GB2265758A

GB2265758A - Temperature controlled radar absorbent material

Info

Publication number: GB2265758A
Application number: GB9205909A
Authority: GB
Inventors: Ellen Dean; Kay Dorothy Waters
Original assignee: CEDARCROFT SERVICES Ltd
Current assignee: CEDARCROFT SERVICES Ltd
Priority date: 1992-03-18
Filing date: 1992-03-18
Publication date: 1993-10-06
Anticipated expiration: 2012-03-18
Also published as: GB2265758B; GB9205909D0

Abstract

A temperature controlled, radar absorbent material includes a screen or filter which includes a front radar absorbent surface layer (1) with a reflective surface layer (4) defining an air gap (5) between the two layers. In order to provide temperature control for the radar absorbent material, heat transfer from the exposed surface of the radar absorbent surface layer (1) is facilitated by the flow of air through transpiration airways (6) connecting the air gap (5) with the ambient air. A further air chamber (7) can be provided behind the reflective surface (4) with heating/cooling fins (8) and an air flow allowed to take place through it to provide temperature control. <IMAGE>

Description

THERMALLY CONDUCTIVE, RADAR ABSORBENT MATERIAL This invention relates to a thermally conductive, radar absorbent material.

There is felt to be a need for a material which can be employed to screen equipment from observation in both the thermal and radar wavebands.

Presently known radar absorbing materials do not conduct heat very efficiently and when exposed to solar radiation they achieve temperatures well in excess of their surroundings. However, materials with good thermal conductivity, which could well be employed as thermal screening, also have high reflectivity in the radar wavebands.

According to the present invention, there is provided a thermally conductive, radar absorbent material including a radar absorbent surface layer incorporating transpiration airways and a reflective surface layer behind the radar absorbent surface defining an air gap therebetween, there being means to induce an air flow in said air gap.

The material may be formed into a panel or a plurality of panels which is/are located in front of a structure to be screened.

The material may further comprise an air chamber behind the reflective surface layer, through which chamber another air flow can occur or be caused. Cooling/heating fins can be located in the chamber.

For a better understanding of the inven ahd'-to show how the same may be carried into effect)-.referenc.ewill now be made, by way of example, to the accompanying drawing, in which:- Figure 1 is a front view of a panel of thermally conductive, radar absorbent material, Figure 1A is an enlarged view of part of Figure 1, and Figure 2 is a diagrammatic, cross-sectional side view of the panel shown in Figure 1.

Referring to the drawing, the thermally conductive, radar absorbent material has a front radar absorbent surface layer 1 which can be a structure manufactured of glass reinforced plastics or similar material which is modified so that it has an electrical resistance of 377 ohms per square unit. Both the bottom 2 and top 3 of the structure formed by the material can also be of similar radar absorbent material.

Behind the radar absorbent surface layer 1 is a reflective surface layer 4 which defines an air gap 5 between the two layers, which are parallel to one another.

As described so far, the structure forms a soecalled "Salisbury" screen or filter, which is known from U.S.-A-2,599,944. A Salisbury screen is formed by a layer of resistive material parallel to and separated from a conducting layer by a distance X. The distance X between the two surfaces is arranged to be one quarter of the wavelength of the radar signal or an odd multiple of a quarter wavelength. The air gap appears as an open circuit which is in parallel with the resistive layer. The effective impedance of the complete assembly is therefore that of the resistive layer and if this is chosen to be 377 ohms per square unit (the impedance of free space) then a good match with free space can be obtained and little energy will be reflected.The screen is based on destructive interference with energy reflected from the resistive sheet cancelling energy reflected from the metal plate and is inherently narrowband.

In order to provide a thermally conductive radar absorbent material, the present invention allows heat transfer from the exposed surface of the radar absorbent surface layer 1 by the incorporation in the layer 1 of transpiration airways 6 connecting the air gap 5 with the ambient air in front of the layer 1. When an air flow is induced in these airways, and therefore in the air gaps, transpiration cooling occurs.

In the embodiment illustrated, a further air chamber 7 is provided behind the reflective surface 4 and an air flow is allowed to take place through the air chamber 7, there being cooling/heating fins 8 in the chamber 7 on the rear surface defining the chamber 7.

The air in the air gap 5 can be heated or cooled to meet the surface temperature requirements and such temperature control can be provided by a variety of means. One possibility is to place one surface of a Peltier effect heat pump in the air flow.

If the gap between the layers 1 and 4 is fixed, then the filter effect of the radar absorbent material is constrained to a narrow bandwidth about the centre frequency. The separation distance is calculated to be (1 + m). 1 /4 wheres = threat wavelength and m = 2,4,6,8 etc. However, in a modification, if a frequency sensor is included within the structure to monitor the frequency and thus the wavelength of the incident wave, and if the outer surface layer 1 can be moved so that the width of the air gap 5 is a variable, dependent on the incident wavelength, then effectively broadband attenuation can be achieved.

The radar absorbent surface layer 1 can be of glass reinforced plastics with an additive of, for example, carbon or nickel, or any other resistive material.

The reflective surface layer 4 can be made of a good electrical conductor such as aluminium, steel or chromium, and it is the rear surface 9 of the structure or material which can incorporate the Peltier effect devices.

It will be appreciated that the transpiration airways 6 need not be substantially normal to the surfaces of the layer 1 as shown but could be arranged obliquely to prevent through radar transmission. Air turbulence would also be enhanced thereby in the air gap 5.

In use, the radar absorbent material would be provided as panels mounted on a structure to be screened and the intention is to provide a radar screened, thermal camouflage so that no uniform thermal identification is given to an imager.

With the present invention, it is possible to (apparently) cool an insulating material, even in sunlight. It will be appreciated that the presently-known conflict between the requirements for radar screening and thermal screening, where the latter needs good heat conductivity and the former needs substantially non-conductive material, is addressedr-by the -presentinvention

Claims

CLAIM8: 1. A thermally conductive, radar absorbent material including a radar absorbent surface layer incorporating transpiration airways and a reflective surface layer behind the radar absorbent surface defining an air gap therebetween, there being means to induce an air flow in said air gap.
2. A material according to claim 1 and further comprising an air chamber behind said reflective surface layer, through which chamber another air flow can occur or be caused.
3. A material according to claim 2, wherein cooling/heating fins are located in said air chamber.
4. A material according to claim 1, 2 or 3, wherein it is formed into a panel or a plurality of panels to be located in front of a structure to be screened.
5. A material according to any one of the preceding claims, wherein said radar absorbent surface is a glass reinforced plastics material.
6. A material according to claim 5, wherein said radar absorbent surface has an electrical resistance of 377 ohms per square unit.
7. A material according to any one of the preceding claims, wherein said reflective surface layer is of an electrically conductive material.
8. A material according to any one of the preceding claims, wherein the distance between said radar absorbent surface layer and said reflective surface layer is adjustable.
9. A material according to claim 8, wherein a frequency sensor is included to monitor the frequency and thus the wavelength of the incident wave of the radar.
10. A thermally conductive, radar absorbent material, substantially as hereinbefore described with reference to the accompanying drawing.