Transmission upper limit of band-pass double-layer FSS and method of transmission performance improvement

The transmission upper limit of a double-layer frequency selective surface (FSS) with two infinitely thin metal arrays is presented based on the study of the general equivalent transmission line model of a double-layer FSS. Results of theoretical analyses, numerical simulations and experiments show that this transmission upper limit is independent of the array and the element, which indicates that it is impossible to achieve a transmission upper limit higher than this one under a given incident and dielectric-supporting condition by the design of the periodic array. Both the applicable condition and the possible application of the transmission upper limit are discussed. The results show that the transmission upper limit not only has a good reachability, but also provides a key to effectively improve the transmission performance of a double-layer FSS or more complex frequency selective structures.