Design of a Dual-Passband Frequency-Selective Rasorber With High Selectivity

A dual-passband frequency-selective rasorber (FSR) with high selectivity is proposed in this letter. By analyzing the equivalent circuit model, the design approach improving the selectivity of the dual passband is introduced. The FSR is formed by a lossy layer and two frequency-selective surface layers. The lossy layer is composed of two parallel resonance units embedded in a rhombic patch, which is connected with four resistors loaded on the T-shape metal strips. The lossless layer consists of two layers of double square slots to provide highly selective passbands with second-order response. The simulated results show that two transmission bands are achieved at 7.2 GHz and 10.8 GHz with the insertion loss of 0.86 dB and 0.57 dB, and the fractional bandwidth for $|S_{11}|$ $< $ $-$10 dB is 94.8%. Both sides of the transmission bands exhibit steep roll-off characteristics, with the coefficient of steep drop being 41.18, 77.78, 31.82, and 17.5, respectively. The unit cell size is $0.18 \lambda \times 0.18 \lambda \times 0.19 \lambda$ ($\lambda$ is the wavelength at the lowest operating frequency). A prototype of the proposed FSR was fabricated and measured. The measured results are consistent with the simulation results.