Active Frequency Selective Surface Research Articles

1‐Bit reconfigurable transmitarray with low insertion loss for wireless power transmission

A 1-bit reconfigurable transmitarray (RTA) is studied in this article for wireless power transmission (WPT). The type of the RTA is micro-element embedded active frequency selective surface (AFSS). First, parameters of the 1-bit RTA unit-cell are obtained at 5.8 GHz through equivalent circuit analysis and HFSS simulation. After optimization, the maximum insertion loss of the unit-cell at 5.8 GHz is 0.96 dB, a 180-degree phase difference can be got when pin diodes reverse and forward biased, and the size of the unit-cell is about 0.42λ0. Second, composing the unit-cell to 11 × 11 AFSS, the beam-steering of ±40° in XOZ-plane and YOZ-plane can be achieved, while feeding layer effect is to be discussed. Finally, the beam-steering performance of this AFSS is verified through testing. The dimensions of each FSS board are 260.9 × 260.9 × 0.8 mm3 and 0/1 distribution is controlled by field-programmable gate array (FPGA). The measured results are basically consistent with the simulation ones in beam steering. The difference of the peak gain in the main direction is analyzed.

Design of a Broadband Switchable Active Frequency Selective Surfaces Based on Modified Diode Model

In this letter, a broadband switchable active frequency selective surfaces (AFSSs) is presented, and the proposed structure is comprised of three metallic layers and two Rogers RT5880 substrates. In order to realize broadband transmission, a second-order bandpass filter is constructed, and the multilayer coupling effect is utilized. A modified equivalent circuit model of the p-i-n diode is proposed to reduce the simulation error and improve the design accuracy. By controlling the bias voltage of the p-i-n diodes, two completely different functions are realized. At the function of dual-polarized shielding, the shielding bandwidth is 2–18 GHz, and the transmission bandwidth is 8.2–10.5 GHz with a fractional bandwidth of 24.6% at the function of dual-polarized transmission. A prototype has been fabricated and measured, and the measurement results are in good agreement with the simulation results, indicating that the AFSSs has good polarization stability for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> -band radar applications.

Imaging Properties of Nonperiodic Time-Varying Active Frequency Selective Surface

The rapid development of artificial electromagnetic (EM) materials provides unprecedented freedom to manipulate EM waves. In recent years, reflective spectrum conversion technology based on EM materials has been widely studied due to its multiharmonic generation capability. The possibility of its application in wireless communication is explored. However, the research on spectrum conversion is mainly based on periodic modulation waveforms. The modulation effect of nonperiodic for EM materials has not been studied so far. In this article, a nonperiodic time-varying active frequency selective surface (AFSS) is proposed to manipulate the spectral distribution of harmonics. By dynamically controlling the amplitude of the reflected echo, a continuous Doppler spectrum is generated. On this basis, the imaging characteristics of nonperiodic time-varying AFSS are analyzed in detail, and the defocusing phenomenon of the target image is discovered. Finally, the radar imaging experiments for nonperiodic time-varying AFSS are conducted to verify the validity of the proposed method.

Fourier Subspace-Based Deep Learning Method for Inverse Design of Frequency Selective Surface

Frequency selective surface (FSS) is critical for electromagnetic (EM) radiation protection due to its high spatial filtering performance, especially for active FSS. Recently, the artificial neural network (ANN) has shown great potential in solving EM inverse problems and rapid industrial design. In such an inverse model with ANN, it establishes the relationship between the given inputs of S-parameters and the desired structure parameters or material parameters. However, faced with applications where S-parameters vary in a large frequency range with different curve shapes, such as multiband microwave devices, equal interval sampling may result in high-dimensional inputs and will require a more complicated neural network. In this work, we present a Fourier subspace-based deep learning method (FS-BDLM) for FSS inverse design, where the dimension of the input is largely reduced by using Fourier subspace to represent the most salient features of the desired S-parameter performance. Compared with existing deep learning methods, the proposed technique makes inverse neural models more compact and more stable to noise contaminations. The validation of the proposed FS-BDLM is conducted both numerically and experimentally through two dual-passband FSS design examples, where the well-designed FSS is fabricated to validate the technique.

Characteristics of an Eight-Quadrant Corner Reflector Involving a Reconfigurable Active Metasurface.

The traditional corner reflector is a type of classical passive jamming equipment but with several shortcomings, such as fixed electromagnetic characteristics and a poor response to radar polarization. In this paper, an eight-quadrant corner reflector equipped with an electronically controlled miniaturized active frequency-selective surface (MAFSS) for X band is proposed to obtain better radar characteristics controllability and polarization adaptability. The scattering characteristics of the new eight-quadrant corner reflector for different switchable scattering states (penetration/reflection), frequency and polarization are simulated and analyzed. Results show that the RCS modulation depth, which is jointly affected by the electromagnetic wave frequency and incident directions, can be maintained above 10 dB in the majority of directions, and even larger than 30 dB at the resonant frequency. Moreover, the RCS adjustable bandwidth can be as wide as 1 GHz in different incident directions.

Active frequency selective surface with switchable response for satellite communications in X and Ka bands

This article proposes an active frequency selective surface (FSS) with application to satellite multi-band antennas in X and Ka bands. The FSS offers two reflective bands at 20 and 30 GHz (downlink and uplink frequencies of broadband communication satellites in Ka-band) under one biasing state, whereas the frequency characteristics are switched in X-band (between 9 and 12 GHz, frequencies used for broadcast satellite services and space exploration) under the second biasing state. The unit cell of the FSS consists of three metallization layers, including four PIN diodes on the bottom layer that provide reconfigurability in X-band. The FSS performance has been analyzed using an equivalent circuit model diagram, which provides a better understanding of the operation principle. The proposed FSS exhibits a robust transmission response for incidence angles up to 40°. Several samples of the FSS formed by a 2 × 4 cell array have been fabricated and measured in a waveguide simulator to confirm the results of the electromagnetic simulations at the lower operating frequencies.

Study of switchable Luneburg lens reflector based on active frequency selective surface

In this article, a transmission and reflection states switchable Luneburg lens (LL) reflector based on active frequency selective surface (AFSS), namely LL-AFSS reflector, is proposed. The reflector is composed of a miniaturized AFSS reflector, which can change the working state, and a LL composed of six layers and five media from inside to outside. By changing the bias voltage of the PIN diode loaded on the AFSS reflector, the transmission and reflection working states of the LL reflector can be freely switched in the frequency range of 3.5–5 GHz. When the PIN diodes are in ON state, the incident electromagnetic wave will not be reflected through the LL reflector. When the PIN diodes are in OFF state, the radar cross section (RCS) generated by the LL reflector to the incident electromagnetic wave reaches about 15 dB within the angle range of −30 ≤ φ ≤ 30°. The electromagnetic numerical simulation and experimental sample verification of LL-AFSS reflector are carried out. The results show that the proposed design method is feasible and the RCS changes of transmission and reflection characteristics of LL reflector are obvious.

Dual-Band Millimeter-Wave Beam Scanning Slotted Square Patch Antenna Based on Active Frequency Selective Surfaces for 5G Applications

This paper is concerned with dual-band electronically beam-scanning for millimeter-wave (mm-wave). A design encompassing slotted square patch antenna with an Active Frequency Selective Surface (AFSS) screen is presented. Each unit cell of AFSS is constructed of spiral arms endorsed with four varactors to enhance a wide tuning range over dual modes. Among many other features, the design supports dual operation bands at 48.5 and 68.5 GHz which is considered without any 90° phase shifter and is excited by a single-feed microstrip line. The overall size of the proposed antenna is only 3.9(3.9(0.1 mm3, which is smaller than some mm-wave antenna modules released recently. The whole structure is an ideal option for wireless applications due to its ability to scan its primary beam in dual-band throughout the whole azimuth plane with a resolution of 60°. The proposed structure can sweep beams throughout the whole 360° angular range and is capable of both dual-band and single-band beam sweeping with high gain, making it an excellent choice for 5G mm-wave applications.

Time-modulated active frequency selective surface absorber/reflector for spectrum conversion

Spectrum conversion involves the movement of a signal from one frequency to another, and it is well-studied in materials interacting with electromagnetic waves from microwave to optical frequencies. In this paper, a time-modulated active frequency selective surface (AFSS) absorber/reflector is proposed to manipulate the spectral distribution of the echo signal. The discrete harmonics based on the period modulation and the continuous spectrum based on the random coding modulation are analyzed in detail. In addition, a simple digital-coding control network based on field programmable gate array (FPGA) in the time-modulated AFSS absorber/reflector (TMAAR) is used to switch between absorbing and reflecting states flexibly. By coding “0” and “1” elements with the controlled sequences, the TMAAR is able to achieve the predefined modulation function for spectrum conversion. On this basis, a prototype of the designed TMAAR is manufactured. The linear frequency modulation radar echo experiments are performed to verify the required function of spectrum conversion.

A Multifunctional and Miniaturized Flexible Active Frequency Selective Surface

In this letter, a flexible multifunctional active frequency selective surface (AFSS) with miniaturized units is proposed, and it is simulated, fabricated, and measured. In the proposed scheme, the flexible AFSS structure adopts p-i-n diodes for function control, realizing the arbitrary selection of electromagnetic-wave transmission and reflection functions and polarization selection function in the range of 8–12 GHz. The flexible multifunctional AFSS structure consists of two frequency selective surface (FSS) layers with p-i-n diodes on the top and bottom, a thin, flexible dielectric layer in the middle, and vias for connecting the FSS layers. The deigned structure does not need additional bias feeders, which can reduce the interference to the transmission performance of AFSS. Finally, it is found that the experimental results of the AFSS structure are in good agreement with the simulation results.

Reconfigurable bandstop filter using active frequency selective surface, design and fabrication

Abstract In this paper a novel FSS array is proposed, that provides dynamic band-gap in C-band. Inside the band-gap, the FSS acts as a bandstop filter. Outside the band-gap the amplitude of the reflected wave from the FSS array decreases. Therefore, the outside band is very useful in radar cross section (RCS) reduction. In this paper, at first a new FSS unit cell is designed, then in order to achieve the maximum bandwidth (1.2 GHz), dimensions of the cell are optimized. In the next step, the FSS cell is equipped with PIN diodes. Turning the diodes ON or OFF, shifts the resonant frequency of the band-gap electronically. When diodes are OFF, the resonant frequency and −10 dB bandwidth of the FSS are 5.23 and 0.9 GHz respectively. When the diodes turn ON, the resonant frequency shifts to 4.75 GHz over a bandwidth of about 1 GHz. While the band-gap is shifted dynamically, the bandwidth is kept wide, which is the novelty of this paper. In order to validate the design process, an array of active cells consisting of 128 pin diodes was designed, fabricated and then tested. Finally, the simulation and measurement results are compared with each other and a good agreement is observed between them.

A Physical Insight Into Reconfigurable Frequency Selective Surface Using Characteristic Mode Analysis

In this letter, we provide a physical insight into the operating mechanism of a reconfigurable bistate active frequency selective surface (AFSS) based on characteristic mode analysis. At first, we study the modal behavior of the unit structure under different load resistances and found that the modal significance bandwidths of the fundamental mode and the high-order modes increase with the load resistance. When the load resistance is very high, the fundamental resonant frequency will move to high frequencies and the high-order modes may not change. Such features can be used in the design of the reconfigurable transmission/reflection frequency selective surface. It is found that the isolation between transparent and reflective states is mainly determined by the radiated field from fundamental mode. At the same time, we provide two improvement approaches to design this kind of frequency selective surfaces. And a reconfigurable AFSS operating at 2.1 GHz is designed, fabricated, and measured. The letter makes sense for the design of reconfigurable bistate AFSSs.

Switchable Polarization-Insensitive Frequency-Selective Surface Reflector/Absorber With Low Profile by Using Magnetic Material

A multifunctional frequency-selective surface reflector (FSSR) is found to be an applicable candidate for the realization of an antenna's radar cross-section reduction in or out of its operating frequency. In this letter, a polarization-selectable in-band switchable FSSR/absorber based on diodes is designed and experimentally demonstrated, which comprises a layer of active frequency-selective surface and a layer of metal-backed frequency-dispersive magnetic material (FDMM). The design realizes the switching functions of broadband absorption band from 3.5 to 12.0 GHz (110% bandwidth) with at least −10 dB reflectivity and a reflection band at 6.2 GHz with two-sided absorption bands (2.2–4.8 GHz, 74.3% bandwidth for the lower absorption band and 7.1–12.9 GHz, 58% bandwidth for the upper absorption band) by manipulating the <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> and <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> states of PIN diodes. Besides, due to the use of FDMM, the thickness of the FSSR is only 0.074 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ_L</i> , making it easy to design the reflective surface of the antenna. A prototype of the proposed FSSR/absorber is fabricated and measured, where reasonable agreement is observed.

A wideband multilayer tunable filter using active frequency selective surface

AbstractA wideband multilayer filter with frequency tunable characteristics is proposed using active frequency selective surface (AFSS). The proposed AFSS includes three metal layers, while the top and bottom metal layers are embedded with varactors. The metal layers are separated by dielectric layers. By simultaneously adjusting the bias voltage loaded on both ends of the varactor diode, the working bandwidth of the AFSS can be continuously tuned. The operating mechanism is studied using an equivalent circuit, which is composed of an LC series resonance circuit and a parallel resonance circuit. The simulation results prove that the proposed AFSS performs wideband filtering in the frequency range of 3.55 to 4.43 GHz without a bias voltage. With a bias voltage of −19 V, the AFSS working passband is tuned to 4.43 to 5.53 GHz. Simulation and experimental results show that the AFSS has wideband filtering characteristics and good tunable performance.

A dual‐function switchable and frequency tunable active frequency selective surface

A dual-function switchable and frequency tunable active frequency selective surface (AFSS) is proposed in this article. The proposed AFSS consists of a dielectric substrate and metal patches on the top and bottom surfaces of the substrate, while varactor diodes and PIN diodes are soldered on the top and bottom layers, respectively. A simple bias network is designed to facilitate the function adjustment of the AFSS. By controlling the OFF and ON states of the PIN diode, the AFSS can switch between filtering and wave absorbing functions. Furthermore, by adjusting the bias voltage of the top varactor diode, the AFSS can achieve continuous frequency tunability. Simulation and experimental results confirm the dual-function switchable and frequency tunable performance of the proposed AFSS.

Beam‐steering transmitarray with active frequency selective surface for wireless power transmission

This letter presents a beam-steering antenna with an active frequency selective surface (AFSS). The proposed AFSS-based antenna consists of a source antenna and an active frequency selective surface as a superstrate. The AFSS unit consists of three layers, where the upper and lower layers are identical and the middle layer has a slight variation. This structure makes it possible to obtain 360° phase variation, which can be achieved by changing the bias voltage at both ends of the varactor diodes in the case of three layers. Its passband is about 40 MHz (from 5.14 to 5.18 GHz). Finally, the proposed unit cell was stacked into a 5 × 5 array; both simulated and measured results demonstrated that the transmitarray has the scan ability of ± 25° at 5.16 GHz. The proposed transmitarray is suitable for dynamic wireless power transmission.

Improved dual-polarized wideband multifunctional switchable absorber/reflector based on active frequency selective surfaces.

An improved dual-polarized multifunctional switchable absorber/reflector with both wideband absorbing and wideband reflecting characteristics is presented in this paper. The proposed structure consists of three parts including a top-layer active frequency selective surface (AFSS) structure, a bottom-layer metal sheet and an air spacer in between. The polarization stability is satisfied by deploying the super-element topology, which contains four similar unitary elements arranged in a 2 × 2 matrix form. The PIN diode is employed as a RF switch in the AFSS structure for the purpose of switching. The bias networks responsible for different polarizations are intentionally separated through via holes. Multifunctional properties with four different operating states can be attained by controlling horizontally- and vertically-loaded PIN diodes independently. In addition, the biggest advantage of the proposed structure lies in its wideband features at both absorbing and reflecting states for different polarizations and incident angles. Finally, a prototype of the design is fabricated and measured to verify the simulation, and a good agreement between the simulated and observational results can be achieved under normal incidence as well as oblique incidence.

A Phase Compensation Beam Switching Antenna Based on Frequency Selective Surface

In this letter, the phase compensation method is used to analyze and design the cylindrical frequency selective surface (FSS), and a beam switching antenna is proposed to verify the correctness of this method. The proposed antenna includes an omnidirectional dipole and a phase adjustable active frequency selective surface around it. To steer and gather the pattern of the dipole, each FSS structure is embedded varactors with an orderly bias voltage under the guidance of the phased compensation method. The proposed antenna achieves the characteristic of a high gain of 13.3 dBi at 2.45 GHz while satisfying the beam switching with a 20° step. The measured results of the fabricated prototype are in good agreement with the simulated ones. This antenna can be widely used to improve the signal distribution and the quality of wireless communication systems.

Beam‐sweeping antenna with beamwidth reconfigurable response

In this work, a simple and compact reconfigurable antenna, whose radiation direction and beamwidth can be manipulated, is proposed based on active frequency-selective surfaces (AFSSs). This design is completed by using a monopole antenna and six reflective AFSS screens, while its reconfigurable behavior is obtained by employing semiconductor switches. By applying different biasing voltages on the diode switches, the operating state of the AFSS screens can be configurated; therefore, the radiation direction and beamwidth of the monopole antenna can be controlled. From 1.8 to 2 GHz, the omnidirectional scanning response can be achieved at xoy-plane with the step of 60° and the maximum radiant efficiency is 96%. Furthermore, the 3 dB beamwidth can also be adjusted from 176° to 120° with the measured gains of 4.58–6.53 dB. The diameter of this structure is only 50 mm, which is a relatively small structural size for an omnidirectional beam-sweeping antenna. Finally, the proposed design is validated by numerical simulations and experimental measurements.

Metamaterial-inspired optically transparent active dual-band frequency selective surface with independent wideband tunability.

This paper presents an optically transparent active bandstop frequency selective surface (FSS) with wideband tunability of two resonance frequencies using the concept of miniaturized element FSS (MEFSS). The proposed design consists of metallic square loop arrays on a new optically transparent substrate as the top layer, a glass interlayer, and periodic patterns of cross dipoles on the substrate as the bottom layer. Two kinds of resonant elements loaded with varactors and the designed bias networks achieve two independent tunable stopbands. The proposed FSS has two large tuning ranges, one is from 1.20 GHz to 2.63 GHz and another is from 2.0 GHz to 5.9 GHz (75% and 99% with respect to the center frequency, respectively). The wideband dual-tuning mechanism is theoretically analyzed and demonstrated by deriving its equivalent circuit (EC) model. The experiment results exhibit reasonable agreement with the numerical simulation responses. This proposed design, with low profile, angular stability, polarization insensitivity, optical transparency, and wideband dual-tunability can play an important role in manipulating electromagnetic wave propagation for manifold applications.