key issues for combining artificial magnetic conductors with tightly coupled ultra-wideband antenna arrays.Specifically,a low-profile tightly coupled phased array has been designed with the capabilities of wide-band wide-angle scanning and 4:1 impedance bandwidth.The main contributions of this thesis are as follows:Firstly,the significance and state of art of low profile ultra-wideband tightly coupled phased array antenna are introduced.Then,taking linear array and rectangular planar array as examples,the radiation pattern of antenna array is derived.Next,the differences between the working principle of traditional broadband antenna and that of broadband antenna based on mutual coupling theory are compared in detail.Secondly,the working principle of reflecting ground AMC is introduced,and the influence of geometric structure and material factors on the reflection bandwidth of the AMC is analyzed.Simulation results show that increasing the permeability of AMC can produce a significant effect on enlarging the in-phase reflection bandwidth.Thirdly,a tightly coupled phased array antenna with the AMC structure has been designed.When performing at 0.5-2.5 GHz,the profile height is only 0.09 times the high-frequency wavelength.In addition,the antenna can achieve ±60° scanning in E-plane and H-plane(VSWR<3).Fourthly,considering the limitation of magnetic medium,a tightly coupling antenna array loaded with the air-filled artificial magnetic conductor structure is proposed.With 4.2:1(0.6~2.5GHz)impedance bandwidth,the proposed antenna is able to scan to ±60° and ±45 ° on E-plane and-H plane(VSWR<3),respectively.In particular,the profile height of the proposed antenna is only 0.254 times of the high-frequency wavelength.Then,the designed antenna elements are used to form an 8 × 8 array for finite array simulation.Due to the fact that the number of array elements is too small,the serious truncation effect has led to the impedance bandwidth shifting.In the end,the impedance bandwidth of 4: 1(0.5 ~ 2GHz)is finally achieved,and the profile height is 0.2 times the high-frequency wavelength.Finally,a low-profile ultra-wideband tightly coupled antenna array with the artificial magnetic conductor structure is fabricated and measured.The test results agree well with the simulation results... |
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