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.

This paper demonstrates a miniaturized flexible multifunctional active frequency selective surface (MAFSS) with excellent angular stability and polarization insensitivity. The proposed structure consists of two metal layers printed on a thin, flexible substrate to achieve curved surface conformal capability. The PIN diodes are welded on two metal layers. By independently switching the bias voltage of PIN diodes, four working states and two functions, including electromagnetic switching and polarization selection, are realized. The unit cell takes itself as a bias feeder to excite the PIN diodes, avoiding the negative impact of the additional feeding network on the overall performance. A spiral wire-based miniaturization technique is utilized to achieve excellent angular stability in the planar and conformal states. On this base, a double-layer symmetrical structure based on the via-connected metal strips is designed to realize polarization insensitivity. The miniaturized unit size can reach 0.082λ ₀ ×0.082λ ₀ ×0.005λ ₀ at the center resonance frequency. The flexible MAFSS prototype is measured in planar and conformal states. The measured operating bandwidth with transmission coefficient difference greater than 10 dB is 2.8 GHz (5.2 GHz ~ 8 GHz). The measured and simulated results have good consistency, demonstrating the effectiveness of the proposed structure.

Synthetic aperture radar (SAR) image modulation is an effective method to protect a target of interest, and thus receives intensive attention. Currently, it still remains challenging to design an image modulation method capable of both flexibility and diversity in a passive way. In this letter, a two-dimensional (2-D) image modulation method based on phase-switched screen (PSS) is proposed against the SAR. The method imposes phase modulation onto the radar reflected signal by the PSS so that 2-D multifalse targets with verisimilar characteristics are formed around the protected target position when received and processed by the victim radar receiver. The theoretical analysis indicates that the amplitude and position of the generated multifalse targets are precisely controlled by the PSS modulating parameters. Moreover, the protected target is concealed and cannot be detected by radar. Simulations and experimental results are conducted to illustrate the validity of the proposed method.