Bandpass Frequency Selective Surface Research Articles

Wideband RCS Reduction Metasurface With a Transmission Window

Metasurfaces have been extensively investigated to achieve radar cross section (RCS) reduction over a wide frequency band. In this article, we propose the design of a novel metasurface with a transmission window within the wide RCS reduction band. The function of this metasurface is similar to rasorbers, whereas the mechanism behind the wideband RCS reduction is scattering cancellation rather than absorption. To this end, the ideal reflection and transmission properties of the metasurface unit cell are firstly revealed. And then an anisotropic unit cell composed of multilayered metallic patterns is designed to obtain the desired reflection and transmission properties. The bottom layer is composed of the typical patch-grid-patch bandpass frequency selective surface, which allows for high transmission in the transparent window and also behaves as a metallic-like reflection sheet in the RCS reduction band. Two metasurfaces, with checkerboard and aperiodic configurations, respectively, were designed, fabricated, and measured, as a proof-of-concept demonstration. A good agreement between the simulations and measured results provides a solid verification for our design. The results here provide an alternative method to open a transparent window within the RCS reduction band and may find potential applications in stealth radomes.

Single- and Dual-Band High-Order Bandpass Frequency Selective Surfaces Based on Aperture-Coupled Dual-Mode Patch Resonators

In this article, a class of bandpass frequency selective surfaces (FSSs) based on aperture-coupled dual-mode patch resonators (AC-DMPRs) are proposed to achieve single- and dual-band high-order filtering responses with low profiles. Initially, a basic resonator of a square patch with diagonal corner truncations is theoretically investigated so as to demonstrate that two orthogonal modes can be simultaneously excited in a single patch resonator. Then, to eliminate the cross-polarized reflection caused by the orthogonal modes, a composite resonator, including four of such corner-truncated patches with 90° rotation between each two adjacent patches, is constructed and analyzed. Furthermore, by arranging two of such composite resonators in a back-to-back manner through coupling apertures on the middle metallic layer, an FSS element of three-layer AC-DMPR is formed. Compared with traditional aperture-coupled patch resonators (AC-PRs), the resonant modes of AC-DMPR are increased twice, thus giving more design flexibility to achieve high-order performance. To validate the design concept, a single-band fourth-order bandpass FSS is designed by introducing magnetic couplings in the AC-DMPRs. Moreover, dual-band second-order bandpass FSSs with magnetic and/or electric couplings in the AC-DMPRs are also designed. Based on the even- and odd-mode analysis method, equivalent circuit models are established to explain the operating principles of the proposed structures. Finally, the designed FSSs are fabricated and measured. Good agreement between the measured and simulated results well validates the conceptual designs.

Tripole type wideband bandpass frequency selective surface for X‐band applications

The study presents a novel single dielectric layer wideband bandpass frequency selective surface (FSS) topology. The structure is designed using a kind of the tripole type FSS which has been analysed for its behaviour. Next, the operating bandwidth has been increased by using the meander lines in the middle of the unit-cell. The proposed FSS structure covers the X-band (45.03% fractional bandwidth) of the electromagnetic (EM) spectrum with a −3 dB transmission bandwidth. Its −1 dB bandwidth extends from 7.93 to 11.93 GHz. The wide operating bandwidth with a single dielectric layered structure makes it suitable for the hybrid radome applications. For the normally incident EM wave, adjacent to the lower and upper cut-off frequencies, the structure offers better than 18 dB attenuation which increases the out of the band rejection. Finally, the proposed structure is fabricated and experimentally verified for the dual polarisation and angle stability up to 45°. Eventually, to prove the novelty in terms of the operating bandwidth and the fabrication simplicity, an exhaustive state-of-the-art comparison is also presented.

Mobile Phone Signals Interference Cancellation With Improved WLAN Transmission and Thermal Insulation of Buildings Using FSS and RC Film

For macro-femto network, intratier and cross-tier interference (CI) must be canceled to achieve the quality of service, better indoor coverage, and higher data rates. Moreover, for better wireless local area network (WLAN) and mobile communication services while ensuring WLAN security, appropriate temperature in modern buildings with reduced energy cost is also required. To address these issues, RadioClear (RC) film can be pasted on an ordinary glass and installed in different sections to cancel the intratier interference between the femto network as well as CI between the femto and macro network. Moreover, this technique can also ensure enhanced WLAN security and thermal insulation in modern buildings. For WLAN transmission, a fractal dual bandpass frequency selective surface (FSS) designed in the RC sheet can be used to improve the WLAN communication in a situation where two or more than two rooms are served through a single WLAN router. Theoretical results are presented which are further verified through the experimental results.

Hybrid Frequency-Selective Rasorber With Low-Frequency Diffusion and High-Frequency Absorption

In this article, a novel frequency-selective rasorber (FSR) is proposed, which provides a high-efficiency transmission band and a broadband radar cross section (RCS) reduction by hybridizing low-frequency diffusion and high-frequency absorption. The FSR comprises three layers. A broadband second-order bandpass frequency-selective surface is designed as the bottom layer, which has a reflection-transmission-reflection frequency response, and acts as an equivalent ground plane for the upper layers outside the transmission band. A circuit analog absorber and a coding metasurface are designed as the middle and top layers, respectively. They use absorption and diffusion to reduce the RCS at high- and low-frequency bands. In the coding metasurface, miniaturized and absorptive techniques are utilized to suppress the harmonic resonances. The whole FSR unit cell exhibits a cross-polarized reflection-transmission-absorption frequency response. A co-polarized transmission magnitude larger than -1.5 dB from 7.4 to 12.1 GHz and a co-polarized reflection magnitude less than -10 dB from 3.1 to 20 GHz are observed in the full-wave simulation under the normal incidence. An FSR sample with 20 x 20 unit cells is designed and measured for verification. The measurement agrees well with the simulation, and it shows a significant monostatic RCS reduction from 3.3 to 20 GHz in comparison with a metallic plane of the same size.

A low profile miniaturized widely‐spaced triband bandpass FSS using coupled resonance

In this article, a triband highly selective widely-spaced bandpass frequency-selective surface (FSS) is presented. The proposed FSS is consisted of five metal layers that are separated from each other by four dielectric substrates. Using coupled resonance between layers, three passbands operating at 11.0, 17.4, and 31.9 GHz are achieved. Meanwhile, the dimension of the unit cell of the FSS can be achieved in 0.139λ0 × 0.139λ0 (λ0 is the wavelength of the first resonant frequency in the free-space), and the overall thickness can be 0.057λ0, exhibiting miniaturization and low profile characteristics. Due to the proposed FSS can provide multitransmission zeros between two adjacent passbands, the relative bandwidth of the spacing between the second passband and the third passband can reach 58.8% from 17.4 to 31.9 GHz. Thus, a widely spaced response is achieved. In practical, the proposed FSS has an important role in the radio cross-sectional reduction of some military systems, such as homing head, which can simultaneously detecting mid-range and long-range targets. Furthermore, the FSS shows the stable response of angles for both TE and TM polarizations. The equivalent circuit model (ECM) is provided to analysis its operating principle. Finally, a prototype of the proposed FSS is simulated, fabricated and measured. The measured results are in good agreement with the simulation ones.

Realization of Circular Slot Frequency Selective Surfaces using Photoplotter and Wet Etching Technique for Terahertz Material Sensing Applications

This paper discusses on the analysis of band pass Frequency Selective Surfaces (FSS) for performance enhancement in material sensing application. Terahertz Spectroscopy has proved to be versatile tool for detection and sensing in measuring non-conductive materials. It is because most of the non-conductive materials have unique molecular resonance that may translate as transmission and absorption of signals within terahertz range. However, the most critical issue in detection and sensing is to improve its sensitivity therefore an extremely low concentration material still can be able to be detected in THz band. Hence, in this paper, a circular slot is modeled on a planar structure of Rogers Duroid 5880LZ substrate with thickness of 508µm using Computer Simulation Technology (CST). The simulation generates a band pass response with transmission magnitude of 0.95 at 0.66THz. Furthermore, photoplotter and wet etching fabrication process is used for the realization of terahertz FSS. Simulated and measured transmission shows a good agreement between 0.5THz to 0.7THz as only 1% shifts in frequency between simulated and measured results. Besides that, the fabrication of circular FSS shows narrower measured bandwidth as compared to its simulated counterpart. Hence, with the limitation of the wet etching to produce micron size structure both simulation and measured result shows good agreement for all the critical issues in this study.

Polarisation‐insensitive dual‐band transmissive rasorber designed on a single layer substrate

In this study, a frequency selective surface (FSS)-based rasorber has been proposed by combining the designs of split-ring absorber and dual bandpass FSS, which are printed on the front and backside of 0.8-mm thick FR-4 substrate, respectively. The proposed FSS-based rasorber is a thin single layer structure and also exhibits the polarisation-insensitive behaviour due to its four-fold symmetry. The split-ring structure at the front contributes to the resonant absorption at 6.2 GHz, while the bandpass FSS structure at the backside provides two transmission windows at 3.1 and 8.4 GHz, respectively. The parameters of the design are optimised such that at the absorption frequency, the bandpass FSS acts as a complete metallic ground. The operating principle of the proposed rasorber is analysed using an equivalent circuit model. The effects of design parameters on the performance of the rasorber are studied, and accordingly an optimum range of lower and upper transmission bands with respect to the absorption band are determined. A 21 × 21 array of the proposed rasorber unit cell is fabricated, and the results are experimentally verified.

Stacked Slotline Structure-Based Unit Cell and Its Application for Synthesis of 3-D Bandpass Frequency-Selective Surfaces

This article presents a basic 3-D frequency-selective surface (FSS) unit cell and its application for synthesis of high-order bandpass filtering response. The proposed FSS unit cell is constructed by stacked slotlines loading with shunted capacitive and/or inductive strip lines, which can be easily fabricated with traditional printed circuit board techniques. First, by analyzing the propagation characteristics of the stacked slotline, it can be found that the stacked slotline is seen as a transmission-line transformer, which can convert spatial electromagnetic waves into guided waves. Then, by loading shunted capacitive and/or inductive strip lines, equivalent capacitors and/or inductors are formed with the slotline section, which can be used to realize desirable resonators for bandpass filtering response. Equivalent circuit models are established and circuit parameters are extracted. With the aids of the generalized filter synthesis theory, the proposed unit cell is first applied to synthesize bandpass FSS with 1/4-wavelength transmission-line inverters. An example of a second-order Chebyshev bandpass FSS with the center frequency of 10 GHz is synthesized and designed. The measured results demonstrate that the desired bandpass filtering response is stable for oblique incidence. Furthermore, to obtain high-order response with a reduced FSS thickness, the proposed unit cell is then applied to synthesize bandpass FSS based on a lumped-element bandpass filter. The detailed synthesis procedure and derivations are given. Another example of a third-order Chebyshev bandpass FSS with the center frequency of 10 GHz is also designed, fabricated, and measured. The measured results show that the desired stable performance is realized and the total FSS thickness is $0.21\lambda _{0}$ ( $\lambda _{0}$ is the free-space wavelength at the center frequency).

Miniaturized-Element Frequency-Selective Rasorber Design Using Characteristic Modes Analysis

A dual-polarization frequency-selective rasorber (FSR) with two absorptive bands at both sides of a passband is presented. Based on the characteristic mode analysis, a circuit analog absorber is designed using a lossy frequency-selective surface (FSS) that consists of miniaturized meander lines and lumped resistors. The positions and values of resistors are determined according to the analysis of modal significances and modal current. After that, the presented rasorber is designed by cascading of the lossy FSS and a lossless bandpass FSS. Equivalent circuits of the FSR are modeled, and surface current distributions of both FSSs are illustrated to explain the operation mechanism. Measurement results show that, under the normal incidence, a minimum insertion loss of 0.27 dB is achieved at a passband around 6 GHz, and the absorption bands with an absorption rate higher than 80% are 2.5–4.6 GHz in the lower band and 7.7–12 GHz in the higher band, respectively. Our results exhibit good agreements between measurements and simulations.

Dual-Band Structure Reused Antenna Based on Quasi-Elliptic Bandpass Frequency Selective Surface for 5G Application

A dual-band structure-reused antenna with quasi-elliptic bandpass feature is proposed in this communication. The antenna consists of a cavity-backed slot array working at $K$ -band (24.2–26.1 GHz) and a frequency selective surface (FSS), which is transparent to the $K$ -band radiation and also reused as a radiator at $S$ -band (2.5–2.7 GHz). The FSS unit cells are designed based on the cross-slot structure. By introducing asymmetric boundaries to each unit cell, the FSS obtains high-frequency selectivity with two transmission zeros located on each side of the passband. The proposed antenna is fabricated and measured. The measurements have a good matching with the simulations.

Helicity‐Dependent Multifunctional Metasurfaces for Full‐Space Wave Control

AbstractFull‐space manipulation of electromagnetic waves with a thin flat plate is particularly intriguing for large‐angle scanning, functionality integration, and data capacity applications. However, majority of the designs to date are confined to linearly‐polarized wave operations; these render the versatile full‐space device operating under circularly‐polarized waves unaddressed due to the critical issue of the geometric phase being hardly decoupled among reflections and transmissions. Herein, a strategy for a helicity‐dependent multifunctional design by sandwiching dual‐layer geometric phase metasurfaces with a bandpass frequency selective surface is reported. The top and bottom metasurfaces are composed of two different types of meta‐structures, split ring resonators (SRRs) and a modified H‐shaped structure, which enable triple‐independent wavefronts at lower and upper frequencies (f1 and f2). At f1, the top and bottom SRRs operate under reflection mode on both sides, and two distinctive wavefronts can be modulated by individually rotating the orientations of SRRs. However, at f2, the modified H‐shaped structure operates in transmission geometry, and additional functionality can be independently modulated by changing their orientations. As a proof of concept, a multifunctional meta‐device is constructed with triple‐versatile functionalities. The approach followed in this study sets up a solid platform for arbitrary helicity‐dependent full‐space integrated devices.

Application of Metaheuristic Optimization Algorithm and 3D Printing Technique in 3D Bandpass Frequency Selective Structure

A 3D band pass frequency selective surface structure with desired center frequency and bandwidth, designed by multi-objective optimization algorithms, is introduced in this paper. The ant colony optimization technique is used to achieve optimal design properties with the required characteristics. The performance of the manufactured structure using the 3D printing method is measured to verify the optimized structure for normal incidence. Come up with the results, the convenience of the optimization algorithm accordingly 3D printing can help in designing and constructing complex 3D structures.

Design of Low-Profile Frequency-Selective Rasorbers Based on Three-Legged Loaded Element

A novel low-profile dual-polarization frequency-selective rasorber (FSR) with a transmissive window in the absorption band is proposed in this paper. Based on the equivalent circuit model (ECM), the principles of the impedance design are theoretically derived. Then, a two-layer structure model is constructed. The top layer is composed of a lossy three-legged loaded element (TLLE), and the bottom layer is composed of a square ring bandpass frequency-selective surface (FSS). Furthermore, the strips are folded to reduce the unit cell size to stabilize the angular response. The maximum stable response angle increases from 20 to 40° due to the miniaturized design under both TE and TM polarization. The experimental results of the prototype are in good agreement with the simulation results, which validates the rationality of our design.

Angularly Stable Bandpass Frequency Selective Surface Based on Metasurface

A novel bandpass frequency selective surface (FSS) working at 2.4 GHz is proposed in this paper based on metasurface with independent-polarization, wide rejection band and good angular stability. The 10-dB upper stopband could cover 3.35-9.4 GHz with a fractional bandwidth of 94.9%. In addition, the filtering response could remain stable under different polarizations and incidence angles. Parameter analysis and equivalent circuit model are illustrated to explain the resonance behavior of the proposed FSS. It is demonstrated that the lower resonant point in the stopband could be tuned independently by changing the length of small rectangular patches in the top layer. While the higher resonant point could be changeable independently by adjusting the diameter of circular gap in the bottom layer. A prototype of the proposed FSS is fabricated and its simulated results are coincided with measurements.

Design of Dual-Polarized Frequency Selective Rasorber With Two Independent Transmission Windows Using Multi-Resonators

In this paper, a dual-polarized frequency selective rasorber (FSR) with two independent transmission windows and three absorption bands is designed and characterized. The proposed FSR is composed of the resistive sheet and the bandpass frequency selective surface (FSS), which are separated by the air spacer. At the top resistive sheet, two concentric modified square loops with different lengths are inserted into the square loop with four lumped-resistor-loaded legs, which provides two independent transmission windows. The bottom bandpass FSS, consisting of the combination of the square slot and the four-leg arrow shaped slot, is adopted to realize the same transmission bands as the upper resistive sheet. Performance of the proposed FSR is investigated via its equivalent circuit model and full-wave simulations. The simulated results exhibit dual-band frequency response, which has two independent transmission windows within three absorption bands. The two pass-bands operate at 8.5GHz and 13GHz with insertion loss of 0.31dB and 0.48dB, respectively. And the three absorption bands are in the range of 4.1GHz-7.5GHz, 9.1GHz-12.3GHz, and 14GHz-19.7GHz, respectively. Also, the band with reflection coefficient lower than −10dB covers from 4.2 GHz to 19.6GHz and the fractional bandwidth is up to 129.4%. Furthermore, prototype of the FSR is fabricated and measured. Good agreements between the measured and the simulated results can be observed.

Quasi-Elliptic Bandpass Frequency Selective Surface Based on Coupled Stubs-Loaded Ring Resonators

In this paper, a novel design of frequency-selective surface (FSS) based on coupled stubs-loaded ring resonators (SLRRs) is proposed. The proposed FSS exhibits quasi-elliptic bandpass filtering characteristic at C-band. And each unit cell of the structure is composed of two SLRRs coupled by a circular aperture. A novel method of transmission line (TL) model is proposed to investigate the operating principle of a single SLRR under normal incidence plane wave. Moreover, the operating mechanism of the proposed FSS is further researched with the aid of the equivalent circuit model (ECM) analysis method. Due to the symmetric and low-profile physical structure, the FSS keeps stable frequency response under oblique incident plane wave for both transverse electric (TE) and transverse magnetic (TM) polarizations. Finally, the designed FSS is fabricated and measured. The measured results agree well with the simulated results. The measured results indicate that the proposed FSS exhibits a dual-polarized and stable bandpass frequency response with two transmission zeros (TZs) at around 5.71 and 6.87 GHz under different incident angles up to 40°.

Harmonic-Suppressed Frequency Selective Rasorber Using Resistive-Film Sheet and Square-Loops Resonator

A harmonic-suppressed (HS) frequency selective rasorber (FSR) based on square-loops resonators and resistive-film sheets is presented in this letter. The FSR is designed by cascading the bandpass frequency selective surface (FSS) and broadband absorber together, which owns a main passband at center frequency of 8 GHz, with harmonic transmission windows suppressed and out-of-band reflection absorbed from 12 to 26 GHz. A lossless FSS with a bandpass response and a lossy absorber with broadband absorption are analyzed by equivalent circuit model, respectively. The frequency range of harmonic suppression covers three octaves of main passband frequency. Besides, the measured results agree well with the simulated ones. The proposed HS-FSR could be used as radome in enhancing an anti-interference performance of a wireless communication system and improving the stealth performance of a radar platform.

Design of frequency selective rasorber with wide transmission/absorption bands

This paper presents a high-frequency transmissive frequency selective rasorber (FSR) with wideband transmission/ absorption properties. It is synthesized by placing a resistive sheet on top of a bandpass frequency selective surface (FSS) separated by an air spacer. The unit cell of resistive sheet comprises the resistor-loaded central tortuous Jerusalem cross. Its central tortuous cross, acting as a parallel LC resonator with a high inductance and a low capacitance, is utilized to realize a wide pass-band with low insertion loss by cutting off resistive sheet’s lossy path over a wide frequency band. The bandpass FSS adopts a triple-layer coupled structure, in which two layers of identical capacitive metallic patches are coupled through an intermediate inductive wire grid. A second-order pass-band with high selectivity is realized by this bandpass FSS. Simulation results show that the fractional bandwidth of the 1 dB transmission band and the 10 dB absorption band is 19.6% and 107.9%, respectively. Meanwhile, since its periodicity is only 8 mm, the proposed FSR also shows good angular and polarization stabilities. A good agreement between the measured and simulated results validates the proposed FSR design.

Design and Synthesis of Band-Pass Frequency Selective Surface With Wideband Rejection and Fast Roll-Off Characteristics for Radome Applications

In this article, we proposed an equivalent circuit-based design to synthesizing second-order band-pass frequency selective surfaces (FSSs) with wide out-of-band rejection and fast roll-off characteristics. An equivalent circuit model (ECM) is established in order to depict the desired frequency response. Then, based on the ECM, a generalized composite structure comprised of three FSS arrays separated by two dielectric substrates is proposed. The main advantage of this method is that it allows us to acquire optimized structural parameters according to the desired frequency response. With this method, an FSS based on the proposed composite structure is designed. The designed FSS can provide a second-order pass-band within the stop-band ranging from 1 to 40 GHz and the pass-band is operating at 10 GHz with 12% fractional bandwidth. Meanwhile, the bandwidth of the two transitional bands on both sides of the pass-band is only 0.26 and 0.6 GHz, respectively. Also, the frequency response of the designed FSS is stable at 60° for both TE and TM polarizations. For further verification, an FSS prototype is fabricated and experimentally characterized. Good agreements between the simulated and measured results can be observed.