Frequency Selective Surface Array Research Articles

Design of Dual-Band Composite Radome Wall With High Angular Stability Using Frequency Selective Surface

The present paper proposes a design method for synthesizing a frequency selective surface (FSS) based dual-band composite radome wall with high angular stability. In the proposed method, a generalized composite structure composed of two FSS arrays embedded within dielectric layers has been adopted to obtain the dual-band second-order band-pass transmission characteristic. To describe this transmission characteristic, a novel equivalent circuit model has been established. By this equivalent circuit model, the optimized structural parameters can be derived from the desired resonant frequency and bandwidth of the pass-bands via a curve-fitting method. Also, discussions and principles for designing the dielectric layers have been provided to achieve the high angular stability. The simulated results show that the designed radome wall can provide two pass-bands operating at 10GHz and 14GHz with a bandwidth of 2GHz and 1.8GHz, respectively. And the two pass-bands are stable at 60° incident angle for both TE and TM polarizations. For verification, a dual-band composite radome wall has been designed, fabricated and measured. Good agreements between the simulated and the measured results can be observed.

Polarization independent quad‐bandpass frequency selective surfaces with wide‐band ratio

A single layer polarization independent quad-bandpass frequency selective surface (FSS) with wide-band ratio is demonstrated theoretically as well as experimentally. The proposed structure passes four frequency bands with wide band ratio. The proposed FSS design is implemented by incorporating alternate arrangement of four units which are rotated 90° clockwise to form a unit cell of metal over a FR4 substrate. The geometrical dimensions of proposed unit cell are optimized and arranged in such a way that the structure possesses the quad bandpass characteristic and aspect dimensions of one unit is 0.11λ × 0.11λ with respect to first resonant frequency. This FSS provides stable response for different angle of incidence in transverse electric (TE) mode and transverse magnetic (TM) mode. To validate the results proposed FSS array has been fabricated and measured in free space environment. The measured results are in good agreement with the simulated results. Excellent stability is also observed for different incident angle.

Reconfigurable and stable frequency selective surfaces on nonuniform and finite arrays

AbstractA reconfigurable frequency selective surface (FSS) is presented based on a nonuniform and finite array of square loop conducting patches printed on a dielectric substrate. The reconfigurability is performed changing the polarization of PIN diodes in the patch elements of the FSS array, from OFF to ON state and vice versa. Simulation is performed by the wave concept iterative procedure (WCIP) formulation and Ansoft HFSS package. Remarkably, a generalization of the WCIP method is proposed to solve scattering problems on FSS structures composed of nonuniform and finite arrays. Prototypes are fabricated for comparison purpose. Simulations and measurements show that the nonuniform and finite FSS has dual band behavior and stable frequency response for different angles of the incident plane wave. A very good agreement between HFSS simulation, WCIP formulation, and experimental results is verified.

Efficient radar backscatter suppression

Researchers at Queen's University Belfast's Centre for Wireless Innovation (CWI) have combined expertise in on-body wireless sensors and frequency selective surfaces to solve a problem limiting the RF performance of medical sensors. Conventional frequency selective surfaces (FSSs) reflect or transmit microwave energy at particular resonant frequencies. However, there is a newer class of periodic structures, thin resistively loaded FSS absorbers, which absorb most of the incident power. This class of microwave absorber has the potential to significantly enhance the RF performance of all existing and future physical layer communication systems used in environments, hostile to propagation. Thin resistivity loaded FSS absorbers have planar, thin and lightweight metal-backed patterned screens, making them particularly suitable for suppressing wave refl ections from metal objects such as wind turbines as well as land, air or space-based vehicles. Normally, FSS arrays are simulated and measured in the ‘far field’ of the excitation antenna. This means that the separation distance is sufficiently large for the periodic absorber surface to be excited by plane waves. For all conventional RF applications this is a reasonable assumption, for example a radar system is generally positioned remotely and far from the structure that it interrogates. However, for healthcare monitoring applications, near field solutions are required as there is close contact between the absorber surface and excitation antenna. In this issue of Electronics Letters, Dr Gareth Conway and colleagues have evaluated the performance of this new class of FSS absorber when in close proximity to an excitation antenna. The thin resistivity loaded FSS consists of a 3 mm thick metal-backed patterned substrate with each unit cell composed of four hexagonal-shaped elements that are similar in size to the antenna. “So, effectively the excitation waves impinge on just one of the nested loops,” explains Conway. “Moreover, in the near field the amplitude and phase of the waves vary spatially across the unit cell. This is totally different to the conventional far field operation of these absorbers, where the wave ‘sees’ an infinite array of unit cells, and the amplitude and phase is the same across each cell.” The results reported show that no detuning is observed at the operating frequency of the antenna when the thin resistivity loaded FSS absorber is placed in very close proximity (λ/30 and λ/5), whereas at such distances a tissue phantom or metal sheet significantly reduces the radiation efficiency of the antenna.. This shows that the FSS structure greatly suppresses radar backscatter and is suitable for use in near field systems, such as wearable systems. Members of the team in the anechoic chamber at the university's Institute of Electronics, Communications and Information Technology. From the left: Dmitry Zelenchuk, Robert Cahill, Gareth Conway and Niamh McGuigan, who is wearing a test device incorporating the FSS. A close-up of a wearable medical monitoring device with the FSS absorber positioned between the device's antenna and the body-side of the device. The human body presents a notoriously difficult propagation environment that is very specific to the individual person. Thin resistivity loaded FSS surfaces decouple the environment surrounding the antenna, such that a tailored electromagnetic barrier is formed. This opens up a large number of new applications in near field environments that were previously “unimaginable”; making wireless performance predictable and reliable regardless of the platform on which the antenna is placed. This is in stark contrast to existing solutions, which cannot deliver this behaviour. In addition to enhancing the performance of on-body wireless sensors, this technology could meet the need to suppress RF energy scattering from satellite platforms, which host large antenna farms. By selective placement of thin resistivity loaded FSS structures on the surface of the space vehicle, radiation pattern degradation and antenna coupling will be reduced, both of which currently require costly solutions. The results obtained from the reported preliminary measurement campaign in this Letter, suggest that this approach has excellent potential for isolating mobile platforms such as the human body. “However, this is only the ‘tip of the iceberg’, researchers have been tackling the problem of electromagnetic scattering from platforms for years, and still continue to do so”, says Conway. “The step change in performance capability of field deployable antennas embedded into electromagnetic control surfaces will allow greater communication link margins, hence facilitating significantly higher quality of service for medical data transfer than is currently envisioned. However, at present, the processes necessary to optimise the design and manufacture of such FSS structures are not well understood. To develop this technology to an acceptable engineering standard, the research must explore the combined spaces between state-of-the-art artificial engineered electromagnetic surface design and new multifunctional additive manufacturing technology. We would be zealous to see the synthesis and demonstration of the operation of a new innovative surface which radically accelerates the application of wireless enabled mobile communication and healthcare sensor technology.”

GSM filtering of horn antennas using modified double square frequency selective surface

In this work, a dual-band frequency selective surface (FSS) is proposed to be placed perpendicularly into the apertures of horn antennas for prefiltering 900 and 1800 MHz GSM signals interfering during the signal reception, with the enhanced return loss, gain, and directivity at the desired frequencies. For this purpose, the microstrip double square loop MDSL is modified in the first stage. As for the second stage, an FSS array (2 × 2) is built up arranging the unit MDSLs in a periodic structure and finally these FSS unit arrays are fixed perpendicularly covering the aperture of a ridged horn antenna which is a part of the available radar system operating between 0.5 and 3 GHz in our laboratory, to construct an integrated module having both bandstop prefilter and horn antenna called “filtenna.” The simulated and experimental results are agreed that the proposed FSS structure attenuates GSM signals at the 900 and 1800 MHz through the high reflection and very poor transmission mechanisms meanwhile enhances return loss characteristics, radiation pattern, and gain of the horn antenna in the desired band. Thus, it can be concluded that these simple microstrip FSS structure can be effectively adapted to the horn antennas which need the GSM prefiltering.

Frequency Selective Surface Structure Miniaturization Using Interconnected Array Elements on Orthogonal Layers

Traditionally, the element of a frequency selective surface (FSS) is rotationally symmetrical and the element arrays in a multilayer FSS are aligned with each other. A new approach to miniaturize the size of the FSS array element is proposed in this paper by interconnecting the array elements only in one direction in a two-layer FSS structure. One layer acts as an enhanced inductor while the other layer provides capacitance. The interconnection between the adjacent array elements changes the equivalent circuit and produces a strong cross-layer capacitance, which lowers the resonant frequency significantly. The dimensions of the miniaturized FSS element are much smaller than the wavelength at the resonant frequency (periodicity $\ll \lambda )$ . The element can also have a low profile since the cross-layer capacitance is stronger with a thinner substrate. The sensitivity to the incident angle of the proposed structure is comparable with traditional ones. A theoretical equivalent circuit model is developed to characterize the structure, based on the analysis of the geometrical configuration of the FSS structure and the electric field distribution on it. The theory was verified by the experimental results.

Frequency‐selective surface‐based chipless passive RFID sensor for detecting damage location

Various types of wireless sensor systems are used for structural health monitoring. Especially, chipless wireless sensors are useful in harsh environmental conditions of high temperature and toxic chemicals. This paper reports a chipless passive structural health monitoring sensor made with frequency-selective surface (FSS) array for detecting damage location. FSS is a kind of microwave meta-material antenna that is selective by its electromagnetic characteristics associated with geometrical shape and electrical impedance. An FSS-array-based wireless sensor was made on a flexible substrate that can be easily attached to arbitrary structures. When microwaves are applied to the FSS via a single horn antenna, the sensor senses its reflection changes, and by scanning the horn antenna, the location of damage is detected. Design, fabrication, and characterization of cross-type FSS array are carried out, and a computer simulation is performed for comparison.

A Low-profile Miniaturized Second-Order Bandpass Frequency Selective Surface

High-order bandpass frequency selective surfaces (FSSs) (N ≥ 1) can achieve high performance with a flat in-band frequency response and fast roll-off. One particular practical issue of designing bandpass FSSs using resonant surfaces is that the thickness of the substrate would be around a quarter of a wavelength. On the other hand, the size of a nonresonant FSS array element is usually large. A new miniaturized FSS capable of exhibiting a second-order bandpass response is proposed in this letter. Two miniaturized resonant surfaces coupled by a nonresonant inductive layer are used to build the proposed FSSs. An FSS operating at around 3.8 GHz is designed to verify the method. The element size is smaller than 0.076λ × 0.076λ for the proposed structure. This is significantly smaller than the element size of second-order FSSs designed using conventional approaches. The overall thickness is less than λ/24, where λ is the free-space wavelength at the resonant frequency. The method could be particularly useful for the design of FSSs at lower frequencies with longer wavelengths.

Design, Prototyping and Measurement of a Cascaded 6-GHz Frequency Selective Surface Array

The design, prototyping, and free-space measurement of a 6-GHz Frequency Selective Surface filter is presented. The prototyping resolution of a large (A4 sheet size) Frequency Selective Surface with small loops as elements is checked, as well as the correlation with measurements performed with a 3-D full-wave solver. The test also involved the effect of cascading two different Frequency Selective Surfaces with a viewpoint towards a narrower frequency range, which provided good results.

Analysis of the Beam Aberration and Transmission Coefficient’s Variation of Frequency Selective Surfaces in Quasi-Optical Feed System

Full-wave simulation is widely adopted for the calculation of the frequency response of frequency selective surfaces (FSSs). Generally, uniform plane wave excitation and an infinite FSS array are assumed, and the simulation results have good agreement with the measurement data. However, the simulation cannot provide accurate determination of the transmission coefficient in the optical path in a quasi-optical system because the incidence wave is Gaussian instead of the uniform plane wave. In the conventional simulation, the mode conversion loss is ignored, and the reflection loss is less than the one for Gaussian beam. It is related to the sensitivity of the FSS to the incidence angle, which results in the beam's aberration and converts the mode of the transmitted beam from the fundamental Gaussian beam mode to higher order of Gauss-Hermite mode. In this letter, the Spatial Spectrum Analysis (SSA) method is applied to calculate beam's aberration, transmission coefficient, and conversion of the transmission beam's mode. The influences of the beam waist radius on the transmission coefficient are analytically discussed. The transmission coefficient and the conversion of the transmission beam's mode in different beam waist radius are measured, which agrees well with the results by Spatial Spectrum Analysis.

Design and measurement of an EM composite material frequency selective surface in C band

An investigation of frequency selective surface made from composite material with cross and Jerusalem cross patches for C band application is presented. The design method selection and its basic theory for analysing the frequency selective surface are introduced. The simulated results from the simulation and measurement results of a designed frequency selective surface array obtained in an anechoic chamber are presented. Their comparison shows a good agreement. The designed composite material frequency selective surface presented in this paper has good electromagnetic characteristics. The 5 dB transmission bandwidth of a single layer cross frequency selective surface is 1 GHz. The double layer frequency selective surface has a better transmission bandwidth than the single one while the Jerusalem cross frequency selective surface design also performs well in terms of frequency responses.

Multiband Metallic Frequency Selective Surface With Wide Range of Band Ratio

A novel metallic multiband frequency selective surface (FSS) is presented. The proposed FSS structure has operation bands from closely spaced to far from each other (wide range of band ratio) and has high selectivity at each operation band. The unit cell comprises four small square slots, each loaded with multiple stub resonators. The stubs are placed in each slot in such a way that the unit cell response to both TE and TM polarizations are the same. To miniaturize the unit cell and cover the lower frequency range, each stub resonator is terminated with a U-shaped section. The length of each stubs within the slot determine the resonant frequency of each passbands. The band ratio can be set from 1.09 (close to each other) up to 2.35 (far from each other). Single, dual, triple, and quad band FSS structures are designed and simulated through HFSS software package. The fabricated dual-band unit cell shows that the proposed FSS structure can provide two close to each other bands, 9.8 and 10.9 GHz. Also, the measured results of a fabricated dual-band ${\bf 17} \times {\bf 17}$ FSS array show a relatively stable response to variations of incidence angle and polarization state.

X-Band Tunable Frequency Selective Surface Using MEMS Capacitive Loads

A tunable frequency selective surface (FSS) based on slotted ground is presented. Tuning of the resonance frequency is achieved by using a metallic MEMS bridge over the slot. The bridge acts as a capacitive load, increasing the equivalent capacitance, and so decreasing the resonance frequency. Electromagnetic and electromechanical simulations are performed to investigate the designed FSS. S-parameter measurements of the FSS unit cell are performed in a waveguide simulator, showing more than 1.7 GHz frequency shift in the X-band, achieved by using only one MEMS bridge. A measured bandwidth of 400 MHz at the resonance frequency of 9.59 GHz is achieved. The designed MEMS bridge benefits from an unconventional method of using SU-8 as the sacrificial layer, resulting in low loss at high frequencies (3.2 dB loss at the resonance frequency of 9.59 GHz). Devices with different heights of the MEMS bridge were fabricated to study the variation in the resonance frequency. The MEMS bridge was tested at fixed heights. Simulated and measured results show excellent agreement. An FSS array is designed based on the FSS unit cell results. The design procedure to maximize the quality factor and controllable frequency range, and improve the radiation characteristics of the FSS array is discussed. Further simulations are performed to examine the performance of the FSS array with regards to grating lobes, oblique incidence and tunability.

A Frequency Selective Surface Based Focal Plane Receiver for the OLIMPO Balloon-Borne Telescope

We describe here a focal plane array of Cold-Electron Bolometer (CEB) detectors integrated in a Frequency Selective Surface (FSS) for the 350 GHz detection band of the OLIMPO balloon-borne telescope. In our architecture, the two terminal CEB has been integrated in the periodic unit cell of the FSS structure and is impedance matched to the embedding impedance seen by it and provides a resonant interaction with the incident sub-mm radiation. The detector array has been designed to operate in background noise limited condition for incident powers of 20 pW to 80 pW, making it possible to use the same pixel in both photometric and spectrometric configurations. We present high frequency and dc simulations of our system, together with fabrication details. The frequency response of the FSS array, optical response measurements with hot/cold load in front of optical window and with variable temperature black body source inside cryostat are presented. A comparison of the optical response to the CEB model and estimations of Noise Equivalent power (NEP) is also presented.

Large area infrared frequency selective surface with dimensions reproducible by optical lithography

An infrared frequency selective surface (FSS) with absorptive resonance near 6.5 μm was fabricated by electron-beam lithography using a patch design with dimensions reproducible by optical-projection lithography. By selective wet etching along with reactive-ion etching, the sample was divided into miniature FSS particles, which were released from the substrate. A large number of such particles could be implemented as a large area, conformal coating. Spectral reflectivity of the full FSS array as well as the FSS particles was measured and compared to electromagnetic simulations. To show the feasibility of this approach, the full array FSS design was fabricated using a g-line (λ = 436 nm) 5× projection lithography stepper and compared to the array fabricated by electron-beam lithography using scanning electron microscopy and Fourier transform infrared spectroscopy. Even though the resolution of the g-line stepper led to a poor fabrication output, the optical resonance was found to be robust, with only slight detuning attributed to the changes in unit cell geometry. This work highlights the utility of optical-projection lithography, coupled with the releasable particle fabrication procedure, to create a large area, conformal coating with specific infrared spectral properties.

Dual Polarized Reconfigurable Frequency Selective Surfaces

Novel band-stop active frequency selective surfaces (FSS) capable of modifying their response for different polarizations are proposed. Geometries based on full rings, and split rings are employed. The designs using full rings are able to tune over a wide frequency range while designs using split rings tune in a narrow frequency band. Both structures use a new biasing methodology which allows independent control of rows and columns of FSS arrays, therefore permitting independent modification of the transmission responses at the vertical and horizontal polarizations. Convoluting the shape of the elements significantly reduces the sensitivity to angle of wave incidence. The aim is to demonstrate a technology that could be used for various applications including modification of the electromagnetic architecture of buildings and the control of electromagnetic wave propagation to improve the efficiency of radio spectrum use. The surfaces incorporate commercially available, low cost, varactor diodes and surface mount resistors. Theoretical and experimental results confirm the operation of the surfaces within the UHF frequency band.

Terahertz FSS for space borne passive remote sensing application

A terahertz frequency selective surface (FSS) which operates inside the spatial filtering front-end of a high sensitivity receiver is proposed and experimentally verified. To achieve this terahertz FSS metamaterial which exhibits extremely low insertion loss and is insensitive to the variations of polarisations and incident angles, the FSS array is constructed by an electrically thick SiO2 layer (quartz wafer) and an ultra-thin high conductivity aluminium layer with periodic aperture unit cells. The micro-machined metallic pattern is shaped by the combination of a sub-wavelength circular ring and a split-ring resonator. Experimental results and predicted spectral responses are shown to be in good agreement over the frequency range of 0.1-0.5 THz.

Influence of defective elements on performance of frequency selective surfaces

The performances of frequency selective surface arrays in which some of the conducting elements at randomly chosen locations were absent or defective are described. The aim was to assess the proportion of defects that can be tolerated when low-cost fabrication techniques are employed.

Zigzag metallic conductors as frequency selective surfaces

Reflection and transmission properties of a new type of frequency selective surface (FSS) made of zigzag metallic elements placed periodically on a dielectric substrate are investigated theoretically for arbitrary polarisation. The electric field integral equation technique with the subdomain overlapping piecewise sinusoidal basis functions are employed to compute the unknown current coefficients induced on the metallic elements. The zigzag FSS arrays act as a low-pass filter in S -(1-5 GHz), X -(8-12 GHz) and K u -(12-18 GHz) bands. Owing to multiple resonances the zigzag FSS arrays can be used in multiband frequency applications. Between the resonances there exist transmission windows that allow full energy transfer. By removing the first and third arms, the power reflection coefficient of three-arm zigzag FSS is compared with a freestanding strip FSS, and it is compatible with the measured and calculated results found in the literature.

Analytical modeling of conformal mantle cloaks for cylindrical objects using sub-wavelength printed and slotted arrays

Following the idea of “cloaking by a surface” [A. Alù, Phys. Rev. B 80, 245115 (2009); P. Y. Chen and A. Alù, Phys. Rev. B 84, 205110 (2011)], we present a rigorous analytical model applicable to mantle cloaking of cylindrical objects using 1D and 2D sub-wavelength conformal frequency selective surface (FSS) elements. The model is based on Lorenz-Mie scattering theory which utilizes the two-sided impedance boundary conditions at the interface of the sub-wavelength elements. The FSS arrays considered in this work are composed of 1D horizontal and vertical metallic strips and 2D printed (patches, Jerusalem crosses, and cross dipoles) and slotted structures (meshes, slot-Jerusalem crosses, and slot-cross dipoles). It is shown that the analytical grid-impedance expressions derived for the planar arrays of sub-wavelength elements may be successfully used to model and tailor the surface reactance of cylindrical conformal mantle cloaks. By properly tailoring the surface reactance of the cloak, the total scattering from the cylinder can be significantly reduced, thus rendering the object invisible over the range of frequencies of interest (i.e., at microwaves and far-infrared). The results obtained using our analytical model for mantle cloaks are validated against full-wave numerical simulations.