The invention relates to reflectors for electromagnetic radiation, particularly those used in the band for radar, which reflectors of often comprise plates extending mutually at right angles.

Such reflectors are often collapsible, though they are not necessarily so. This provides for relatively easy storage and transport, but often it is desirable to give this option, without destroying the reflective efficiency of the reflectors.

It is accordingly an object of the invention to seek to provide a reflector which may be dismantled.

According to the invention there is provided a reflector coπprising three plates arranged mutually at right angles, the periphery of each plate extending at right angles to the plane of the adjacent plate.

The apparatus may comprise a plurality of plates adapted to be assembled together to provide a reflector the plates each having two boundary sheets spaced apart and connected by a core, at least one plate having part of one sheet and an associated core part removed at or adjacent an edge whereby to receive an adjacent edge part of a second plate so that the assembled first and second plates extend in planes substantially at right angles.

Using the invention it is possible to provide a reflector in which the plate components slot together and take apart.

There may be an adhesive to maintain the plates in a desired spatial relationship.

The core may be a honeycomb.

According to a second aspect of the invention there is provided a reflector assembled from reflector apparatus as hereinbefore defined in the ircmediately preceding paragraphs.

A reflector may be placed assembled or dismantled in packaging.

The reflector may be supported on a support stand which may or may not be within the packaging.

In order to provide that the reflector may assume a desired attitude in use in relation to a substrate on -which it is placed, the support stand, which may be a tripod or jack, may be adjustable.

There may be a level monitoring means such as a spirit level for monitoring the level of the reflector during adjustment of the tripod.

The spirit level may be permanently mounted on the reflector, when the reflector is removable from the packaging prior to use.

Alternatively, the -spirit level may be on the packaging. This may be particularly applicable where the packaging is opaque to incident electromagnetic radiation such as radar beams so that the reflector does not have to be removed from the packaging in order to function. In this case, packaging may be in the form of a box of any suitable material, such as fibre-glass, polyethylene or the like. The box, with the reflector inside it, may also be filled with means to avoid condensation, such as foamed plastics material.

The box may be in two parts, which are mutually rotatable as by swivelling so that the two parts may be swivelled to one position for storage and/or transport, and then swivelled to a second position for use to reflect incident radar energy. In this case one

rt of the reflector may be in one part of the box and the other part may be in the other part of the box, and the two parts may be so arranged that they are rotatable with respect to one another so that in the second position of the box parts, the reflector parts may be in the correct orientation to provide the required reflection.

There may be means such as a latch or lock whereby the two box parts may be locked in one, both or other of the two positions.

Where the reflector is not removed from the box in use, the box may be stored or transported with a plurality of other boxes containing identical or other reflectors. If a plurality of boxes are then transported together for use, for example in a truck or container, it is merely necessary to remove a box at a particular location and set it up for use there, without the trouble of unpacking the reflector.

The effects may be dismantled to form a flat pack.

It will be understood that the reflector can be assembled from the reflector apparatus so that a reflected radar beam gives the impression of having been reflected from a suitable desired object, such as an aircraft, field gun, rocket launcher, battle tank and the like.

A reflector embodying the invention is hereinafter described, by way of example, with reference to the accompanying drawings.

Fig. 1 shows the reflector in perspective view;

Fig. 2 shows a perspective view of part of a sheet of metal used in the reflector of Fig. 1; and

Fig. 3 shows a second reflector according to the invention.

Referring to the drawings, there is shown a reflector 1 made up of reflector apparatus in the form of metal plates 2, 3 which comprise two substantially parallel boundary sheets 4 spaced apart and connected by a reflective core 5. At least one plate 2, 3 is cut away through one sheet 4 and the core 5 but not through the other or facing sheet, to form a blind slot 6 adjacent an edge, so that another plate may be slid into the slot 6 (which is of just sufficient width to receive it), so that it is supported on the uncut plate. In this way, the cut plate is provided with reflective material even at the slot 6, so desired reflective levels are maintained.

The plates may then be readily dismantled, although it will be understood that the plates may be permanently secured together as by using adhesive.

The assembled reflector may be removably or permanently mounted in a box as described herein, on a support means such as a tripod or jack 7.

Suitable metal for the plates 2 and 3 may have the following material performance data:

NS 4 (!_ hard) aluminium Weight 12.5 mm thick = 4.6 Kg/m ² Core shear strength 2.2 MN/m ² (ribbon direction)

1.5 MN/m ² (transverse)

Core shear modulus 440 N/m ² (ribbon direction)

295 N/m ² (transverse)

Core compressive strength 4.0 MN/m ² Board skin modulus 69 GN/m ²

(tensile strength 200 MN/m ² )

(elongation 5% to fracture)

Thermal conductivity K (through board) 1.7 W/m°C

Coefficient of thermal -6 expansion (along board) 23 x 10 /°C

The spirit level may operate in two planes, to provide for levelling in two planes. Where the reflector may be removed from the box, or not, there may be two two-plane spirit levels, one on the reflector itself and one on the outside of the box.

In Fig. 1, the reflector comprises two parts 11 and 12. The first part 11 has a square base plate 13 of βOOrrm sides which is horizon¬ tal, and the two vertical plates 2 each of 300mm height and 600mm length arranged mutually at right angles on the base plate 3 to form an arrangement which is symmetrical about the two vertical plates. The three plates thus divide the volume they occupy into four cubic volumes of 300mm sides.

The -second reflector 12 is of similar shape to the first 11, but with linear dimensions increased by the ratio 1.414 (the square root of 2). A ratio of 4/3, i.e. 1.333, may in practice be a suitable approximation. Furthermore, the vertical plates 2 of the second reflector are arranged at 45° to the vertical plates of the first reflector, The first reflector 11 is mounted on top of the second reflector 12 and the second reflector 12 is mounted on a stand 19 to keep the member 7 separated from the ground.

Each reflector 11 and 12 can be formed from three separate plates which can be packed flat for transit and fitted together at the operation station. The two vertical plates can have a slot cut along half the length of their intersecting line so that they can be fitted together interdigitally, and the lower edges of the two vertical plates can be provided with lugs which fit into corre¬ sponding recesses or holes in the base plate. The two reflectors can therefore be transported to site in a flat pack and easily assembled to the operative condition.

Each reflector has a reflecting lobe extending in each of four mutually perpendicular directions. The twisting of the first refle- tor in relation to the second reflector directs the lobes of the second reflector in different and intermediate directions from the first, thus giving a more uniform reflectivity to the combined reflector.

The reflector shown in Fig. 3 again comprises two portions 21, 22. Each portion comprises two rectangular plates 23, 24 intersecting along a horizontal line 25 joining the mid points of opposite sides, together with a third plate 26 arranged mutually at right angles to the first and second plates, the third plate extending within only two opposite included angled of the four included angles subtended by the intersecting first two plates. As will be seen from Fig. 3, the third plate portions 26 extend on either side of the line of intersection 25 of the first two plates, but not above and below. The first and second plates 23, 24 are inclined to the vertical at 45°, but the third plate 26 is vertical. The two third plate portions each comprises a square with its corner cut off, thus having a boundary 27 which extends at right angles to the plane of the adjacent first or second plate for a distance equal to half the side of the square, the free ends of the two boundaries 27 being joined by an intermediate boundary 28 which is parallel to the diagonal adjoining the ends of the complete sides of the square.

The second reflector 22 is similar to the first 21 and of the same size, but the line of intersection 25 between the first two plates in the first reflector 21 is arranged at 45° to the corresponding line 25' of the second reflector 22. Detachable legs 31 are mounted as extensions of the first and second plates to support the lower reflector 22 away from the ground. The first reflector 21 is mounted on the second reflector 22.

The embodiment of Fig. 3 can be constructed with its three sets of plates packed together flat for transit and subsequently assembled. For example, the two rectangular plates 21, 22 could be slotted along half the line of intersection and fitted together inter- digitally and the third plate portions 26 provided with lugs which fit into corresponding recesses or holes in the first and second plates 21 and 22. The third plate portions 26 keep the first and second plates 21 and 22 at the desired right angle separation. The legs 31 can be detachably connected to the lower parts of the first and second plates 21 and 22 in any suitable manner.

The first plate could be formed in two halves, each half being hinged to the second plate along the line of intersection. In transit, the first plate halves would be hinged flat against the second plate and in operation the first plate halves would be hinged at right angles to the second plate and kept in that pos¬ ition by insertion of the third plate as described above.

The arrangement of the plates as a rectangle or truncated rectangu¬ lar square wall within the angle included by the other two walls has been found to give a greater reflection efficiency for a given tolerance in manufacture and assembly compared to walls which are right angled triangles in the included angle between the other two walls. To give the same reflection efficiency, it has been found that a rectangle or truncated rectangle wall of smaller area (and hence weight) is required compared to a right angled triangular wall.

Using the invention, it is possible to provide a reflector compris¬ ing three plates arranged mutually at right angles, the periphery of each plate extending at right angles to the plane of the adjac¬ ent plate. The boundary may extend continuously until it meets with another boundary so defined, so that the plate included within the angle formed by the other two plates will be rectangular. Alterna¬ tively, a corner of the rectangle can be cut off, so that the two boundaries so defined do not meet, but are instead joined by an

intermediate boundary and this intermediate boundary may be at 45° to the two boundaries so defined. One of the three plates forming the reflector may extend only within one or some included angles subtended by the two other plates.

An even further improvement may comprise providing two such reflec¬ tors one mounted beside the other, a line of intersection between two plates of one reflector being rotated through a non-right angle, preferably 45°, in relation to the corresponding line of the other reflector. The two reflectors may be of equal pattern, but one reflector may have a smaller size than the other, for example the linear dimension of one reflector may be 0.7071 (the square root of ) of the linear dimension of the other reflector.