Interrupted-sampling repeater jamming (ISRJ) provides a novel coherent-jamming mode against wideband radar. ISRJ allows the single-antenna jammer to periodically sample and repeat a fraction of the intercepted signal, which reduces the sampling rate and achieves transmit-receive isolation. The coherent-jamming signal generated by ISRJ can form multiple verisimilar false targets when received and processed by the victim radar receiver. Moreover, some false targets can precede the real target. This paper surveys the use of ISRJ in linear frequency modulated (LFM) radar jamming. The theory and application of ISRJ has been researched for more than one decade, but what is missing is a completed summary for the framework of this technique. In this paper, mathematic principles of ISRJ against LFM radars, which utilize matched-filter processing, stretch processing, and range-Doppler processing, are developed. The unique jamming effects in radar systems are focused on when the interrupted sampling frequency of the jammer is smaller than the bandwidth of the radar signal. Specifically, the false-target characteristics, including amplitude, space distribution, and phase, are discussed, respectively. On this basis, the key jamming elements, which determine these false-target characteristics, are pointed out and analyzed in detail. At last, simulation and real data are used to verify the correctness of the analyses. Experimental results highlight the potential application of the proposed jamming mode.

Switchable active frequency selective surface (AFSS) absorber/reflector can work in opposite scattering state and be widely applied in the stealth field. At present, abundant researches on material design have been conducted to obtain excellent electromagnetic (EM) properties. However, there are few studies on the modulation effect of switchable AFSS absorber/reflector on the EM wave. In this paper, a time-domain digital-coding AFSS absorber/reflector is proposed to manipulate the spectral distribution of harmonic. The accurate control of each harmonic is achieved through AFSS modulating parameters. Then, a simple controller based on FPGA is designed. By coding “0” and “1” elements with controlled sequences, the AFSS can be switched flexibly in both absorbing and reflection states. On this basis, the imaging characteristics of time-domain digital-coding AFSS absorber/reflector are further analyzed, and distance transformation phenomena are described in detail. The SAR experiments are performed to demonstrate the required function of distance transformation.

Time-modulated metasurfaces (TMMs) can achieve extraordinary manipulation of electromagnetic (EM) waves and hold remarkable potential in future information fields. However, many limiting factors, including angular stability, polarization insensitivity, and conformal ability, remain unconsidered for the TMM. Moreover, it is a significant challenge to achieve independent control of orthogonal polarization via the TMM while achieving conformal structure. In this article, a flexible multifunctional TMM (MTMM) with polarization control for conformal applications is proposed to manipulate the spectrum distribution. It can simultaneously achieve amplitude, polarization, and frequency control in a single structure with excellent angular stability and conformality. A Doppler effect with multiple discrete frequency components is implemented based on fast temporal modulation. Furthermore, a conformal MTMM for radar application is explored. It possesses the advantage of manipulating radar echo signals in a wide-angle domain. The modulation effect of the conformal MTMM on the radar is analyzed in detail. Multiple discrete false harmonic peaks are generated. Finally, the MTMM characteristics are tested, and the radar experiments are performed, verifying the validity of the structural design and theoretical method.

The rapid development of artificial electromagnetic (EM) materials provides unprecedented freedom to manipulate EM waves. In recent years, reflective spectrum conversion technology based on EM materials has been widely studied due to its multiharmonic generation capability. The possibility of its application in wireless communication is explored. However, the research on spectrum conversion is mainly based on periodic modulation waveforms. The modulation effect of nonperiodic for EM materials has not been studied so far. In this article, a nonperiodic time-varying active frequency selective surface (AFSS) is proposed to manipulate the spectral distribution of harmonics. By dynamically controlling the amplitude of the reflected echo, a continuous Doppler spectrum is generated. On this basis, the imaging characteristics of nonperiodic time-varying AFSS are analyzed in detail, and the defocusing phenomenon of the target image is discovered. Finally, the radar imaging experiments for nonperiodic time-varying AFSS are conducted to verify the validity of the proposed method.

The discovery of electromagnetic (EM) metasurfaces has significantly advanced the free manipulation of EM waves. Further, the harmonic phenomena based on time-modulated metasurfaces (TMMs) have received much attention in communication and radar detection applications, which can reduce the efficiency of radar target detection. However, existing research on time-varying metasurface has neglected its potential to manipulate the scattering characteristics of polarized radar targets. When a linear polarization wave is incident on a metasurface, it will be reflected into two orthogonal polarization states of EM waves, thus directly affecting the polarized scattering of the target. Active Polarization conversion metasurfaces (APCM) can manipulate the polarization of electromagnetic waves, aligning well with polarimetric radar. In this work, a dual-polarization radar target feature manipulation (Pol-TFM) method based on APCM is proposed, which enables manipulation of the harmonic and energy distribution of radar echo signals in multiple polarization channels. Through this processing, virtual targets generated by harmonic effects become observable on the dual-polarized high resolution range profile (HRRP). This method also allows for the position manipulation of the virtual targets. Finally, echo signal experiments are conducted in dual-polarimetric channels to verify the effectiveness of the proposed method.