GB2258951A

GB2258951A - Reflection-reducing coating

Info

Publication number: GB2258951A
Application number: GB8915777A
Authority: GB
Inventors: Joachim Kaiser
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1988-07-13
Filing date: 1989-07-10
Publication date: 1993-02-24
Also published as: FR2687015A1; GB8915777D0; IT8920821A0; IT1235729B

Abstract

A coating for the reduction of the reflection power of a body 1 in relation to electromagnetic radiation consists of a layer 2 deposited on the surface of the body, the layer allowing the passage of the incident electromagnetic radiation and having a thickness which corresponds approximately to a quarter of the wavelength which the radiation assumes inside the layer, and the layer consists of a material which, in the frequency range of the incident electromagnetic material, has an anomalous dispersion characteristic. The material is preferably so chosen that its refractive index in the frequency range of this radiation is approximately proportional to the reciprocal of the frequency of this radiation. An impedance film 3 and/or a metallic conducting film (15, Fig 2 not shown) may also be provided. <IMAGE>

Description

Reflection-reducing coating The present invention concerns a coating for the reduction of the reflection power of bodies in relation to electromagnetic radiation, and more particularly a layer deposited on the surface of the body, the layer allowing the passage of electromagnetic radiation and having a thickness corresponding approximately to a quarter of the wavelength which the incident electromagnetic radiation assumes inside the layer.

To decrease the reflectivity of materials in relation to electromagnetic radiation, many differing coatings have already been suggested. Apart from the so-called gradient-layers, in which the refractive index of the layer varies from the inside to the outside, there are also the quarter wavelength layers of the type mentioned above. An aggravating disadvantage of these lastmentioned coatings is that the condition that the layer thickness corresponds precisely to a quarter of the wavelength is fulfilled, strictly speaking, only for one particular frequency, so that reflection damping is effected only in a very narrow frequency band. To broaden this effective band-range, there has thus been suggested inter alia the use of the previouslymentioned gradient layers, of multiple layers, or a covering with a film with a complex surface impedance, i.e. with an antenna or aerial structure.

The present invention seeks to provide a coating of the type mentioned in the introduction with which a broad band reflection damping for electromagnetic radiation can be achieved.

According to the present invention, there is provided a coating for reducing the reflection power of bodies in relation to electromagnetic radiation in the form of a layer deposited on the surface of a body, the layer allowing the passage of electromagnetic radiation and having a thickness corresponding approximately to a quarter of the wavelength which the incident electromagnetic radiation assumes within the layer, wherein the layer consists of a material which, in the frequency range of the incident electromagnetic radiation, has an anomalous dispersion characteristic.

In this way, it can be arranged that the effective layer thickness of the coating material, with reference to the wavelength of the incident electromagnetic radiation, remains approximately constant throughout a larger frequency range. In particularly an optimal effect is obtained when, in an advantageous development of the invention, the coating is so chosen that the variation of the real component of the refractive index with frequency is approximately proportional to the reciprocal of the frequency. Coatings in accordance with the invention thus have the advantage of a simple construction together with a larger effective band range. Only a simple layer is required and no gradient in the refractive index is necessary.

For matching to the environment and the substrate to be coated, the quarter wavelength layer can be covered, in a further development of the invention, on the front and/or rear side additionally with an impedance film.

The film thickness of the impedance film is to be chosen such that it is very much lower than the wavelength of the electromagnetic radiation so that interference within the film layer can be neglected.

Finally, for the reflection- free masking of metallic components, which are arranged inside a body of a nonmetallic material, coatings according to the invention can be supplemented by a metallic conducting film which is arranged between the substrate and the quarter wavelength layer.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Fig. 1 shows a section -through a surface zone of a body with a quarter wavelength layer; and Fig. 2 shows a section through a second body.

In the arrangement shown in Fig. 1, the reflectionreducing layer 2 is deposited on a metallic substrate 1 which in this case forms the outer wall of a body. In this embodiment of the invention, there is desired a damping of the reflection in the range of microwave radiation, i.e. a radiation in the wavelength range of a few centimetres.

The coating consists of a material which, in the frequency range of the microwave radiation, has an anomalous or abnormal dispersion characteristic, the real part of this refractive index n for this radiation being roughly proportional to the reciprocal of the frequency f n o( f It is thus ensured that the condition regarding layer thickness is adhered to over as large as possible a frequency range. For matching of the coating to the surrounding air, which has a characteristic impedance of 377 Ohm, the layer is covered at its exterior with an impedance foil 3, of which the surface resistance also amounts to 377 Ohm.

The body shown in Fig. 2 has a jacket 11 of a nonmetallic material. In order to achieve, in this case too, a reflection-free masking of a metallic component 14 arranged inside the body, there is introduced in this embodiment between the coating 12 and the non-metallic jacket 11, a metallic'conducting film 15. Also in this embodiment an impedance film 13 is deposited on the exterior of the coating 12, the impedance film having a purely ohmic surface impendance.

Although in the above-described embodiments, the electromagnetic radiation to be masked lay in the microwave range, the invention is not limited to this spectral range. It can also be applied to other ranges of the electromagnetic spectrum,provided that for the relevant range there can be found materials which have in this spectral range the described anomalous dispersion characteristic. Anomalous dispersion always occurs in frequency ranges in which the material also displays absorption. More precisely : The anomalous dispersion is associated with the absorption band.

Further improvements regarding the increasing of the effective band width as well as the effective range of angle of incidence can be found by the use of impedance films with complex surface impedance, i.e. antenna structures in combination with coatings according to the invention. In this connection, it is also possible, to increase the effectivity, to provide multiple-layers or layer thicknesses deviating from a quarter wavelength. In each of these cases it is important that, due to the suitably-chosen layer material with anomalous dispersion characteristic, the effective layer thickness, measured in terms of wavelengths of the incident electromagnetic radiation in the layer material, remains largely constant over a wider frequency range.

Claims

1. A coating for reducing the reflection power of bodies in relation to electromagnetic radiation in the form of a layer deposited on the surface of a body, the layer allowing the passage of electromagnetic radiation and having a thickness corresponding approximately to a quarter of the wavelength which the incident electromagnetic radiation assumes within the layer, wherein the layer consists of a material which, in the frequency range of the incident electromagnetic radiation, has an anomalous dispersion characteristic.

2. A coating according to claim 1, characterised in that the layer consists of a material the refractive index of which in the frequency range of the incident electromagnetic radiation is approximately proportional to the reciprocal of the frequency of the radiation.

3. A coating according to claim 1 or 2, wherein on the exterior of the layer there is deposited an impedance film

4. A coating according to any of claims 1 to 3, wherein between the substrate and the layer there is arranged an impedance film.

5. A coating according to claim 3 or 4, wherein the impendance film has a purely ohmic surface impedance.

6. A coating acccrding to claim 3 or 4, wherein the impedance film has a complex surface impedance.

7. A coating substantially as herein described with reference to Fig.l or Fig. 2 of the accompanying drawing.

8. A body having a coating as claimed in any preceding claim.