Design of a salisbury screen absorber using frequency selective surfaces to improve bandwidth and angular stability performance

A new design method that greatly enhances the reflectivity bandwidth and angular stability beyond what is possible with a simple Salisbury screen is described. The performance improvement is obtained from a frequency selective surface (FSS) which is sandwiched between the outermost 377 Omega/square resistive sheet and the ground plane. This is designed to generate additional reflection nulls at two predetermined frequencies by selecting the size of the two unequal length printed dipoles in each unit cell. A multiband Salisbury screen is realised by adjusting the reflection phase of the FSS to position one null above and the other below the inherent absorption band of the structure. Alternatively by incorporating resistive elements midway on the dipoles, it is shown that the three absorption bands can be merged to create a structure with a - 10 dB reflectivity bandwidth which is 52% larger and relatively insensitive to incident angle compared to a classical Salisbury screen having the same thickness. CST Microwave Studio was used to optimise the reflectivity performance and simulate the radar backscatter from the structure. The numerical results are shown to be in close agreement with bistatic measurements for incident angles up to 40 degrees over the frequency range 5.4-18 GHz.