IL96185A - Radar-absorbing material and its use for camouflaging against radar detection - Google Patents

Abstract

2.1 The complicated layer structure, high density and low mechanical resistance of radar-absorbing materials should be avoided. 2.2 The material with radar-absorbing properties has a binder layer with high electrical resistance and a content of 10-60% by volume of electrically conducting particles having a conductivity of 1-100 ( OMEGA .m)<-1> whose maximum and minimum linear dimensions are comparable and whose minimum linear dimension is 1-7 mm. The particles are uniformly distributed in such a way that the direction of their minimum or maximum linear dimension is essentially perpendicular to the layer and this determines the layer thickness. The layer is on a substrate which is virtually completely reflective in the cm and mm electromagnetic wave range. 2.3 Camouflage against radar investigation. <IMAGE>

Description

D' n >"y 'i A η ιιοηίί »m>Bin a"_ja yi>i-i 10m Radar-absorbing material and its use for camouflaging against radar detection Wilh. Becker Industrielack GmbH and Gerd Hugo C. 81983 RADAR ABSORBING MATERIAL AND ITS USE FOR CAMOUFLAGE AGAINST RADAR DETECTION The invention relates to a material having radar absorbing properties in the region of the electromagnetic cm- and mm- waves, which, owing to its special features, is especially suitable as a coating for radar camouflage of mobile or fixed objects. For special uses the material can also be manufactured in the form of a foil or mat, which can be applied onto mobile or fixed objects, for example by glueing.

Camouflaging of military objects against radar detection is based on application on objects to be protected of a material whose reflection degree R for radar waves in the respective frequency range is sufficiently low. For objects to be protected on land the typical requirement is R < 0.1 or expressed in decibels, <-10 dB. Fulfillment of this requirement throughout a sufficiently broad frequency range is an important characterising feature for a radar absorber (band broadness). In the ideal case, a sufficiently low value for R should be achieved for all wavelengths below a limit wavelength λσ which is characteristic for the respective absorber type.

In DE-OS 15 91 114 there is described a pyramid absorber made of foam material, whose electric losses are achieved by incorporation of graphite or carbon black. The disadvantage of this arrangement resides in the fact that the ratio between the absorption thickness and the limit value of the wavelength d/xQ is as a rule greater than 1. If an object were to be camouflaged with this arrangement against radar detection, an extremely thick layer would have to be applied which, according to the principle of this absorber must have a very small density ( foam material ) and thus can not be subjected to mechanical load (stress). It would therefore be impossible to walk on an object thus protected, which is, for example, necessary in a tank.

From DE-OS 33 07 066 there is known a radar absorbing arrangement which is characterised by a multi-layered structure with increasing concentration (also known as "radar sump" ) made of electrically conductive particles or ferrite filling materials. The disadvantage of this arrangement resides in the extremely complicated structure of the layers juxtaposed on each other. A wide band absorption of, for example, 10 to 100 GHz with continuous 10 dB attenuation, is only possible in this arrangement with very thick layers.

From DE-AS 27 15 823 there is known a wall for absorption of electromagnetic waves, in which ferromagnetic materials are used for radar absorption. The disadvantage of this arrangement is the fact that it can be used only in frequency ranges below 10 GHz, because the ferromagnetic materials are known to have only a very weak absorption above 10 GHz, which absorption becomes nil with increasing frequency. Moreover, the high specific weight of the materials is of disadvantage considerably restricting the possibilities of use.

DE-OS 35 07 889 describes a coating in which radar absorbing active compounds are embedded. The conductivity of the active material as well as its dimensions were not specified. Such known coatings could at best absorb radar waves only in a very narrow frequency range.

It is an object of the invention to provide a material having radar absorbing properties, which is suitable for radar camouflage over a broad frequency range, is easy to apply and can withstand high mechanical stresses.

This object is achieved by the present invention which provides a material having radar absorbing properties in the region of electromagnetic cm- and mm-waves, and characterised in that it comprises a) a layer of a binder material with an electric Q resistance of >10 Ω cm and a dielectric constant Gr in the range of 1.5 to 4 and, embedded therein, 10 to 60% volume-%, calculated on the total volume of the layer, of first electrically conductive particles with a specific conductivity of 1 to 100 (Qm)~ , containing a binder material having an electric resistance of >10^ Ohm x cm and a dielectric constant £r in the range of 1.5 to 5 and, dispersed in the binder material, second electrically conductive particles with a specific conductivity of 1000 to 62 x 108 (Om)"1, which cause the conductivity of the first electrically conductive particles by contact and mutual chain formation, the maximal and minimal linear dimension of the first electrically conductive particles being about 1 to 7 mm, and these first electrically conductive particles being uniformly distributed throughout the volume of the binder material in such manner, that the direction of their maximal or minimal linear dimension extends substantially perpendicular to the layer and the linear dimension which determines the perpendicular direction, determines the thickness of the layer, and all the first electrically conductive particles have approximately the same specific conductivity and dimensions; and a substrate which is nearly completely reflective in the range of the electromagnetic cm- and mm-waves, on which said layer is applied either directly or over one or more intermediate layers.

In accordance with the invention, the above defined layer can be applied on to the object to be camouflaged, to the extent that its surface is nearly or completely reflective in the range of the electromagnetic cm- and mm-waves. Alternatively, the substrate or object can be provided with an adhesive coating or a corrosion protective coating which, after the final coating layer is applied, becomes an intermediate layer. It is also possible to first apply the layer on a substrate such as a foil or mat, which satisfies the reflection requirement. Thereafter, such a foil or mat can be used for covering the object to be camouflaged or can be attached to the object to be camouflaged, for example by glueing.

In accordance with the invention there is applied to the reflecting substrate a layer containing electrically conductive particles with a specific conductivity of 1 to lOO(nm)-1. Preferably the specific conductivity is 1 to 20 (Qm)"1.

The particles advantageously have minimal and maximal linear dimensions which are similar to each other, i.e. are of comparable orders of magnitude. In the ideal case, the particles should thus have a spherical or nearly spherical shape. However, this shape is not absolutely necessary and e.g. block-, lens-, plate- cylinder- and similar shapes are suitable, having maximum and minimum linear dimensions between 1 and 7 mm.

The electrically conductive particles consist of non-conductive plastic materials, into which electrically conductive materials are incorporated. Examples of such plastic materials are polyvinylchloride, polyethylene, ethylvinyl-acetate or synthetic materials such as synthetic resins, natural resins and rubber, as can be used, , for the binder materials defined below, the conductive material incorporated therein being, for example, carbon black. In fact, the particles used can consist of the same material as the binder in which they are dispersed, but these particles must contain electrically conductive materials. The conductive particles can be produced, for example, in the form of granulates from the respective plastic material containing conductive materials. Suitable are, for example, also particles of intrinsically conductive synthetic materials, such as polypyrroles or polyanilines, to the extent that their structure meets the specified requirements of the invention.

The abovementioned electrically conductive particles in the layer of the radar absorbing material according to the present invention, are embedded or dispersed in a binder material with high electrical resistance which is practically non-conductive.

The binder material is selected in accordance with the desired properties and is not subject to any limitation. For example, synthetic materials, synthetic resins, natural resins and rubbers such as used, for example, in the paint and varnish industry are suitable. Especially suitable are binder materials which do not contain solvents and consist of film-forming materials, which can be cured upon addition of reactive substance, such, as, for example, conventional two-component coating materials, which consist, for example, of a fluid epoxy resin and a suitable curing agent therefor. However, it is also possible to use binder materials as contained in conventional coating materials containing solvents and also coating materials based on aqueous dispersions. Moreover, materials which can be applied in the form of melts are also suitable as binder materials. Examples of suitable binder materials - and curing-type materials for the two-component - coating materials are epoxy resins cured with amines, amides, polyamides or isocyanates; polyesters containing OH-groups and/or polyethers containing OH-group and/or acrylic resins containing OH-groups, cured with isocyanates; unsaturated acrylic resins and/or unsaturated polyesters, cured with peroxides. The binder materials can be chosen in accordance with the desired load resistance of the finished coating.

The electrically conductive particles are uniformly distributed throughout the volume of the binder material contained in the layer of the radar absorbing material in accordance with the present invention. In the ideal case, the embedded conductive particles are distributed in such manner, that the centres of gravity of these particles are situated at equal distances from the respective centres of gravity of the neighbouring particles. This ideal case can of course be achieved in practice only approximately. In principle it is sufficient if a statistical distribution is achieved. This statistical distribution is sufficient especially in those instances in which the maximal and minimal linear dimensions of the electrically conductive particles are equal or only slightly different from each other.

The thickness of the layer of radar absorbing material in accordance with the present invention containing the electrically non-conductive binder material corresponds either to the minimal or the maximal linear dimension of the conductive particles contained therein. The conductive particles contained in this layer are oriented in principle in such a manner that their minimal or maximal linear dimension extends perpendicularly to the spreading direction of the layer. Thus, the conductive particles are in principle arranged within the layer in juxtaposed position and not in mutually superimposed position.

Any method for producing the layer containing the electrically conductive particles can be used, which permits substantially uniform distribution of the electrically conductive particles in the binder and enables a coating method achieving such thicknesses of the layer obtained which correspond either to the minimal linear dimension perpendicular to the spreading direction of the layer of the particles "lying" therein, or to the maximal linear dimension perpendicular to the spreading direction of the layer of the particles "standing" therein perpendicular to the spreading direction of the layer. In principle, for particles whose minimal and maximal linear dimensions hardly differ from each other, dispersion of the particles in a solution of the binder material in an organic solvent, in an aqueous dispersion of the binder material or in a melt of the binder material and subsequent application of the so-obtained coating in a layer whose thickness corresponds to the dimension of the particles, for example, by spreading or by knifing, are suitable. Thereafter, curing can follow, for example by elimination of the solvents or the water. It is however also possible, to apply the particles, at the desired distribution, e.g. by glueing on a foil, for example a foil consisting of the binder material or a foil made of nearly completely reflective material or directly on the object to be camouflaged, if its surface is nearly completely reflective, and thereafter to apply the resin in dissolved, dispersed or molten form, at the required layer thickness, followed by conventional curing.

The binder material used in the layer of the material in accordance with the present invention, can be mixed with conventional additives, for example thixotropy conferring agents to improve its rheological properties during coating, materials having flame-retardant properties and other conventional additives, as usually added to coating agents, binding agents or thermoplastic resins. Such additives can be dissolved or dispersed in the binder material. They can also be present in the form of particles.

It is especially favourable, in accordance with the present invention, to use a binder material whose dielectric constant corresponds to the dielectric constants of air or vacuum which are about 1. However, in view of the fact that the binder materials and thermosplastic resins normally have dielectric constants in the order of 3-5, it is possible, in accordance with the present invention, to add to the binder materials and resin, particulate materials, which lower the dielectric constant. Examples of such particulate materials, which can be added to the binder materials are gas-filled (for example air filled) hollow spheres, such as gas-filled, (for example air filled) glass beads, and particulate foam materials. The dimensions of such particulate materials can be similar to those of the electrically conductive particles used, but should preferably be less. Instead of gas-filled hollow spheres it is also possible to include a gas, for example air in the form of bubbles, in the binder material. The effect' thus achieved is similar to that obtained by using hollow gas-filled spheres or particulate foam material.

Further particulate materials, for various purposes, may also be added to the binder material. The dimensions of such materials depend on their geometric shape, and as indicated above, could usually be as large as the electrically conductive particles used, but preferably less. Fibre type materials may also have larger linear dimensions. Thus, for example, pigments can be used which can also be electrically conductive. The electric conductivity of such pigments is, for example, in the range of 10"8 to 100 (Qm)-1, especially from 10~8 to 10~4 (Qm)-1. It is furthermore possible to add particulate materials having flame retardant or thixotropic properties or cause magnetic losses. Materials which cause magnetic losses are, for example, ferrites and ferritic materials, which have an improved absorption at lower frequencies in the range of about 10 GHz, i.e. in the cm-wave region. However, it is also possible to use particulate materials which improve the hardness, such as, for example, short-cut fibres, having a length of, for example, 0.1 to 20 mm. Such fibres can also be electrically conductive such as, for example, carbon fibres. Electrically conductive short cut Q fibres have, for example, a specific conductivity of 10 ° up to 100 (Om)"1, especially from 10~8 to 10~4 (Om)"1. The use of conductive fibres not only improves the hardness, but also makes it possible to obtain displacement of the frequency range of the radiation to be absorbed.

The layer of the radar absorbing materials in accordance with the present invention containing the binder material is applied either directly on or at a small distance from the substrate, which is nearly completely reflective in the electromagnetic cm- and mm-wave range. "At a small distance" means that the reflective material is provided, for example, with a coating, such as a ground coating. However, it is advantageous that the conductive particles of the layer containing the binder material be at the smallest possible distance from the reflecting substrate. Conductive contact is however not necessary.

The reflecting substrate is preferably an electrically conductive material with a specific conductivity of more than 1000 (Qm)"1. This may be for example, the surface of the object to be camouflaged optionally provided with a coating or a ground layer, or it may also be a substrate in the form of a foil or mat, which is suitable for covering or applying on to the object to the protected, for example, by glueing.

In accordance with another embodiment the material according to the present invention having radar absorbing properties can be provided, on the surface of the binder-material containing layer, with one or more additional layers having a total thickness of less than 1mm, preferably less than 0.1mm, whereby the surface of the total layer exhibits camouflaging properties in the spectral range of 0.1 to 14um or in a partial range thereof. Such layers thus provide additional camouflage in the region of the visible waves or IR-waves. For example, the surface can be low emitting in the spectral range of thermal infrared of 4-14 urn, with an emission factor smaller than 0.9, preferably smaller than 0.7.

The accompanying Fig. 1 represents an example of a material with radar absorbing properties according the present invention. It shows a cross-section through such a material .

In the figure, (1) is the binder material with high electric resistance, in which electrically conductive particles (2) with a specific conductivity of 1-100 (Ωιη)-^ and a diameter of 1-7 mm are dispersed. (3) represents the electrically conductive materials, which are dispersed in a second binder material ( 4 ) having a high electric resistance, so as to form the particles (2), and (5) represents particles (2) of an intrinsically conductive synthetic material. (6) indicates the thickness of the material ( 1 ) which roughly corresponds to the diameter of the particles (2). (7) represents further electrically conductive or non-conductive particulate materials, which are incorporated into the material ( 1 ) . ( 8 ) represents an upper layer with camouflaging properties in the spectral range of 0.1-14 urn and (9) represents a layer which is nearly completely reflective in the region of the electromagnetic cm- and mm-wave.

Camouflage over a wide range of frequencies is obtained by the material having radar absorbing properties in accordance with the present invention. There is achieved a very small value of the degree of reflection R for radar waves for all wavelengths over a wide range of wavelengths. Moreover, a low value of the ratio d/X0, is achieved, so that a thin radar absorber is obtained which permits the use of low amounts of material, low surface weights and low costs.

A surface camouflaged with a dielectric material according to the invention can be walked on and resists high mechanical stresses. If the material is intended, for example, for a lower frequency limit of 10GHz, the thickness of the material is only 3.5 mm, which means that the ratio of the thickness of the material to the limit wavelength d/X0 is nearly 0.1. Application of radar camouflage onto a surface is simple. The dielectric material in accordance with the invention can be applied in a single operation, by knifing a single layer onto the surface to be camouflaged. Owing to the typical specific weight of the material according to the invention (of about 0.4 to 2.5, preferably 0.6 to 1.3), there exist no limitations of the range of use. It absorbs radar radiation over a wide frequency range.

Example 1 50 g of commercially available carbon black-filled polyvinyl chloride (PVC) particles, having a cylindrical shape of 2 mm diameter and 2.5 mm length, are uniformly distributed on a metal plate having the dimensions of 24 x 24 cm. The specific conductivity of the PVC/carbon black particles was about 5 (Ωπι)"■·. The resulting radar reflex attenuation, depending on the frequency, was as follows: Frequency [GHz] 18 35 51 94 Attenuation [-dB] 6.5 22 13 9.5 However, the carbon black-filled PVC particles were only loosely distributed over the plate and could, therefore, not be used in this form for radar camouflage. The particles were dispersed in a commercial two-component epoxy resin and spread on the metal plate. The composition was as follows: 72 g Epoxy resin 30 g Curing agent 50 g PVC/carbon black particles The mass was uniformly distributed on a metal plate of the dimensions of 24 x 24 cm, cured and measured by radar. The thickness of the cured layer was about 2mm. The resulting radar reflection attenuation, depending on frequency, was as follows: Frequency in [GHz] 10 15 18 Attenuation [-dB] 10 14 20 Example 2 Micro hollow glass spheres with an average specific density of 0.24 were added to the composition in accordance with Example 1. The composition was as follows: 72 g Epoxy resin 30 g Curing agent 17 g Micro hollow glass spheres 50 g PVC/carbon black particles The mass was uniformly distributed on a metal plate of 24 x 24 cm, cured and measured by radar. The thickness of the cured layer was about 2 mm. The resulting radar reflection attenuation, depending on the frequency, was as follows: Frequency in [GHz] 18 35 51 94 Attenuation [-dB] 12 9 10 11 In all pertinent frequency ranges the attenuation was about -10 dB.

Example 3: In this Example spherical or cylindrical shaped particles with a diameter of about 3.5 mm of an ethylene vinylacetate copolymer were used having a pigment carbon black concentration of about 35%, of a conductive carbon black preparation (Corex P of Degussa). The specific conductivity of the particles was about 10 (Qm)-1. A mixture of the following composition was prepared: 66 g Epoxy resin 27.5 g Curing agent 20 g Micro hollow glass spheres 60 g Ethylene vinyl acetate carbon black particles.

The mass was uniformly distributed on a metal plate of 24 x 24 cm, cured and subjected to broad band radar. The thickness of the layer was about 3.5 mm. The resulting radar reflection attenuation is represented in Fig. 2.

In the frequency range of 10 GHz a radar reflection attenuation of more than 20 dB could be measured.

At all higher measuring frequencies the attenuation was still better than 10 dB.

Comparative Test 1 The same mixture as in Example 3 was prepared, the ethylene vinyl acetate/carbon-black particles being however comminuted to a diameter of 0.3 mm. The mass was again distributed on a metal plate of 24 x 24 cm, so that a layer of 3.5 mm thickness was obtained. After curing, the plate was measured over a broad band frequency by radar. The resulting radar reflection attenuation is represented in Fig. 3.

At 10 GHz no considerable absorption could be measured. Up to about 30 GHz the absorption was less than 10 dB and was about 7 to maximum 8 dB. At 35 GHz the absorption was better than 20 dB. At higher frequencies the absorption was however again less than 10 dB. This arrangement is thus not suitable for a broad band radar absorber.

Comparative Test 2 As a further comparative test to Example 3 the same amount of conductive carbon black (Corex P) as used in the mixture of Example 3 was directly dispersed in the binder of the radar coating and was applied at a thickness of 3.5 mm on a metal plate of 24 x 24 cm. The amount of ethylene vinyl acetate contained in the composition of Example 3 was replaced by a corresponding amount of epoxy resin.

The specific conductivity of the conductive carbon-black particles was about 5,000 (Qm)~^. The diameter of the particles was about 5-50um. The composition was as follows : 97.2 g Epoxy resin 40.5 g Curing agent 20 g Micro hollow glass spheres 21 g Conductive carbon-black (Corex P) After curing, the layer was measured by radar at a broad band range. The resulting radar reflection attenuation is represented in Fig. 4. Throughout the whole measuring range the radar reflection attenuation was less than IdB and consequently completely unsuitable for radar coating.

Example 4 Similar to Example 1 PVC/carbon black-particles with a diameter of 2.5 mm and a length of 2.5 mm were used. In addition to the PVC/carbon black-particles short cut carbon fibres having a length of 0.1-lmm and a specific conductivity of 10"^ (Om) -1 were dispersed in an epoxy resin. The composition was as follows: 66 g Epoxy resin 27.5 g Curing agent 20 g Micro hollow glass spheres 1.5 g Short-cut carbon fibres 60 g PVC/carbon black-particles The mass was uniformly distributed on a metal plate of 24 x 24 cm, hardened and measured by radar. The thickness of the hardened layer was about 2.5 mm. The resulting radar reflection attenuation, depending on the frequency was as follows: Frequency in [GHz] 18 35 51 94 Attenuation [GHz] 6 12 14.5 15.5 Example 5 Flame-retardant aluminium hydroxide was added to the PVC/carbon black-particles of 2.5 mm diameter used in Example 4, the composition of the mixture being as follows: 50 g Commercial polyurethane binder 41 g Curing agent 10 g Microhollow glass spheres 50 g PVC/carbon black-particles 10 g Aluminium hydroxide The mass was uniformly distributed on a metal plate of 24 x 24 cm, hardened and measured by radar. The thickness of the hardened layer was about 2.5 mm. The resulting radar reflection attenuation depending on the frequency was as follows: Frequency in [GHz] 18 35 51 94 Attenuation [GHz] 8 10 17 15 Example 6 The same mixture as in Example 5 was used but instead of aluminium hydroxide metal platelets consisting of an electrically conductive Mu metal, an alloy with especially high permeability, were added. The composition of the mixture was as follows: 55.2 g Epoxy resin 23 g Curing agent 17 g Micro hollow glass spheres 15 g Mu metal platelets 46.5 g PVC/carbon black particles of 2mm diameter The mass was uniformly distributed on a metal plate of 24 x 24 cm, hardened and measured by radar. The thickness of the hardened layer was 2mm. The resulting radar reflection attenuation, depending on the frequency was the following: Frequency in [GHz] 7 12 18 35 Attenuation [-dB] 12 14 10 10

Claims (10)

1. A material having radar absorbing properties in the range of electromagnetic cm- and mm-waves, characterised in that it comprises a) a layer of a binder material with an electric resistance of >10¾cm and a dielectric constant er in the range of 1.5 to 4 and, embedded therein, 10 to 60% volume-%, calculated on the total volume of the layer, of first electrically conductive particles with a specific conductivity of 1 to 100 (Qm)-1, containing a binder material having an electric resistance of >10 Ohm x cm and a dielectric constant Gr in the range of 1.5 to 5 and, dispersed in the binder material, second electrically conductive particles with a specific conductivity of 1000 to 62 x 108 (Qm)-1, which cause the conductivity of the first electrically conductive particles by contact and mutual chain formation, the maximal and minimal linear dimension of the first electrically conductive particles being about 1 to 7 mm, and these first electrically conductive particles being uniformly distributed throughout the volume of the binder material in such manner, that the direction of 961 85 /2 - 24 - their maximal or minimal linear dimension extends substantially perpendicular to the layer and the linear dimension which determines the perpendicular direction, determines the thickness of the layer, and all the first electrically conductive particles have approximately the same specific conductivity and dimensions; and b) a substrate which is nearly completely reflective in the range of the electromagnetic cm- and mm- waves, on which said layer is applied either directly or over one or more intermediate layers.

2. A material according to Claim 1 having radar absorbing properties in the range of electromagnetic cm- and mm-waves, characterised in that in the layer of binder there are incorporated in addition to the electrically conductive particles, further electrically conductive and/or non- conductive particulate materials having smaller linear dimensions than the first particles.

3. A material according to Claims 1 or 2, having radar absorbing properties in the range of electromagnetic cm- and mm-waves characterised in that on said layer (a) one or more further layers of a total thickness of less than 1 mm are applied, the surfaces of these additional layers having camouflaging properties in the spectrum range of 0.1 to 14 urn or in partial portions of this spectrum range.

4. A material according to Claims 1-3, having radar absorbing properties in the range of cm- and mm-waves, characterised in that the first particles have a spherical or nearly spherical shape, or a block-, lens-, plate- or cylinder shape.

5. A material according to one or more of the preceding claims, having radar absorbing properties in the range of cm- and mm-waves characterised in that the first particles have a specific conductivity of 5 to 20 (Qrn)"1.

6. A material according to Claims 2 to 5, having radar absorbing properties in the range of cm- and mm-waves, characterised in that, in addition to the first particles there are incorporated in the binder material further particulate materials in the form of gas filled hollow bodies and/or of gas bubbles with smaller linear dimensions than the first particles.

7. A material according to any of Claims 1 to 6, having radar absorbing properties in the region of cm- and mm-waves, characterised in that the substrate is the object to be camouflaged, or a foil or mat which can cover the object to be camouflaged or which can be applied to such object.

8. A method for camouflaging an object against radar detection, characterised in that a) a layer of a binder material with an electric g resistance of >lCrncm and a dielectric constant 6r in the range of 1.5 to 4 and, embedded therein, 10 to 60% volume-%, calculated on the total volume of the layer, of first electrically conductive particles with a specific conductivity of 1 to 100 (Om)-1, containing a binder material having an electric resistance of >10^ Ohm cm and a dielectric constant er in the range of 1.5 to 5 and, dispersed in the binder material, second electrically conductive particles with a specific conductivity of 1000 to 62 x 108 (Om)"1, which cause the conductivity of the first electrically conductive particles by contact and mutual chain formation, the maximal and minimal linear dimensions of the first electrically conductive particles being about 1 to 7 mm, and these first electrically conductive particles being uniformly distributed throughout the volume of the binder material in such manner, that the direction of their maximal or minimal linear dimension extends substantially perpendicular to the layer and the linear dimension which determines the perpendicular direction, determines the thickness of the layer, and all the first electrically conductive particles have approximately the same specific conductivity and dimensions; b) is applied onto an object being a substrate, which is nearly completely reflective in the range of electromagnetic cm- or mm-waves, directly or on top of one or several intermediate layers provided on the substrate, or c) is applied onto a foil or mat constituting a substrate which is nearly completely reflective in the range of electromagnetic cm- or mm-waves, and thereafter the foil or mat is applied on the object to be protected.

9. A method according to claim 8 characterised in that a layer as defined in claims 2-7 is applied.

10. Use of a coating based on a binding material q having an electrical resistance of >10 Ωαη and a dielectric constant €r of 1.5 to 4 and, embedded therein, 10 to 60% volume-%, calculated on the total volume of the layer, of first electrically conductive particles with a specific conductivity of 1 to 100 (Qm)"1, containing a binder - 28 - 96185/2 material having an electric resistance of >109 Ohm x cm and a dielectric constant €r in the range of 1.5 to 5 and, dispersed in the binder material, second electrically conductive particles with a specific conductivity of 1000 to 62x10s (Qm)"1 which cause the conductivity of the first electrically conductive particles by contact and mutual chain formation, the maximal and minimal linear dimensions of the first electrically conductive particles being about 1-7 mm, and all the first electrically conductive particles having approximately the same specific conductivity and dimensions, for the manufacture of materials with radar absorbing particles, substantially as described in the specification . For the Applicants, DR. REINHOLD COHN AND PARTNERS

81983. J JT/p rg/29. 11 . 1994