Recently, the problems about the absorption and the interference shielding of the electromagnetic wave have been very important issues for commercial and military purposes. The main purpose of stealth technique is to reduce the Radar Cross Section (RCS) of the aircrafts or warships. Many studies about the Radar Absorbing Materials (RAM) and the Radar Absorbing Structures (RAS) have been performed for many years. This study deals with the RAS having sandwich structures in the X-band (8.2~12.4GHz) frequencies. Generally, sandwich structures have an advantage of high flexural stiffness to weight ratio over laminated composites. Glass/epoxy composites containing conductive carbon blacks and carbon/epoxy composites were used for the face sheets. Polyurethane (PU) foams containing multi-walled carbon nano tube (MWNT) were used for the core. Their permittivities in the X-band were measured using the transmission line technique. The reflection loss characteristics for multi-layered sandwich structures were obtained using the theory about transmission and reflection in a multi-layered medium. Three cases were selected for fabrication of specimens. Their reflection losses in the X-band were measured using the free space technique. Experimental results were in good agreements with simulated ones in 10 dB absorbing bandwidth but center frequencies of them had small changes.
