Stealth technology encompasses various techniques to minimize the detection of acoustic, visual, infrared, and electromagnetic (EM) wave signals. Specifically, to minimize the radar cross section (RCS) of the radar system, radar-absorbing structures (RAS) capable of absorbing EM waves are essential. RAS composed of composites can be damaged, and both their structural and EM performances become important issues in RAS studies. Therefore, in this study, EM wave interactions in damaged specimens were evaluated and visualized using a scanning free-space measurement (SFM) system. First, the change in the EM performance due to damage was analyzed using Al-alloy 6061 and Teflon specimens. When the damage size was less than one-half wavelength (30 mm and 20 mm) in both the X (8.2 similar to 12.4 GHz) and Ku (12.4 similar to 18 GHz) bands, the return loss of Al-alloy 6061 was close to 0 dB and the real part of the permittivity was between 2 and 2.1. On the other hand, when the damage size was more than one-half wavelength, the EM performance in terms of the return and insertion loss and the EM properties of the material (such as the permittivity) were changed in both Al-alloy 6061 and Teflon specimens. Second, a nickel fiber-based RAS with lightning strike-induced damage was also tested by the SFM system, and a behavior similar to that of non-stealth specimens was observed. The RAS was also tested by a pulse-echo ultrasonic propagation imager (PE-UPI) system to investigate the internal damage induced by the lightning strike. Much larger delaminations than the penetration occurred at different plies of the RAS. Eventually, it was found that not only the penetration damage but also the delaminations predominantly affected the EM performance, which caused the overall stealth degradation.