Metasurface Doppler Cloak for Broadband Radar Stealth

Concealing an aircraft is a multi-faceted endeavor, notably involving radio and infrared frequencies. In a radar stealth context, it often translates to the reduction of the radar cross-section (RCS). However, other routes that take advantage of radar signal processing exist. For instance, a solution has recently been developed, which consists in compensating the motion-induced Doppler shift with a time-modulated metasurface since Doppler radars filter out static targets to avoid being swamped by radar clutter (buildings, trees, etc.). Such a coating, referred to as a Doppler cloak, is able to compensate any frequency shift. However, frequency-modulated radar signals require a broadening of the frequency conversion bandwidth of existing Doppler cloaks, which are all designed for harmonic signals. In this work, the focus is thus placed on a broadband Doppler cloak able to suppress the Doppler information over a wide frequency range. To achieve this, the reflection coefficient of a varactor diode-loaded metasurface is linearized in time to obtain a linear phase ramp necessary to shift the frequency of impinging waves. Numerical and experimental validations are performed using frequency-modulated continuous wave (FMCW) broadband radar signals over the VHF-UHF range.