According to that article, the most common use of a Luneberg lens is as a reflector, returning the RF back to its source, and I would guess that is what you wou

According to that article, the most common use of a Luneberg lens is as a reflector, returning the RF back to its source, and I would guess that is what you would want it to do when your goal is to enhance its visibility to conventional radar. I do not think this would help passive radar, however, even if the lens functions as a retro-reflector at the frequencies used by the passive radar, as the sources and detector are typically a long way apart. These lenses presumably have some isotropic scattering at the frequencies used by this passive radar, and the question is, how much?