Comparison of FSS Topologies for Maximising the Bandwidth of Ultra-Thin Microwave Absorbers

The maximum obtainable reflectivity bandwidth is compared for circuit analogue absorbers patterned with two different types of frequency selective surface (FSS). Metal backed resistively loaded FSS designs with thickness in the range of λ/18 to λ/14, were investigated to identify the crossover point below which the bandwidth of structures patterned with nested loops is narrower than a much simpler arrangement consisting of an array of patch elements. The results show that the deployment of a multi-resonant loop FSS, which are widely used to enhance the bandwidth of this class of microwave absorber, is undesirable below a threshold thickness where it is impossible to merge the individual absorption bands resulting from the nested loops. Numerical simulations are compared with radar backscatter measurements that were performed at normal incidence over the frequency range 7-14 GHz.