Water-based thermally controlled multifunctional switchable frequency selective rasorber.

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.

A frequency selective rasorber (FSR) with three transmission bands and three absorption bands is proposed in this paper. The FSR is constructed via cascading a lossy resistive surface and a multiple bandstop frequency selective surface (FSS) separated by air spacer. The transmission characteristics are obtained by the impedance poles of the interdigital capacitor-meander inductor resonators in the resistive surface and also the rectangular loop resonators in the FSS. The absorption bands are achieved using the ohmic loss of the resistors in the resistive surface along with the reflections of the FSS. The physical principle of operation of the multiband FSR is explained based on an equivalent circuit model. A prototype is fabricated and measured. The experimental results exhibit three transmission bands at 6.1 GHz, 8 GHz and 10 GHz with insertion loss below 0.9 dB, 0.8 dB and 0.8 dB, respectively. The effective absorption bands are in range of 7.05 GHz - 7.55 GHz, 8.87 - 9.35 GHz and 12.59 GHz - 14.22 GHz, respectively. The simulation results show the angular stability of the FSR response. In addition, the effect of FSR dimensions on the transmission/absorption bands is studied.

<p indent="0mm">Against the background of precision payload mounting plates with complex heat sources in high-precision and high-stability spacecraft, an intelligent design method for functionally graded structures based on physics-informed neural networks is presented. Independent of thermal control devices, the local temperature in the load installation area and the fundamental frequency of the structure are regulated by optimizing the material gradient distribution. A loss function is established that includes the heat-conduction governing equation, local temperature and fundamental frequency optimization objectives, and material volume fraction constraints, considering multiple heat source distributions. The inverse problem with material gradient distribution as a design variable is solved using physics-informed neural networks. The results show that a reasonable design of the material gradient distribution can accurately regulate the local temperature at specific locations and the fundamental frequency without relying on external thermal control devices. The proposed method is expected to reduce the need for thermal vibration control devices for spacecraft structures and improve structural thermal-mechanical stability.

This paper presents a frequency-selective rasorber which has two absorption bands on the both sides of a tunable passband. An original rasorber with fixed-passband is designed by cascading of an absorptive frequency-selective surface (FSS) and a lossless bandpass FSS. The absorptive FSS is realized with tripole elements and the lossless bandpass FSS is achieved with tripole ring slot. Varactors and biasing networks are inserted for tuning the passband. The simulated results show that the center frequency of passband is altered from 4.5 to 6 GHz with the capacitance of varactors changing from 0.2 to 0 pF. The frequency-selective rasorber with a fixed-passband is fabricated and measured to verify the design.

A design to achieve frequency-selective rasorber (FSR) and reflector with two-sided absorption bands is proposed. The theoretical analysis and the calculation method for the structure parameters are presented by utilizing the equivalent circuit model. The reflector and the rasorber are designed in simple 2-D structures, consisting of only one layer of resistive sheet and a pure ground or a bandpass frequency-selective surface, respectively. Based on the analysis of the condition for realizing two-sided absorption, we found that the parameters of each element in the resistive sheet are correlative and proportional to each other. The calculation method of actual physical dimensions of the proposed structure is provided, so the element sizes of unit cell structure can be calculated and designed to approximate the desired values. Hence, based on these proposed methods, the perfect two-sided absorption FSR or reflector can be realized directly when the deduced relationship of the parameters is satisfied. Therefore, the approach of impedance matching becomes much easier. The simulation results show the two wideband absorption bands located above and beneath the reflection/transmission band. An FSR and a reflector are designed, fabricated, and measured. The measurement results match well with the simulation results and verify the designing principle. The FSR realizes the two-sided absorption bands at 2.1–5.1 and 11.9–17.5 GHz and a transmission band at 9.7 GHz. The reflector shows a wide reflective bandwidth and two wide absorption bands at 2.8–6.5 and 12–17.4 GHz.

Professor Yongmeng Liu from the Harbin Institute of Technology in China talks about the research behind his Letter ‘Optical-transparent Wi-Fi bandpass mesh-coated frequency selective surface’ on page 381. Professor Yongmeng Liu Our group's work is focused on designing novel frequency selective surfaces (FSSs) as security wallpaper for optical windows of electromagnetic filtering and interference shielding. A FSS is an array of periodic apertures perforated on a conducting sheet or metallic patches which exhibits bandpass or bandstop responses. A traditional FSS generally has very low optical transmittance and cannot be easily applied as a dual-mode filter to achieve simultaneous high optical transparency and Wi-Fi bandpass filtering. The solution is to create novel periodic structures that have unique and fascinating features attracting us to this field of research. We have reported a FSS periodic structure fabricated using a UV lithography technique to achieve simultaneous high optical transparency and Wi-Fi bandpass filtering. The FSS is a resonance cavity structure with a periodic array of apertures removed from an optically transparent conductive mesh coating. Simulation and experimental results show that the resonance centre frequency of the mesh-coated FSS is about 5.3 GHz with transmittance loss of −2dB, while its optical transmittance is greater than 87.5% in a waveband of 1.35 to 3.3 µm. Such a FSS has the merits of ameliorable electromagnetic shielding effectiveness and improvable optical transmittance by optimising the thickness of the mesh layer. Our FSS can be used as security wallpaper for optical windows to solve the problem of transmission security of Wi-Fi communications. Generally, transparent conductive mesh coating is widely used to provide both strong RF/microwave shielding and optical transmittance, however this seriously attenuates Wi-Fi transmittance. A bandpass FSS is used to provide spatial filtering, but suffers from low optical transmittance due to the large fraction of the area obscured by metallic films. Neither of these existing technologies can meet the dual requirement of optics/Wi-Fi filtering for security wallpaper. It is a significant advancement to develop a periodic structure to achieve simultaneous optical transparency and stable Wi-Fi bandpass. One potential application is as security wallpaper for securing confidential business data. Another application is to enhance the transmission of the specific signal of interest through optical windows. State-of-the-art FSS technology includes indium tin oxide (ITO). This kind of ITO-FSS is used for energy saving windows since the ITO layer reflects a significant portion of long and short infrared wave energy. However, the restrictive relationship between conductivity and optical transparency of ITO is the fatal flaw in this implementation. We are working on a project to design conductive metallic meshes to shield radio-frequency/microwave interference for optical windows to optimise mesh patterns and parameters to improve optical transmission and to minimise the high-order optical diffraction effect, as well as enhance electromagnetic shielding effectiveness. Challenges will be to further optimise the structure of the mesh-coated FSS to improve the performance, specifically the sheet resistance, bandwidth and the transmission stability of incidence angles and polarisations. The FSS has high sheet resistance which will impact Wi-Fi transmission, however the FSS has adjustable thickness to enhance its conductivity and improve Wi-Fi transmission performance while having no impact on optical transparency. Additional challenges include developing ways to integrate the mesh-coated FSS with optical windows of existing buildings. Fortunately we can fabricate the FSS on flexible PET films and add the composite films to each side of existing optical windows. Over the next few years novel periodic electromagnetic structures will integrate metamaterials, active circuit elements and other new techniques to develop security wallpapers to protect communications security. The trend of periodic electromagnetic structures for electromagnetic filtering and shielding will evolve from expensive, heavy and passive to low-cost, lightweight, active and self-tuning. We would like to see increasing numbers of new periodic electromagnetic structures being developed throughout numerous frequency ranges, perhaps even the entire electromagnetic spectrum. We believe all these beautiful periodic electromagnetic structures will be brought into real-world use and will make our lives better!

A strategy for folded reflectarray with low detectability and enhanced gain is presented. For this purpose, the folded reflectarray is engineered by combining a polarization-conversion focusing metasur face, a frequency selective rasorber (FSR), and a diffusion metasurface. The FSR is composed of a lossy layer and a single-polarized I-shaped slotted frequency selective surface (FSS) between which the diffusive metasurface is inserted for in-band and out-of-band cross-polarized low radar cross-sectional (RCS) reduction. Therein, the antenna exhibits a transmission window and two out-of-band absorptions due to the FSR for copolarized radiation. In contrast to detection, the system operated for both wave incidence. For cross-polarized wave incidence, a wideband RCS reduction is enabled due to the wave-diffusive function of the inserted metasurface. The half profile is guaranteed since the wave bounces two times between the FSS and focusing metasurface. The numerical and experimental results coincide well, both indicating that the low-RCS folded reflectarray achieves a peak gain of 23.5 dB at 10.2 GHz, corresponding to an aperture efficiency of 37.8%. Besides, the antenna achieves 10 dB monostatic RCS reduction within 4.6–8.8 GHz and 9.6–13.8 GHz under copolarization detection and a 10 dB monostatic RCS reduction from 8.4 to 19 GHz under cross-polarization detection. Moreover, good invisibility is also achieved for bistatic RCS reduction.

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.

Sandwich structures with frequency selective surfaces have been widely used to fabricate low-observable radomes. Traditional frequency selective surfaces were made of metals, such as copper and aluminum. There were problems when used the frequency selective surfaces with the sandwich structures, such as a bonding layer and thermal mismatch. Because of the thermal mismatch between the sandwich structures and the metals, mechanical properties of the sandwich structures usually decreased. To avoid these disadvantages, a new type of composite frequency selective surface was put forward in this study. A four-legged slot-array frequency selective surface and a square-aperture array frequency selective surface were fabricated by cutting carbon fiber/epoxy composites. Free-space method and finite-element method were carried out to evaluate the electromagnetic transmission characteristics of the frequency selective surface specimens, respectively. Results show that the composite frequency selective surface with four-legged slot array can realize the function of frequency selection, and its minimum transmission loss can be decreased by increasing the electrical conductivities of the composite material, adjusting the thickness, and increasing the aperture-to-cell ratio of it. For the composite frequency selective surface with square-aperture array, grating lobes come closely after its resonant frequency, although its minimum transmission loss is very close to zero.

In the abstract, a frequency selective rasorber (FSR) with a wide passband and double polarization reconfigurability is suggested. Two layers make up the FSR: the first is a lossy frequency selective surface (FSS), which is composed of energy-absorbing meander lines and lumped resistors. Characteristic mode analysis can be used to locate the lumped resistors. The lossless FSS layer, which has PIN diodes for reconfigurable functionalities, is a further layer. For a broad passband, the lossless FSS has three layers. The lower band's absorption bands run from 2.73GHz to 3.76GHz, while the upper band's absorption bands span 6.07GHz to 7.94GHz when PIN diodes are off. The reflect band, on the other hand, covers the frequency range of 4.21GHz to 5.88GHz and has a value greater than −3dB. The pass-band with insertion loss less than −1dB ranges from 3.82GHz to 5.3GHz when the PIN diodes are turned on. The absorption band spans from 5.9GHz to 7GHz concurrently.

A patch-slot-patch frequency selective surface (FSS) exhibiting bandpass response is proposed in this paper. The FSS unit cell is composed of two cross-dipole shaped patches and a cross-dipole shaped slot in between. The proposed FSS exhibits bandpass response with two closely spaced transmission poles around 3 GHz and 3.5 GHz. A 3-dB transmission bandwidth of upto 20% is achieved for the FSS. The proposed design has a thickness on the order of 0.032 λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> . The paper presents with parametric studies how the poles can be tuned individually. The transmission phase of the FSS varies linearly towards zero with frequency that makes the FSS useful in designing Filter-Antenna when placed as a superstrate above a broadside radiator.

In this paper, a novel frequency-selective rasorber (FSR) which has a transmission band between two absorption bands is proposed. The FSR is composed of curved metal sheets with lumped resistors printed on a dielectric substrate and a frequency selective surface (FSS) used as the ground plane of the absorber. An equivalent circuit model of the structure is given for analyzing its operation mechanism. A good stability of the transmission and two-band absorption under the incident angle from 0° to 30° for TE and TM polarizations has been obtained.

Abstract： The absorption–transmission–absorption–transmission (A-T-A-T) type Frequency Selective Rasorber (FSR), featuring dual absorption bands and dual transmission passbands, has attracted considerable attention for modern electromagnetic applications. However, most existing A-T-A-T FSR designs cannot simultaneously achieve a wide absorption bandwidth and low insertion loss, which limits their practical use in high-performance systems. To overcome this challenge, a polarization-insensitive and miniaturized A-T-A-T type FSR with wide absorption bandwidth and low insertion loss is proposed. The lossy layer consists of metallic patterns loaded with four resistors, which broadens the lower-frequency absorption band. The lossless layer is designed as a bandpass frequency selective surface incorporating a cross-shaped slot and four rectangular apertures, generating two transmission poles for dual-band transmission. The proposed structure operates over 3.59–18.6 GHz (135.29%), exhibiting two absorption bands of 3.59–13.7 GHz (116.95%) and 16.21–17.13 GHz (5.52%). In addition, two transmission passbands are obtained at 15.05 GHz and 18.6 GHz, with minimum insertion losses of 0.59 dB and 0.58 dB, respectively. Furthermore, the proposed design provides polarization insensitivity, stable performance under oblique incidence up to 45°, and compact dimensions with a thickness of 0.078λ and a unit-cell size of 0.014λ². To verify its practical feasibility, a 20 * 20 array prototype was fabricated and measured. The measured results agree well with the simulated ones, confirming the effectiveness and reliability of the proposed design. Owing to its balanced and comprehensive performance, the proposed A-T-A-T type FSR is a promising candidate for dual-band radiating system shielding, radar cross-section (RCS) reduction, and electromagnetic interference (EMI) suppression.

A new bandpass frequency selective surfaces (FSS) is designed and the composite system (patch antenna covered by an FSS) is analyzed using the finite difference time domain (FDTD) method. Far-field patterns of the patch antenna, with and without FSS radomes, are obtained and compared with each other. The feed is modeled by using a coaxial line extension, which enables us to compute the return loss of the patch and the composite system accurately. Numerical results show a frequency shift in the return loss response of the patch in the presence of the FSS cover.

In this letter, a novel bandpass frequency selective surface (FSS) with miniaturized periodic element is presented. The proposed FSS is printed on one face of a single-layer substrate with a relative permittivity of 2.65. The novel FSS has promising miniaturization characteristics with the unit-cell dimension 0.058λ×0.058λ, where λ represents the free-space wavelength corresponding to the resonant frequency. The FSS exhibits excellent stability under different polarizations and incident angles. Both the simulation and measurement results validate the stable performance of this FSS.