Miniaturized Frequency-Selective Rasorber With a Wide Transmission Band Using Circular Spiral Resonator

This paper presents a miniaturized frequency-selective rasorber (FSR) that has a wide transmission band with low insertion loss and a wide absorption band below the transmission band. The FSR is composed of a resistive sheet and a bandpass frequency-selective surface (FSS). The unit cell of the resistive sheet is a resistor-loaded hexagonal metallic loop, each side of which is inserted with a circular spiral resonator (CSR) in the center. The CSR is equivalent to a parallel LC circuit that has a high inductance and low parasitic capacitance. At a high frequency of 10 GHz, the CSR resonates to be infinite impedance, around which a 0.5 dB transmission band of 8.68-11.34 GHz is produced. The bandpass FSS is a triple-layer FSS in which two layers of identical hexagonal patches are coupled through a layer of hexagonal aperture; it has a fast rolloff 0.5 dB transmission band of 8.2-11.33 GHz, which almost coincides with that of the resistive sheet. The 1 dB transmission band of the FSR by placing the resistive sheet on the bandpass FSS is 8.3-11.07 GHz. At low frequency, the FSR performs as an absorber, and the 10 dB absorption band is 2.4-7.1 GHz. In addition, due to its tiny physical sizes of only 1.2 mm x 1.6 mm, the CSR can be viewed as a lumped element circuit with a high inductance, in which the parallel resonance is almost independent to the incident angle. The transmission performance of the FSR has a good independence of polarizations and incident angles. A prototype of the proposed FSR is fabricated and measured to validate the design.