GB2181898A

GB2181898A - Electro-magnetic wave absorbers

Info

Publication number: GB2181898A
Application number: GB08624509A
Authority: GB
Inventors: Vivian Butler; Howard John Farr
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1985-10-21
Filing date: 1986-10-13
Publication date: 1987-04-29
Anticipated expiration: 2006-10-13
Also published as: GB8624509D0; GB2181898B

Abstract

An electro-magnetic wave absorber surface 1 having enhanced radar absorption characteristics over normal surfaces is coated with fibre absorbers 5 in random orientations. The fibres may comprise metal, metal plated polymers or glass, silicon carbide or carbon and are deposited in an adhesive layer 3. <IMAGE>

Description

SPECIFICATION Electo-magnetic wave absorber surface The present invention relates to electro-magnetic wave absorber surface and more particularly to such absorbers comprising orientated fibres to provide a broad band absorber.

The use of fibre absorbers is known in the art but it is important that the alignment of the fibres is such that a component of their direction is parallel to the electric field of the incoming electro-magnetic wave. Thus, the fibres should be either randomly oriented to absorb incoming electro-magnetic waves at various incident angles or specifically oriented to absorb waves from a particular incident angle. The frequencies of incident waves that are absorbed are dependent upon fibre size and concentration.

It is an object of the present invention to provide a method of applying fibre absorbers to a surface to improve absorption of a broad band of electro-magnetic wave frequencies.

According to one aspect of the present invention there is provided an electro-magnetic wave absorber surface comprising a substrate surface, an adhesive layer and a multitude of fibre absorbers, the adhesive layer covering the substrate surface and the multitude of fibre absorbers oriented in the adhesive layer.

The fibre absorbers may be randomly oriented or arranged in a uniform manner.

The fibre absorbers may be blown on to the adhesive layer or electro-statically deposited.

The length, diameter, shape and resistivity of the fibre absorbers may be varied according to the requirements.

According to a second aspect of the present invention there is provided a method of producing an electro-magnetic wave absorber comprising coating a surface with an adhesive layer, depositing resistive fibre absorbers on said surface and allowing or forcing setting of the adhesive.

In one method the fibre absorbers are deflocculated and blown on to the adhesive layer.

In an alternative method electrostatic deposition of the fibres is employed.

In the alternative method the resistive fibre absorbers may be combed into a random distribution or exposed to variable electrostatic fields to give a random disposition.

Embodiments of fibre absorbers in accordance with the present invention will be described by way of example only with reference to the accompanying drawing of which: Figure 1 shows a surface covered with absorbant fibres in accordance with the present invention with the fibres blown on to the surface; and, Figure 2 illustrates a surface covered with a fibre absorber produced using electrostatic techniques.

Consider Fig. 1, a target surface 1, such as on a military vehicle, is coated with an adhesive or sealing layer 3 which may be activated thermally, atmospherically or optically. Fibre absorbers 5 of either uniform or of variable dimensions are defiocculated and blown onto the adhesive 3 whilst still in the liquid state whereby they assume a random distribution in three dimensions. The adhesive 3 is activated and the fibres 5 held substantially in their respective orientations thereafter. As the fibres are randomly oriented a proportion of the fibres will present a component of their direction parallel to the electric field of an incoming electro-magnetic wave arriving at the surface 1 at any angle of incidence and will thus absorb incident electromagnetic wave in the complete incident hemisphere.

It will be appreciated that the fibre absorbers 5 can be made of materials with high or low electrical resistance such as metal, metal plated polmer or glass, silicon carbide and carbon have various shapes crinkled or straight.

Referring to Fig. 2, the fibre absorbers 5 are deposited in an adhesive layer 3 on a surface 1 similar to Fig. 1 by electrostatic means. The fibre absorbers 5 arrange themselves to be substantially normal to the surface 1 due to electrical depole interactions etc, by charging the surface 1 and fibres 5 to a suitable potential difference. The fibre 5 again can be of variable dimensions and area populations to suit particular performance objectives and are substantially fixed in their orientations after the adhesive 3 is activated.

With fibres 5 specifically oriented as in Fig.

2 at a particular angle to the surface the effective incident angles are severely limited however the performance of the absorbing surface at these incident angles is improved over the randomly oriented fibres.

Randomisation of the fibres 5 can be achieved also by combing an electric field variation which can also be used to uniformly angle the fibres 5 with respect to the surface as the fibres 5 will align themselves in an electric field.

Fibre absorbers 5 of particular orientation may usefully be employed in reducing the effect of permanent features such as ship superstructures and the like or permanent radar installations associated therewith.

Fibre absorbers of random orientation are most useful in reducing the profile of military vehicles and the like from potentially unfriendly radar transmissions.

1. A method of producing an electro-magnetic wave absorber surface comprising coating a substrate with an adhesive layer, depositing resistive fibre absorbers on said substrate and allowing or force setting the adhesive layer.

2. A method of producing an electro-magnetic wave absorber surface as claimed in

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Electo-magnetic wave absorber surface The present invention relates to electro-magnetic wave absorber surface and more particularly to such absorbers comprising orientated fibres to provide a broad band absorber. The use of fibre absorbers is known in the art but it is important that the alignment of the fibres is such that a component of their direction is parallel to the electric field of the incoming electro-magnetic wave. Thus, the fibres should be either randomly oriented to absorb incoming electro-magnetic waves at various incident angles or specifically oriented to absorb waves from a particular incident angle. The frequencies of incident waves that are absorbed are dependent upon fibre size and concentration. It is an object of the present invention to provide a method of applying fibre absorbers to a surface to improve absorption of a broad band of electro-magnetic wave frequencies. According to one aspect of the present invention there is provided an electro-magnetic wave absorber surface comprising a substrate surface, an adhesive layer and a multitude of fibre absorbers, the adhesive layer covering the substrate surface and the multitude of fibre absorbers oriented in the adhesive layer. The fibre absorbers may be randomly oriented or arranged in a uniform manner. The fibre absorbers may be blown on to the adhesive layer or electro-statically deposited. The length, diameter, shape and resistivity of the fibre absorbers may be varied according to the requirements. According to a second aspect of the present invention there is provided a method of producing an electro-magnetic wave absorber comprising coating a surface with an adhesive layer, depositing resistive fibre absorbers on said surface and allowing or forcing setting of the adhesive. In one method the fibre absorbers are deflocculated and blown on to the adhesive layer. In an alternative method electrostatic deposition of the fibres is employed. In the alternative method the resistive fibre absorbers may be combed into a random distribution or exposed to variable electrostatic fields to give a random disposition. Embodiments of fibre absorbers in accordance with the present invention will be described by way of example only with reference to the accompanying drawing of which: Figure 1 shows a surface covered with absorbant fibres in accordance with the present invention with the fibres blown on to the surface; and, Figure 2 illustrates a surface covered with a fibre absorber produced using electrostatic techniques. Consider Fig. 1, a target surface 1, such as on a military vehicle, is coated with an adhesive or sealing layer 3 which may be activated thermally, atmospherically or optically. Fibre absorbers 5 of either uniform or of variable dimensions are defiocculated and blown onto the adhesive 3 whilst still in the liquid state whereby they assume a random distribution in three dimensions. The adhesive 3 is activated and the fibres 5 held substantially in their respective orientations thereafter. As the fibres are randomly oriented a proportion of the fibres will present a component of their direction parallel to the electric field of an incoming electro-magnetic wave arriving at the surface 1 at any angle of incidence and will thus absorb incident electromagnetic wave in the complete incident hemisphere. It will be appreciated that the fibre absorbers 5 can be made of materials with high or low electrical resistance such as metal, metal plated polmer or glass, silicon carbide and carbon have various shapes crinkled or straight. Referring to Fig. 2, the fibre absorbers 5 are deposited in an adhesive layer 3 on a surface 1 similar to Fig. 1 by electrostatic means. The fibre absorbers 5 arrange themselves to be substantially normal to the surface 1 due to electrical depole interactions etc, by charging the surface 1 and fibres 5 to a suitable potential difference. The fibre 5 again can be of variable dimensions and area populations to suit particular performance objectives and are substantially fixed in their orientations after the adhesive 3 is activated. With fibres 5 specifically oriented as in Fig. 2 at a particular angle to the surface the effective incident angles are severely limited however the performance of the absorbing surface at these incident angles is improved over the randomly oriented fibres. Randomisation of the fibres 5 can be achieved also by combing an electric field variation which can also be used to uniformly angle the fibres 5 with respect to the surface as the fibres 5 will align themselves in an electric field. Fibre absorbers 5 of particular orientation may usefully be employed in reducing the effect of permanent features such as ship superstructures and the like or permanent radar installations associated therewith. Fibre absorbers of random orientation are most useful in reducing the profile of military vehicles and the like from potentially unfriendly radar transmissions. CLAIMS

2. A method of producing an electro-magnetic wave absorber surface as claimed in claim 1 wherein the resistive fibre absorbers are deposited on the substrate by deflocculating and blowing them onto the adhesive layer.

3. A method of producing an electro-magnetic wave absorber surface as claimed in claim 1 wherein the resistive fibre absorbers are deposited by electrostatic attraction.

4. A method of producing an electro-magnetic wave absorber surface as claimed in claim 3 wherein the deposition of resistive fibre absorbers is made random by varying an imposed electro-magnetic field across the substrate.

5. A method of producing an electro-magnetic wave absorber surface as claimed in any one of claims 1 to 3 wherein the resistive fibre absorbers are randomly distributed by combing the fibres prior to completion of setting the adhesive.

6. An electro-magnetic wave absorber surface comprising a substrate surface, an adhesive layer and a multitude of fibre absorbers, the adhesive layer covering the substrate surface and the multitude of fibre absorbers oriented in the adhesive layer.

7. An electro-magnetic wave absorber surface as claimed in claim 6 wherein the fibre absorbers are randomly oriented.

8. An electro-magnetic wave absorber surface as claimed in claim 6 wherein the fibre absorbers are arranged in a uniform manner.

9. An electro-magnetic wave absorber surface as claimed in claim 6, claim 7 or claim 8 wherein the fibre absorbers are formed from electrical resistance material such as silicon carbide.

10. An electro-magnetic wave absorber surface as claimed in any one of claims 6 to 9 wherein the fibre absorbers are made of low electrical resistance material such as a metal.

11. An electro-magnetic wave absorber surface as claimed in any one of claims 6 to 10 wherein the absorber fibres are straight.

12. An electro-magnetic wave absorber surface as claimed in any one of claims 6 to 10 wherein the fibre absorbers are crinkled.

13. An electro-magnetic wave absorber surface substantially as hereinbefore described with reference to the accompanying drawings.

14. A method of producing an electromagnetic wave absorber surface substantially as hereinbefore described with reference to the accompanying drawings.