GB2319117A

GB2319117A - Materials which absorb electromagnetic energy

Info

Publication number: GB2319117A
Application number: GB7906287A
Authority: GB
Inventors: Sydney Clifford Woolcock
Original assignee: EMI Ltd
Current assignee: EMI Ltd
Priority date: 1978-02-22
Filing date: 1979-02-22
Publication date: 1998-05-13
Anticipated expiration: 1999-02-22
Also published as: GB7906287D0; GB2319117B

Abstract

In order to reduce radar reflections a material has a graded intrinsic impedance normal to its surface. An example of the material is carbon fibre material having a smaller density of fibres near the surface than elsewhere. The air intake duct of a jet engine may be made of the material to reduce radar reflections.

Description

"IMPROVEMENTS IN OR RELATING TO MATERIALS WHICH ABSORB ELECTROMAGNETIC ENERGY" The present invention relates to a material which absorbs electromagnetic energy especially radar waves and to an object comprising such a material.

It is desirable to reduce the radar cross-sectional area of military equipment. In order to do this, it is known to cover, or line, dominant sources of reflection with a radar absorber, e.g. a lossy ceramic material However, there are problems of for example weight and mechanical strength with such a material. Alternatively reflections can be reduced by carefully shaping various parts of the equipment; but, this cannot always be done as the shape of a part of the equipment is often vital to its function.

According to one aspect of the present invention, there is provided a material having a greater intrinsic impedance in the region of a surface of the material than in a region more remote from that surface due to variation of the composition of the material at least in a direction normal to that surface.

An example of such a material is carbon fibre material in which the density of the fibres is smaller in the said surface region than in the said more remote region.

According to another aspect of the invention, there is provided a duct comprising said material, the surface defining the inner surface of the duct.

For a better understanding of the present invention, its application to an aircraft will now be described by way of example only.

The level of the radar cross-section of a modern aircraft in the centimetre and millimetre bands, at least away from the broadside aspect, mostly results from contributions from radar aerials, bulkheads, radomes, the cockpit, stores and in particular from the engines and their air intake and jet-pipe assembles.

For radar to fuselage main-axis angles of up to 70 deg. from nose-on, the engines and their intakes can be dominant sources of reflection especially if some care is taken to reduce the reflections from other regions e.g. from forward facing radar assemblies by use of radar absorber and from cockpits of optically transparent but radar reflective metallising of the canopy.

By careful shaping, the reflection from the lip of an intake can be reduced, but unfortunately the majority of the reflection arises in consequence of electromagnetic waves travelling down the intake and being reflected from discontinuities within the duct or from the engine. It is essential, for aerodynamic reasons, that the intake is not screened by other parts of the aircraft. Also, because of the possibility of bird strikes, it is not possible to insert in the intake a metal grid to act as a radar screen.

As with a radar waveguide, the method of propagation down the intake duct can be described in terms of rays of energy suffering many reflections from the walls during transmission along the duct towards and away from the engine. It has been suggested previously that a lossy ceramic material liner could be used as a radar absorber.

To investigate the effectiveness, in reducing radar crosssection, of lining intakes with absorber, backscatter measurements have been made using a high grade scale model of an aircraft with and without its intakes lined with absorber.

(There were no stores on the model aircraft). At X-band (full scale equivalent) and using vertical linear polarisation the reflectivity of the aircraft was reduced by a factor of Ii from to 55 deg.

about 5 deg./from nose-on for the radar sightline near the wing plane after adding absorber. For horizontal linear polarisation the azimuth angle over which such a large reduction was achieved was more limited being 5 deg. to 35 deg. Even so, there is a high probability of detection by a radar over this aspect range and so the results are of great significance.

There are problems of weight and mechanical strength associated with the use of a liner in the air intake. Another problem is that small pieces of material breaking off could cause serious damage to the engine.

Thus, in this exemplary application of the invention, instead of using a liner in a metal air intake duct, the air intake duct is formed of monolithic carbon fibre material according to the said one aspect of the invention. This carbon fibre material has a smaller density of carbon fibres in the skin region of the material, extending to a depth of about 0.5 cm from the inner surface of the duct, than in the rest of the material. This permits some penetration of the electromagnetic field into the material and limited absorption of the energy of the field occurs due to the presence of carbon fibres in the skin region.

If a uniformly dense concentration of carbon fibres were used throughout the material, its reflecting properties would be similar to those of a metal duct. By decreasing the concentration of fibres adjacent to the surface of the material, that surface is made slightly lossy, thus reducing the -, reflectivity of the material. Only a small surface reflection loss is necessary since a radar wave, considered as a ray, undergoes multiple reflections in the duct as described earlier. About 20% absorption of the energy in an incident ray is more than sufficient. The exemplary material according to the invention is capable of providing sufficient mechanical strength to meet the mechanical constraints of aircraft design as well as absorbing electromagnetic energy.

Claims

What we claim is:

1. A material having a greater intrinsic impedance in the region of a surface of the material than in a region more remote from that surface due to variation in the composition of the material at least in a direction normal to that surface.

2. An air intake duct for a jet engine substantially as hereinbefore described.

2. A carbon-fibre material according to Claim 1 in which the density of the fibres is smaller in the said surface region than in the said more remote region.

3. A duct comprising the material of Claim 1, or 2, the said surface defining the inner surface of the duct.

4. A duct according to Claim 3, which is the air intake duct of a jet engine.

5. A material substantially as hereinbefore described.

Amendments to the claims have been filed as follows What we claim is:1. An air intake duct for a jet engine the duct being formed of monolithic carbon fibre material in which the region adjacent the inner surface of the duct has a smaller density of carbon fibres than regions more remote fro the surface, thereby to make said surface region slightly lossy and to reduce the reflectivity of the material.