Salisbury Screen Research Articles

Salisbury Screen Absorbers of Dielectric Lossy Sheets of Carbon Nanocomposite Laminates

In this paper, we present an innovative radar absorber composed of a dielectric spacer and a dielectric lossy sheet (DLS), instead of the resistive sheet of the conventional Salisbury screen absorber. We propose carbon nanocomposites as DLS to utilize their structural robustness and advantages in large-area applications, while maintaining a relatively precise thickness. These are laminates made of E-glass fabric/epoxy prepregs containing carbon black (CB), carbon nanotube (CNT), and carbon nanofiber (CNF). The spacer is a pure E-glass fabric/epoxy composite laminate. The optimal design process based on a numerical model of the complex permittivity of the carbon nanocomposites, and a genetic algorithm shows that the real part of the complex permittivity of the sheet is closely related to the reduction of the spacer thickness. In contrast, the variation of the imaginary part is very marginal. The design results show that, in the X-band, the absorber thickness is 3.1 mm for the CB-composites, 2.76 mm for the CNT-composites, and 2.5 mm for the CNF-composites. The 10-dB bandwidth of every absorber is about 3.8 GHz. In the Ku-band, the absorber thickness is 1.95 mm for the CB-composites, 1.85 mm for the CNT-composites, and 1.7 mm for the CNF-composites. The 10-dB bandwidth of every absorber is broader than 5.5 GHz.

Performance enhancement of Salisbury screen absorber using resistively loaded spiral FSS

AbstractIn this article, we show that the −10 dB reflectivity bandwidth of a Salisbury screen operating at normal incidence is increased by 56% when a lossy spiral shaped frequency selective surface is inserted between the outermost 377 Ω per square resistive sheet and the ground plane. Moreover, this structure exhibits significantly better angular stability and is suitable for suppressing radar backscatter simultaneously in the TE and TM plane. Reflectivity predictions for normal and 40° incidence are shown to be in close agreement with bistatic measured results over the frequency range 5.4–18 GHz. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26040

Design of a Salisbury screen absorber using frequency selective surfaces to improve bandwidth and angular stability performance

A new design method that greatly enhances the reflectivity bandwidth and angular stability beyond what is possible with a simple Salisbury screen is described. The performance improvement is obtained from a frequency selective surface (FSS) which is sandwiched between the outermost 377 Ω/square resistive sheet and the ground plane. This is designed to generate additional reflection nulls at two predetermined frequencies by selecting the size of the two unequal length printed dipoles in each unit cell. A multiband Salisbury screen is realised by adjusting the reflection phase of the FSS to position one null above and the other below the inherent absorption band of the structure. Alternatively by incorporating resistive elements midway on the dipoles, it is shown that the three absorption bands can be merged to create a structure with a -10 dB reflectivity bandwidth which is 52% larger and relatively insensitive to incident angle compared to a classical Salisbury screen having the same thickness. CST Microwave Studio was used to optimise the reflectivity performance and simulate the radar backscatter from the structure. The numerical results are shown to be in close agreement with bistatic measurements for incident angles up to 40° over the frequency range 5.4-18 GHz.

Suppression of Surface Wave from Finite Conducting Surfaces with Impedance Loading at Margins

The use of impedance loading to suppress edge scattering of surface wave from finite conducting surface is proposed. Electromagnetic back-scattering from the finite conducting surfaces with impedance loading at margins is analyzed for horizontal and vertical polarizations, and compared with that of the wave scattering from the target of the same shape but having no impedance loading at margins. The location of impedance loading is determined by analyzing the Poynting flux of surface wave on the surface. The impedance loading at margins can suppress surface waves from finite conducting surfaces, which can be regarded as a two-dimensional (2-D) version of the one dimension (1-D) Salisbury screen. By changing the impedance value and the width of the impedance loading boundaries, and keeping the center frequency invariable, the relationship between these parameters and the suppression effect of edge scattering is established. The margin impedance loading is effective for plane wave incidence at large angles (low grazing angles) with the considered finite conducting surfaces. Such a way of margin impedance loading can remarkably reduce the mono-static radar cross sections (RCS) at low frequency band, and the method of the absorbing coating is proved not effective enough.

Multiband electromagnetic wave absorber based on reactive impedance ground planes

A novel structure able to perform multi-band absorption is presented. As is well known, the most famous and simplest radar absorbing material is the Salisbury screen. Improved solutions, aimed at a thickness reduction, have been presented in the literature, conceptually based on metamaterials and employing properly designed high-impedance surfaces (HIS) in practice. The absorber presented here combines the working principles of the previously mentioned designs in order to join the two types of absorber into a single structure. The novel design comprises a single resistive sheet mounted at a fixed distance from a reactive surface. The latter is a composite layer of a frequency selective surface printed on a grounded dielectric slab acting as an artificial magnetic conductor at lower frequencies, while responding as a perfect electric conductor at higher frequencies. The distance of the resistive sheet can therefore be dimensioned so as to recover the Salisbury functions in the upper band, while operating as a λ/10 absorber in the low-frequency regime. The structure is explained by means of a simple circuital model and measured data are reported to verify the proposed concepts. It is finally shown that low-frequency resonance can be advantageously adjusted by introducing some active elements in the HIS.

Effect of short carbon fibers and MWCNTs on microwave absorbing properties of polyester composites containing nickel-coated carbon fibers

Multiphase composite materials filled with multiwall carbon nanotubes (MWCNTs), short nickel-coated carbon fibers and millimeter-long carbon fibers with various weight fractions and compositions are developed and used for the design of wide-band thin radar-absorbing screens. The effective complex permittivity of several composite samples is measured in the frequency range from 8 GHz to 18 GHz. The obtained results show that the addition of the MWCNTs into the mixture allows tuning the EM properties of the composite filled with the short nickel-coated fibers. Numerical simulations are also performed in order to design new radar-absorbing shields. Single-layer and bi-layer thin dielectric Salisbury screens are designed to exhibit minimum reflection coefficient at 10 GHz and at 15 GHz, and maximum bandwidth at −10 dB. It results that the total thickness of the screen can be reduced below 2 mm by using a lossy sheet made with the composite filled with MWCNTs and nickel-coated carbon fibers, whereas the bandwidth at −10 dB can exceed 6 GHz in a bi-layer structure.

Capacitive Circuit Method for Fast and Efficient Design of Wideband Radar Absorbers

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A simple, fast and efficient method for designing wideband radar absorbers is proposed. The idea is to modify the circuit analog absorber method without perturbing the bandwidth. This is done by utilizing the asymptotic behavior of such an absorber at low frequency and replacing the band-stop resonating frequency selective surfaces with low-pass capacitive ones, which can be synthesized by square patches. It is shown that higher frequencies are not influenced by these modifications. A thin wideband capacitive circuit absorber (CCA) is presented with 28% reduction of thickness and 57% increase of bandwidth in comparison to the Salisbury screen. It is also explained why some optimized metamaterial designs fail to compete with the CCA method. For high permittivity layers, it is shown that the CCA is a better solution than the Jaumann absorber and improvements both in thickness and bandwidth are possible. A three layered ultrawideband CCA is presented with total thickness of 15.1 mm. </para>

Salisbury screen with reduced angular sensitivity

A new Salisbury screen absorber design is presented, which yields a significantly reduced sensitivity to angle of incidence. This is achieved by using an artificial magnetic ground plane, which is designed to scatter the reflected signals in a conjugate phase relationship with respect to the change in the electrical spacing between the ground plane and the resistive screen. Numerical and experimental results in the range 8-12 GHz are used to demonstrate that scan compensation can be obtained over the angular range 0-40deg thus removing the shift in the resonant frequency that occurs in conventional Salisbury screen absorbers when operated off normal incidence.

Low profile Salisbury screen radar absorber with high impedance ground plane

The radar absorbing properties of a Salisbury screen are investigated for a structure which is constructed using a high impedance ground plane. Simulated and experimental results in the frequency band 8-12 GHz show that this physical arrangement can be used to obtain a significant reduction in the physical thickness of the absorber when the specified -10 dB reflectivity bandwidth is in the range 18-37%.

WAVELENGTH SELECTIVE BOLOMETER DESIGN

Multi-color narrow-band Salisbury Screen and Jaumann Absorbers combined with optimized thick Si 3 N 4 support layers are designed for wavelength selectivity in 7~14µm wavelength band. The Jaumann Absorbers are adopted as a vertically ‘stacked’ pixel structure to save space and enhance resolution compared against ‘tiled’ structure (pixels lying in a common plane).

EMC Impact of Advanced Carbon Fiber/Carbon Nanotube Reinforced Composites for Next-Generation Aerospace Applications

This paper presents a comparative analysis of the electromagnetic properties of new composite materials that are of interest to future aircraft/aerospace structures. The fabrication process of single-phase and new multiphase micro/nanocomposites is described. Carbon black, carbon fibers, and multiwall carbon nanotubes are randomly mixed into an epoxy resin matrix at various weight fractions and compositions. The experimental characterization in the frequency range 8-18 GHz shows that the dispersion characteristics of short-carbon-fiber-reinforced composites can be properly controlled by the addition of nanopowders and nanotubes into the mixture. Numerical simulations demonstrate the feasibility of the fabricated materials for the design of new electromagnetic micro/nanostructured shields and radar-absorbing laminates. Thin dielectric Salisbury screens are especially designed to exhibit minimum reflection coefficient at 15 GHz. It shows that the total thickness of the screen can be reduced below 2 mm by using a lossy sheet made of three-phase composites.

A NOVEL TECHNIQUE FOR IMPROVING THE PERFORMANCE OF SALISBURY SCREEN

The most common and simple structure to reduce the level of the reflected power from a metallic surface is the single layer structure known as Salisbury screen which is a sheet of porous material impregnated with graphite and spaced a quarter-wavelength off a metallic backing plate. The main disadvantage of this mechanism is the narrow frequency bandwidth. Many techniques have been reported to improve the working frequency bandwidth but with some degradation in the other technical properties of the overall structure. In this paper a novel technique has been introduced based on a spatial kind of material called the circuit analog screen. Theoretical analysis shows that the bandwidth of the reflected power will be improved if the graphite sheet of the Salisbury screen has been loaded by circuit analog screen with spatial geometry with spatial parameters.

Oblique Incidence Performance of a Novel Frequency Selective Surface Absorber

Oblique incidence performance of a novel two-layer absorb/transmit frequency selective surface (FSS) is investigated. The FSS has good frequency stability for both horizontally and vertically polarized waves incident normally or at oblique angles. Due to its transmission for 900/1800/1900 MHz mobile bands and good absorption for 5 GHz waves, it has the potential as a security wall or isolator for 5 GHz WLAN systems. The absorption in the stop band helps reduce additional WLAN multipath fading caused by conventional reflecting FSS designs. The first layer of the FSS consists of conventional conducting cross dipoles having a circular aperture in the centre, while the second layer uses resistive cross dipoles. Moreover, the conducting cross dipoles have been sandwiched between two dielectric sheets to achieve a stable response for different angles of incidence. The periodicity of both FSS layers is the same while the distance between the two layers is reduced to one eighth of the free-space wavelength. This reduction leads to a more compact design compared to the conventional Salisbury screen, while still achieving acceptable absorption in the stopband. Both theoretical and experimental results are presented to confirm the performance of the absorb/transmit FSS.

Absorbing properties of frequency selective surface absorbers with cross-shaped resistive patches

Recently, frequency selective surfaces (FSSs) have been found frequent uses in the radar absorbing materials (RAMs). The results in open literatures all show that FSSs can modify and improve the absorbing performances of RAMs, but the FSS absorber with resistive patches is rarely reported. In this paper, the absorbing properties of FSS absorber with cross-shaped resistive patches were studied by finite element method. The calculated results indicate that lattice type, element spacing, cross dipole size, patch sheet resistance and thickness of dielectric substrate all can affect the absorbing properties of the FSS absorber, which are in agreement with experimental results. It is also found that the absorbing performances of the FSS absorber are very different from the classic Salisbury screen, and the absorption properties would be beyond the Salisbury screen if proper parameters were chosen.

Scattering from Small Salisbury Screens

This paper presents radar cross section results from a method of moments model of scattering from small Salisbury screens. Both normal and off-normal incidence is considered. The resistivity and separation of the resistive sheet and conductor are optimized for a square Salisbury screen having a side length between 0.5/spl lambda/ and 7.5/spl lambda/.

Magnetorefractive cell on basis of the Salisbury screen

Recently, it has been shown that the magnetorefractive effect (MRE) can be significantly enhanced if a layer of material exhibiting MRE is placed inside a photonic crystal to form a defect mode. We propose to use for the MRE enhancement the Salisbury screen as a competitive and much more simple scheme. Since MRE is mostly due to a change of light absorption under magnetization the three-layer Salisbury structure optimized for highest possible absorption inside magnetic layer provides MRE at least two orders of magnitude greater than that for single layer and of the same value or even greater than that for 37-layer photonic crystal.

Tailoring silicon radiative properties

We study several schemes to enhance the emission of radiation of silicon surfaces in the infrared. First, we investigate the emission pattern of microscale lamellar gratings ruled on doped silicon which is substantially modified due to the excitation of surface plasmons. In addition to their remarkable spectral selectivity, those sources emit 50% more than a plane interface of bulk doped silicon. An interferential antireflection system similar to a Salisbury screen is also considered. This broadband emitter allows a significant enhancement of the total emitted power compared to the plane interface. These results may have broad applications in sensing and radiative cooling.

Magnetic Salisbury screen and its high-frequency response

We have studied the possibility of constructing magnetic Salisbury screen, and the frequency response of reflectivity of the screen in microwave frequency range. We have found that magnetic Salisbury screen has a lower reflectivity when magnetic susceptibility satisfies χ″χ′. If the frequency response of magnetic susceptibility is in the relax type of domain wall resonance, we could make filmtype Salisbury screen, in which the thickness of the screen may be in the order of micrometer or even submicrometer. The reflectivity of the screen depends on the characteristic impedance zr of the magnetic film that is determined by ferromagnetic resonant frequency and susceptibility under a static field. The frequency response of reflectivity shows that the magnetic film Salisburyscreen has a broadband frequency range for absorbing microwaves well.

High frequency response of Salisbury screen and its related factors

In this work, the high-frequency response of the Salisbury screen made of metallic thin film was investigated. The formulas of reflectivity and bandwidth coefficient were derived. Theoretical and numerical studies show that the spectrum of reflectivity presents a resonance type, which is symmetric to the resonance frequency. The reflectivity is determined only by the normalized surface resistance α of the metallic film at resonance, and by both α and wave impedance η2 of the isolation layer at other frequencies. For the bandwidth coefficient Δ, it is determined by the examined reflectivity Γa, η2 and α. Bandwidth coefficient can be improved by changing the wave impedance η2, and tuning the surface resistance to the optimum value αc which is correlated to Γa and not equal to the one in most cases. It is also noted that the thickness of the metallic thin film should be in the order of sub-micron or nanometers.

A single-layer tuneable microwave absorber using an active FSS

An experimental single-layer active microwave absorber in described. The absorber is a planar structure based upon the topology of a Salisbury screen, but in which the conventional resistive layer is replaced by an active frequency selective surface (FSS) controlled by pin diodes. The resulting structure has superior reflectivity-bandwidth characteristics compared to conventional passive absorbers of corresponding thickness. Measured data are presented and show that the reflectivity response of the absorber can be controlled over the frequency band from 9 to 13 GHz.