| Electromagnetic(EM)stealth technology can be implemented to decrease the effective scattering section of military target,which realizes a corresponding reduction of enemy radar detection range.The penetration capability and survivability of military weapons can be significantly improved with the use of EM stealth method.With the fast development of radar detection technology,however,the circumstances of military equipment are getting more complicated and changeable,conventional stealth methods cannot efficiently meet the needs of current and future practical applications.Therefore,how to design a flexibly tunable and wideband Radar Cross Section(RCS)reduction technique,and the exploration of new EM stealth technology will be increasingly urgent and necessary.EM surface is defined as a novel artificial-designed periodic structure,which can flexibly control the EM response of the surface.Hence,EM surface has broad application prospects in the fields of RCS reduction and exploration of new EM stealth technology.In this dissertation,tunable methods on the scattering characteristics of EM surfaces and the spread spectrum stealth mechanism of the modulated echo signal are studied.The main contents of this work are divided into four parts:two controllable degrees of freedom(polarization state and operational frequency)of metamaterial absorber,the wideband tunable reflection performance of active frequency selective surface(AFSS)absorber,ultra-wideband RCS reduction based on artificial chessboard and tunable absorber,different effective stealth mechanisms of the modulated echo signal.The main contributions and innovations of this dissertation are as follows:1.An active absorbing structure with two electrical control degrees of freedom(polarization state and operational frequency)is presented.Firstly,different metallic patterns are combined to achieve effective absorption at three frequency points with compact and polarization insensitive properties.The equivalent circuit model,multi-reflection interference,and effective media theory are used to analyze the resonant triple absorption behavior.Next,one PIN diode is inserted in the unit cell to control the working frequency and polarization state simultaneously.The conduction condition of induced current can be changed by switching the PIN diode between ON and OFF states,then the resonant frequency and polarization property are controlled accordingly.Besides,the current distributions are studied under different states in simulation,which validate the corresponding relationship between local resonant modes and relative absorption performance.Meanwhile,the absorption measurement in anechoic chamber has demonstrated the flexible manipulation and effective absorption of the designed active metamaterial absorber.2.An AFSS absorber with wideband tunable bandwidth is designed.The inserted diode can be equivalent as a variable resistor under different forward bias.Dynamical control of the bias voltage for diode can change the induced current distributions and path length,then the absorption response of the AFSS absorber will be altered accordingly.Meanwhile,the analysis of different loss ratios contributing to the total incident energy reveals that the resistive absorption from diodes should be responsible for the high absorption performance.Hence,it is an effective method to control the absorption response of the proposed AFSS absorber by changing the bias voltage on the diode.Measured results show that the active absorber can achieve an adjustable low reflection from 2.5 to 9.1 GHz.The proposed tunable method can benefit the research on transient modulated echo,and it is also a promising solution to design ultra-wideband structure with flexibly tunable performance.3.An ultra-wideband RCS reduction method based on phase interference cancellation of chessboard and tunable absorption of AFSS absorber is proposed.In higher frequency range,the diagonally arranged chessboard is constructed of two different kinds of Artificial Magnetic Conductors(AMCs),which exhibits a wideband effective phase difference between the square loop and circular patch unit cells.Simulated results show that the RCS reduction is more than 10 d B in the frequency range from 14.6 to 36.5 GHz,and the space diffusion effect of EM energy contributes to the low scattering property.Concerning the lower frequency band,a square loop AFSS absorber integrated with PIN diodes is designed to achieve a low reflection level from 4.2 to 14.9 GHz.The RCS reduction effect in the lower frequency band should be attributed to tunable ohmic absorption,i.e.,the equivalent resistance effect of PIN diodes in the bottom AFSS absorber under different bias voltages.The multilayer composite structure is designed with the chessboard and AFSS absorber stacked in vertical direction,and the ultra-wideband property can be realized by overlapping theses two different low reflection effects.Experimental results exhibit a variable RCS reduction of more than 10 d B with an ultra-wideband frequency bandwidth from 4.4 to 36.1 GHz.4.Based on the effective combination of target EM scattering theory and radar signal processing principle,a novel modulation spread spectrum stealth technology is proposed in this dissertation.The previous studies on different kinds of tunability have contributed to the analysis of reflection magnitude and phase from the designed composite structure.By analyzing the echo magnitude-phase information change with the transient time-varying property of A.C.energized signal,the reflection modulation effect in frequency domain is calculated theoretically.From the target echo measurement using spectrum analyzer,the modulation spread spectrum characteristics are observed at different center frequencies.Meanwhile,the suppressions at carrier frequency points are compared under the A.C.energized and non-energized situations.Besides,the reduction levels under square wave signals with different duties are deeply studied.In addition,the comprehensive analyses of the modulation spread spectrum phenomenon,in-band residual power,harmonic feature and the effect of energized signal on the echo spectrum characteristics are investigated under different loaded signals.From the theoretical calculation,compared to unmodulated echo,the reductions of in-band residual power and output signal-to-noise ratio in modulated echo signal can achieve 9.4 d B and 12 d B,respectively,which have demonstrated the comprehensive stealth performance of the proposed novel EM stealth approach. |
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