Investigation into the use of active frequency selective surfaces to extend the absorption bandwidth of a conventional Salisbury screen absorber
It is well accepted that the absorption bandwidth of a metal back-plane absorber, built with either dielectric or magnetic materials, is inherently narrow. It is also well known that in order to increase the absorption bandwidth, the absorber thickness must be increased through decreasing the permittivity or permeability of its spacer. This improved performance, however, comes at a cost. The absorption bandwidth is increased at the expense of not only an increase of absorber thickness, specially at lower frequencies, but also the yielding of a mechanically weaker structure. The most important implication of the former is that there is a tradeoff between absorber thickness and absorption bandwidth. These two conflicting absorber properties are, however, of equal importance since the optimum absorber is one which has a small thickness as well as a wideband absorption response. This inherent trade-off is due to the fundamental frequency limitations imposed by the constitutive parameters of materials and is more detrimental at microwave frequencies. The aim of the research programme described in this thesis was thus to investigate the use of adaptive complex impedance structures, in the form of active frequency selective surfaces (AFSSs), to extend the absorption bandwidth of a small thickness Salisbury screen absorber, thus addressing directly the aforementioned by minimizing the trade-off that exists between absorber thickness and absorption bandwidth.