Reactance-slope-engineered broadband frequency-selective rasorber based on a multi-order equivalent-circuit model

A broadband frequency-selective rasorber (FSR) incorporating a lossy layer with a second-order auxiliary resonant network is proposed to realize broadband transmission and dual-band absorption within a compact configuration. A multi-resonator equivalent-circuit model is developed to describe the electromagnetic coupling between the lossy layer and a broadband bandpass frequency-selective surface, thereby supporting impedance regulation and spectral shaping of the absorption–transmission–absorption response. The auxiliary resonant branches and capacitive-compensation branch provide additional tuning degrees of freedom for broadening the transmission window while retaining out-of-band absorption. The proposed FSR exhibits a −3 dB transmission band from 6.2 to 10.1 GHz, corresponding to a fractional bandwidth of 47.9%. Absorptivity exceeding 80% is achieved over 2.3–5.7 and 10.4–12.0 GHz, corresponding to fractional bandwidths of 85.0% and 14.3%, respectively. The principal spectral features remain identifiable for incidence angles up to 40°, although the upper absorption band becomes more polarization-sensitive at large angles. A prototype comprising a 15 × 15 array of unit cells was fabricated and measured. The measured responses reproduce the principal trends predicted by the full-wave simulations, supporting the proposed circuit-model-guided design methodology.