Patent Publication Number: US-11658424-B2

Title: Deployable reflector for an antenna

Description:
TECHNICAL FIELD 
     The present invention relates to deployable reflectors for antennas. 
     BACKGROUND 
     Deployable structures are widely used in satellites and other space applications. Such structures allow the physical size of an apparatus to be reduced for loading into a payload bay of a launch vehicle. Once in orbit and released from the payload bay, the structure can be deployed into a larger configuration to increase the overall dimensions of the apparatus. For example, deployable structures may be capable of being unfolded, extended or inflated. 
     Deployable antenna reflectors have been developed which comprise a deployable backing structure and a metal mesh. The deployable backing structure forms the metal mesh into a parabolic shape, to act as a reflector in an antenna. The deployable backing structure serves two purposes: firstly, it provides a mechanism to deploy the metal mesh once in orbit; and secondly, it provides a thermo-elastically stable platform for the reflector. Since the metal mesh possesses no inherent stiffness, a complex collection of tensioning elements and cable network structures are thus required to shape the metal mesh in-situ into its desired configuration. 
     Conventional mesh-based deployable reflectors suffer from a number of drawbacks. The cable network only shapes the metal mesh locally, at the points where the cables attach to the mesh, creating pillowing and faceting effects in all other areas of the metal mesh. As a result, the final shape of the reflector may only approximate an ideal paraboloid. Also, cable network structures are complex to design and manufacture, and can increase the risk of entanglement during deployment. 
     The invention is made in this context. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a deployable reflector for an antenna, the deployable reflector comprising a deployable membrane configured to adopt a pre-formed shape in a deployed configuration, and an electrically conductive mesh disposed on a surface of the membrane such that in the deployed configuration, the conductive mesh adopts the shape of the membrane and forms a reflective surface of the reflector wherein the electrically conductive mesh is configured to permit relative lateral movement between the electrically conductive mesh and the membrane during deployment of the reflector. 
     In some embodiments according to the first aspect, the membrane comprises an open-cell woven material. For example, the open-cell woven material may have a triaxial weave structure. In some embodiments, the open-cell woven material comprises a weave of para-aramid fibres embedded in a silicone matrix. 
     In some embodiments according to the first aspect, the electrically conductive mesh is arranged to be disposed on a convex surface of the deployable membrane in the deployed configuration, such that during deployment of the reflector the deployable membrane presses into and deforms the electrically conductive mesh into the pre-formed shape. 
     In some embodiments according to the first aspect, the membrane is formed of material that is transparent to electromagnetic radiation at radio-frequency wavelengths. 
     In some embodiments according to the first aspect, the electrically conductive mesh is configured to permit relative lateral movement between the electrically conductive mesh and the membrane during deployment of the reflector. 
     In some embodiments according to the first aspect, the deployable membrane is a first membrane, and the electrically conductive mesh is disposed between the membrane and a second membrane. 
     In some embodiments according to the first aspect, the deployable reflector comprises a plurality of first connecting members configured to connect the mesh to the membrane. 
     In some embodiments according to the first aspect, each first connecting member comprises a flexible connector in the form of a loop configured to secure one or more fibres of the mesh to the membrane. 
     In some embodiments according to the first aspect, each first connecting member is formed of an elastic material capable of stretching to permit relative lateral movement between the mesh and the membrane. 
     In some embodiments according to the first aspect, a length of the loop in each first connecting member is longer than a minimum distance required to encircle the one or more fibres of the mesh, such that slack in the loop can be taken up during relative lateral movement between the mesh and the membrane. 
     In some embodiments according to the first aspect, the deployable reflector further comprises a plurality of second members passing through the electrically conductive mesh, each one of the plurality of second members being connected to the first and second membranes to maintain a spacing between the first and second membranes during deployment of the reflector. 
     In some embodiments according to the first aspect, the membrane is configured to provide a continuous three-dimensional curved surface for shaping the electrically conductive mesh in the deployed configuration. 
     In some embodiments according to the first aspect, the deployable reflector is configured as a shaped reflector for a contoured-beam antenna, wherein in the deployed configuration the three-dimensional curved surface of the membrane includes a plurality of regions of different curvatures so as to produce a beam having an irregular pattern. 
     According to a second aspect of the present invention, there is provided an unfurlable antenna comprising a deployable reflector according to the first aspect. 
     In some embodiments according to the second aspect, the unfurlable antenna further comprises a backing structure configured to deploy the deployable reflector. 
     According to a third aspect of the present invention, there is provided a satellite comprising an unfurlable antenna according to the second aspect. 
     According to a fourth aspect of the present invention, there is provided a method of manufacturing a deployable reflector for an antenna, the method comprising pre-forming a deployable membrane on a mould, such that in a deployed configuration the membrane adopts the shape of the mould, and disposing an electrically conductive mesh on the self-supporting membrane such that in the deployed configuration, the conductive mesh adopts the shape of the membrane and forms a reflective surface of the reflector, wherein the electrically conductive mesh is configured to permit relative lateral movement between the electrically conductive mesh and the membrane during deployment of the reflector. 
     In some embodiments according to the fourth aspect, pre-forming the deployable membrane comprises laying an open-cell woven material on the mould, applying a gel to the open-cell woven material, before or after laying the open-cell woven material on the mould, and curing the gel to form a solid matrix around the open-cell woven material, whilst the membrane remains on the mould. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG.  1    is a cross-sectional view illustrating a layer structure of a deployable reflector for an antenna, according to an embodiment of the present invention; 
         FIG.  2    illustrates a triaxial weave structure of a membrane layer in the deployable reflector of  FIG.  1   , according to an embodiment of the present invention; 
         FIG.  3    illustrates a reflector antenna comprising a deployable reflector, according to an embodiment of the present invention; 
         FIG.  4    illustrates a contoured-beam antenna comprising a deployable shaped reflector, according to an embodiment of the present invention; 
         FIG.  5    illustrates a satellite comprising the contoured-beam antenna of  FIG.  4   , according to an embodiment of the present invention; 
         FIG.  6    is a flowchart showing a method of manufacturing a deployable reflector for an antenna, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, only certain exemplary embodiments of the present invention have been shown and described, simply by way of illustration. As those skilled in the art would realise, the described embodiments may be modified in various different ways, all without departing from the scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
     Referring now to  FIG.  1   , a cross-sectional view of a layer structure of a deployable reflector  100  for an antenna is illustrated, according to an embodiment of the present invention. The deployable reflector  100  comprises a first membrane  101 , a second membrane  103 , and an electrically conductive mesh  102 . The electrically conductive mesh  102  is disposed between the first membrane  101  and the second membrane  103 . 
     In the present embodiment the first membrane  101  is a deployable membrane. ‘Deployable’ means that the first membrane  101  can be collapsed into a compact stowed configuration, and subsequently unfolded into a deployed configuration. Antennas in which the reflector itself can be unfolded during deployment are commonly referred to as ‘unfurlable’ antennas. Accordingly, in embodiments of the present invention, the primary reflector of an unfurlable antenna may comprise the first membrane  101 . The deployable membrane may also be referred to as an ‘unfurlable’ membrane. The first membrane  101  is configured to adopt a pre-formed shape in the deployed configuration. For example, to form a reflector for a parabolic antenna, the first membrane  101  can be pre-formed on a parabolic mould with the correct geometric properties. In the deployed configuration, the first membrane  101  may be capable of maintaining the reflector  100  in the desired three-dimensional shape by shaping the electrically conductive mesh  102 . 
     The electrically conductive mesh  102  is disposed on a surface of the first membrane  101  such that in the deployed configuration, the conductive mesh  102  adopts the shape of the membrane  101  and forms a reflective surface of the reflector  100 . The electrically conductive mesh  102  may be configured to permit relative lateral movement between the electrically conductive mesh  102  and the first and/or second membrane  101 ,  103  during deployment of the reflector. For example, the electrically conductive mesh  102  may be free to slide over the surface of the first and/or second membrane  101 ,  103  to permit relative lateral movement between the electrically conductive mesh  102  and said first and/or second membrane  101 ,  103 . Alternatively, the surface of the electrically conductive mesh  102  may be connected to the adjacent surface of the first and/or second membrane  101 ,  103  by one or more adhesive or mechanical joints that permit relative lateral movement of the two surfaces during deployment. Such joints may also be referred to as linkages, connectors or tethers. Since the electrically conductive mesh  102  acts as the reflective surface and gives the reflector  100  the necessary reflective properties, it is not necessary for the first and second membranes  101 ,  103  to be formed of reflective material. 
     By permitting relative lateral movement, the deployable reflector can be made less susceptible to damage during deployment by reducing stresses in the mesh  102  and/or the first and second membranes  101 ,  103 . Also, by permitting relative lateral movement between the mesh  102  and the first and/or second membranes  101 ,  103 , the antenna can accommodate different rates of thermal expansion between the differing materials of the mesh  102  and the first and second membranes  101 ,  103  when the antenna is subjected to thermal cycling once deployed in space. 
     In the present embodiment the electrically conductive mesh  102  is arranged to be disposed on a convex surface of the deployable first membrane  101  in the deployed configuration, such that during deployment of the reflector  100  the first membrane  101  presses into and deforms the electrically conductive mesh  102  into the pre-formed shape. In this way, the electrically conductive mesh  102  can be placed under tension by the first membrane  101  in the deployed configuration, and tensile strain in the electrically conductive mesh  102  can assist in holding the mesh  102  against the convex surface of the first membrane  101  in the deployed configuration so that the mesh  102  adopts the same shape as the deployed first membrane  101 . 
     In embodiments in which the electrically conductive mesh  102  is disposed on the convex side of the first  101  membrane, electromagnetic radiation received or transmitted by the antenna must pass through the first membrane  101  before being reflected by the electrically conductive mesh  102 . In such embodiments the first membrane  101  can be formed of material that is RF transparent to electromagnetic radiation at radio-frequency (RF) wavelengths. Here, ‘RF transparent’ means that the first membrane  101  exhibits negligible losses and negligible additional reflections at RF wavelengths, such that the presence of the first membrane  101  has little or no impact on the performance of the antenna. By forming the first membrane  101  from a low RF loss material, the reflecting efficiency inherent to the conductive mesh  102  can be maintained. 
     In some embodiments, the electrically conductive mesh  102  and the deployable membrane  101 ,  103  may be arranged such that in use, incident electromagnetic radiation is reflected by the mesh  102  before reaching the membrane  101 ,  103 . For example, in some embodiments the electrically conductive mesh  102  may be disposed on the concave surface of the deployable membrane  101 ,  103 , such that incident electromagnetic radiation is reflected by the electrically conductive mesh  102  without passing through the deployable membrane  101 ,  103 . In such embodiments the performance of the antenna may not be dependent on the RF properties of the deployable membrane  101 ,  103 , and accordingly the deployable membrane  101 ,  103  may be formed from RF reflective material or from RF transparent material. 
     The second membrane  103  may also be a deployable membrane. In some embodiments the first and second membranes  101 ,  103  may be formed from the same material as each other and may have the same, or similar, thicknesses. For example, the first and/or second membrane  101 ,  103  may be formed from an open cell woven material. In other embodiments the first and second membranes  101 ,  103  may be formed from different materials to each other, and/or may have substantially different thicknesses. Providing a second membrane  103  can offer more accurate control over the shape of the reflector  100  in the deployed configuration. In some embodiments the second membrane  103  may be omitted. 
     The deployable reflector  100  of the present embodiment comprises a plurality of first connecting members  106 ,  107  connecting the mesh  102  to the first membrane  101  or the second membrane  103 . In some embodiments a first connecting member  106 ,  107  may connect the mesh  102  to both the first membrane  101  and the second membrane  103 . The first connecting members  106 ,  107  can be formed as adhesive or mechanical joints, as described above. Each first connecting member  106 ,  107  connects part of the mesh  102  to a point on the surface of the first or second membranes  101 ,  103 , whilst permitting a certain amount of lateral movement between the mesh  102  and the first and second membranes  101 ,  103 . 
     In the present embodiment each first connecting member  106 ,  107  comprises a flexible connector in the form of a loop, which is wrapped around one or more fibres of the mesh  102  and secures the one or more fibres to the first and/or second membrane  101 ,  103 . For example, both ends of the loop may be embedded in a matrix material of the first or second membrane  101 ,  103  as shown in  FIG.  1   , or may pass through the membrane  101 ,  103  and be secured on an opposite side of the membrane  101 ,  103 . In some embodiments, relative lateral movement may be permitted by making each loop  106 ,  107  from an elastic material capable of stretching to permit the mesh  102  to slide across the surface of the first or second membrane  101 ,  103 . In some embodiments, relative lateral movement may be permitted by making each loop  106 ,  107  longer than a minimum distance required to encircle the one or more fibres of the mesh  102 , such that a certain amount of slack is provided in the loop  106 ,  107  which can be taken up during lateral movement of the mesh  102  relative to the first or second membrane  101 ,  103 . 
     In the present embodiment the deployable reflector  100  further comprises a plurality of second connecting members  104 ,  105  passing through the electrically conductive mesh  102 . Each one of the plurality of second connecting members  104 ,  105  is connected to the first and second membranes  101 ,  103  so as to maintain a spacing between the first and second membranes  101 ,  103  during deployment of the reflector  100 . For example, the second connecting members  104 ,  105  may be connected to the first and/or second membrane  101 ,  103  by embedding the ends of the second connecting members  104 ,  105  in the matrix of the membrane  101 ,  103  when forming the membrane  101 ,  103 . Alternatively, recesses for receiving the second connecting members  104 ,  105  may be formed in a surface of one of the membranes  101 ,  103  during or after forming the membrane  101 ,  103 , and the second connecting members  104 ,  105  may subsequently be secured in the recesses using suitable adhesive. As a further alternative, the second connecting members  104 ,  105  may be connected to the first and/or second membrane by suitable mechanical means. For example, a thread may be formed on an end of each second connecting member  104 ,  105 , which may pass through a hole in one of the membranes  101 ,  103  to allow the second connecting member  104 ,  105  to be secured by a nut screwed on to the thread. 
     The second connecting members  104 ,  105  tie the first and second membranes  101 ,  103  together to prevent the first and second membranes  101 ,  103  from moving apart from one another as the reflector  100  is deployed. The second connecting members  104 ,  105  help to prevent faceting and pillowing in the electrically conductive mesh  102  by ensuring that the mesh  102  remains tightly held between the first and second membranes  101 ,  103 . In embodiments in which a second membrane  103  is omitted, the second connecting members  104 ,  105  may be omitted. Furthermore, in embodiments in which the second membrane  103  is omitted and first connecting members  106 ,  107  are provided, the first connecting members  106 ,  107  may only connect the mesh  102  to the first membrane  101 . 
     Referring now to  FIG.  2   , a triaxial weave structure of a membrane layer in the deployable reflector of  FIG.  1    is illustrated, according to an embodiment of the present invention. The structure shown in  FIG.  2    may be used for one or both of the first and second membranes  101 ,  103  in  FIG.  1   . In the present embodiment the membrane layer  101 ,  103  comprises an open-cell woven material which has a triaxial weave structure. The woven material comprises a plurality of woven fibres  201  orientated along three principal axes. The fibres  201  may be embedded in a matrix material  202 . In the present embodiment, a triaxial weave of para-aramid fibres  201  embedded in a silicone matrix  202  is used. For space applications, a space-grade silicone may be used for the matrix  202 . 
     Triaxial weave materials are capable of being formed into any arbitrary three-dimensional shape, and so can accurately conform to the contours of a mould on which the first or second membrane  101 ,  103  is formed. However, due to the open-cell structure, triaxial weave materials generally have poor reflective properties, particularly at RF wavelengths. Accordingly, in some embodiments of the present invention a triaxial weave material can be combined with an electrically conductive mesh to provide a reflector which exhibits accurate shape control in the deployed configuration together with low RF losses. 
     In other embodiments the membrane may be formed from another suitable material other than triaxial weave, for example a knitted fabric. The membrane may be formed from material that exhibits high drapability. Here, ‘drapability’ is used in the conventional sense to refer to the ability of a material to deform under its own weight. 
     A material with high drapability can be capable of forming complex three-dimensional curved shapes without creasing. The drapability of a material may be quantified using the drape coefficient (DC), wherein a material with high drapability has a low DC, indicating that the material can easily deform over complex curves without creasing. The maximum acceptable DC for the material from which the membrane is formed may vary between embodiments, according to the particular pre-formed shape that the membrane is required to adopt. For example, in embodiments of the invention the membrane may comprise a material with sufficiently high drapability to be able to deform into the desired pre-formed shape without creasing. 
     Referring now to  FIG.  3   , a reflector antenna  300  comprising a deployable reflector  310  is illustrated, according to an embodiment of the present invention. The reflector antenna  300  comprises the deployable reflector  310 , an antenna feed  320 , and a secondary reflector  330 . In this embodiment, the deployable reflector  310  forms the primary reflector of the antenna  300 . In other embodiments the secondary reflector  330  may be omitted, such that the primary reflector  310  directs the beam directly into the antenna feed  320 . 
     In the present embodiment, the membrane  101  of the deployable reflector  310  is configured to provide a continuous three-dimensional curved surface for supporting the electrically conductive mesh  102  in the deployed configuration. 
     By ‘continuous’, it is meant that all areas of the electrically conductive mesh  102  are supported by part of the membrane  101 . Using a continuous membrane  101  can provide the most accurate control over the shape of the reflector  310  in the deployed configuration. 
     However, in other embodiments some parts of the electrically conductive mesh  102  may not be directly supported by an underlying membrane  101 . For example, in some embodiments the membrane  101  may include one or more apertures for reducing the overall mass of the antenna  300 , with the conductive mesh  102  spanning the aperture to provide a continuous reflective surface. Such an arrangement may be used in applications where it is necessary to reduce the mass of the antenna as far as is possible, and in which a decrease in performance due to the loss of accurate shape control in the region of the aperture is an acceptable compromise. 
     The antenna  300  may also comprise a backing structure  340  for automatically deploying the reflector  310 . For example, the backing structure  340  may comprise an elastic frame  341  anchored to the reflector  310  at certain points via cables  342 . The elastic frame  341  can be folded into a compact stowed configuration, along with the deployable reflector  310 . When a restraining force on the backing structure  340  is released, the elastic frame  341  automatically unfolds and pulls the deployable reflector  310  into the deployed configuration. Backing structures for deploying and supporting reflectors are known in the art, and a detailed description will not be provided here so as not to obscure the present inventive concept. 
     Conventional backing structures are highly complex, as the structure is required to hold the reflector in the desired shape once deployed. In contrast, in embodiments of the present invention a deployable reflector comprises a membrane which automatically adopts the desired shape of the reflector. In this way, the shape of the reflector  310  in the deployed configuration can be controlled by the self-supporting membrane  101 ,  103 , instead of being controlled by the backing structure  340 . 
     In embodiments of the present invention, the backing structure  340  is therefore not required to accurately control the shape of the reflector  310  once deployed, and only needs to apply sufficient force to unfold the reflector  310 . Accordingly, the complexity of the backing structure can be significantly reduced in comparison to conventional designs, reducing the overall size and mass of the antenna assembly comprising the reflector  310  and the backing structure  340 . It will also be appreciated that since the membrane automatically adopts the pre-formed shape in the deployed configuration, the electrically conductive mesh layer  102  does not suffer from pillowing or faceting, in contrast to conventional deployable mesh-based antennas in which the shape of the mesh is controlled by a complex cable network structure. 
     Furthermore, although a backing structure  340  for deploying the reflector  310  is illustrated in  FIG.  3   , in some embodiments the backing structure  340  may be omitted. For example, in some embodiments the elastic strain energy stored in the stowed reflector  310  may be sufficient to cause the reflector to automatically unfold and deploy, particularly in zero-gravity environments. Furthermore, in some embodiments the first membrane  101 , and/or the second membrane  103  if present, may be capable of supporting the reflector  100  in the desired pre-formed shape in the deployed configuration, and hence may be referred to as a ‘self-supporting’ membrane. However, if the reflector  310  is to remain in the stowed configuration for a relatively long time period, matrix creep may reduce the total elastic energy stored in the self-supporting membrane  101 ,  103 . Accordingly, a backing structure  340  may be provided to be certain that sufficient force will be available to deploy the reflector  310 . 
     Referring now to  FIG.  4   , a contoured-beam antenna  400  comprising a deployable shaped reflector  410  is illustrated, according to an embodiment of the present invention. Like the reflector antenna  300  of  FIG.  3   , the contoured-beam antenna  400  also comprises an antenna feed  420  and a secondary reflector  430 . In the present embodiment the shaped reflector  410  is substantially parabolic, but includes a plurality of regions of different curvatures  411  so as to produce a beam having an irregular pattern. The regions of different curvature  411  can be configured to produce a beam with any desired shape, for example to allow the reflector to be focused on specific countries and continents.  FIG.  5    illustrates a satellite  500  comprising the contoured-beam antenna  400 , in which a downlink beam  510  with an irregular pattern is produced. 
     Previously, conventional shaped reflectors have only been achieved in solid dish architectures using complex manufacturing methods. In the embodiment shown in  FIG.  4   , a shaped reflector is achieved by combining a deployable membrane  101 ,  103  with an electrically conductive mesh  102  as shown in  FIG.  1   . The arbitrarily shaped pre-formed membrane  101 ,  103  distorts the metal mesh  102  into the same shape as the pre-formed membrane  101 ,  103  in the deployed configuration, thus achieving a shaped deployable reflector  410 . For example, a triaxial weave material as shown in  FIG.  2    may be used to form an arbitrarily shaped pre-formed membrane. Triaxial weave is particularly suitable for use in deployable shaped reflectors such as the one illustrated in  FIG.  4   , since triaxial weave is capable of being formed into complex shapes. 
     Referring now to  FIG.  6   , a flowchart showing a method of manufacturing a deployable reflector for an antenna is illustrated, according to an embodiment of the present invention. The method involves pre-forming a deployable membrane on a mould, followed by disposing an electrically conductive mesh on the membrane. Consequently, in the deployed configuration, the conductive mesh will adopt the shape of the membrane and can act as the reflective surface in an antenna. 
     First, in step S 601  an open-cell woven material is laid on the mould. For example, a triaxial weave may be used, as described above with reference to  FIG.  2   . Next, in step S 602  a gel is applied to the open-cell woven material, for forming the matrix. Depending on the embodiment, the gel may be applied before or after laying the open-cell woven material on the mould. Therefore in some embodiments, step S 602  may be performed before step S 601 . Then, in step S 603  the gel is cured to form a solid matrix around the open-cell woven material, whilst the membrane remains on the mould. In this way, the membrane is pre-formed so as to automatically adopt the same shape as the mould in the deployed configuration. Then, in step S 604  the electrically conductive mesh is then disposed on the membrane in such a way as to permit relative lateral movement between the electrically conductive mesh and the membrane during deployment of the reflector, as described above. 
     Whilst certain embodiments of the invention have been described herein with reference to the drawings, it will be understood that many variations and modifications will be possible without departing from the scope of the invention as defined in the accompanying claims.