This invention relates to electromagnetic reflectors, especially radar reflectors.

Radar reflectors commonly take the form of trihedral or corner reflectors which comprise three radar reflective planar panels that are arranged mutually perpendicular to one another with right corners meeting at a vertex so that radar energy incident on the inner surfaces are focused and redirected back towards the source. It is known to arrange a plurality of such corner reflectors in an assembly of a linear form or with a central axis so as to reflect radar energy incident over a wide angular range.

It is also known to provide radar reflectors that are foldable or compressible so that they can be stored in a compact form ready to be erected when they are needed. In some designs, the reflective panels are rigid and are hinged together so that they can be collapsed, in other designs the reflective panels are formed of flexible sheet material that is secured to an erectible frame

that tensions the panels into the required form when deployed. Springs or other actuator means are provided to assist erection of these reflectors, and where erectible frames have been used these are either mechanical or pneumatic. Typical examples of foldable radar reflectors are shown in U.S. Patent No. 4028701 and U.K. Patents Nos. 2188783-A and 2189079-A.

Disclosure of the Invention

An object of the present invention is to provide an improved radar reflector that can be folded into a compact form and rapidly deployed.

According to the present invention, an electromagnetic reflector comprises an array of pairs of panels of electromagnetic reflecting material arranged about a central axis with the panels of each pair perpendicular to one another and meeting along a linear join, and with the joins of said pairs of panels lying on the sides of a polygon about said central axis.

Preferably, said panels are rigid and each pair is hinged together along said join so that they can

be folded together, whereby the whole array can be collapsed and erected along the line of said central axis.

Further, two or more similar arrays can be stacked together along the same central axis with the adjacent edges of the panels of adjacent arrays hinged together so that the whole assembly can be collapsed and erected as a single unit.

Preferably, each pair of panels is provided with a pair of intermediate panels of electromagnetic reflective material, one towards each end of said join, and each connected between said panels so as to form a corner reflector. Where said pair of panels are hinged together, each intermediate panel is designed so as to fold or collapse between said pair of panels. For example, each intermediate panel may be a rigid panel of two hinged parts with the hinge axis intersecting said join. These intermediate panels enhance the reflective properties of the array by forming corner reflectors and also serve to maintain the perpendicular relationship of said pair of panels for optimum reflecting characteristics.

The adjacent ends of said pairs of panels along the sides of said polygon may abut in the erected state of the array, although if intermediate panels are provided as described above, only one means is needed to define the erected state of the array.

The collapsible and erectible form of the array may be provided with resilient means that is loaded when the array is collapsed and which serves to erect the array when it is deployed. For example, tension springs may be provided between opposite pairs of panels around the sides of said polygon and act on a line through the respective joins so as to pull them together. Alternatively, one or more coiled compression springs may be provided to act in the direction of said central axis.

It will be appreciated that a radar array according to the invention may have as many radar reflecting faces as required around the sides of a polygon, each face comprising a right angled re-entrant channel formed by a pair of panels or a plurality of similar channels arranged alongside

one another so that the overall array takes the form of a bellows with concertina type walls that can collapse into a flat pack form. Such an assembly is relatively simple to construct using rigid panels hinged together, and can be simply adapted to incorporate springs or actuator means to make it rapidly erectible.

Description of the Drawings

Figure 1 is a side elevation of a collapsible radar array according to the invention,

Figure 2 is a plan view of the array of Figure 1,

Figure 3 is a plan view similar to that of Figure 2 but with the array collapsed,

Figure 4 is a vertical section through the array of Figure 1 showing the spring means,

Figure 5 is a vertical section similar to that of Figure 4 but with the array collapsed,

Figures 6, 7 and 8 show plan views of sub- assemblies used in the construction of the array

of Figures 1 to 5.

Best Mode of Carrying Out the Invention The illustrated array is constructed from three different types of sub-assembly each of which can be formed flat and either folded out of a single sheet or preferably made from a series of rigid panels hinged together. The sheet material or panels are composed of radar reflective material and may be metal or metal coated or incorporating a metal mesh or lattice or any other known radar reflective material. The hinges are preferably also formed from radar reflective material.

The section of Figure 4 allows the sub-assemblies to be most readily identified as A, B and C and these are shown flat in Figures 6, 7 and 8, respectively.

Sub-assembly A, used for the top and bottom of the array comprises a central square panel 1, a first trapezium-shaped panel 2 hinged along each side 3 of the square panel 1 so that the short sides 4 diverge outwards at an angle 125 degrees to the side of the square, and a second trapezium-shaped

panel 5 hinged along the outer long side 6 of each panel 2 so that the short sides 7 converge outwards at an angle of 135 degrees to the long side 6 of the panel 2. As shown in Figure 4, a sub-assembly A at the top of the array has the first panels 2 arranged downwards and the second panels 5 directed inwards parallel to the central square panel 1. A similar sub-assembly A is inverted and used for the bottom of the assembly.

Sub-assembly B is used as an intermediate connecting member between the other sub-assemblies A and C, and comprises a pair of hinged panels 8, 9 with the same shape as the panels 2 and 5 of sub-assembly A. As shown in Figure 4, a sub- assembly B is connected to each of the second panels 5 of sub-assembly A with the correspondingly shaped panel 9 in face-to-face contact with panel 5 and with panel 8 angled inwards. In the erected state shown in Figure 4, the pairs of panels 2 and 9 each include a right angle and together form a continuous outer surface.

Sub-assembiy C comprises a central square panel 10

of the same size as panel 1, four outer rectangular panels 11, each hinged along one side 12 of the panel 1, and each of the same width as panel 8, and eight right-angled isosceles panels 13, each hinged by its hypotenuse along a short side 14 of the panel 11 so that pairs of panels 13 meet edge to edge and form a triangular infill in the corners between the rectangular panels 11. As shown in Figure 4, each of the panels 11 is connected in face-to-face contact with the outer face of a panel 9 of the intermediate sub- assemblies B and the isosceles panels 13 are hinged outwards away from the panel 9 so that they are normal to the panel 9.

A second sub-assembly C is connected to the first sub-assembly with the central square panels 10 in face-to-face contact and with the outer rectangular panels 11 angled away from the adjacent panels 11 at a right angle and with the isosceles panels 13 hinged into the same normal plane as the adjacent panels 13 so as to meet edge-to-edge with these edges 14 being hinged together. In this manner, the pairs of hinged panels 13 hold the adjacent panels 11 at right

angles to one another and form a corner reflector with the panels 11 towards a respective end of the panels 11. Thus, pairs of panels 11 around the four sides of the array form a radar reflecting, re-entrant channel 15 meeting at the four corners and bridged by triangular infill panels 13 towards each corner.

A second pair of sub-assemblies C are similarly connected together to form a second re-entrant channel 15' around its four sides, and this is connected to the first pair of sub-assemblies C along adjacent respective edges by intermediate pairs of sub-assemblies B. A panel 9 of each sub- assembly B is connected to the inside face of each panel 11 of sub-assembly C, and the panels δ are connected together face-to-face directed inwardly.

The bottom of the array is completed by the sub- assembly A which is connected to the adjacent sub- assembly C along its four edges by four sub- assemblies B in the same way as at the top.

The whole array is collapsible along its central vertical axis x - x by virtue of the hinged

inter-connections of the panels, as shown in Figure 5. The array is maintained in its erected state by pairs of tension springs 16 connected between the inwardly directed panels 8 across the centre of the array so as to pull these panels inwards and open the channels 15, 15'. As shown in Figures 2 and 3, the springs 16 are arranged in crossed pairs between opposite pairs of panels 8.

As shown in Figure 2, when the array is erected, the short sides 4 of the panels 2, the short sides 7 of the panels 5, and the short sides of panels 9 all engage to lock the array in its fully erected state. However, the isosceles panels 13 also serve this function, and thus it may be preferred to arrange that the isosceles panels 13 serve this function exclusively and lock in their planar vertical state before said short, sides engage, thereby ensuring the optimum reflecting characteristic of the corner reflectors formed by these isosceles panels.

In alternative embodiments of the invention, the array can be extended by simply adding further pairs of sub-assemblies C that each produce an

additional re-entrant channel 15.

Further, whereas the illustrated embodiment is an array with four sides, it will be appreciated that the same design principles can be employed to construct an array with three sides or five or more sides, the resulting structure still being of a collapsible bellows or concertina form.

In other embodiments of the invention, the tension springs 16 could be replaced by coil compression springs, preferably, pack-flat helical springs, that are located within the array to act along the line of the central vertical axis x - x. In the illustrated embodiment a helical spring could be provided between the top assembly A and assembly C, between the two central assemblies C, and between the bottom assemblies A and lower assembly C. Alternatively, if cut outs are formed in the central square panels 10, a single helical spring could be provided over the whole height between the top and bottom assemblies A.