Active Frequency Selective Surface Research Articles

An ultra‐thin switched state active frequency selective surface absorber with wide bandwidth using semi‐analytical method

AbstractUsing low‐voltage forward biased PIN diodes, an ultrathin switched states active frequency selective surface based microwave absorber is designed. Using a semi‐analytical method, the NXP#BAP70‐03 PIN diode‐based single polarised unit‐cell of the active FSS absorber is rigorously analysed and its equivalent circuit model is developed. The unit‐cell of the structure is selected as such to increase the envelope of the operating bandwidth and thus a total measured operating bandwidth extending from 1.70 to 11.36 GHz with reflectivity ≤−10 dB and the fractional bandwidth of 148.4% is achieved. This envelope also contains a 21.05% minimum, and 94.76% maximum fractional bandwidths corresponding to their resonance frequencies. It has been achieved by the absorber with the unit‐cell size of 0.113λL × 0.113λL × 0.048λL where λL is the wavelength corresponding to the lowest operating frequency. The structure has been fabricated and experimentally verified for the normal and the angular incidences of the electromagnetic wave up to 30°. An exhaustive state‐of‐art comparison has also been made to demonstrate the novelty of the proposed work. Due to its low thickness of 0.048λL, and wide envelope of the operating bandwidth; it is a potential candidate for a smart stealth system, electromagnetic camouflage, and adaptive radar absorbing materials.

Second order frequency selective surface design multi-functional at two bands

This paper proposes an active frequency selective surface which furnishes three bands of higher order filtering characteristics. It controls multi-functional features, at two bands (centered at 2 GHz and 4 GHz), namely electromagnetic response and polarization of plane wave. Nine discrete filtering outputs are independently controlled through switching in bias supply to four PIN diodes embedded in two layers of AFSS unit cell. Two multifunctional bands hold stability upto 45° angles under all polarizations of wave. Passive band at around 5.9 GHz contributed by all the independent binary combinations exhibits stability upto 45° angles of TE wave only. The equivalent circuit method is used to explain and verify the designed structure. Finally, the AFSS structure is fabricated and reliability of the simulation results is verified through experiments.

Design of a broadband multifunctional switchable absorber/reflector based on active frequency selective surfaces

Metamaterial absorbers are candidates for stealth systems due to their excellent wave modulation capabilities. With the rapid development of radar detection technology, the switching ability of metamaterial absorbers is becoming more demanding. A broadband multifunctional switchable absorber/reflector based on active frequency selective surfaces (AFSS) is presented in this paper. The proposed structure comprises an absorbing layer, a switching layer, and a ground with air spacers sandwiched between each layer. The PIN diodes can realize significant broadband conditioning performance between absorption and reflection functions by switching four operating states: ‘00’, ‘01’, ‘10’, and ‘11’. The bias network of this structure features horizontal and vertical configurations that independently coordinate the two polarization directions. It is worth noting that the bias network of the designed structure is very simple to minimize the impact on the structure. Distributions of surface current, an equivalent circuit model, and structural parameters are analyzed for a better professional insight into the working mechanism of the absorber/reflector. Finally, a prototype is fabricated and measured to verify the simulation.

A Polarization-Independent Multifunctional Active Frequency Selective Surface With Compound Modes of Absorption, Transmission, and Reflection

A Polarization-Independent Multifunctional Active Frequency Selective Surface With Compound Modes of Absorption, Transmission, and Reflection

SPICE‐based modeling of a polarization‐independent active frequency selective surface based tunable absorber with high angular stability

AbstractNowadays, demand for active frequency‐selective surfaces (FSSs) is increasing due to their reconfigurable characteristics. Numerical simulations of reactive elements require a model that can replicate their physical realization. Traditionally, reactive elements are modeled as approximated lumped circuits during electromagnetic simulations. However, such lumped circuits might fail to replicate device characteristics at high frequencies, leading to approximate results. This paper presents an alternative method to model reactive elements using their simulation program with integrated circuit emphasis models (SPICE) in circuit co‐simulations. The proposed modeling method is investigated by realizing a varactor‐loaded active FSS‐based tunable absorber. Simulated results indicate that the proposed absorber provides tunability in a wide frequency range from 0.54 to 2.09 GHz under normal incidence when electrostatic bias varies from 0 to 28 V. Moreover, polarization‐independent and exhibits stable frequency response for oblique incidences up to 80° for dual‐polarizations. These characteristics make it a potential candidate for stealth applications.

FPGA-Controlled Dual-Band Multifunctional Active Frequency Selective Surface

FPGA-Controlled Dual-Band Multifunctional Active Frequency Selective Surface

A Multifunctional Active Frequency Selective Surface With Scattering Diffusivity and Transmission Passband Tunability

A Multifunctional Active Frequency Selective Surface With Scattering Diffusivity and Transmission Passband Tunability

A Trifunctional Active Frequency Selective Surface for 6G Shielding Applications

A Trifunctional Active Frequency Selective Surface for 6G Shielding Applications

Active frequency selective surfaces: a systematic review for sub-6 GHz band

Abstract Radar absorption structures made of an active frequency selective surfaces (AFSS) have enormous potential in the aviation, naval, and other industries. In this research paper, a systematic review (SR) is carried out in the field of the AFSS to bring uncertainties, obstacles, challenges, classifications, applications, and design issues that arrive in the development of the sub-6 GHz architecture. To bias the AFSS component, as per the signal requirements, a unique set of circuits (PIN diode) is required, with ON and OFF state and a transmission zone. The bandwidth of which is determined by the bias voltage supplied. It can behave as a complicated hybrid impedance structure by providing ON and OFF biasing voltage to a PIN diode embodied in an FSS structure. Higher manufacturing costs of AFSS components, more significant complexities involved, a large amount of power consumption, and reactive impedance losses are some common limitations faced while implementing and designing an AFSS. Many envisioned problems are corrected with the AFSS design, current or creative implementations, and processing parameters are investigated progressively. It implies that new AFSSs will be an alternative to regular FSSs in the future. This paper is based on Kitchenham’s three-phase review procedure and supplements it with results, views, and recommendations from other leading experts in the field.

A wideband and high‐gain switched‐beam antenna system using active frequency selective surface

AbstractAntennas using the active frequency selective surface (AFSS) can achieve fast beam‐scanning ability at a low cost. In this study, a switched beam antenna with a high gain and 360° coverage in the horizontal plane is proposed. Twelve columns of AFSS form a cylindrical cavity and are transmissive or reflective to the incident electromagnetic waves under the control of PIN diodes. Benefiting from a coaxial continuous transverse stub array placed at the cavity centre and using 4 units to generate a high gain omnidirectional radiation, the proposed antenna can realise a highly directive switched beam. An antenna prototype is designed, fabricated and measured. Results show that it achieves a |S11| < −10 dB bandwidth of 12.9% and is able to sweep its beam around the azimuth plane with a high gain up to 13.6 dBi at 5.8 GHz working frequency.

A Switchable A-T-A Rasorber With Polarization Selectivity and High Roll-Off Characteristics

Switchable rasorber has attracted a great deal of interest in recent years due to its in-band shielding ability. In this work, a methodology for designing a switchable rasorber with high roll-off and polarization selection characteristics is proposed. The device can be switched between rasorber, polarization selection and absorber modes, and composes of two resistive layers (a passive and an active resistive layer) and a frequency selective surface (FSS). Transmission zeros (TZs) are introduced in the FSS design to improve the roll-off characteristic of its transmission band. The PIN diodes on the active resistive layer and FSS can be independently controlled in the orthogonal polarizations to realize the polarization selectivity. The simulation results indicate that the device exhibits a 27.5% fractional transmission bandwidth (3.42 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 4.51 GHz) under rasorber mode. In addition, the fractional absorption bandwidth is as large as 142.9% (1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 6 GHz) under absorber mode. A prototype is fabricated and measured to validate the proposed design methodology. This work is promising for designing metamaterials (MMs) with excellent flexibility and high performance.

Microwave Common-Frequency Absorption/Transmission Mode Conversion Based on Active Components

To overcome the limitations of traditional frequency selective surfaces in flexible state switching at the same frequency, this paper proposes a novel active frequency selective surface (AFSS) that simultaneously exhibits conversion between absorption and transmission modes at the same frequency band. By welding PIN diodes on the bottom structure of the AFSS, the conversion between band-pass (transmission) and band-stop (absorption) filters can be controlled electronically. In the common-frequency switch between absorption and transmission modes, the impedance matching of the AFSS is attained by altering the capacitance value of the varactors embedded on the top structure of the AFSS. The functionalities of the proposed AFSS design are investigated by full-wave simulations (in HFSS software) at 11.4 GHz. Furthermore, the operating principle is analyzed using an equivalent circuit model (in AWR software). To verify the concept, a prototype is manufactured, and the responses of mode switching are measured by adjusting the bias voltage. The measurement result is consistent with the simulation analysis. Owing to the tunability of the varactors, the structural asymmetry is compensated to achieve 80% absorptivity and transmissivity within a field of view of ±35°. The developed AFSS structure is highly valuable to be used in scenarios such as antenna domes, etc.

Active frequency selective surface-based 360° beam steering in two switchable frequency bands independently

ABSTRACT A reconfigurable antenna with frequency agility and pattern-switching ability is presented in this paper. The proposed design comprises a planar and compact frequency reconfigurable monopole antenna and an active frequency selective surface (AFSS) with reconfigurable transmission and reflection properties in dual bands. The AFSS screen is divided into four sections and each section is composed of four FSS elements with only one PIN diode in each unit cell. The monopole antenna, which is situated at the center of the box-shaped AFSS to illuminate , can reconfigure its resonating frequency between 2.5 and 3.6 GHz with the help of only one PIN diode. With the help of this transformation ability of AFSS behavior electronically, the pattern can be steered across the entire X–Z plane in eight steps at two different operating frequencies independently. This antenna can be very useful in various wireless applications such as drones, small base stations, etc.

Switchable Polarization Insensitivity Radar Absorber/Reflector Based on Active Frequency Selective Surface

In this article, a switchable polarization-insensitive lower thickness active frequency selective surfaces (AFSSs) absorber/reflector is proposed. The proposed structure consists of the top AFSS layer, which contains four patches and four pairs of PIN diodes in parallel with resistors, the upper substrate I, a metal ground, the bottom substrate II, and a simple bias network behind the bottom substrates that effectively reduce unnecessary disturb. The PIN diodes are controlled by the bias networks through the metalized via holes to obtain the switchable properties. The polarization-insensitive performance is realized by symmetrical structure. The resistors in parallel with PIN diodes load between every two patches to enlarge the resonant bandwidth. One remarkable feature of the structure exists in it can be switched in a variety of operating states to control the visibility of the target in radar systems. When it operates as an absorber, it has a stable absorptivity, which is more than 85% in the frequency range of 12.6–25 GHz with a small radar cross section. The equivalent circuit model, surface loss density, and surface circuit distribution are provided to assist in understanding the physical property of the entire design. A configuration of the structure is fabricated and the measurement results demonstrate a reasonable agreement with the simulate ones, thus confirming the proposed concept.

Wideband Reconfigurable Reflectarray Based on Reflector-Backed Second-Order Bandpass Frequency Selective Surface

In this communication, a wideband reconfigurable reflectarray based on reflector-backed active second-order bandpass frequency selective surface (FSS) is presented. The reflector is composed of periodic short-circuited parallel plate waveguide (PPW) and the FSS is composed of stacked non-resonant metallic elements separated by thin dielectric substrates. By integrating microwave varactors in the capacitive layers of FSS, more than 270° continuous phase tunability is achieved within a fractional bandwidth of 14%. A 1-D reflectarray prototype operating at C-band is fabricated and measured. The experimental results show that it can achieve ±55° beam scanning coverage. Symmetric beam steering is observed due to the center-fed configuration. With advantages of low cost and simple structure, the proposed reflectarray can be potentially used in wideband wireless communication and radar systems.

A Flexible Multifunctional Active Frequency Selective Surface With Angular and Polarization Stability for Curved Surface Conformal

This article demonstrates a miniaturized flexible multifunctional active frequency selective surface (MAFSS) with excellent angular stability and polarization insensitivity. The proposed structure consists of two metal layers printed on a thin, flexible substrate to achieve curved surface conformal capability. The PIN diodes are welded on two metal layers. By independently switching the bias voltage of PIN diodes, four working states and two functions, including electromagnetic (EM) switching and polarization selection, are realized. The unit cell takes itself as a bias feeder to excite the PIN diodes, avoiding the negative impact of the additional feeding network on the overall performance. A spiral wire-based miniaturization technique is utilized to achieve excellent angular stability in the planar and conformal states. On this base, a double-layer symmetrical structure based on the via-connected metal strips is designed to realize polarization insensitivity. The miniaturized unit size can reach <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.082\lambda _{0} \times 0.082\lambda _{0} \times 0.005\lambda _{0}$ </tex-math></inline-formula> at the center resonance frequency. The flexible MAFSS prototype is measured in planar and conformal states. The measured operating bandwidth with a transmission coefficient difference greater than 10 dB is 2.8 GHz (5.2–8 GHz). The measured and simulated results have good consistency, demonstrating the effectiveness of the proposed structure.

Time-Varying Reflectivity Modulation on Inverse Synthetic Aperture Radar Image Using Active Frequency Selective Surface

Interrupted-sampling repeater modulation (ISRM) can generate multiple virtual target images by sampling and forwarding radar signals to confront the inverse synthetic aperture radar (ISAR). However, the repeating delay is unavoidable and the system is complex. Moreover, numerous works mainly discuss the ISAR image deception based on periodic modulation, and the modulation effect is relatively limited. Active frequency selective surface (AFSS) can perform intermittent amplitude modulation to the reflected signal by varying target reflectivity, thus, it can achieve the function of ISRM. In this paper, the possibility of AFSS applied in the multiple virtual target generation is discussed, and an ISAR image-modulation method is proposed. The radar signal is periodically modulated by varying target reflectivity, which is accomplished by the AFSS-loaded PIN diodes. On this basis, the AFSS modulating frequency is randomly varied in a slow time domain. When the modulation signal is processed by the ISAR system, the produced target image is unfocused and cannot be recognized by radar. Simulation results are utilized to demonstrate the effectiveness of the proposed method.

A method for transforming 2D imaging features with the AFSS‐loaded rotating Luneberg lens reflector

The micro-motion target can perform micro-Doppler modulation on the electromagnetic wave due to motion components in different directions, leading to the defocus effect of the target imaging feature along the azimuth direction. This phenomenon has been widely concerned and investigated in the field of target recognition and anti-recognition. However, the micro-motion target fails to transform image features along the distance direction according to its slow mechanical modulation speed. In comparison, electromagnetic metasurfaces can flexibly control the characteristics of electromagnetic waves with faster modulation speed through electronic control. It can realize the image feature transformation along the range direction. Inspired by this idea, a joint modulation method of rotational micro-motion and electronic control through an active frequency selective surface (AFSS)-loaded rotating Luneberg lens reflector is proposed. The signal models of micro-motion modulation and non-periodic AFSS modulation are established separately. On this basis, the imaging properties of the joint modulation are further analysed and the two-dimensional (2D) defocusing phenomenon of the image is discovered. Moreover, the simulation of the measured synthetic aperture radar (SAR) data with different modulation methods is conducted to demonstrate the effectiveness of the proposed method.

A Low-Cost Beam Steering Antenna for Indoor Wireless Communication

Low-cost beam coverage is one of the main goals of indoor wireless communication. We propose a low-cost beam steering (BS) antenna by using a hemispherical active frequency selective surface (AFSS) and its embedded varactors. At the same time, in the 3-D spherical coordinate system, we propose a universal design method for this type of antenna based on the principle of phase compensation. The designed BS antenna can realize energy gathering and beam switching in the quasi-full airspace on the ground. BS without phase shifter and reasonable allocation of energy brings the advantages of low cost. The proposed approach provides a feasible scheme for improving the robustness and energy saving of wireless communication.

A high angle stable and polarization symmetric dual band reconfigurable frequency selective surface

Abstract A dual band reconfigurable active frequency selective surface is designed which resonates at 2.5 and 3.6 GHz as transmission pole frequencies. Lower and upper pass bands have −3 dB bandwidths of 0.4 and 0.5 GHz, respectively. Two notch frequencies are configured from 4.71 to 3.5 GHz and from 2.78 to 2.45 GHz. Dimensions of the unit cell are optimized to λ 0/10 calculated at the lowest resonance. The proposed design exhibits insignificant variation at oblique excitation angles until 75° of dual polarized plane wave. An equivalent circuit model is developed for optimization and vivid understanding of the structure behaviour. A prototype of the designed structure is fabricated and the measured response is in satisfactory agreement with the simulated transmission response. High angle stability, low insertion loss at both the transmission pole frequencies, dual band pass/stop tuning for single and dual polarized wave with use of only two varactor diodes are eminent features of the design.