Single-layer quad-band FSR with enhanced angular stability for S- and C-band applications

A single-layer, polarization-insensitive frequency-selective rasorber (FSR) with quad-band absorption performance is proposed. The presented FSR is developed by integrating a quad-band resonant absorber and a single bandpass frequency-selective surface (FSS) on opposite sides of an FR4 substrate. In the absorption bands, the top layer effectively absorbs electromagnetic waves, while the backside FSS functions as a reflector. In the transmission band, the top layer becomes nearly transparent, allowing efficient wave passage through the backside FSS. The combination of the two layers results in effective multi-functional performance. The top layer achieves over 90% absorption at 2.14, 2.88, 6.02, and 7.32 GHz. The backside FSS generates a transmission band at 4.54 GHz with low insertion loss. The design exhibits enhanced angular stability up to 50° for both TE and TM polarizations, with polarization insensitivity ensured through four-fold rotational symmetry. The physical mechanism is analyzed using an equivalent circuit model and effective-medium analysis. A 15 × 15 prototype has been fabricated and measured, showing excellent agreement with simulations. This resistor-free single-layer FSR is suitable for radar cross-section (RCS) reduction, electromagnetic interference (EMI) mitigation, and multi-functional radome applications in S- and C-bands.