Design of a frequency selective rasorber for wideband transmission and dual-band absorption in L-S-C bands

This work investigates the design of a frequency selective rasorber (FSR) operating in L, S, and C bands with an absorption-transmission-absorption response, characterized by a wide transmission window. The FSR structure comprises a three-layer FSS stack-up with two air gaps in between, presenting a considerably reduced thickness of 31.275 mm (0.156λ_L, free space wavelength at the lowest operating frequency). The design process exploits an equivalent circuit analysis supported by accurate numerical simulations, providing comprehensive guidelines and optimizing the computational burden. Accurate numerical simulations and measurements on fabricated PCB prototypes were carried out, and a satisfying agreement was obtained. The proposed FSR solution achieves a -3 dB transmission window from 2.75 to 5.55 GHz, i.e. a 68% fractional bandwidth. Conversely, the -10 dB absorption bands extend from 1.5 to 2.75 GHz for the lower absorption band (LAB), and from 5.55 to 8 GHz, for the upper absorption band (UAB), with an overall 137% fractional bandwidth for the - 10 dB reflection coefficient. Additionally, the structure shows stable performance for oblique incidences up to 30° and achieves the widest transmission bandwidth with the thinnest electrical profile among previous literature results. These remarkable capabilities will help advancing the FSR state-of-art, promising significant contributions to applications spanning from electromagnetic interference (EMI) shielding to low-observability antenna systems.