# Retrospective Spectrum-Conversion Method Based on Time-Modulated Van Atta Array
## 1 Introduction
On this basis, a novel wireless communication system based on digital-coding metasurface is proposed to transmit real-time signals with excellent performance, which significantly simply the architecture of modern communication systems [31; 32]. However, in the above method, the modulation system and manipulation schemes are complex because the pin diodes within the unequal unit or column are controlled by different waveform. At the same time, the prior information of the incoming wave often needs to be acquired to support the pointing of the reflected wave. The [PERSON] array is an excellent option to realize spectrum conversion in a wide-angle domain. It is a new beam adaptive passive antenna array based on phase conjugation technology [33; 34], which has been widely used in target calibration, satellite communication, and radio frequency identification fields [35; 36]. The array is simple in design and automatically reflects the incident EM wave toward the source direction without prior knowledge, so it can accurately achieve directional backscattering characteristics of incident waves at different angles [37]. At present, abundant research has been performed to obtain wide operating bandwidth and strong energy. [PERSON] [38] proposes the circular polarization characteristics of planar passive Van Atta arrays for vehicle radar applications. The designed array utilizes the baseband coaxial line as the feedback network, which can further expand the working bandwidth of the array. However, the above research on the array mainly focuses on its static EM properties, while the dynamic modulation effect on electromagnetic (EM) waves is rarely reported. [PERSON] [39] describes a retro-reflective Van Atta array to modulate radar signal in a large angle range. [PERSON] [40] reports experimental results on a completely passive antenna carpet device inspired by the retroreflector configuration of the Van Atta array. A large electric triangular bulge is designed on the ground plane, its sides covered by two conventional patch antenna arrays. The antennas are connected to each other in a Van Atta style connection, which ensures the recovery of the reflection angle and simulates the reflection of the ground plane at different lighting angles. [PERSON] [41] discusses a possible antenna-based cloak that uses a triangular bulge in one ground plane obscured by a pair of conventional half-wavelength dipole arrays. It simulates the reflection of the ground plane over the operating band of the antenna element frequency. Due to the adaptive characteristics of the proposed configuration, a wide angular bandwidth is achieved. Inspired by the above ideas, a spectrum-conversion method with flexible harmonic modulation capability in oblique incidence direction is reported in this paper. The designed structure is a Van Atta array based on time modulation. On the basis of wide-angle backtracking characteristics, the on-off control of Van Atta array is realized in combination with electronically controlled microwave switching elements, and the dynamic modulation effect of EM wave is finally realized. The array structure includes eight columns of rectangular series resonant microstrip patch antenna elements, connected microstrip transmission lines, and single-pole single-throw (SPST) switches based on PIN diode. Excited by a rectangular pulse of bias signal, the array is able to intermittently switch between high-scattering and low-scattering states.