WO2004079862A1

WO2004079862A1 - Electromagnetic wave absorbent panel

Info

Publication number: WO2004079862A1
Application number: PCT/EP2004/002373
Authority: WO
Inventors: Geoffrey Crossley; Barry Chambers
Original assignee: Edm Spanwall Limited
Priority date: 2003-03-07
Filing date: 2004-03-05
Publication date: 2004-09-16
Also published as: GB0305235D0

Abstract

One embodiment of the invention provides an electromagnetic wave absorbent panel comprising an electromagnetic wave attenuating layer. The attenuating layer comprises a plurality of electrically resistive fibres carried by a dielectric medium. The electrically resistive fibres are distributed in the dielectric medium such that they are electrically insulated from one another. The fibres are dimensioned so that, during use, they act as a respective Hertzian dipole. The panel may be a Salisbury screen, or modified Salisbury screen type panel, operable on the quarter wavelength principle.

Description

ELECTROMAGNETIC WAVE ABSORBENT PANEL

Field of the Invention

The present invention relates to electromagnetic (EM) wave absorbent structures, and in particular a microwave absorbent panel for use in the cladding of buildings or other structures, in order to suppress radar reflections therefrom.

Background to the Invention

The use of radar absorbent materials (RAM's) has become commonplace for a wide variety of both military and civil applications. One such civil application, with which the present invention is particularly concerned, is the cladding of terminal buildings, hangers, cargo centres and similar large scale buildings or structures in close proximity to airport control radar.

Such buildings or structures in close proximity to radar antennas reflect radar signals in different directions from the antenna "pointing direction", giving rise to "ghost" aircraft radar returns. These produce radar images to air traffic controllers of aircraft in the direction of tne antenna when pointing at the building, which in practice, due to reflection from the building, are in a completely different angular position in the sky. Hence a ghost image of the aircraft appears, as the radar will pick-up the real aircraft image when the radar antenna is pointing directly at the aircraft on its true beating, and therefore a single aircraft becomes two images on the radar control screen, between which the controller cannot distinguish the real image. The ability of radar operators to safely handle traffic at airports is therefore severely compromised, as any ghost aircraft occupy air space that cannot be allocated to other real aircraft. The problem may be further complicated by multiple reflections from adjacent buildings, giving rise to multiple ghost images.

In order to counter this problem, structures or buildings may be covered with a RAM in order to eliminate any hazardous reflections. The RAM may be formed integrally with the facade of the building, or may take the form of a demountable system of panels which can be retro-fitted to the facade. In either case, the RAM utilised is conventionally of the type which operates on the quarter wavelength principle and is commonly known as a Salisbury screen or modified Salisbury screen (MSS) type absorber. Such absorbers are constructed so as to provide a high level of absorption for a specific frequency of EM waves, or a narrow bandwidth of such EM waves, and are thus well suited for use with fixed frequency secondary radar systems utilised at commercial airports. Such EM wave absorbers are capable of relatively high EM wave absorption levels.

Typical Salisbury screen type panels comprise a layer of electrically conductive material, usually comprising carbon/graphite coated or impregnated "lossy" fabric or foam, spaced from a reflective backing by a layer of low dielectric material, such as a mineralwool or plastic. The layer of.low dielectric material should have an electrical thickness of approximately one quarter of the wavelength of the incident EM waves in the propagation direction, such that the panel achieves attenuation of the EM waves by both energy dissipation within the "lossy" layer, and cancellation of the incoming and emergent wave components at the surface of the panel when these components are 180° out of phase with one another. Hence this type of absorber is said to operate on the "quarter wavelength principle". The EM wave absorbent or lossy material is generally in the form of a thin resistive sheet across which current can flow, being induced by the passage of the EM waves through the lossy layer. The induced current is only permitted to flow across the lossy layer by the provision of a suitably high concentration of carbon/graphite particles, or the like, in or on the lossy layer to achieve electrical conductance. As current flows in this carbon/graphite doped lossy layer, it is dissipated in the form of heat, which heat is itself dissipated into the spacing layer of dielectric material. It is therefore essential in these known screen type absorbers that high concentrations of carbon are utilised to achieve conductance in the lossy layer in order to achieve attenuation of incident EM waves.

Such a panel configuration is described and illustrated in United States Patent US 5121122 and United States Patent US 5083127. The resistive sheets or layers in these panel constructions are designed to have a surface resistivity of approximately 377 ohms per square, which is equivalent to the characteristic impedance of free space, in order to minimise reflections from incident EM waves, although a partial reflection does still occur. The transmitted portion of the EM wave undergoes multiple reflections within the panel, each time passing tlirough the lossy layer, thereby^" attenuating the EM wave. The panel dimensions and characteristics, such as permittivity and permeability, are chosen such that the sum of the emergent waves is equal in amplitude, while being 180° out of phase with, the reflected component of the incident EM wave, thereby resulting in total destructive interference between these non-absorbed or non-attenuated components of the EM wave. Conventional lossy layers are relatively difficult to manufacture and handle. In addition, because they are usually non-porous, a conventional lossy layer needs to be separately bonded to adjacent layers within an EM wave absorbent structure. This can comprise the integrity of the structure. Further, conventional lossy layers are relatively heavy.

It would be desirable therefore to provide an improved EM wave absorbent structure which mitigates the problems outlined above.

Summary of the Invention

Accordingly, one aspect of the invention provides an electromagnetic wave absorbent structure, preferably in the form of a panel, comprising an electromagnetic wave attenuating layer, the attenuating layer comprising a plurality of electrically resistive elements carried by a dielectric medium, the electrically resistive elements being distributed in said dielectric medium such that they are electrically insulated, or substantially electrically insulated, from one another.

In preferred embodiments, the length of said electrically resistive elements is, by an order of magnitude, less than the wavelength of the electromagnetic waves to be absorbed, during use, by said structure and is preferably such that the electrically resistive elements act, during use, as a respective Hertzian dipole.

It is preferred that the electrically resistive elements each comprise a respective electrically resistive fibre or strand, preferably carbon fibre, or chopped carbon fibres. hi preferred embodiments, the dielectric medium carries said electrically resistive elements at a density of approximately between 0.125 g/m² and 1 g/m², and preferably approximately 0.625 g/m².

The dielectric medium is preferably porous and advantageously comprises a non- woven fabric (NWF), for example non-woven glass fibres. The dielectric medium may carry between approximately 0.25 and 2% resistive elements by weight, more preferably 1 to 2% by weight, and most preferably 1.25% by weight.

In the most preferred embodiment, the structure further comprises an electromagnetic wave reflecting layer, the structure being dimensioned and arranged so that electromagnetic waves incident upon the structure during use travel through the structure a distance corresponding to approximately one quarter of the wavelength of the incident electromagnetic waves before impinging upon said reflecting layer and, upon reflection by said reflecting layer, travel through the structure a distance corresponding to approximately one quarter of the wavelength of the incident electromagnetic waves before emerging from said structure, the electromagnetic attenuating layer being arranged with respect to the reflecting layer such that said incident electromagnetic waves pass through said attenuating layer before impinging upon said reflecting layer and/or after being reflected by said reflecting layer. The structure may therefore be said to be operable on the quarter wavelength principle and may be generally described as a Salisbury screen or modified Salisbury screen type EM wave absorber.

Other preferred features of the invention are recited in the dependent claims and further advantageous aspects of the invention will become apparent, to those ordinarily skilled in the art upon review of the following description of a specific embodiment and with reference to the accompanying drawings.

Brief Description of the Drawings

The present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 illustrates a sectioned side elevation of a preferred embodiment of an EM wave absorbent structure according to one aspect of the present invention;

Figure 2 illustrates a plan view of an EM wave absorbent or lossy layer forming part of the panel illustrated in Figure 1;

Figure 3 illustrates a microscopic view of the surface of the lossy layer illustrated in Figure 2, showing the structure thereof; and

Figure 4 illustrates a graph detailing the levels of absorption of incident EM waves striking the panel of Figure 1 at various angles of incidence.

Detailed Description of the Drawings

Referring now to the accompanying drawings, there is illustrated a preferred embodiment of an EM wave absorbent structure in the form of a panel, generally indicated as 10. The panel 10 is particularly, hut not exclusively, suited for use in cladding buildings (not shown) or other sfructures in order to suppress radar reflections therefrom. The panel 10 comprises an outer layer or skin 12, a first spacing layer 14, an EM wave attenuating layer 16, a second spacing layer 18 and a reflective layer 20, all bonded together to form a composite sandwich, or multilayer, structure.

In a preferred application, the panel 10 forms the outer surface of any building or structure (not shown) to which the panel 10 is fitted, and so the outer skin 12 is advantageously provided as an environmental barrier protecting the other components of the panel! 0 from environmental degradation, in particular the ingress of moisture. The outer skin 12 may also be flame retardant. The outer skin 12 may comprise a composite material, or any other suitable dielectric material, which permits transmission of EM waves, in particular microwaves. In order to assist in the balancing, or cancellation of, reflected waves, the relative permittivity, or dielectric constant, of the outer skin 12 is advantageously greater than the relative permittivity of both the first and second spacing layers 14, 18 (it is preferred that the relative permittivity of the spacing layers 14, 18 is substantially equal to 1). In addition to preventing the spread of flame and the ingress of moisture to the panel 10, the characteristics of the outer skin 12, and in particular its relative permittivity and/or thickness, may be used as design variables when creating the panel 10. The outer skin 12 advantageously enhances the performance of the panel 10^~by increasing its effective operating bandwidth for incident EM waves, as well as the range of angles of incidence of the EM waves striking the panel 10 with which the panel 10 is effective. In the preferred embodiment, which is designed for use with commercial airport secondary radar systems operating at a receive frequency of around 1.09 GHz, the outer skin 12 may be formed from Bauclad™, and may be approximately 6mm thick. Both the first and second spacing layers 14, 18 are preferably formed from an electrical insulating material, or dielectric material, for example mineralwool or plastics. It is preferred that the electrically insulating material is porous. Preferably, electrically insulating material is also thermally insulating and, in the preferred embodiment, may comprise Rockwool™

The reflective layer, or ground plane, 20 is formed from a material which reflect substantially all of the EM waves incident thereon. In the preferred embodiment, the reflective layer 20 comprises sheet steel, although any other EM wave reflecting material may be used. In preferred embodiments, the reflective layer 20 is integrally formed with the panel 10 but in alternative embodiments (not illustrated) the reflective layer may be provided by a structure, e.g. a building wall, on which the panel is mounted during use.

The overall thickness or depth of the panel 10 is chosen, as will be described in more detail hereinafter, to achieve interference attenuation of EM waves. The interference attenuation works in combination with attenuation achieved by the EM wave attenuating layer, or lossy layer 16, again as will be described hereinafter in more detail. The overall thickness of the panel 10 is determined by the combined thickness of the layers 12, 14, 16, 18 although, in practice, it is convenient to determine the overall thickness by selecting a desired combined thickness of the first and second spacing layers 14, 18. In the preferred embodiment, the combined thickness of the first and second spacing layers 14, 18 is approximately 100mm.

In the illustrated embodiment, the lossy layer 16 is spaced apart from the reflective layer 20 by the second spacing layer 18 and from the outer skin 12 by the first spacing layer 14 and may located substantially midway between the outer skin 12 and the reflective layer. In alternative embodiments, the lossy layer 16 may adopt alternative locations with respect to the reflective layer 20 and the outer skin 12 (when present) and need not necessarily be midway between the two.- In some embodiments, the lossy layer 16 need not be spaced-apart from both the outer skin 12 (when present) and the reflective layer 20.

Referring still to Figure 1, an incident EM signal or wave 28, on striking the outer skin 12, is split into two components, a reflected component 30, and a transmitted component 32. The transmitted component 32 passes through the first spacing layer 14, and then through the lossy layer 16, which attenuates the transmitted component 32 as will be described hereinafter. The transmitted component 32 then travels on through the second spacing layer 18, to strike the reflective layer 20. The transmitted component 32 is reflected by the reflective layer 20 as a reflected component 34, which passes back through the lossy layer 16, further attenuating the EM wave. On reaching the outer skin 12, the reflected component 34 is again reflected back towards the lossy layer 16, although a small component of the reflected component 34 escapes from the panel 10 as an emergent component 36. This process is repeated a large number of times until the final reflected component 34, having passed through the lossy layer 16, fully escapes from the outer skin 12 as a final emergent component 36. It will be appreciated that a far greater number of reflections occurs within the panel 10 than is illustrated in Figure 1, including, for example, reflections between the lossy layer 16 and reflective layer 20, and between the lossy layer 16 and outer skin 12.

The panel 10 is dimensioned to define a path of length L along which the transmitted component 32 travels within the panel 10. Similarly, the reflected component 34 travels along a path of length L within the panel 10. In Figure 1, the path taken by the transmitted component 32 is shown from the outer skin 12 to the reflective layer 20 and the path taken by the reflected component 34 is shown from the reflective- layer 20 to the outer skin 12 (although each path length may also include the thickness of the outer skin 12). The panel 10 is designed for a given operational frequency, or frequency range, such that the electrical path length L for both the transmitted component 32 and reflected component 34 is approximately one quarter of a wavelength of the incident wave 28. Hence, each emergent component 36 is approximately 180° out of phase with the partial reflection 30, since each emergent component 36 has travelled within the panel 10 a distance of a half of the wavelength of the incident wave 28. Hence, the panel 10 may be said to be operable on the quarter wavelength principle. It will be apparent that the angular orientation of each path with respect to the panel 10 depends on the angle of incidence of the EM wave 28. Accordingly, the panel 10 is optimised for attenuating EM waves 28 having a particular angle of incidence (typically the angle at which EM waves are most commonly expected to impinge upon the panel 10 during use). EM waves 28 arriving at other angles of incidence are still attenuated by the panel 10 although not so effectively. One reason for this is that the length of the path along which the transmitted components (or reflected components) of such EM waves travel between entering the panel 10 and striking the reflective layer 20 (or between striking the reflective layer 20 and leaving the panel 10) is a poorer approximation of one quarter of the wavelength of the EM wave than is the case for those EM waves that are incident at the angle for which the panel 10 is particularly designed. Typically, a transmitted or reflected wave component may travel a distance of between 0.18 and 0.36 times the wavelength of the incident EM wave 28 depending on its angle of incidence. By way of example, and referring to Figure 4, plots A to E are shown in which reflectivity performance in decibels (dB) of the panel 10 is plotted (along the vertical axis) against the frequency in gigahertz (GHz) of the incident EM waves over a range of operating frequencies which includes 1.09GHz (being the frequency of airport secondary radar systems). Plots A to E represent a respective angle of incidence of EM waves 28 on the panel 10. It will be seen that, at the desired operational frequency of 1.09GHz, the panel 10 provides attenuation in excess of — 15dB for a wide range of angles of incidence (5° to 65°).

The attenuation effected by the lossy layer 16 also ensures that the combined magnitude of the plurality of emergent components 36 is substantially equivalent to that of the partial reflection 30. Therefore, the partial reflection 30 and plurality of emergent components 36 substantially cancel each other out by means of destructive interference. In this way, the panel 10 substantially prevents reflection of incident EM waves 28.

Referring to Figures 2 and 3, the configuration and operation of the lossy layer 16 is entirely different to that of conventional Salisbury screen or MSS type absorbers. The lossy layer 16 comprises a dielectric medium 22 which contains or carries a plurality of electrically resistive-elements, or Hertzian dipoles.

Preferably, the electrically resistive elements comprise fibres 24 or strands of electrically resistive material. In the preferred embodiment, the fibres 24 take the form of carbon fibres, especially chopped carbon fibres. The fibres 24 are sufficiently spaced apart from one another so as to be electrically insulated or isolated from one another by the dielectric medium 22. Hence, the surface resistivity of the lossy layer 16 is effectively infinite and prevents the flow of current across the lossy layer 16 (this is in contrast to a conventional carbon doped lossy layer wherein the carbon particles are contiguous across the medium so that the lossy layer serves as an electrically conductive layer). The length of each fibre 24 is such that each fibre 24 acts as a short or Hertzian dipole, or doublet, when excited by EM waves or radiation. To this end, the fibres 24 are preferably shorter in length than the wavelength of the incident EM waves 28 by an order of magnitude, typically 10 to 100 times shorter. By way of example, where the incident EM waves comprise radar signals at approximately 1.09GHz, each carbon fibre 24 is preferably of a length in the range of 3mm to 25mm, most preferably being approximately 25mm in length.

As each fibre 24 effectively acts as a Hertzian dipole, current is induced in each fibre 24 as the transmitted and reflected components 32, 34 of the EM waves 28 pass through the lossy layer 16. Thus the energy of the EM wave is converted into current in the fibres 24. The electrical resistance of each fibre 24 causes the alternating current therein to be dissipated by the fibres 24 as heat, which heat is itself dissipated into the surrounding dielectric medium 22. Hence, the fibres 24 have an attenuating affect on the transmitted component 32 and reflected component 34 as they pass through the lossy layer 16. The lossy layer 16 may therefore be said to comprise an EM wave, or energy, absorbent or attenuating layer.

In the preferred embodiment, carbon fibres 24 are present in the lossy layer 16 at a concentration of approximately between 0.25% and 2%, and most preferably approximately 1.25% by weight, or 0.625g/m². It is preferred that the fibres 24 are randomly, or substantially randomly, orientated with respect to one another in the layer 16. In alternative embodiments, the fibres 24 may be formed from any other suitable electrically resistive material or lossy attenuating material, for example silicon carbide; a "conducting polymer" (for example polypyrol, polyanaline, or the like); or other fibres (for example glass, nylon, or the like) coated in a conducting polymer.

It is preferred that the dielectric medium 22 comprises a porous material and in particular a non-woven fabric (N F). The dielectric^' medium 22, in the preferred embodiment, comprises non- woven glass fibres 26 secured together by means of a conventional binder (not shown). Conveniently, the binder also holds the fibres 24 within the dielectric medium 22. The dielectric medium 22 typically comprises a thin or tissue-like layer in the order of, for example, 0.5mm to 1mm in thickness. In preferred embodiments, the thickness of the dielectric medium 22 and therefore of the layer 16 is approximately 0.65mm or 0.77mm.

In alternative embodiments, the dielectric medium 22 may comprise any other suitable material. For example, the non-woven glass fibres 26 could be replaced with alternative fibres, such as any suitable polymeric fibres, preferably held together as a non- woven sheet or layer by a suitable binder (not shown). The lossy layer 16 could alternatively be provided by suspending the fibres 24 in a liquid/flowable dielectric or electrically insulating material (not shown), for example a polymer emulsion (not shown) or the like, and applied to the interface between the first and second spacing layers 14, 18. Such a configuration would require very accurate application of the liquid/flowable insulating material in order to achieve the desired concentration and .dispersion of the fibres 24. The concentration or density of fibres 24 in the lossy layer 16 is very low in comparison with the density of carbon particles in a conventional lossy layer. Accordingly, the lossy layer 16 is relatively lightweight and flexible. Moreover, the preferred porous nature of the lossy layer 16 enables same to be aggressively adliered to the first and second spacing layers 14, 18, during the manufacture of the panel 10, by a suitable adhesive (not shown) - the porosity of the layer 16 allows adhesive to soak into the layer 16 between the non- woven fibres 26, thereby achieving an excellent bond therewith. It is further preferred that the spacing layers 14, 18 are also formed of porous material so that the adhesive or binder may also soak tlierethrough. Aggressive bonding of the various components of the panel 10 is highly desirable especially where the end use of the panels 10 is as cladding (not shown) which is suspended, in use, in a vertical position and which must therefore be capable of withstanding the structural loads thereon. Coupled to this, the lossy layer 16, being lightweight, adds negligible weight to the panel 10 thereby keeping at a minimum the self weight of each panel 10, and so the structural loads imposed on the various components thereof.

The physical characteristics of the lossy layer 16 are also extremely beneficial during the fabrication of the panels 10. The negligible weight of the lossy layer 16 which, in the preferred embodiment illustrated is approximately 50g/m², lends ease of handling and positioning, while its flexibility allows the lossy layer 16 to be provided as a continuous roll of flexible non- woven fabric which lends itself to be quickly and accurately cut to size.

The panel 10 may thus be consistently manufactured to accurate design specifications or tolerances, with verifiable radar performance, and then fitted by conventional cladding means (not shown). It will be apparent to those ordinarily skilled in the art that the lossy layer 16 described here may alternatively be used in EM wave absorbent structures other than the Salisbury screen, or modified Salisbury screen, type panel described above.

The present invention is not limited to the embodiments described herein, wliich may be amended or modified without departing from the scope of the invention.

Claims

CLAIMS:

1. An electromagnetic wave absorbent structure comprising an electromagnetic wave attenuating layer, the attenuating layer comprising a plurality of electrically resistive elements carried by a dielectric medium, the electrically resistive elements being distributed in said dielectric medium such that they are electrically insulated from one another.

2. A structure as claimed in Claim 1, wherein the length of said electrically resistive elements is, by an order of magnitude, less than the wavelength of the electromagnetic waves to be absorbed, during use, by said structure.

3. A structure as claimed in Claim 1 or 2, wherein said electrically resistive elements are dimensioned to form a respective Hertzian dipole.

4. A structure as claimed in any of claims 1 to 3, wherein said electrically resistive elements each comprise a respective electrically resistive fibre or strand.

5. A structure as claimed in Claim 3, wherein said electrically resistive elements each comprise a respective carbon fibre.

6. A structure as claimed in any preceding claim, wherein, each resistive element is of a length between 3mm and 30mm, more preferably between 15mm and 30mm, and most preferably 25mm.

7. A structure as claimed in any preceding claim, wherein the dielectric medium carries said electrically resistive elements at a density of approximately between 0.125 g/m² and 1 g/m², and preferably approximately 0.625 g/m².

8. A structure as claimed in any preceding claim, wherem said resistive elements are substantially randomly oriented relative to one another.

9. A structure as claimed in any preceding claim, wherein the dielectric medium is porous.

10. A structure as claimed in any preceding claim, wherein the dielectric medium comprises a non- woven fabric (NWF).

11. A structure as claimed in Claim 10, wherein the dielectric medium comprises non-woven glass fibres.

12. A structure as claimed in Claim 10 or 11, wherein said non- woven fabric has a weight of between approximately 30 - 70g/m², more preferably 40 - 60g/m², most preferably approximately 50g/m².

13. A structure as claimed in any preceding claim, wherein said dielectric medium carries between approximately 0.25 and 2% resistive elements by weight, more preferably 1 to 2% by weight, and most preferably 1.25% by weight.

14. A structure as claimed in any preceding claim, wherein the electromagnetic attenuating layer has a substantially infinite surface resistivity.

15. A structure as claimed in any preceding claim, further comprising an electromagnetic wave reflecting layer, the structure being dimensioned and arranged so that electromagnetic waves incident upon the structure during use travel through the structure a distance corresponding to approximately one quarter of the wavelength of the incident electromagnetic waves before impinging upon said reflecting layer and, upon reflection by said reflecting layer, travel through the structure a distance corresponding to approximately one quarter of the wavelength of the incident electromagnetic waves before emerging from said structure, the elecfromagnetic attenuating layer being arranged with respect to the reflecting layer such that said incident electromagnetic waves pass through said attenuating layer before impinging upon said reflecting layer and/or after being reflected by said reflecting layer.

16. A structure as claimed in Claim 15, further comprising a first spacing layer of dielectric material provided between the attenuating layer and the reflecting layer.

17. A structure as claimed in Claim 15 or 16, further comprising a second spacing layer of dielectric material provided beyond the attenuating layer with respect to the reflecting layer.

18. A structure as claimed in Claim 16 or 17, wherein the, or each spacing layer is porous.

19. A structure as claimed in any one of Claims 15 to 18, further comprising an outer layer of dielectric material spaced apart from said reflecting layer by a distance approximately equal to one quarter of the wavelength of said incident electromagnetic waves in the direction of propagation.

20. A sfructure as claimed in Claim 19, wherein the relative permittivity of said outer layer is greater than the relative permittivity of the or each spacing layer.

21. A structure as claimed in any one of Claims 15 to 20, wherein the layers are generally parallely disposed with respect to one another.

22. A shTicture as claimed in any one of Claims 18 to 21, wherein at least some of the layers are bound together by adhesive soaked tlirough at least one layer.

23. An electromagnetic wave absorbent panel comprising an electromagnetic wave absorbent structure as claimed in any preceding claim.