Frequency Selective Rasorber Research Articles

Frequency-Selective Rasorber Based on High-Q Minkowski Fractal-Shaped Resonator for Realizing a Low Radar Cross-Section Radiating System

In this article, a novel design of compact frequency-selective rasorber exhibiting absorption–transmission–absorption characteristics and having high passband selectivity has been proposed. A square-loop resonator with mounted lossy elements serves as the elemental broadband absorber from 4 to 12 GHz. The transmission frequency at 8.4 GHz is realized by printing a Minkowski fractal-shaped resonator with a high-Q factor on the resistive layer, which provides a transmission pole at the frequency corresponding to the passband of a double-cross-slot-shaped bandpass layer. The functioning of the proposed frequency-selective rasorbers (FSR) is explained by a corresponding equivalent circuit model. The proposed FSR exhibits higher selectivity at the passband, thus making it a suitable candidate for shielding the narrowband radiating system. Measurements performed on the fabricated prototype of a 17 × 17 unit cell array provides experimental validation. Further, a low radar cross-section antenna is realized by integrating a patch antenna with the proposed FSR, which achieves an average out-of-band monostatic radar cross-section (RCS) reduction of 11.92 and 5.04 dB in the lower (4–7.5 GHz) and upper (9.2–10.8 GHz) absorption bands, respectively, while maintaining the other antenna parameters. Furthermore, the bistatic total RCS reduction of 77% and 60% are achieved in the lower and upper frequency bands, respectively.

Design of a Frequency Selective Rasorber With a Waveform Selective Passband

In this letter, a frequency selective rasorber with a waveform selective passband (FSR-WSP) is proposed based on the combination of frequency selective absorber and waveform selective surface techniques to achieve the multiple functions, including absorbing stealth, energy protection, and waveform selection. The structure achieves more than 90% absorptance in the range of 8.4–18.5 GHz (relative bandwidth reaches 75.1%) and a waveform selective transmission band around 3.8 GHz. For waveform selective function, the transmittance reaches more than 80% when a continuous wave is incident, while the transmittance is less than 25% when a high-power short pulse wave is incident. The absorption characteristics and waveform selective transmission mechanism are analyzed and validated by full-wave simulation and cosimulation. Finally, the FSR-WSP is fabricated and measured to verify its functionality.

A Highly Selective Rasorber With Ultraminiaturized Unit Based on Interdigitated 2.5-D Parallel Resonator

A high selectivity frequency selective rasorber (FSR) with an ultraminiaturized unit based on 2.5 dimensional (2.5-D) parallel resonator (PR), exhibiting low insertion loss passband between two absorption bands, is investigated. The lossy unit is realized by inserting a 2.5-D strip-type PR structure into the center of each side of the metal square ring and loaded with resistors connected at the four corners. The novel 2.5-D PR consists of interdigitated capacitors and strip metal wire connecting the other side of the lossy layer obtained by using metallized vias. The 2.5-D PR can effectively alleviate the congestion of the single-sided structures and achieve a high degree of miniaturization by means of tortuous extension inductance structure; as an additional feature, the values of L and C can be independently adjusted to determine the passband frequency allowing to provide additional degree of freedom to the design. An equivalent circuit model is proposed to analyze its operating principle. The dimensions of the miniaturized unit are 0.13 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> × 0.13 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> × 0.16 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> (being <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> the free space wavelength at the passband). A transmission window with low insertion loss of 0.125 dB is obtained at 4.65 GHz under normal incidence. The fractional bandwidth for −10 dB reflection is about 96%. 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.

Active Frequency Selective Rasorber Based on Fiber-Reinforced SiO$_{2}$ Ceramic Matrix Composite

The radome absorber, always be defined as rasorber, is investigated based on the SiO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{2{\text{f}}}$</tex-math></inline-formula> /SiO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula> fiber-reinforced ceramic matrix composite (CMC) for high-strength environment. The prominent advantage of using CMC as the substrate is that it can significantly improve the power capacity and environmental tolerance of the rasorber. The surface roughness and the dielectric stability are considered key factors in the electromagnetic-material codesign. Silica sol is used to densify the surface of fiber CMC for screen printing, and the process is characterized by scanning electron microscopy. The passband can be controlled by integrating p-i-n diodes on the screen-printed silver paste. The equivalent ground condition for the bandpass reflector is theoretically analyzed, and ideal integration of absorbing and transmitting functions is achieved. Afterward, a CMC-based rasorber is manufactured. A dual-polarized transmission window at 1.6 GHz can be triggered, while a low radar cross section (RCS) band is designed from 2.7 to 4 GHz. Transmission and RCS characteristics of the CMC rasorber under 25–700 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> C environment are both analyzed. The tensile, compressive, and flexural strength of the proposed CMC rasorber are tested, and the electromagnetic properties are verified using compact antenna test range.

Ultra-wide transmission band frequency-selective rasorber using 2.5-D miniaturized structures.

In this paper, a miniaturized frequency-selective rasorber (FSR) with an ultra-wide transmission band (| S21 | <1 dB) is presented. The FSR is made up of a lossless multi-layer cascaded frequency-selective surface (FSS) with a 2.5-D lossy layer above, separated by an air gap. Modified metallic Jerusalem crosses loaded with lumped resistors serve as the unit cell of the lossy layer. An equivalent parallel LC circuit with high inductance and extremely low parasitic capacitance is introduced by zigzag strips and vias, which broaden the transmission band significantly. The equivalent circuit model (ECM) is constructed to provide further insight into the structure. Simulation results show that the proposed FSR exhibits an ultra-wide transmission band of 1-dB from 6.20 to 17.97 GHz (97.4%) with a minimum insertion loss of 0.074 dB. Meanwhile, an 80% absorption band covering 2.58-4.13 GHz is achieved below the passband. For demonstration, an experimental prototype of the 2.5-D FSR is fabricated and measured, and the simulated and measured results are reasonably consistent.

Ultrathin and Simple 3-D Rasorber Based on Ferrites With Embedded Epsilon-Near-Zero Waveguides

An ultrathin and simple three-dimensional (3-D) frequency-selective rasorber with a passband located between two absorption bands is proposed. The absorption bands are obtained by employing commercial magnetic materials in the absorption channel and the passband is realized using epsilon-near-zero tunneling waveguides. The attractively ultrathin and simple feature is achieved by utilizing tunneling effect at the cutoff frequency of metallic waveguides with arbitrary length, permitting the overall thickness shrink into the same as that of the absorber. A prototype with a thickness of only 0.108 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ_c</i> ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ_c</i> is the free-space wavelength at the transmission frequency) is fabricated and measured, exhibiting a transmission band with −3 dB fractional bandwidth (FBW) of 35.7% and a minimum insertion loss of 1 dB, and an FBW of 135.6% for reflection coefficient less than −10 dB. The lumped-components-free and lithography-free features greatly facilitate the fabrication, which is promising in practical low-cost applications.

A Compact Ultrawideband Frequency Selective Rasorber With Hybrid 2-D and 3-D Structure

A compact ultra-wideband frequency selective rasorber (CUWFSR) with hybrid two-dimensional (2-D) and 3-D structure is proposed in this letter. The wide absorption band is obtained by the multiple resonances excited by two dipole resonators and the hybrid structure. By inserting split-ring resonator into the short dipole to bypassing the losses in the passband, an FSR with an internal transmission window and ultrawideband absorption performance is finally realized. The proposed rasorber shows a fractional bandwidth of 140.8% with a transparent window from 14.2–15.8 GHz. Then an improved design which involves two approaches of bypassing the losses of the lossy layer is also provided. Finally, an FSR with a middle passband at 15 GHz and a fractional bandwidth of 137.3% for the absorption rate better than 90% is achieved. The prototype of the improved CUWFSR is fabricated and measured for validation.

A Multifunctional Hybrid Frequency-Selective Rasorber With a High-Efficiency Cross-Polarized Passband/Co-Polarized Specular Reflection Band

A novel multifunctional hybrid frequency-selective rasorber (MHFSR) which shows a broadband high-efficiency cross-polarized passband/co-polarized specular reflection band besides a wideband radar cross section (RCS) reduction is designed and analyzed. The passband/reflection band is realized by the bottom transmissive polarization converter (TPC) which exhibits a broadband cross-polarized transmission window for the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> -polarized incidence and a co-polarized specular reflection window for the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula> -polarized incidence. The design methodology and four-port equivalent circuit model (ECM) are proposed and investigated. By arranging the top layer in the checkerboard-like distribution, the −1 dB cross-polarized transmission band is 9.5–11.7 GHz, and the 10 dB monostatic RCS reduction band is 2.8–20 GHz with a 151% fractional bandwidth (FBW) for the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> -polarized incidence. While for the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula> -polarized incidence, two 10 dB monostatic RCS reduction bands locate at 3.3–8.5 and 15–20 GHz, and the middle −1 dB co-polarized reflection bands locate at 10–11.5 GHz. Meanwhile, the periodicity of the proposed MHFSR is only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0{.}07\lambda _{L} $ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{L} $ </tex-math></inline-formula> is the free-space wavelength at the lowest working frequency (2.8 GHz). The experimental results of the fabricated sample agree with the simulation results well, which proves the design validity. The proposed design combines the design of FSR, diffusion-like metasurface, and TPC to present an extra cross-polarized passband/co-reflection band besides the ultrabroadband RCS reduction.

An Ultrathin Frequency Selective Rasorber Based on Electromagnetic Composite Absorption

In this article, an ultrathin frequency selective rasorber (FSR) based on electromagnetic composite absorption is proposed. At first, a subwavelength unit (SLU) for electromagnetic composite absorption is designed and its mechanism is discussed under the excitation of electric field ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E$ </tex-math></inline-formula> -field) and magnetic field ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula> -field) of incident electromagnetic waves separately. Next, a single layer ultrathin frequency selective absorber (UFSA) is constructed by eight SLUs, which are arranged by mirror symmetry and 90° rotation symmetry. Then, two UFSAs are designed, which work at different frequencies. The proposed ultrathin FSR, which is constructed by simply stacking two UFSAs, has a transmission band (2.09–2.69 GHz) and two absorption bands (two absorptive peaks above 90% at 2.05 and 2.78 GHz) on both sides of the transmission band. Importantly, compared to the traditional FSR, the profile of the FSR is low and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.039\lambda _{c}$ </tex-math></inline-formula> . Finally, to verify the design and analysis, the proposed FSR was fabricated and measured. A good agreement between the results of the experiment and simulation can be obtained accordingly.

A Wideband Frequency-Selective Rasorber With Rectangular Spiral Resonators

This letter presents a frequency selective absorber (FSR) with a passband and two wide absorption bands. The structure is composed of a resistive layer and a bandpass frequency selective surface (FSS) layer. The resistive layer is a combination of metal lines with loaded resistors and rectangular spiral resonators, which generate parallel resonances. Square slots are designed in the bandpass FSS to produce parallel resonance inside the passband and perform as a metal plane outside the passband. A nearly transparent window can be obtained by adjusting the parallel resonant frequencies of two layers to be consistent. The simulation results show that the 3 dB transmission band ranges from 6.5 to 7.76 GHz, and the minimum insertion loss is 0.05 dB at 7.09 GHz. The dual-polarized FSR presents stable operating characteristics and realizes two absorption bands on both sides of the passband, and the fractional bandwidth of the reflection coefficient lower than −10 dB is up to 127%, which ranges from 2.69 to 12.04 GHz. In addition, the FSR is manufactured and measured, and the measurement results are consistent with the simulation results.

An ultra‐wide passband frequency‐selective rasorber with high transmission

Abstract A frequency‐selective rasorber (FSR) with an ultra‐wide transmission band and low‐insertion loss based on compound spiral resonator is presented. The structure consists of the top lossy layer and bottom lossless layer. The frequency‐selective surface is adopted as the lossless layer to obtain an ultra‐wide transmission window. The top lossy layer is made up of the compound spiral resonator and I‐shaped metallic strips with four lumped resistors in this design. The equivalent circuit model is constructed to expound the operating mechanism of this FSR. The 1 dB transmission bandwidth is achieved from 6.98 to 12.21 GHz (54.5%) with the minimum insertion loss of 0.13 dB at 10.40 GHz. Moreover, the bandwidth of S11 &lt; −10 dB is obtained in 4.64–12.21 GHz with a fractional bandwidth of 89.8%. Furthermore, the designed FSR has a good oblique incident angle of 30° and polarization stability. Finally, the simulation results are proved by the experimental results, which confirm the feasibility of the design.

High-performance dual-band frequency-selective rasorber based on cascaded metasurface

Frequency-selective rasorbers (FSRs), which have absorptive and transmissive properties at different frequencies, are crucial in a wide range of applications in communication and radar engineering. However, the FSRs currently available usually exhibit a single high-performance transmission band, which cannot meet the rising demands for modern multiband communication/radar systems. In this article, we propose a dual-band frequency-selective rasorber that utilizes a cascaded metasurface and is designed with an equivalent circuit model and parameter optimization. In addition, a prototype dual-band frequency-selective rasorber is fabricated and characterized. Microwave measurements present two highly transparent peaks located near 11.5 and 17.5 GHz with an insertion loss of around 0.5 dB. Also, there are two absorption bands with absorptivities higher than 80% at 3.9–10.2 GHz and 13–15 GHz. Such low insertion loss for both transparent bands is achieved by optimizing the position of resistors through mode analysis. Our approach facilitates the manufacture of high-performance multi-band frequency-selective rasorbers for use in multi-band communication systems.

A wide inter-absorption dual-transmission dual-polarized frequency selective rasorber based on SRRs

ABSTRACT In this letter, a dual-polarized dual-transmission frequency selective rasorber (FSR) geometry is demonstrated. The top layer of the FSR is built of a cross-dipole design loaded with chip resistors for obtaining a wideband absorption. In addition, two different split ring resonator patterns are printed at each side of the substrate connected by metallized vias to achieve two in-band transmission responses. The bottom layer is designed from a slot geometry to exhibit two transmission bands similar to that of the top layer. The overall FSR structure exhibits a −10 dB reflection response ranging from 2.0 GHz to 8.15 GHz (having a fractional bandwidth of 121%), with transmission peaks appearing at 4.2 GHz and 6.2 GHz having insertion losses of 2.3 dB and 2.9 dB, respectively. The unit cell topology is miniaturized with dimensions of 0.1λL×0.1λL, λL being the free space wavelength at the lowest operating frequency. The proposed FSR also satisfies polarization-insensitive characteristic and angular stability behavior for differentmodes. The working principle behind such wideband absorption and in-band transmission phenomena are analyzed and an equivalent circuit model is presented. A prototype of the proposed FSR is manufactured and measured, confirming the simulated responses.

A Dual-Polarized Frequency-Selective Rasorber With a Switchable Wide Passband Based on Characteristic Mode Analysis

This study proposes a dual-polarized frequency-selective rasorber (FSR) with a switchable wide passband. The FSR is made up of two layers: the first is a lossy frequency-selective surface (FSS), which is made up of meander lines and lumped resistors that absorb energy. The position of the lumped resistors can be determined via characteristic mode analysis. Another layer is a lossless FSS with PIN diodes for the reconfigurable function. The lossless FSS is made up of three layers for a wide passband. When PIN diodes are turned off, the absorption bands in the lower band vary from 2.73 to 3.76GHz, while the upper band ranges from 6.07 to 7.94 GHz. Meanwhile, the reflect band with a value greater than −3 dB spans the frequency range from 4.21 to 5.88 GHz. When the PIN diodes are turned on, the passband with insertion loss less than −1 dB extends from 3.82 to 5.3 GHz. At the same time, the absorption band ranges from 5.9 to 7 GHz. Due to the nonlinearity of PIN diodes, there is a certain difference between the simulated and the measured results. Also, we analyzed the influence of the inductance value of PIN diodes on the whole structure.

A frequency‐selective rasorber with wideband absorption and in‐band transmission using resistive ink

AbstractA polarization‐insensitive frequency‐selective rasorber (FSR) with wideband absorption and in‐band transmission characteristics has been presented and investigated in this letter. The unit cell of the proposed FSR comprises one lossy and one lossless resonator, both printed on FR4 substrates and segregated by an air spacer. Resistive ink patterns are deposited in the top lossy layer to realize a wideband absorption, displaying significant improvement over the existing FSR structures. The absorption bandwidths are obtained from 1.31 to 3.22 GHz and 4.88 to 6.69 GHz having fractional bandwidths of 84.32% and 31.28%, respectively. An in‐band transmission response is occurred, due to the bottom lossless layer, at 4.05 GHz between the two absorption bands with a small insertion loss. The equivalent circuit model is used to develop the proposed FSR and the design variables are formulated accordingly to generate the desired passband and stopband characteristics. Variation in the incident and reflected/transmitted wave properties have also been studied and the topology is found to be polarization‐insensitive as well as angularly stable. Finally, the structure has been fabricated and measured to validate the FSR design principle and its performance.

An ultra‐wideband dual‐band hybrid frequency‐selective rasorber

A hybrid frequency-selective rasorber (HFSR) with two broadband passbands and high selectivity based on serpentine line resonator (SLR) is proposed, which has low insertion loss (IL) in two passbands. The HFSR is constituted by a polarization layer, lossy layer, and lossless layer. The polarization layer is formed by a pair of arrows to realize the polarization conversion at the high-frequency band. The lossy layer is composed of the SLR and metallic strips with four resistors, and it is adopted to absorb the electromagnetic waves and supply two broadband transparent windows. The lossless layer is a second-order bandstop frequency-selective surface, which consists of three metal layers of the patches and square rings. A four-port equivalent circuit model with absorbing function is excavated to interpret the working principle of the designed HFSR. The simulation results indicate that the transmission window with 1 dB IL is achieved in 4.57–5.86 GHz and 10.65–14.43 GHz corresponding to a 24.7% and 30.1% fractional bandwidth (FBW), respectively. Besides, the co-polarized reflection coefficients less than −10 dB are obtained from 4.57 GHz to 17.37 GHz, which has a 116.6% FBW. In addition, the physical sample of the proposed HFSR is fabricated and tested. The test results are consistent with the simulation results.

Chiral Absorber-Based Frequency Selective Rasorber With Identical Filtering Characteristics for Distinct Polarizations

In this article, we propose a new paradigm to design frequency-selective rasorber (FSR) with two-sided efficient absorptions and broadband transmission in a chiral structure. The proposed FSR is implemented via combining two chiral absorber layers with a bandpass frequency-selective surface (FSS). An absorptive split-ring resonator (ASRR) inserted with a resistive film is designed to provide efficient spin-sensitive absorption. Then, four ASRRs are arranged to form a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$C_{4}$ </tex-math></inline-formula> symmetric split-ring absorber (SRA) with identical absorption for distinct polarized incident waves. The physical mechanisms of spin-selective absorption and polarization-insensitive absorption are discussed theoretically. The broadband bandpass FSS is introduced to provide a transparent window and ground plane for the absorbers. A prototype is fabricated and measured to verify the feasibility of the method. The simulation and experimental results manifest over 90% absorptive bands of 1.5–3.6 and 15.2–21.6 GHz and a transmission band of 8–12 GHz with insertion loss less than 2 dB. Compared with previous FSRs, the proposed FSR has a broadband transmission, efficient absorptions with high-frequency selectivity, and easy geometric structure.

Broadband-Transmissive, Frequency-Selective Rasorber Design Using Characteristic Mode Analysis

This article designs a frequency-selective rasorber (FSR) with a broadband transmission window. It is synthesized by a broadband absorber and a frequency-selective surface (FSS). The resistive layer achieves broadband absorption by introducing a tortuous Jerusalem cross load with lumped resistors, and the lossless FSS adopts a three-layer metal structure to realize the broadband transmission window. The absorption mechanism of the resistive layer is analyzed using the theory of characteristic mode. The position of the resistor is determined according to the analysis of mode current distribution and parameter optimization. Prototypes of the structure were fabricated and measured, and the simulation results show that the 1 dB transmission window and the absorption band with |S11|&lt; −1 dB are 36.4% and 97%, respectively. Therefore, the designed FSR has potential application prospects in electromagnetic stealth technology and radar cross-section reduction.

Broadband Absorptive Frequency-Selective Rasorber Based on Multilayer Resistive Sheets Using Multilayer Resonator

In this article, the frequency-selective rasorber (FSR) built on multilayer resistive sheets, which achieves a broad absorption band, was proposed. First, the detailed analysis of the absorption and transmission performance for FSR with the single-layer/multilayer resistive sheets based on the equivalent circuit model (ECM) was demonstrated, which proved that the FSR with the multilayer resistive sheets can obtain a broader absorption band. Second, the multilayer resonator (MR) of a smaller size and higher design freedom compared with single-layer resonator was proposed. Then, the FSR on the basis of multilayer resistive sheets using MR was designed. It consists of the three-layered metallic structures and separated by an air spacer. For the top and middle layers, the lumped-resistor-loaded metallic dipole with the MR inserted into the center was applied to generate a wide absorption band and transparent window, respectively. For the bottom layer, the square-slot frequency-selective surface was optimized to match the transmission band of the resistive sheet. Simulated and measured results show that the broad absorption band and transmission band with low insertion loss have been realized, and the proposed FSR yields the widest relative bandwidth <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\vert S_{11}\vert \le -10$ </tex-math></inline-formula> dB of 145.2%.

Realization of optically transparent and broadband frequency selective rasorber using thin copper wire

Numerous studies have been made to design broadband frequency selective rasorber (FSR). However, it is still a challenge to realize optically transparent FSR, due to the difficulty in realizing high transparency, high-quality factor(Q) parallel resonance, and low-Q series resonance simultaneously. Here, we propose a new method to design transparent FSR for the first time. To break through the technical barrier, a thin copper wire with a different configuration method is utilized to design lossy layer, which can realize high transparency, in-band transmission, and out-of-band absorption concurrently. The proposed FSR is configured by cascading a transparent lossy layer above a transparent frequency selective layer with an air space to separate them. We adopt the equivalent circuit model, electric field distribution, and loss distribution to reveal the physical mechanism of the FSR. The simulation results show that the transmission loss is 1 dB at 8.3 GHz, and the reflection amplitude is less than −10 dB in 2.7–8.9 GHz. The FSR can achieve 96.3% optical transparency theoretically. A prototype is fabricated and measured to validate the proposed design. The measured results achieve good agreement with the simulated results. The proposed FSR can act as a stealth radome for transparent components of ships and aircrafts. The application scenarios can be extended to communication and imaging systems.