WO1996000992A1

WO1996000992A1 - Energy absorbing arrangements

Info

Publication number: WO1996000992A1
Application number: PCT/GB1995/001516
Authority: WO
Inventors: Steven Brian Morris
Original assignee: Gec-Marconi Limited
Priority date: 1994-06-28
Filing date: 1995-06-28
Publication date: 1996-01-11
Also published as: GB9412957D0; GB2293273A; GB9513173D0; AU2799895A

Abstract

An energy absorbing arrangement suitable for use in anechoic chambers comprises an assembly of elongate planar strips arranged in two sets of parallel strips in a crossing relationship. The strips carry energy absorbing material and in one preferred embodiment consist of cardboard coated with carbon black. The assembly may be easily manufactured as a large number of strips may be produced in one cutting step. Inexpensive material may also be used and by using an arrangement of this type complex impedance profiles may be produced.

Description

Energy Absorbing Arrangements

This invention relates to energy absorbing arrangements and more particularly, but

not exclusively, to arrangements for absorbing microwave radiation.

It is desirable for many applications to prevent the reflection of microwave radiation

at a surface. For example, the radiation pattern of an antenna may be determined by placing

it within an anechoic chamber. Radar absorbing material covers the floor, walls and ceiling of the chamber to absorb incident radiation which might otherwise be reflected from these surfaces and interfere with the pattern being monitored.

One known form of radar absorbing material suitable for use in anechoic chambers comprises foam material which is impregnated with a dielectrically lossy material, such as carbon black. The front surface of the foam material at which radiation is incident is configured as an array of tapering projections, which may be pyramidal in shape for example. The tapered projections present a gradual change in impedance to incident

radiation in a direction through the material from its front to its rear. This minimises reflection at the boundaries between the absorbing material and surrounding air. Resistive heating occurs and the energy is thus absorbed by the material.

The foam may be cut to the required profile from a block of material having dimensions corresponding to the maximum sizes of the finished array. However, the material in the interstitial regions between the projecting portions is lost and thus this method is relatively expensive in material terms. More complicated cutting profiles may be used to reduce wastage of material during manufacture but these techniques tend to be labour intensive and require more sophisticated machinery for implementation.

The present invention arose from attempt to provide an energy absorbing arrangement

having improved characteristics and ease of fabrication compared to known arrangements. It

is particularly applicable for the absorption of microwave energy and for use in anechoic chambers but it is envisaged that the invention may also be suitable for the absorption of other forms of energy and for other uses.

According to a first aspect of the invention, there is provided an energy absorbing arrangement comprising a plurality of planar elongate members and energy absorbing material arranged to present to incident energy an electrical impedance with changes along a direction from the front to the rear of the arrangement, the material being distributed in a plurality of discrete planes extending substantially parallel to the direction along which the impedance changes and the material being incorporated within the material of the members and/or laid down on the members, the members having slots therein for locating them relative to one another in a crossing relationship.

The front of the arrangement is that part on which incident radiation first impinges.

By employing the invention, a required impedance profile may be designed and

fabricated relatively easily. The absorbing material is distributed in planes and it is thus necessary only to control its configuration in two dimensions rather than the three previously required. The change in impedance may therefore also be accurately controlled. An arrangement i.i accordance with the invention may be arranged to present a very complex

change in impedance profile to incident radiation by suitably configuring the absorbing material in each of the planes in which it is distributed. For example, the impedance may be

arranged to vary in an exponential manner through the depth of the arrangement. Tapered

projections having such a shape would be difficult to produce using the previously known

technique described above. In other arrangements the impedance profile may be such that it

would be impossible to implement it, or an equivalent to it, using conventional approaches.

For example, the absorbing material could be distributed in patterns of vertical and horizontal strips, which are not necessarily joined, in the discrete planes, being more closely spaced from one another towards the rear of the arrangement.

An arrangement in accordance with the invention may also be more iightweight, because of its structure, than previously known arrangements of the foam type having an array of solid projecting elements. This makes it easy to assemble and fit.

Furthermore, by employing the invention, materials may be used which are cheaper than those presently in use whilst still enabling the arrangement to have similar performance characteristics. In one advantageous embodiment, the arrangement consists of cardboard

coated with a resistive ink. A great advantage of the invention is that a kit of the component parts can be flat packed for shipping and easily assembled where required.

In one embodiment in accordance with the invention the energy absorbing material is arranged in planes in a configuration which corresponds to sections through a notional solid element. The solid element may be pyramidal in configuration and, for example, the absorbing material equivalent to one such element may be distributed in a set of five adjacent

planes parallel to one another and another set of five adjacent parallel planes orthogonal to

the first set. In such a case, the absorbing material in the central plane of each set is

extensive over a greater depth than that distributed in other planes. By arranging the energy

absorbing material in such a fashion, the changing impedance which incident energy meets is similar to that which would be offered by the equivalent solid foam element.

Another embodiment of the invention includes absorbing material distributed in planes corresponding to sections through a notional solid projecting element having faces at

45° to columns and rows of an array of a plurality of such elements.

In may be advantageous to arrange the absorbing material in planes such that the equivalent solid element would have exponentially curved sides.

The absorbing material may be arranged to represent planes through an array of solid elements. In such an arrangement, each discrete plane of the material contributes to a

number of elements of the array.

Absorbing material may be impregnated in the material of the elongate members or may be combined with it during the manufacture of the members. For example, the members may be of cardboard and the absorbing material may be carbon black. When the cardboard constituents are in a pulp state prior to manufacture in sheet form, carbon black may be added to the pulp such that the resulting sheet is inherently energy absorbing. Alternatively, carbon black could be impregnated in cardboard after it has been manufactured into sheet form. It may be possible to control the impregnation of absorbing material in the members

so that the required change in impedance is produced by having a non-uniform distribution. However, it may be advantageous in some arrangements that the members themselves are

configured in an appropriate profile so that when they are assembled the resultant

arrangement exhibits the necessary impedance change through its depth. This technique

gives a more controllable distribution of the absorbing material and is more easily carried out

because the distribution is governed by the shape of the members which can be accurately produced by cutting, for example. It is only necessary to ensure that the absorbing material is uniformly distributed over the area of the members.

The absorbing material may be included as curved planar members which are arranged concentrically with one another, for example. In another embodiment, conductive ink, or some other suitable material, is printed on a continuous sheet to obtain a desired pattern and the sheet cut to a plurality of lengths which are then formed into cylinders. The cylinders may then be arranged parallel to one another in a surrounding frame. They can be of different heights and/or diameters and arranged in a non-ordered fashion. They may be included in the cells defined by the crossing elongate members.

In another embodiment of the invention, absorbing material is carried by a plurality

of planar substrates. The material may be deposited, for example, by a printing method in which a conductive ink is laid down over the surface of a substrate. The absorbing material

may be deposited over substantially the whole area of the substrate which is then cut to shape to give the required change in impedance when assembled with the other substrates. In another embodiment of the invention, the absorbing material is deposited on only parts of a substrate, the deposition pattern being chosen to give the desired profile of the absorbing material in the completed arrangement. In this arrangement, a plurality of substrates may be employed which are uniform in shape, for example, they may all be rectangular whilst

carrying different patterns of absorbing material. The front edges of the substrates in the

completed arrangement could then be arranged to lie substantially in the same plane. If the

substrates are located sufficiently close together or are of a rigid robust material, this arrangement could be suitable for covering the floor of an anechoic chamber if it is strong enough to support the weight of a person. This may be possible even when the substrate material is in itself not particularly strong, for example, if it is cardboard.

The absorbing material may be deposited on one or both surfaces of the substrates.

In some arrangements in accordance with the invention, it may be preferable to deposit a second energy absorbing material onto the first. For example, the second material may be magnetically lossy, such as a ferrite. The magnetically lossy material could be arranged along the edge of a substrate remote from its front face, such a configuration being particularly advantageous where low frequency microwave radiation is to be prevented from being reflected.

It may be desirable in some circumstances to have a planar member which includes

absorbing material and which also acts as a substrate for other absorbing material

subsequently deposited thereon.

In one embodiment, the elongate members are arranged in two orthogonal sets. In another preferred alternative, the members are arranged as three sets of planes which

intersect at 60°. This configuration offers better angle of incidence performance at

perpendicular polarisation for some frequencies than the orthogonal structure. In

perpendicular polarisation the incident and reflected rays lie in the plane of incidence, as

shown in Figure 18a where E is the electric vector perpendicular to the plane of incidence and parallel to the plane of reflection, and H is the magnetic vector and lies in the plane of

incidence. Figure 18b illustrates parallel polarisation in which the incident and reflected rays

lie in the plane of incidence. In the orthogonal structure, there is a minimum in the reflection

coefficient when the spacing between the planes, adjusted for angle of incidence, is equal to

a multiple of a half wavelength. In another alternative the substrates are arranged as four sets of planes, the sets being arranged as two pairs of two orthogonal sets, one pair being rotated through 45° with respect to the other pair. The planes of each set may be regularly spaced apart and the spacing may be the same for each set. In an advantageous arrangement, however, the distance between the planes of the sets in one pair is d and that between those of the other pair is

In another embodiment, the substrates may be arranged to define the sides of open boxes or cylinders, being arranged concentrically within one another and not crossing. However, this latter arrangement is structurally not as strong as the preferred arrangement and is less easy to fabricate. Also, the preferred arrangement gives greater flexibility in

suitably profiling the energy absorbing material to give a desired change in impedance.

A plurality of tubes having a printed resistive pattern thereon could be used to reduce the difficulties with angle of incidence performance as this also presents a plurality of planes to incident radiation.

Preferably, where the energy absorbing material is included with two sets of

members, the first set has slots within which members of the second set are engaged to locate

them relative to one another and where more than two sets are required, the number and

locations of the slots are selected accordingly. Slots may be included in only one of the sets of members or may be included in all the sets, in which case they may be mutually interengagable.

In one embodiment of the invention, the discrete planes in which the absorbing

material is distributed are arranged as to a regular matrix and a planar energy absorber, such as a Jauman absorber, is at the rear of the arrangement. Thus, where two sets of elongate members are arranged orthogonal to one another, the members of each set are spaced from adjacent ones of that set by the same distance to give an array of open cells having a square cross-section. Where the substrates are arranged as three sets at 60° , and those of each set are spaced at regular intervals, this also leads to a regular matrix but its repeat pattern is more complex. The absorber at the rear of the arrangement reduces the magnitude of the peak of reflection which otherwise appears at a particular frequency. However, in other

arrangements, it may be advantageous to have a non-uniform spacing of substrates to obtain

a good angle of incidence performance.

The edge of areas of energy absorbing material may be a random undulating or sawtooth pattern to prevent or reduce any diffraction effects which might otherwise occur if the edges were straight. The variation in the edge shape from that required for a particular ideal impedance profile may be chosen to be sufficiently small that the performance of the

arrangement is not significantly compromised.

Advantageously, the regions between adjacent planes of absorbing material are left

open to give a lightweight structure. However, in some applications it might be desirable to fill them with some other material which is transparent to the energy which is to be absorbed

to obtain additional rigidity and structural strength.

Anechoic chamber materials of known types are arranged to present a smoothly

varying impedance with distance into the absorber which gives the absorber a low frequency limit. In an advantageous embodiment of the invention, the energy absorbing material is arranged so as to present an electrical impedance which changes abruptly in a direction from the front to the rear of the arrangement.

By employing this feature, low frequency performance may be enhanced if a resonance resulting in destructive interference can be introduced. The abrupt change or changes in impedance can be chosen to give reflections with a particular amplitude and

phase. These reflections, including that from the rear surface, can be arranged to give nulls at particular frequencies and enhancement of low frequency performance. The high frequency performance will be affected but this may be minimized by having the front part of

the absorber as a gradual transition.

In one embodiment of the invention, an electrical element is carried by a substrate which includes energy absorbing material. Electrical elements, or parts of elements, may be printed on the substrate. In one embodiment, a printed conductive track on one substrate defines an inductive loop to facilitate coupling to the magnetic field.

In one particularly advantageous embodiment of the invention, the energy absorbing

arrangement includes a cover over the front of the arrangement which is transparent to

energy which is to be absorbed. This gives a smooth surface which is desirable where, say, it is wished to test an antenna within an anechoic chamber under dust free conditions. Also, if the cover is placed on the floor, it provides a walkway for personnel carrying out the tests. The cover can have location means which co-operates with spaces between the members to position it.

According to a second aspect of this invention, an energy absorbing arrangement comprises energy absorbing material arranged to present to incident energy an electrical impedance which changes along a direction from the front to the rear of the arrangement, the material being distributed in a plurality of discrete planes extending substantially parallel to the direction along which the impedance changes and there being non-regular spacings between the planes.

According to a feature of this invention, a kit comprises components of an

arrangement in accordance with the invention and includes elongate members having slots

therein and carrying or incorporating absorbing material.

According to a feature of this invention, a method for manufacturing an energy absorbing arrangement in accordance with the invention includes the steps of: providing a plurality of slotted elongate strips; incorporating energy absorbing material with the strips;

and assembling the strips together to form the arrangement, the absorbing material being in a

distribution such that the impedance changes in a direction from the front to the back of the

arrangement.

The energy absorbing material may be arranged in the desired distribution by

impregnating or printing it on the sheet material or the strips in a particular pattern. Alternatively it may be uniformly laid down over the sheet or strips and the configuration of the strips defines the required distribution.

Preferably, the plurality of strips is cut from a planar sheet in a single operation. If the strips are suitably arranged on the planar sheet, they can be cut with only small wastage of material. The planar sheet may be of a material which incorporates radar absorbing material or may be impregnated with such material prior to the cutting operation. Alternatively, or in addition, such material may be deposited on the sheet, for example by printing, prior to it being cut. In another embodiment of the invention, the energy absorbing material is laid down on the strips after they are cut, but this approach tends to be less

convenient.

It is preferred that the strips include slots orthogonal to their length. When the strips are assembled together, the slots are arranged to co-operate with strips in a crossing direction

so as to hold the assembly in place. Some ways in which the invention may be performed are now described by way of example with reference to the accompanying drawings, in which:

Figure 1 is an explanatory diagram;

Figures 2a and 2b schematically illustrate steps in manufacturing an assembly in accordance with the invention;

Figure 3 schematically shows in perspective view an element of the assembly constructed from the elements shown in Figures 2a and 2b;

Figure 4 is a plan view of the partial assembly of Figure 3;

Figure 5 is an explanatory diagram of another radar absorbing array configuration;

Figures 6a and 6b schematically illustrate steps in manufacturing another assembly in accordance with the invention;

Figure 7 is a schematic exploded view of part of a further assembly in accordance

with the invention;

Figures 8 and 9 schematically illustrate components of arrangements in accordance

with the invention; Figure 10 illustrates in perspective and partially broken away a further assembly in

accordance with the invention;

Figure 11 is a plan view of another arrangement in accordance with the invention;

Figure 12 is part of an arrangement in accordance with the invention;

Figure 13 schematically illustrates strips included in another assembly in accordance

with the invention;

Figure 14 shows an arrangement in accordance with the invention and Figures 15a and 15b illustrated strips used to fabricate it;

Figure 16 illustrates an arrangement using four sets of planes arranged in two orthogonal pairs; and

Figures 17a and 17b illustrate patterns used where abrupt changes in electrical

impedance are required.

A known type of radar absorbing arrangement suitable for use in anechoic chambers

employs a foam material which is impregnated with carbon black or some other dielectrically

lossy material. A block 1 of impregnated foam is profiled, as shown in Figure 1, to present a plurality of projections 2 to radar frequency energy incident from the direction shown by the arrow. The projections 2 are pyramidal spikes in shape having triangular faces 3, the base of each of which is parallel to one of the end faces 4 and 5 of the block 1 as shown. The array of tapering projections 2 results in incident radiation impinging on a gradually changing

impedance as it travels from the front surface of the block 1 to its rear face. The radiation

reacts resistively with the lossy material to produce heating, dissipating the energy and

minimising reflections.

In order to understand how the elements of an arrangement in accordance with the invention are configured, the corner element 6 of the block 1 is considered. A plurality of sections may be taken at intervals through the element 6 in planes parallel to the plane of the end face 5. Figure 1 illustrates five sections taken at equal intervals through the element 6.

The central section A has a triangular upper portion adjoining a lower rectangular portion, the height of the section A being the same as the depth of the element 6 plus the depth of its base. The sections immediately adjacent to section A, shown as sections B, include sloping portions 7 corresponding to the sloped sides of the pyramidal element and a flat top side 8. The two outermost sections C include sloping sides 9 and a top side 10, which is longer than the side 8 of section B. Similar sections may be taken parallel to the face 4 in an orthogonal direction to sections A, B and C. As the pyramids of the array are regular, these sections are of the same configuration as the corresponding sections already described . These sections

parallel to the face 4 are illustrated as A', B' and C.

In an assembly in accordance with the invention, two sets of substantially planar elongate members having shapes corresponding to a plurality of the sections shown in Figure 1 are arranged in an orthogonally crossing relationship to produce an open matrix defining an array of energy absorbing elements which is equivalent in performance to the array illustrated in Figure 1.

In this embodiment of the invention, the planar elongate members are fabricated from

cardboard which is initially in the form of a large rectangular sheet as shown at 11 in Figure

2a. The sheet 11 is firstly impregnated or coated with carbon black or some other suitably

lossy material. It is then placed on a flat surface and cut into the required elongate members.

This is carried out in a single action by a tool pressed downwardly through the sheet 11 , the lines of the cut being shown. A plurality of strips is produced, shown as A, B and C each of which corresponds in configuration to a plurality of sections designated by the same letters in Figure 1. Each strip also includes slots 12 extending inwardly from the base portion, there being five slots per element and the centre one being longer and the end ones being shorter. The assembly in accordance with the invention requires a second set of elongate strips corresponding to the sections A', B' and C as shown in Figure 1. These are produced by taking another sheet 13 of cardboard, also impregnated or coated with carbon black, as shown in Figure 2b. In a single operation, a cutting tool is pushed downwardly through the sheet 13 to define five strips. These strips also include slots 14 therein which in this case extend from the profiled edge inwardly. A number of sets of elongate members are fabricated in this manner to produce sufficient for the complete arrangement. Of course,

more than five strips may be produced from a single sheet and each strip may contribute to

more than three elements.

The elongate members produced by the operations described with reference to Figures 2a and 2b are then assembled as shown in Figure 3, the strips shown in Figure 2a being uppermost and those in Figure 2b, shown shaded in Figure 3 for clarity, lowermost. The slots in each set of strips are arranged to co-operate with one another to locate the members and fix them in position. Figure 4 illustrates the assembly in plan view from which it can be seen that the strips are arranged in a regular matrix to define cell. An energy

absorber 15, such as a Jauman absorber, is positioned at the rear of the assembly.

In another arrangement in accordance with the invention, the strips are fabricated

from a material such as glass fibre mixed with resin and carbon black so that the material is inherently lossy. In this case the sheets of material need only be cut and then assembled.

Figure 5 illustrates in perspective view another known radar absorbing material in which an impregnated foam material 16 is formed into a plurality of projections 17. The projections 17 are each defined by four diamond shaped faces 18 which are orientated at 45 with respect to end faces 19 and 20 and to the columns and rows in which the projections are distributed. As in Figure 1, sections through the corner element are considered, these being taken parallel to the end faces 19 and 20 and shown as A,A', B,B', C and C. In this case each section has a sloping top sides and the angle at which these meet and their length is the same for each section. The sections are configured as triangular portions on rectangular bases, the depth of the base being greater for the central sections A and A'.

Figures 6a and 6b illustrate the cuts required to produce two sets of elongate strips,

each strip representing sections corresponding to solid elements of the type illustrated in

Figure 5. When the two sets of strips have been cut, they are assembled to form an open matrix in a similar manner to that described above with reference to Figure 3. In the arrangements illustrated in Figures 2, 3, 4 and 6, the elongate members themselves comprise a material which is lossy over its whole surface area. By suitably

profiling the edges of the strips, the required gradation in impedance is obtained.

In another embodiment of the invention, elongate members are used which are of a

non-lossy material but bear a printed pattern of, for example, a conductive ink which gives

the required conductivity at the lossy part of the surface. In this particular embodiment, each

strip is rectangular and the required profiling is obtained by selecting an appropriate pattern for the material as it is laid down on the substrate. Different patterns are applied to different strips depending on the change in impedance required. Figure 7 illustrates in an exploded

perspective view an arrangement in which two sets of elongate members 21 and 22 are of a non-conductive material and have a rectangular configuration. The slots are omitted for clarity. Prior to assembly, the strips are printed with conductive ink 23 in triangular patterns corresponding to the sections shown in Figure 5.

The strips are arranged in a crossing relationship to define elements of equivalent performance and configuration to those solid elements shown in Figure 5. Each such element is defined by parts of five strips from each set. Thus, as illustrated, the upper four members 23, together with next adjacent one from that set (not shown) combine with the

lower five strips 21 to define an end element.

In this arrangement, the completed assembly is sufficiently rigid to support the weight of a person and also the front edges of the strips are all in substantially the same plane. Hence this may be used for flooring in anechoic chamber. If desired a cover 24 may be included over the front face of the assembly and resting on it, the cover being transparent

to the energy to be absorbed to prevent reflections from it. Projections 24a locate the cover

in the cells defined by the strips. The cover may be opaque to visible radiation and is

particularly suitable for applications in which a chamber must be kept completely dust free,

for example, space applications.

In another embodiment of the invention, strips similar to those in the arrangement of Figure 7 are employed. One of the strips is illustrated in Figure 8 and is rectangular bearing a patterned layer 25 of absorbing material on its surface. The slots have been omitted in the drawing for the sake of clarity. In this embodiment another layer 26 is printed on top of the previously laid down layer 25. The second layer 26 includes ferrite material and is located along the edge of the strip which abuts a rear conductive plate of the arrangement when assembled. The ferrite material is suitable for the absorption of low frequency energy.

Figure 9 shows another elongate member used in the arrangement which incorporates printed patterns 27 of conductive ink to provide the energy absorbing material. The member 26 also includes printed circuit elements 28 on its surface. These elements may be resistive

or inductive.

With reference to Figure 10, another arrangement, shown partly broken away, includes conductive loops 29 printed on surfaces of the arrangement and included within

cells 30.

Figure 11 illustrates in plan view another arrangement in accordance with the invention in which two sets of elongate members are arranged orthogonally to one another to define planar depositions of energy absorbing material. In this case, the members are not

regularly spaced but are closer together to define more rapidly changing impedance. In this

case, the sections are chosen to present areas of substantially exponential change in

impedance to incident energy.

Another advantage of this approach is that the non-regular spacing of the edges of energy absorbing material reduces any diffraction effects which might arise with a regular array and it is not necessary to include a rear absorber perpendicular to the planes of

absorbing material.

Figure 12 illustrates one strip 31 of an arrangement similar to that shown in Figure 7. The edge 32 of the absorbing material 33 nearest the front of the assembled arrangement generally follows a straight line, which is shown dotted. However, the edge includes random deviations from the straight line. Other strips of the arrangement also carry absorbing material having front edges which vary in a non-related manner. Thus the arrangement reduces reflections which might arise from coherent diffraction from regularly spaced planes of absorbing material. The edges are shown to consist of a series of short straight lines. However, they could vary in a smoothly curving fashion.

Figure 13 schematically illustrates some of a plurality of strips 34 which are

assembled in an orthogonally crossing relationship to fabricate another arrangement in

accordance with the invention. Each strip is a planar sheet of substantially non-energy absorbing material on which

is printed conductive ink 35 in a pattern of horizontal and vertical lines (as shown). The top

of each strip as illustrated corresponds to the front face of the arrangement when assembled

and the bottom to the rear face. The pattern of lines becomes denser in a direction towards

the bottom of the strips.

This arrangement is not of the type in which planes of absorbing material represent planes through notional solid elements. The designer is able to explore possibilities other than those which are equivalent to the tapered projections already known and is not

restrained to producing configurations which are capable of being reproduced in solid forms.

In another embodiment (not shown) the planar sheet includes apertures therethrough. It may, for example, be formed by taking a sheet on which is printed a continuous, uniform layer of conductive ink and then making apertures in it to provide a lattice of conductive material. In this arrangement, the conductive material includes joins to connect parts of the pattern together. If it is wished to produce a lattice similar to the patterns illustrated in

Figure 13 in which some conductive parts are isolated from other parts of the pattern, a

framework of non-absorbing material may be used.

The pattern need not be of straight, vertical and horizontal distributions of material. The material could be curved in the plane, for example as a spiral, and other orientations could be used instead.

With reference to Figure 14 which is a plan view, an energy absorbing arrangement includes absorbing material laid down on three sets 36, 37 and 38 of substrates. The

substrates of each set are parallel to one another and equally spaced. The sets 36, 37 and 38 are arranged at 60° to each other in a crossing relationship. Figures 15a and 15b illustrate

strips used to fabricate the array of Figure 14. To manufacture the array, the same number of

the type shown in Figure 15a, those of Figure 15b with the slots at the front and those of

Figure 15b with the slots at the back are required.

With reference to Figure 16 which again is a plan view, four sets 39, 40, 41 and 42 of substrates carrying absorbing material are employed. The substrates of each set are arranged parallel to each other and are equally spaced. Two sets 39 and 40 are arranged orthogonally to each other and for both sets, the spacing between the substrates is distance d. The other pair of sets, 41 and 42 are also arranged orthogonally to each other and in this case the spacing between their substrates is Via². The pair of sets 39 and 40 is orientated at 45° to the other pair 41 and 42.

For some arrangements, it is desirable to incorporate abrupt changes in impedance.. This may be done by printing patterns of absorbing material 43, 44 onto substrates such as those illustrated in Figures 17a and 17b. In these, the front part of the pattern 43a and 44b exhibits a smooth charge in impedance to minimize the reduction in performance of higher

frequencies.

Claims

1. An energy absorbing arrangement comprising a plurality of planar elongate members and energy absorbing material arranged to present to incident energy an electrical impedance which changes along a direction from the front to the rear of the arrangement, the material being distributed in a plurality of discrete planes extending substantially parallel to the direction along which the impedance changes and the material being incorporated within the material of the members and/or laid down on the members, the members having slots therein

for locating them relative to one another in a crossing relationship.

2. An arrangement as claimed in claim 1 wherein the energy absorbing material is laid down non-uniformly on the members.

3. An arrangement as claimed in claim 2 wherein the members are substantially identical in

shape.

4. An arrangement as claimed in claim 1 wherein the energy absorbing material is

substantially uniformly distributed on the members which are of different configurations.

5. An arrangement as claimed in any preceding claim wherein a second energy absorbing material is deposited on the first mentioned absorbing material.

SUBSTITUTE SHEET (RULE

26) 6. An arrangement as claimed in claim 5 wherein the second energy absorbing material is magnetically lossy and deposited on surfaces at the rear of the arrangement.

7. An arrangement as claimed in any preceding claim wherein the members are arranged in

three sets of parallel planes, the sets being arranged at 60° to one another.

8. An arrangement as claimed in any one of claims 1 to 6 wherein the members are arranged

in four sets of substantially parallel planes, the sets being arranged as two pairs of two

orthogonal sets, one pair being rotated through 4 ? with respect to the other pair.

9. An arrangement as claimed in claim 8 wherein the distance between parallel planes of the sets in one pair is d and that between those of the other pair is y/2cr .

10. An arrangement as claimed in any preceding claim wherein at least some of the discrete planes are regularly spaced from one another in at least one direction and including a planar energy absorber at the rear of the arrangement and perpendicular to the said plurality of discrete planes.

11. An arrangement as claimed in any one of claims 1 to 9 wherein the discrete planes are

non-regularly spaced from one another in substantially all directions.

12. An arrangement as claimed in any preceding claim and including cylindrical tubes located between at least some of the discrete planes.

13. An arrangement as claimed in claim 12 and wherein absorbing material is incorporated within the material of the tubes and/or laid down on it.

14. An arrangement as claimed in any preceding claim wherein the energy absorbing

material is arranged in planes in a configuration which corresponds to sections through a notional solid element.

15. An arrangement as claimed in claim 14 wherein the energy absorbing material is arranged in planes in a configuration which corresponds to sections through an array of notional solid elements.

16. An arrangement as claimed in any preceding claim wherein the energy absorbing material is arranged so as to present an electrical impedance which changes abruptly in a direction from the rear of the arrangement.

17. A arrangement as claimed in claim 16 wherein the material is arranged such that there is a gradual change in electrical impedance near the front, and an abrupt change near the rear,

of the arrangement.

18. An arrangement as claimed in any preceding claim wherein the front edge of absorbing material deviates from an ideal configuration.

19. An arrangement as claimed in any preceding claim wherein an electrical component is carried on a substrate surface.

20. An arrangement as claimed in any preceding claim and including a cover over the front

of the arrangement which is substantially transparent to energy which is to be absorbed.

21. An arrangement as claimed in claim 20 wherein the cover includes location means

extending into cells defined by the members to position it.

22. An energy absorbing arrangement comprising energy absorbing material arranged to

present to incident energy an electrical impedence which changes along a direction from . ,e front to the rear of the arrangement, the material being distributed in a plurality of discrete planes extending substantially parallel to the direction along which the impedence changes and there being non-regular spacings between the planes.

23. An arrangement as claimed in claim 23 wherein the absorbing material is laid down on and/or incorporated in the material of substantially cylindrical tubes arranged in a non- ordered fashion.

24. An arrangement as claimed in claim 22 or 23 wherein the absorbing material is laid

down on and/or incorporated in the material of planar elongate members.

25. A kit comprising components of an arrangement as claimed in any preceding claim and

including elongate planar members having slots therein and carrying or incorporating

absorbing material.

26. A method for fabricating an energy absorbing arrangement in accordance with any

preceding claim including the steps of: providing a plurality of slotted elongate strips;

incorporating energy absorbing material with the strips; and assembling the strips together to

form the arrangement, the absorbing material being in a distribution such that the impedance changes in a direction from the front to the back of the arrangement.

27. A method as claimed in claim 26 wherein the plurality of strips are cut from a planar

sheet in a single operation.

28. A method as claimed in claim 27 wherein the strips include slots orthogonal to their length and the slots are used to position the strips during assembly.