Physical Selection and Verification of a Trusted Source Signal Using an Angle-Selective Metasurface

The development of physical-level information security methods allows us to supplement traditional cryptographic tools with instruments directly related to signal propagation conditions and the spatial location of a signal's source. Previous work by the authors demonstrated that when transmitting commands to a group of unmanned aerial vehicles (UAVs), the coordinates of a trusted radio transmitter or the direction to it can be determined based on joint measurements by several UAVs during secure data exchange within the group, for example, over fiber-optic lines. For practical purposes, protecting a command channel requires high localization accuracy: if the operator's location is known a priori and the system operates within direct radio visibility, it is sufficient to establish that the received signal belongs to a trusted angular sector. However, determining the direction alone does not solve the problem of selecting the desired signal in the presence of intentional jamming or spoofed signals. In this paper, we propose the use of an angle-selective metasurface for the second level of protection. A two-channel scheme with a reference receiving channel and a channel with a flat metasurface is proposed. A periodic unit cell for the 6 GHz band was developed, and full-wave simulations were performed in Ansys HFSS. A pronounced S₂₁(f, θ) dependence was obtained in the 5.8-6.2 GHz band, with incidence angles varying from 0° to 15°. The maximum local contrast is approximately 5.67 dB, and the band-averaged absolute change relative to normal incidence is approximately 2.1-4.7 dB. It is shown that due to the non-monotonicity of S₂₁(f, θ), the metasurface should be used not as a standalone absolute angle meter but as a physical selector and signal verifier after pre-determining the trusted direction.