Studies On The Airborne Low Frequency Antenna And Low Scattering Antenna Based On The Characteristic Mode Theory

With the rapid development of modern military technology and information technology,air strike forces have gradually become the dominant forces in the modern military.Military aircrafts,including stealth fighters,play key roles in the battlefield.As an indispensable part of the aircraft's multifunctional integrated electronic system,the design of airborne antennas,especially the design of airborne low-frequency antennas and low-scattering antennas,still faces many challenges.Based on the requirements of research projects,this work focuses on the airborne low-frequency antenna and low-scattering antenna using the theory of characteristic mode(CM).The dissertation mainly includes two parts.The first part is the airborne low-frequency antenna design using the CM theory.It covers the design of airborne low-frequency multi-antenna system and the design of airborne low-frequency beam scanning antenna array.The second part is the low scattering antenna design using the CM theory.The major work and achievements of this dissertation are summarized as follows:1.The CM theory including the formula derivation,modal characteristics,and definitions and physical meanings of CM-related quantities for the perfectly electrically conducting(PEC)bodies and PEC structures in multilayered medium is summarized.To better illustrate the implementation of the CM theory in antenna designs,a CM-based method of dual polarized slot antenna design is presented.This method shows a systematic design flow of the bandwidth-enhanced dual polarized slot antenna using CMs.It is not necessary to consider the form and position of the feeding structure at the initial stage of the antenna design.Valuable information of radiating modes provides guidelines for optimization of the shape and size of the slot itself,and consequently improve the antenna performance.The proposed antenna design method is systematic and offers clear physical explanations for the radiation mechanism.It can be applied to the airborne slot antenna designs and slot antennas on other conducting platforms.2.An approach for the design of aircraft-integrated multiantenna system based on the theory of characteristic modes(CMs)is proposed.By investigating the CMs of the aircraft body in the entire HF band,three types of radiation patterns are synthesized at three frequency bands centered at 8 MHz,18 MHz,and 28 MHz,respectively.The synthesis is achieved through linear weighting combinations of the first few dominant CMs,and a multi-objective evolutionary algorithm is implemented to seek the complex weighting coefficients in each band.Following the theoretically synthesized radiating currents,three pairs of exciters are then designed and flush mounted on optimal locations over the aircraft.Furthermore,properly designed LC matching networks are introduced in each exciter to get good impedance matching in each band.Because of the large physical size of the aircraft,scaled model of the proposed aircraft-integrated multi-antenna system was fabricated and experimentally verified.Measurement results show that the impedance bandwidth and realized gain of the airborne multiantenna system proposed in this paper are far superior to traditional HF airborne antennas.3.An approach for the design of VHF aircraft-integrated antenna arrays with scanning beams by using the characteristic modes(CMs)of the aircraft platform is proposed.The CMs of the aircraft platform provide the modal significances,modal currents,and modal radiation fields of each mode.An in-depth investigation over the useful CMs information is performed to prove the feasibility of VHF band antenna array design using the CMs of the platform.Scanning radiation patterns with horizontal polarization are then synthesized from these CMs.The scanning capability and polarization constraint make it more challenging than CM based fixed power pattern synthesis.The polarized scanning beam synthesis is carried out with a multi-objective evolutionary algorithm.A trade-off among the synthesized current distributions for different scanning angles is made to determine the optimal current excitation locations for antenna array development.Meanwhile,a novel bandwidth enhanced triple-couple-line coupling element is proposed to allow platform flush mounted designs.A scaled model of the proposed aircraft-integrated antenna array system was fabricated to experimentally verify the effectiveness of the proposed approach.The experimental results show that the airborne antenna array has low cross polarization,high gain,and beam scanning characteristics.It has obvious advantages in the design of the airborne low frequency antenna array.4.A systematic approach for in-band scattering cross-section(SCS)reduction of a U-slot patch antenna is proposed.Characteristic mode(CM)analysis is conducted on the U-slot patch antenna to visualize the resonant behavior of the radiating patch.By examining the modal current distributions,an impedance loading location for SCS reduction is determined.Value of the loading impedance is directly calculated from the proposed approach to cancel out the scattering mode SCS and residual SCS.Prototypes of the U-slot patch antennas with reduced SCS and the reference antenna are fabricated.The experimental results show that a maximum in-band SCS reduction of 26 dB has been achieved with the proposed approach.