This communication presents a novel frequency-selective surface (FSS) with high in-band transmission at high frequency and wideband absorption at low frequency. It consists of a resistive sheet and a metallic bandpass FSS separated by a foam spacer. The resistive element is realized by inserting a strip-type parallel LC (PLC) structure into the center of a lumped-resistor-loaded metallic dipole. The PLC resonates at the passband of the bandpass FSS and exhibits an infinite impedance, which splits the resistive dipole into two short sections per the surface current; this setup allows for high in-band transmission at high frequency. Below the resonance frequency, the PLC becomes finite inductive and the entire FSS performs as an absorber with the metallic FSS as a ground plane. The surface current distribution on the resistive element can be controlled at various frequencies via the PLC structure. The wideband absorption and high in-band transmission of the proposed design are verified by both numerical simulation and experimental measurements. The potential extension to polarization-insensitive designs is also discussed.

A novel frequency-selective rasorber (FSR) is proposed in this paper which has a nearly transparent window between two absorption bands. The FSR consists of a resistive sheet and a bandpass frequency-selective surface (FSS). The impedance conditions of absorption/transmission for both the resistive sheet and the bandpass FSS are theoretically derived based on equivalent circuit analysis. The insertion loss of FSR at the resonant frequency of lossless bandpass FSS is proven to be only related to the equivalent impedance of the resistive sheet. When the resistive sheet is in parallel resonance at the passband, a nearly transparent window can be achieved regardless of lossy properties. An interdigital resonator (IR) is designed to realize parallel resonance in the resistive element by extending one finger of a strip-type interdigital capacitor to connect the two separate parts of the capacitor. The IR is equivalent to a parallel LC circuit. Lumped resistors are loaded around the IR to absorb the incident wave at lower and upper absorption bands. With the bandpass FSS as the ground plane, the absorption performances at both the lower and upper bands around the resonant frequency are improved compared to a metal-plane-backed absorber structure. The FSR passband is designed at 10 GHz with an insertion loss of 0.2 dB. The band with a reflection coefficient below -10 dB extends from 4.8 to 15.5 GHz. A further extension to dual-polarized FSR is designed, fabricated, and measured to validate the proposed design.

This paper presents a miniaturized frequency selective rasorber (FSR) that has a wide transmission band with low insertion loss (IL) and a wide absorption band below the transmission band. The FSR is composed of a resistive sheet and a bandpass FSS. The unit cell of the resistive sheet is a resistorloaded hexagonal metallic loop (HML), 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 high frequency of 10 GHz, the CSR resonates to be an 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 roll-off 0.5-dB-transmission band of 8.2- 11.33 GHz, which almost coincide 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. Besides, due to its tiny physical sizes of only 1.2mm×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 incident angle. The transmission performance of the FSR has a good independence of and polarizations and incident angles. A prototype of the proposed FSR is fabricated and measured to validate the design.

In this paper, we propose a method for designing checkerboard surfaces using strategically placed artificial magnetic conductors (AMCs) for wideband radar cross section (RCS) reduction. The theoretical analysis shows that the reflection phases of the two AMCs should be linear. The cancelation conditions of checkerboard structure are analytically derived using an equivalent transmission line model, and the ideal impedance conditions of the two AMCs are obtained, which serve as guidance to select patterns of two AMCs unit. A planar checkerboard surface with two kinds of AMC elements is designed for wideband RCS reduction. A 10 dB RCS reduction is observed for approximately 91.5% of the relative bandwidth (3.77-10.14 GHz) under normal incidence. A prototype of the proposed checkerboard surface is fabricated, and the measurements results show good coincidence with the synthetic studies. The bistatic RCS of the checkerboard surface is considered for both transverse electric and transverse magnetic polarizations under oblique incidence.

A novel frequency-selective rasorber (FSR) with dual-band transmission and wideband absorption property is proposed in this letter. The FSR is a two-layer structure composed of a lossy resistive sheet at the top and a lossless bandpass frequency selective surface (FSS) at the bottom, separated by an air spacer. The element of the resistive sheet is a lumped-resistor-loaded metallic dipole with a dual-resonance structure inserted in the center. The dual-resonance structure is realized by three different series LC structures in parallel, based on the Foster reactance theorem. There are two impedance poles between the three impedance zeros of the series LCs, where two transparent windows can be obtained for the lossy resistive sheet. The bandpass FSS is a simple dual-band FSS by etching two slots with different shapes and dimensions in a metallic plane. The FSR has realized two transmission bands at 7.7 GHz and 12.6 GHz, and the band with $\rm \vert S11\vert < -10 dB$ is 3.7-11.1 GHz. The design has been validated by full-wave simulation and experimental measurement.

This paper proposes a wide transmission band frequency selective rasorber (FSR) based on convoluted resonator (CR). The FSR consists of a two-layered metallic structure with an air gap in the middle. At the top layer, the lumped-resistor-loaded metallic dipole with a CR inserted into the center is applied to generate wide absorption and transmission bands. At the bottom layer, the wideband FSS is realized by a triple-layer metallic structure in which two layers of identical square patches are coupled through a layer of the square aperture. The full-wave simulation results show that the wide transmission band with low insertion loss and wide absorption band are realized. To validate the performance of the proposed FSR, the prototypes are fabricated and measured, and reasonable agreement between the simulated and measured results is obtained. The measured results show that the transmission band with insertion loss less than 1dB is covered from 9.00GHz-12.63GHz with 33.5% relative bandwidth. Compared with the previous FSRs with wide transmission band, the proposed FSR yields the widest relative bandwidth, better oblique performance, simpler design.