Patent Publication Number: US-2012026056-A1

Title: Radio antenna with improved decoupling angles

Description:
TECHNICAL FIELD 
     The present invention relates to the field of reflector radio antennae, and concerns in particular an antenna for a spacecraft, such as a telecommunications satellite. 
     STATE OF THE PRIOR ART 
     The antennae of spacecrafts must satisfy specifications notably concerning the reflectivity of their reflectors, but also the mechanical properties of the fastenings of the reflectors to the spacecrafts, which are subject to the vibratory, acoustic and dynamic stresses caused by space launchers. These antennae must also satisfy specifications concerning their thermoelastic properties in orbit. 
     Since the level of acoustic stresses caused by the launchers is very difficult to predict, it is preferable that these antennae should be almost insensitive to acoustic efforts, in order to limit the risks of under-dimensioning or over-dimensioning of the reflectors&#39; fastenings to the spacecrafts. 
       FIGS. 1 and 1   a  represent an example of a radio antenna  10  ( FIG. 1 ) for a telecommunications satellite, operating at frequencies of between 12 GHz and 18 GHz approximately (Ku band), of a known type. 
     Reflector  12  of antenna  10  includes a body of the sandwich type formed from a honeycomb structure on to which are affixed a front skin—commonly called the active skin—and a rear skin, where each of these skins consists of a sheet of carbon fibres sunk in an epoxy resin. 
     Body  14  of reflector  12  is supported by a rigid rear structure  16  of this reflector. The rear structure  16  is, for example, formed from tubular elements positioned in a hexagon shape centred on an axis of the reflector. In the example represented in the figures these tubular elements have, seen in section, a rectangular shape. 
     Rear structure  16  is connected to the rear skin of body  14  by angles  18  ( FIG. 1   a ) capable of providing the mechanical properties of the antenna at launch and insertion into orbit of the satellite fitted with this antenna, and also a thermomechanical decoupling between reflector  12  and rear structure  16  when the satellite is in orbit. In addition, the rear structure  16  is supported by a support arm  19  intended to provide the connection between the antenna  10  and the satellite. 
     The carbon fibres of the sheets of the abovementioned front and rear skins are positioned in the form of triaxial fabrics which are characterised by near-isotropic mechanical properties, and by the presence of through-perforations which are regularly distributed over their surface. 
     These perforations allow the mass of the reflector to be reduced, and communicate with cells in the honeycomb structure, such that this type of reflector is insensitive to vibratory stresses, particularly to acoustic stresses at the launch of the satellite fitted with the antenna  10 . 
     The composite materials used in these antennae generally make them very light, which constitutes an essential advantage in the field of space applications. 
     However, the reflectivity properties of the perforated reflectors of the type described above are not satisfactory at frequencies of approximately between 20 GHz and 40 GHz (Ka band). 
     Solutions have been proposed, which consist, using an antenna of the type described above, in reducing the dimensions of the perforations of the active skin, or even in replacing the perforated active skin by an unperforated skin, but the antennae obtained in this manner have proved to be too sensitive to acoustic stresses. 
     Moreover, at these higher frequencies, the tolerances relative to the profiles of the reflectors are stricter, leading to more severe requirements in terms of manufacturing precision, and of stability over time of the reflectors, typically of the order of 30 μm RMS, which should be compared with 150 μm RMS in the case of satellites operating at the lower frequencies of the Ku band. 
     And the sandwich structures of the type described above, which include perforated skins formed from a single sheet of composite material, do not easily allow the criteria inherent to operation in the Ka band to be satisfied. 
     SUMMARY OF THE INVENTION 
     One aim of the invention is notably to provide a simple, economic and efficient solution to these problems, allowing the abovementioned disadvantages to be avoided. 
     Its goal is notably a radio antenna for space satellite, capable of operating at the frequencies of the Ka band, and satisfying the requirements imposed on this type of antenna, notably in respect of the sensitivity of the antenna to the vibratory stresses caused by the launchers, the precision of manufacture of the profile of the antenna&#39;s reflector and the stability of this profile over time and, generally, the antenna&#39;s thermomechanical properties in orbit. 
     The invention proposes to this end a radio antenna, particularly for a spacecraft, including a reflector and means of support of this reflector, where the reflector includes a body able to reflect radio waves, and a rigid rear structure supported by the means of support and connected to the body by decoupling angles distributed around an axis of the body, and each including a first base attached to the body of the reflector, a flexible metal blade or a second base attached to the rigid rear structure, and a central metal blade connecting the abovementioned first base to said flexible metal blade or to said second base, and able to dampen a transverse component of vibrations of the body. 
     According to the invention, each of said decoupling angles includes, at one at least of its ends, a layer of an elastic material able to dampen at least an axial component of vibrations of the body. 
     In addition, the abovementioned layer of elastic material is interposed between the first base and the body, or between said flexible metal blade or said second base and the rigid rear structure. 
     Each decoupling angle can thus include either a single layer of elastic material positioned at one of the ends of the angle, or two layers of elastic material respectively positioned at both ends of the angle. 
     The layer of elastic material of each angle enables the impact of vibratory stresses, notably acoustic stresses, on the means of support of the antenna&#39;s reflector to be reduced substantially. 
     This enables the level of mechanical properties required for the means of support to be limited, thus making the dimensioning of these means of support easier. 
     In a preferred embodiment of the invention, the reflector&#39;s body includes a solid skin, i.e. one which is not perforated. 
     The great dampening capacity of the angles, as a consequence of their layer of elastic material, indeed makes possible the use of a solid front skin, capable of giving the reflector optimal properties of reflectivity, whilst limiting the risks of under-dimensioning of the reflector&#39;s means of support. 
     In the preferred embodiment of the invention said elastic material has a Young&#39;s modulus of between 0.25 MPa and 1 MPa, a traction resistance of between 0.1 MPa and 0.5 MPa, and a breaking elongation of between 20% and 40%. 
     The layer of elastic material of each angle is thus capable of dampening optimally the vibratory stresses to which the antenna is likely to be subject, particularly when this antenna is fitted to a spacecraft. 
     In the preferred embodiment of the invention, said elastic material is a foam, and includes at least one compound belonging to the group of polyimides. 
     Each angle can also include a sandwich structure including two composite material skins affixed either side of said layer of elastic material. 
     This can, in particular, allow the angles to be attached to the reflector by a method similar to a method habitually used for attaching the angles of the reflectors of the conventional type described above, which may be of substantial economic advantage. 
     As a variant, the elastic material may include an adhesive including an elastomer, silicon or polyurethane compound. 
     When the antenna is fitted to a spacecraft, the elastic material is chosen so as not to deteriorate at space operational temperatures in orbit, and more specifically at temperatures of between −180° C. and +200° C. 
     In the preferred embodiment of the invention, the front skin and the rear skin are made from a composite material including fibres sunk in a hardened resin. 
     These fibres are advantageously carbon fibres positioned so as to optimise the isotropy of the mechanical and thermal properties of these skins. 
     To accomplish this said fibres can, for example, be positioned in the form of two sheets of taffeta fabrics intersecting at angles of more or less 45 degrees, or in the form of three to six sheets of layers of fibres draped symmetrically) (0°, +60°, −60°). 
     These manners of positioning of the fibres also allow the precision and the stability of the profiles of the skins to be improved compared to the skins with a single sheet of conventional reflectors. 
     Generally, the antenna is advantageously configured to operate in a predetermined band of frequencies of the microwave spectrum, where this band of frequencies can in particular be within the Ka band. 
     The use of an unperforated active face, made possible by the invention, is indeed particularly advantageous in the Ka band, as was explained above. 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
       The invention will be better understood, and other details, advantages and characteristics of it will appear, on reading the following description given as a non-restrictive example, and with reference to the appended illustrations, in which: 
         FIG. 1 , which has already been described, is a schematic perspective view of a radio antenna of a known type; 
         FIG. 1   a,  which has already been described, is a larger-scale view of detail Ia of  FIG. 1 ; 
         FIG. 2  is a view similar to that of  FIG. 1   a , of a radio antenna according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
       FIG. 2  represents a part of reflector  20  of a radio antenna according to an embodiment of the invention. 
     This reflector  20  is to a large extent of the same type as reflector  12  of the prior art represented in  FIGS. 1 and 1   a , but reflector  20  includes a body  22  with a solid front skin, and decoupling angles  24  of a new type, in accordance with the invention. 
     In what follows, the terms “front”, “rear” and “side” are used in reference to the antenna&#39;s transmission direction. 
     In a known manner, the shape of body  22  of reflector  20  is roughly that of a paraboloid of revolution around an axis of the reflector. 
     The front skin (not visible in  FIG. 2 ) of body  22  is made from a conventional composite material, of the type including a fabric of structural fibres, for example carbon, sunk in an epoxy or comparable resin. 
     The structural fibres of the front skin are woven so as to provide an optimal isotropy of the mechanical properties of front skin  22 , and such that front skin  22  is solid. To accomplish this the structural fibres are, for example, positioned in the form of two sheets of taffeta fabrics intersecting at angles of 45 degrees, or in the form of three to six sheets of layers of fibres draped symmetrically) (0°, +60°, −60°). This type of structure notably enables the precision and the stability over time of the profile of the front skin to be optimised. 
     Body  22  also includes a rear skin  28  which is made from a composite material comparable to the one, described above, of the front skin, which thus has the same advantages. 
     Reflector  20  includes a rigid rear structure  28  formed of tubular elements  29  of roughly rectangular section, and similar to rear structure  16  of reflector  12  of the prior art. 
     Rear structure  28  is connected to body  22  of the reflector by angles  24 , which each include a central metal blade  30 . An end of blade  30  includes a first base  32  for attachment to rear skin  26  of body  22 , and another end of blade  30  is attached to a flexible metal blade  34  attached to a side face  36  of a tubular element  29  of rear structure  28 . 
     In a known manner, flexible blade  34  and, to a lesser degree, central blade  30 , allow by their elasticity the transverse component, i.e. that perpendicular to the axis of body  22 , of vibrations of this body  22  to be dampened. 
     According to the invention, each angle  24  also includes a layer of elastic material  38 , interposed between base  32  of the angle and rear skin of body  22 , to dampen the axial component of any vibrations of body  22 . 
     In the embodiment represented in  FIG. 2 , elastic material  38  is a polyimide foam chosen such that it does not deteriorate at temperatures of between −180° C. and +200° C., and in order to satisfy the space standards relative to degassing, typically specifying a total mass loss (TML) of less than 1% approximately. 
     Said foam is also chosen to have thermomechanical properties such that the foam affects the thermomechanical properties of reflector  20  as little as possible. In particular, the foam is chosen to have the lowest possible thermoelastic coefficient. 
     In addition, the polyimide foam has a density of between 10 kg/m 3  and 20 kg/m 3 , a traction resistance of between 0.1 MPa and 0.5 MPa, a Young&#39;s modulus of between 0.25 MPa and 1 MPa, and a breaking elongation of between 20% and 40%. The abovementioned physical parameters are chosen in accordance with the level of dampening and mechanical decoupling required between body  22  and rear structure  28  of the reflector. 
     As variant or in addition, each angle  24  can include a layer of elastic material of the type described above, interposed between central blade  30  of angle  24  and rear structure  28  of the reflector. 
     In this case, it is preferable that each angle  24  is attached to a front face  40  of a tubular element  29  of rear structure  28 , for example by a second base similar to the abovementioned first base  32 , and connected to the end of central blade  30  opposed to said end comprising the first base  32 . The layer of foam can thus be interposed between the second base and front face  40  of tubular element  29 , to allow satisfactory dampening of the axial component of vibrations of body  22 . 
     As another variant, the layer of elastic material may be incorporated in a sandwich structure, and may in particular be inserted between two solid skins, for example of a type comparable to the type of the skins of body  22 . With respect to the attachment of angles  24  to rear skin  26  of the reflector, this characteristic notably allows a method of attachment to be used similar to a conventional method of attachment of angles of reflectors of a known type. 
     It is also possible, without going beyond the scope of the invention, to replace the polyimide foam by a flexible adhesive consisting of elastomer or silicon, or again consisting of polyurethane. 
     In the represented embodiment, rear structure  28  is of the tubular type, but the invention is also compatible with rear structures of other types, such as flat, paraboloid or comparable structures, for example of the composite sandwich type. 
     The shape of the front skin of the reflector can, of course, be different from the one described above as an example, without going beyond the scope of the invention.