GB2192308A - Energy reflective bodies - Google Patents

Abstract

Radar energy reflective bodies of flying machines such as helicopters include radar absorbent material (RAM) distributed across the effective aperture of reflection of the body in such a manner that reflection of the energy outside of specular reflection tends to be reduced.

Description

SPECIFICATION Improvements in or relating to the reflectivity of metallic bodies This invention relates to the reflectivity of metallic bodiesand is particularly concerned with the reflec tiofl:.hy'target metallic bodies of a plane or cylindrical nature,orapproximationsthereto, of an incident inferrogaSsag radar beam which causes the radar beam. to bare-radiated in a preferred direction in a so-called specularmannerwhich also has sidelobe re-radiation in otherdirections due to the dimensionai effects ofthe reflective surface. It is such sidelobe re-radiation which gives rise to an echo characterised by glints which enables a targetto be detected.

This effect is illustrated in Figures 1 and 2 of the accompanying drawings in which, Figure 1 illustrates a cross-section of a typical helicopter body illuminated bythe general R.F. field of a radar beam and Figure 2 graphically illustrates the distribution of the E field of reflected power across the angular volume normal to the reflecting surface.

Referring to Figure 1, as the structureofthe helicopter body is represented at 1. This is shown as illuminated bythe general R.F. field 2 of a radar beam of uniform intensity so thatthe structure reflects power specularly, as represented in an idealised way, in the direction 3. The reflecting surface may be construed as having an idealised aperture as dimension D.

The illumination of an apertureof dimension Dwill cause itto reflect exactly as though it were the reflector of an antenna of corresponding aperture D illuminated uniformly by its feed withoutataper which is characteristic of such il lu mination, so that the aperture behaves characteristically as a uniformly illuminated antenna aperture having a radiation pattern asshown in Figure 2 being the well known sWxX E field distribution of power across the angularvolume normal to the reflecting surface.Thus illumination which is incidentwill excite the reflection of the surface in an aperture as described above and the re-radiation will have amplitude characteristics in accordance with Figure 2 so that in addition to the radiation in the main direction 3 shown by the main lobe there is also the reflections of reduced amplitude in the so-called sidelobes, whose distribution generally characterises an echo by glints because oftheir uncontrolled and seemingly random occurrence arising from the great plurality of angles and indeed ofthe plurality of portions ofthe aircraft structure.

The present invention seeks to provide an improved reflective body which reflection outside ofspecular refled ioll:is relGvely reduced.

Acco;rdiftgothis invention, an energy reflective body: having. an effective aperture of reflection is previdesiwberein material absorbent of said energy is distributed across said aperture in such mannerthat reflection of said energy outside ofspecular reflection tendsto be reduced.

Expressed in otherterms, according to this invention a reflective body having an effective aperture of reflection is provided wherein material absorbent of said energy is distributed across said aperture in such mannerthat reflectivity within said aperture is reduced so that the reflectivity has a substantially guassian or normal distribution across said aperture.

Normally said energy will be radio energy and said absorbent material will be radar absorbent material (RAM) as known peruse.

Typically, said effective aperture will be substantial ly circular in which case the reflecting surface may be provided with absorbent material in the form of a plurality of lines with a radial nature but with the length of the lines and closeness of the spacings between lines such asto give a near exponential density ofthe application ofthe material from the periphery ofthe aperture.

Again where the aperture is substantially circular, lines of absorbent material may be provided on the reflective surface, which extend circumferentiallywith the spacing between lines reduced towards the periphery ofthe aperture.

Preferably, the proportion of reflected power atthe centre is near 100% and atthe periphery is near zero.

Again where said aperture is substantially circular said absorbent material may be provided in the form of a radial and circumferentially mesh dimensioned to provide an inner exponential density ofthe application ofthe material from the periphery ofthe reflector.

Said absorbent material may also be applied in the form of discrete elements of circular or other shapes, such as diamond, square, triangular, orcrystalform, with a density of application which is exponentially reduced towards the centre of the apertu re. In this last mentioned casethe individual elements should be sufficiently small asto be well belowthe dimension of a wavelength of the shortest wavelength of energy which is to be reflected by said body.

Said absorbent material may also be provided as a continuous layer of material having a thickness which reduces in exponential fashion towards the centre of the aperture.

Whilst not limited thereto the present invention is particularly applicable to flying machines such as helicopters.

The invention will now be described in greater detail with reference to Figures 3 to 6 ofthe accompanying drawings in which Figures 3 and 4 are illustratory diagrams, Figures 5 and 6 illustrate various embodiments of the present invention.

Referring to Figure 3, as illustrated it is characteristic of antenna practice that a feed of an antenna aperture istapered or shaped so as to follow some distribution which is not uniform but is greater at the centre of an aperture by as much as an order greaterthan the illumination at the edges of the said aperture. Such a distribution of illuminating power results in the almost complete suppression ofsidelobes in the radiated energy pattern as indicated by the so-called normal distribution in the illustration in Figure 3.

Whereas such an idealised distribution of the illumination results in a beam of exactly the same character being substantially without any sidelobes, it has been found thatthe same overall resuit may be achieved in a uniformly iliuminated aperture by the control ofthe reflectivity ofthesurfacewhich is illuminated so that the reflection has a distribution acrossthe aperture which follows the same ideal shape, as illustrated in Figure 4. Here the reflection at the edge ofthe aperture is reduced to substantially zero and is at maximum at the centre so thatthe re-radiated pattern from the reflector is appropriately shaped thus reducing the sidelobes from a typical performance such as that shown in Figure2to a performance such as is shown in Figure 3.In other words, the reflectivity within the aperture is reduced so that the reflectivity now has a substantially guassian or normal distribution across the aperture.

Referring to Figure 5, forthe sake of simplicity a circular aperture is illustrated which is divided in six different sections from A to Fwhich for the purposes of illustration are differently treated by the application of radar absorbent material to the reflecting surfaces.

In segment A, the reflecting surface is sprayed with a plualityoflineswith a radial nature but length ofthe lines and closenessofthespacing being such as to give a near exponential densityofthe application of the materialfromthe periphery of the reflector so that the approximate amount of power is reflected in the desired distributed manner.

In segment B, lines of radar absorbent material are sprayed on to the reflected surfaces circumferentially andthespacingthere is reducedtowardsthe periphery sothatthe proportion of reflected power at the centre is near 100% and at the edge is zero.

Whilst not illustrated, a combination ofthe two techniques illustrated in segmentsAand Bcould be made so asto produce a radial and circumferential mesh of radar absorbent material applied to the surfaces ofthe reflector which naturally lends itself to the application of a so-called absorbing paint.

Segment C illustrates the application ofthe paintin dots of circular or other shapes such as diamond, square, triangular or indeed in any other shapes such as those illustrated in Figure 6(a), (b) or (c). These shapes are sufficiently small asto be well below the dimension of a wavelength atthefrequenciescontem- plated. The density of application of the pattern of shapes of radar absorbent material is exponentially reduced towards the centre ofthe reflecting surfaces as shown in section C of 5.

Sectors D, E and F illustrate the application of radar absorbent material which has a discrete thickness of its own as a sheet material. In sector Dthe radar absorbent material is shown applied in the forms of discs distributed exponentially so as to achieve the desired reduction of reflecting capability at the periphery.

Sector E illustrates another method employing thorn shaped wedges cutfrom absorbent material thus providing a dual ofthe lines of paint-like absorber used in Sector A.

Sector F illustrates the radar absorbent material whose thickness is reduced towards the centre of the reflecting surface in order to achieve a similar result.

Whilstin relationto Figure 5, differentsegments of the aperture are described as being differently treated (which is possible) in practice it would be normal to treat the whole ofthe aperture in like manner.The aperture is shown segmented in Figure5 merely to i illustrate some of the techniques which may be applied in carrying outthe invention.

Claims (12)

CLAIMS 1. An energy reflective body having an effective aperture of reflection wherein material absorbent of said energy is distributed across said aperture in such mannerthat reflection of said energy outside of specular reflection teneisto be reduced. 2. A reflective body having an effective aperture of reflection wherein material absorbent of said energy is distributed across said al3ature in such mannerthat reflectivity within said aperture is reduced so thatthe reflectivity has a substantially gaussian or normal distribution across said aperture 3. A reflective body as claimed in clairt or2and wherein said energy is radio energy wand sam absorbent material is radar absorbent material'(tRAM). 4. A reflective body as claimed in anvolithe preceding claims and wherein said efftedive:aperture is substantially circular and the reflectingsurface is provided with absorbent material in the form of a plurality of lines with a radial nature butwfththe length ofthe lines and closeness of the spacings between lines such as to give a near exponential density ofthe application ofthe material from the periphery of the aperture. 5. A reflective body as claimed in any ofthe preceding claims 1 to 3 wherein said effective aperture is substantially circular and wherein lines of absorbent material are provided on the reflective surface, which extend circumferential ly with the spacing between linesreducedtowardsthe periphery ofthe aperture. 6. A reflective body as claimed in any ofthe preceding claims and wherein the proportion of reflected poweratthe centre is near 100% and at the periphery is nearzero. 7. A reflective body as claimed in any ofthe preceding claims 1 to 3 wherein said aperture is substantially circular and said absorbent material: is provided in the form of a radial and circumferentially mesh dimensioned to provide an inner exponential density of the application ofthe material from the periphery ofthe reflector. 8. A reflective body as claimed in any ofthe preceding claims 1 to 3 and wherein said absorbent material is applied in the form of discrete elements of circular or other shapes, such as diamond; square, triangular, or crystalform,with a density, of application which is exponentially reduced towards the centre of the aperture. 9. A reflective body as claimed in,any/Qft-he preceding claims 1 to 3 and wherein.saici absorbent material is provided as a continuQeL$1!a.yerof material having a thickness which reducesin. e2en0nential fashion towards the centre ofth,eaperture. 10. A reflective body as claimed in any of the preceding.claims and compnisirlgl aflying machine. 11. A helicopter havinga reflective surface sub- stantially as herein described with reference to Figures 3to 6 ofthe accompanying drawings. New claims or amendmentsto claims filed on All claims superseded. New or amended claims:

1. A radio wave energy reflective body having an effective aperture of reflection wherein material absorbent of said energy is distributed across said aperture in such manner that reflection of said energy outside of specular reflection tendsto be reduced irrespective ofany pola risation that said incident energy may have.

2. A reflective body as claimed in claim 1 having an effective aperture of reflection wherein material absorbent of said energy is distributed across said aperture in such manner that reflectivity within said aperture is reduced so that the reflectivity has a substantially gaussian or normal distribution across said aperture.

3. A reflective body as claimed in claim 1 or 2 and wherein said absorbent material is radar absorbent material (RAM).

4. A reflective body as claimed in any ofthe preceding claims and wherein said effective aperture is substantiallycircular and the reflecting surface is provided with absorbent material intheform of a plurality oflineswith a radial nature butwith the length ofthe linesand closeness ofthespacings between lines such as to give a near exponential density of the application ofthe material reducing from the periphery ofthe aperture.

5. A reflective body as claimed in any ofthe preceding claims 1 to 3wherein said effective aperture is substantially circularandwherein lines of absorbent material are provided on the reflective surface, which extend circumferential ly with the spacing between lines reduced towards the periphery of the aperture.

6. A reflective body as claimed in any ofthe preceding claims and wherein the proportion of reflected poweratthe centre is near 100% and atthe periphery is nearzero.

7. A reflective body as claimed in any ofthe preceding claims 5 to 3wherein said aperture is substantially circular and said absorbent material is provided in the form of a radial and circumferentially mesh dimensioned to provide an inner exponential density ofthe application ofthe material from the peripheryofthe reflector.

8. A reflective body as claimed in any ofthe preceding claims 1 to 3 and wherein said absorbent material is provided as a continuous layer of material having a thickness which reduces in exponential fashion towards the centre of the aperture.

9. Areflective body as claimed in anyofthe preceding claims 1 to 3 and wherein said absorbent material is applied in the form of discrete elements with a density of application which is exponentially reduced towards the centre of the aperture.

10. A reflectoras claimed in claim 9 and wherein said elements are of circular, diamond, square, triangularorcrystalform shape.

11. A reflective body as claimed in any of the preceding claims and comprising a flying machine.

12. A helicopter having a reflective surface sub- stantiallyas herein described with reference to Figures3to 6 of the accompanying drawings.