GB2256751A - Reconfigurable reflectors - Google Patents

Abstract

A reconfigurable reflector 1 is formed by a conductive elastic sheet 4 linked to actuators 7 at a number of points. To reduce pillowing the conductive elastic sheet has bending stiffness introduced locally around each point by discrete stiffening elements 19. <IMAGE>

Description

Improvements To Reconfigurable Reflectors This invention relates to reconfigurable reflectors and relates specifically to reconfigurable reflectors used in reconfigurable antennas.

Reconfigurable antennas are antennas able to alter the beam pattern they generated during operation. This is achieved by using a reconfigurable reflector in the antenna.

A reconfigurable antenna of this type is shown in Figure 1, this comprises a reflector 1 and microwave feed 2 mounted on a rigid frame 3. The reflector 1 is formed by a elastic electrically conductive sheet 4 secured at its edges to a rigid rim 5. A grid of rigid members 6 support a plurality of actuators 7 each of which is linked to a single point at the sheet 4. Typically the actuators are electric motors and the links are formed by wires and the conductive sheet is formed of knitted molybdenum fibre with a gold plating.

The beam pattern is controlled by the actuators 7 moving their respective links and so moving their respective points on the sheet 4. As the points are moved, the sheet 4 changes shape and so the beam pattern produced by the microwaves from the feed 2 is altered.

A problem with systems of this type is so called pillowing of the sheet forming the reflector surface.

This effect is shown in Figures 2A and 2B which show a small antenna formed by an elastic conductive sheet 8 secured to a rigid square frame 9. The flexible sheet 8 is moved by four actuators (not shown) linked to points 10 on the sheet 8. The points 10 are in a square grid. In practice move than four actuators would generally be used, but four are sufficient for an example.

The figures show the antenna from the rear with the points 10 positioned by the actuators to form an approximately parabolic reflector. Ideally the sheet 8 should be shaped as shown in Figure 2A into a parabolic reflector.

In practice it is found that the actual shape of the sheet is as shown in Figure 2B. The sheet pillows between the points 10. In a practical system with more actuators pillowing will occur between all of Xhe actuator controlled points across the array.

This pillowing introduces errors into the field across the antenna aperture, limiting the gain and generating high sidelobes. Normally it is preferred to arrange the actuator controlled points on any reconfigurable reflector antenna in a regular grid this will result in the pillowing being a regular periodic error in the antenna surface position and as a result its high sidelobes will be generated as very high sidelobes at specific angles rather than a random scattering. Clearly the limited gain and high sidelobes are a problem.

This invention was intended to overcome this problem, at least in part.

This invenion provides a reconfigurable reflector formed by an elastic conductive sheet having discontinuous bending stiffness.

The provision of bending stiffness in the conductive sheet reduces the amount of pillowing.

Antennas employing the invention will now be described by way of example only with reference to the accompanying diagrammatic examples in which; Figure 2A shows an ideal reconfigurable antenna forming an approximately parabolic reflector, Figure 2B shows a real reconfigurable antenna using an elastic mesh to form an approximately parabolic reflector, Figures 3A to 3F show reconfigurable reflectors according to the invention with progressively larger uniform mesh bending stiffnesses forming approximately parabolic reflectors, Figure 4 shows how bending stiffness can be added uniformly across a mesh surface, Figure 5 shows a mesh for use in a reconfigurable reflector antenna according to the invention having a plurality of discrete stiffener elements, Figures 6A to 6D show reconfigurable reflectors according to the invention using a mesh as shown in Figure 5 with progressively larger stiffener stiffness forming approximately parabolic reflectors, identical parts having the same reference numerals throughout.

It has been realised that pillowing occurs because the conductive sheet, which has always been formed by an elastic mesh, has very little bending stiffness. That is, if a sheet of the mesh is layed out flat in a plane it has very little resistance to being folded about an axis lying in that plane.

Thus one way of reducing pillowing is to increase the bending stiffness of the entire conductive sheet forming the antenna reflector.

The resulting antenna profiles are shown in Figures 3.

Figures 3A to 3F show the profiles of antenna reflectors formed by a conductive elastic sheet 11 having increasing bending stiffness secured to a rigid square frame 12 and moved by four actuators limited to points 13 in a square array.

In Figure 3A the sheet 11 has a low stiffness value, it can be seen in comparison with Figure 2B that the amount of pillowing has been reduced.

The degree of pillowing is further reduced in Figures 3B to 3F as sheets with higher and higher bending stiffnesses are used. By the very high bending stiffness sheet of Figure 3F pillowing has been largely removed.

It can be shown that pillowing would be reduced by the greatest amount and the ideal profile of Figure 2A approached most closely if the sheet has infinite bending stiffness.

One disadvantage of this solution is that as the bending stiffness of the sheet increases so do the actuator loads, requiring stronger actuators and linkages and a stronger supporting frames.

One way of increasing the bending stiffness of the sheet is shown in Figure 4. This comprises a regular mesh of relative stiff wires 14 extending across the antenna and intermeshed with the knitted elastic mesh 15.

Where the knitted elastic mesh 15 is formed by knitted gold plated molybdenum fibre a suitable material for the wires 14 is gold plated steel.

Although relatively simple such an arrangement is difficult to construct and suffers from the problem that as the antenna changes shape the elastic mesh 15 stretches while the stiff wires 14 do not, as a result the mesh 15 must slide relative to the wires 14 which can wear off the gold plating resulting in passive intermodulation due to dissimilar metals in the antenna reflector being in contact.

Another approach to reducing pillowing which avoids these problem is to add selective stiffening around the points at which the actuators are linked to the flexible sheet with the stiffness introduced in directions radially aligned around each attachment point.

Referring to Figure 5 a conductive elastic mesh 16 of knitted gold plated molybdenum fibre is attached to a plurality of actuators at points 17 by link wires. Each actuator is directly attached to a stiffener element 19 on the opposite side of the mesh 16 to the actuator 17, that is the stiffeners 19 are on the illuminated side of the mesh 16 when the antenna reflector is in use.

The stiffener elements 19 are cruciform and planar in shape with four arms 19A of equal length, are formed by being punched out from a phosphor bronze sheet and are P.T.F.E. coated.

The effect of the stiffener elements can be seen in Figure 6A where an antenna reflector is formed by a flexible mesh 16 held in a rigid square frame 20. Four actuators are linked to the mesh 16 at points 17 in a square grid and a stiffener element 19 is provided at each point 17. The degree of pillowing is considerably less than in Figure 2B where an unstiffened mesh is shown.

In Figures 6B to 6D stiffening elements with progressively greater stiffnesses are used with correspondingly lower levels of pillowing.

However, unlike the uniformly stiffened mesh, the closest approach to the ideal parabolic shape is not when the stiffness of the stiffener elements is infinite, because very high stiffener stiffnesses cause distortion of the mesh, but at a particular value of stiffener stiffness which will depend in practice on the other physical parameters of the antenna.

Although there is some relative movement between the mesh and the stiffener elements, there is less relative movement than in the uniform stiffening with wires example.

The stiffener elements are coated with P.T.F.E. to reduce the friction between mesh and stiffener and thus reduce the wear produced by their relative movements. The P.T.F.E.

also insulates the stiffener from the mesh, preventing contact between dissimilar metals, which can produce interference by passive intermodulation. The P.T.F.E.

coating could be replaced by a coating of another plastic or gold plating.

The reflector must be physically elastic and microwave reflecting, although a knitted gold plated molybdenum mesh is one example of such an element any other material meeting these criteria could be used, such as a metal loaded rubber sheet or a rubber sheet bearing a plurality of discrete resonant elements.

The examples shown are arrays of four actuator controlled points on a mesh held in a square frame and are used to form parabolic reflectors. Reflectors of other shapes could of course be formed by appropriate movement of the actuators, other frame shapes could also be used. It would even be possible to use a non rigid frame and alter the frame shape to control the reflector shape in addition to the actuators. Other numbers of actuators could also be used. In general a much larger number of actuators will be necessary in a practical system.

The cruciform stiffener elements shown could be replaced by other shaped elements, examples include maltese crosses, or eight pointed asterisks.

The stiffener elements could interlock or overlap if this was necessary to prevent excessive distortion of the mesh at the discontinuities formed by the stiffener element edges.

Although the stiffener elements described are formed from metal sheet they could be formed in other ways, such as from plastics material or metal wires.

The wire links connecting the actuators to the reflector could be replaced by rigid rods.

Although only a single reflector is shown, reflectors of this type could of course be used in multiple reflector antennas such as Cassegrain antennas.

The stiffener elements described are on the illuminated or front face of the conductive elastic sheet, it would of course be possible to have them on the other, rear, face of the sheet provided an appropriate method of attachment of the sheet to the stiffener was used.

Claims (8)

1. A reconfigurable reflector formed by an elastic conductive sheet having discontinuous bending stiffness.

2. A reconfigurable reflector as claimed in claim 1 where the conductive sheet is formed by a first continuous element having very little bending stiffness and a number of discrete second elements spaced across the sheet and having a much larger bending stiffness than the first continuous element.

3. A reconfigurable reflector as claimed in claim 1 where the first continuous element comprises a knitted conductive mesh.

4. A reconfigurable reflector as claimed in claim 2 or claim 3 where the conductive sheet is linked to actuators at a number of points and a discrete second element is associated with each of these points.

5. A reconfigurable reflector as claimed in claim 4 where the second elements add bending stiffness to the conductive sheet radially about each point.

6. A reconfigurable reflector as claimed in claim 5 where the second elements are cruciform in shape.

7. A reconfigurable reflector as claimed in any of claims 2 to 6 where the first continuous element is between the second discrete elements and the actuators.

8. A reconfigurable reflector substantially as shown in or as described with reference to any of Figures 5 or 6A to 6D of the accompanying drawings.