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A millimeter-wave passive Van Atta retrodirective antenna (RDA) array is presented in substrate integrated waveguide technology. The proposed RDA design is based on alternating SIW slot antenna arrays whose longitudinal slots are etched in a continuous ground plane covering the entire circuit thus minimizing the diffraction at the edges of the feeding network. Owing to its closed guiding medium and high Q factor, the SIW feed topology provides an optimum coupling to the radiating elements with minimum losses and spurious feed radiation allowing a simple passive design with enhanced radar cross section (RCS). Two SIW-based prototypes are presented in both single- and double-layer configurations at 30 GHz. While the single layer design can be extended to larger RDAs with minimum losses, the double-layer configuration is exploited to provide a more compact design that reduces the scattering of the structure while providing the same re-radiated pattern of the corresponding single layer configuration. Simulated and measured results are in good agreement showing more than 10 dB enhancement in the monostatic RCS of the passive RDA over an angle of from boresight, compared to the equal-size reference rectangular metallic sheet.