A dual-polarized frequency-selective rasorber (FSR) with a wide high-transmission passband and two absorption bands is proposed and investigated in this paper. The proposed FSR, comprised of a lossy and a lossless layer, is formed by introducing a broadband transmission window into a wider absorption band. Simple guidelines based on impedance analysis are proposed, and the wide high-transmission passband is realized by connecting multiple coplanar parallel resonances (MCPRs) in series which introduces a large impedance in a wide band for the lossy layer. In this design, the lossy layer and lossless layer consist of a square ring absorber with multiple interdigital resonators (IRs) and a multi-layer broadband frequency selective surface (FSS) respectively. The simulation results indicate that the 1dB transmission window can be obtained from 8.12GHz to 11GHz with a 30.4% fractional bandwidth (FBW), and the bandwidth of reflection below -10dB is up to 111% from 4GHz to 14GHz. Finally, the proposed structure is fabricated and measured, both the simulation and experimental results agree well, which suggests that the concept of connecting MCPRs in series can generate a broadband transmission window effectively.

In this paper, a novel water-based reconfigurable frequency selective rasorber (FSR) at microwave band is proposed, which has a thermally tunable absorption band above the transmission band. By breaking through the technical barriers of traditional FSR reconfigurable feature design, we have achieved water-based FSR for the first time in a public report. The water-based FSR consists of a bandpass frequency selective surface (FSS) and a 3D-printed container. The water substrate is filled into the sealed space constructed by the above two structures. Numerical results show that the FSR can achieve absorption with high absorptivity from 8.3 to 15.2 GHz, and obtain a transmission band of 5.2 to 7.0 GHz. The minimum insertion loss of the transmission band reaches 0.72 dB at 6.14 GHz. In addition, the FSR has the reconfigurable characteristics of absorbing or reflecting electromagnetic waves by filling with water or not. Due to the Debye model of water, the absorption band can be adjusted by water temperature, while the passband remains stable. At last, the designed FSR based on water has been fabricated, and experimental results are presented to demonstrate the validity of the proposed structure.

A novel hybrid reflection method is proposed to realize wideband gain enhancement and radar cross section (RCS) reduction properties for Fabry-Perot (FP) antennas. This method adopts two kinds of Frequency Selective Surface (FSS) units, which are etched on both sides of a single-layer substrate with chessboard arrangement to operate separately as a Hybrid Surface (HS) and a Reflective Surface (RS), to form a Hybrid Partially Reflecting Surface (HPRS). The two FSSs with different positive reflection phase gradient bands and reflection amplitudes collaborate to expand the 3-dB gain bandwidth and improve the gain enhancement on the RS. Meanwhile, the different compensation phases of the two FSSs can redirect reflected waves to reduce the RCS on the HS, and phase cancellation is applied between the retransmitted waves and reflected waves to further reduce the RCS. The simulated and measured results verify the correctness of the proposed method. The prototyped FP antenna has a measured 10-dB impedance bandwidth of 8.64-12.07 GHz (33.1%), a 3-dB gain bandwidth of 8.6-11.1 GHz (25.4%) with a peak gain of 17.08 dBi at 9.5 GHz, and an RCS reduction bandwidth of 8.6-14 GHz with a peak RCS reduction of 19.5 dB.

A novel dual-polarized frequency-selective polarization converter (FSPC) with low-backward scattering is proposed by combing a band-pass frequency selective surface (FSS) with a polarization converter. The FSPC, comprised of the top polarization conversion layer and the bottom multi-layer band-pass FSS, is formed by introducing two polarization converter bands on both sides of the broadband passband. The four-port equivalent circuit model cascading FSS and polarization converter is proposed and investigated. It is numerically demonstrated that a flat 1dB transmission window can be obtained from 8.5GHz to 11GHz with a 25.6% fractional bandwidth (FBW), and the bandwidth of reflection below -10dB is up to 92% from 5.6GHz to 15.13GHz. Moreover, the proposed device can achieve two polarization conversion bands (5.66GHz-6.9GHz and 12.8GHz-15.2GHz) with the polarization conversion ratio over 90%. Besides, by arranging the proposed structure in a checkerboard-like distribution, the backward scattering energy can be reduced in a wide frequency band ranging from 4.5GHz to 16GHz. Both simulation and experimental results are in good agreement, which demonstrates our design strategy. Compared with the conventional polarization converters, the proposed design presents an extra frequency-selective performance and hence can be a great substitute for the frequency selective rasorber (FSR).

A miniaturized high selectivityfrequency selective rasorber (FSR) with a wide interabsorption high transparent bandpass based on multiple 2.5-dimensional (2.5-D) resonators is investigated. The main contribution is the achievement of a wide and high transparent passband by combining the large inductance of parallel resonators (PRs) and cascading two 2.5-D PRs, which also provide the function of miniaturization. The 2.5-D PR is implemented by connecting the interdigitated capacitor and the metal meandered strip-line on both surfaces of a lossy layer through metalized vias. The inductor and capacitor values of 2.5-D PR can be individually adjusted to shift the passband's central frequency. The lossy element is realized by inserting two 2.5-D strip-type PRs at the center of each side of a meandered cross-dipole loaded with two resistive sheets on both sides. An equivalent circuit model is proposed to analyze its operating principle. The dimensions of the miniaturized element are 0.12 λ _L × 0.12 λ _L × 0.118 λ _L . While maintaining a good wide passband (−1dB relative bandwidth is 21.2%), the miniaturized FSR satisfies the characteristic of polarization insensitivity (TE and TM), and angular insensitivity (up to 45°). A prototype of miniaturized FSR has been manufactured and measured, showing a reasonable agreement with simulations.

In this study, a new broadband and conformal metamaterial absorber using two flexible substrates was proposed. Simulation results showed that the proposed absorber exhibited an absorption band from 6.08 to 13.04 GHz and a high absorption of 90%, because it was planar. The absorber was broadband as its relative absorption bandwidth was 72.8%. Moreover, the proposed absorber was insensitive to the polarization of the TE and TM waves. The absorber was ultra-thin; its total thickness was only 0.07l at the lowest operating frequency. Furthermore, different regions of absorption can be adjusted by lumping and loading two resistors onto the polyimide film, respectively. Moreover, compared with the conventional microwave absorber, the absorption bandwidth of the proposed absorber can be broadened and enhanced when it was bent and conformed to the surface of objects. Experimental and simulation results were in agreement. The proposed absorber is a promising absorbing element in scientific and technical applications because of its broadband absorption, polarization insensitivity, and flexible substrates.