Abstract:
A microwave channel multiplexing device comprises several elementary filters connected in parallel with a common output port by way of a transverse waveguide, each filter comprising a lower end fixed to a support common to all the filters and an upper end away from the support, an external peripheral wall, at least one internal cavity defining an internal channel, a signal input connected to the internal cavity and a signal output connected to the transverse waveguide. The multiplexing device furthermore comprises a conducto-radiative device coupled mechanically and thermally to at least two filters, the conducto-radiative device comprising at least one thermally conducting plate, and linked to the external peripheral walls of each of said at least two filters, the plate being fixed at the level of the upper end of the filters. The invention applies to the field of satellite telecommunications and more particularly to signals repetition devices aboard satellites.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to foreign France patent application No. 0904212, filed on Sep. 4, 2009, the disclosure of which is hereby incorporated by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a thermally optimized microwave channel multiplexing device and to a signals repetition device comprising at least one multiplexing device. It applies notably to the field of satellite telecommunications and more particularly to signals repetition devices aboard satellites. 
       BACKGROUND OF THE INVENTION 
       [0003]    As represented for example in  FIG. 1 , a repetition device  1  aboard a satellite  2  generally comprises microwave signal transmit and receive chains intended to convey, amplify and route the signals between a terrestrial station and users located in specific geographical zones. On reception, the signals received by the receive antenna  3  are sent to a receiver  4  by way of a receive filter  5  and then amplified by amplifiers  6  and re-transmitted, after passing through a transmit filter  7 , by a transmit antenna  8 . For technical amplification reasons, before amplification, the bandwidth of the signal received is divided into several sub-bands of reduced width equal to those of the user channels by way of a demultiplexing device  9  conventionally called an IMUX (for Input Multiplexor), and after amplification, the amplified signals are recombined into a single broadband signal. The recombining of the signals into a single output broadband signal is generally carried out by means of an output multiplexing device  10  conventionally called an OMUX (for Output Multiplexor) which comprises several elementary filters  11 , each elementary filter having a predefined central frequency and bandwidth. 
         [0004]    As represented for example in  FIG. 2 , each filter  11  comprises a signal input  13  and a signal output  14 , the filters being connected in parallel with a common output port  15  by way of a transverse waveguide  16 , called a manifold, which links together the outputs  14  of all the channels. Each filter  11  comprises at least one resonant internal cavity or several resonant internal cavities coupled together, for example by way of coupling irises so as to form a channel in which the RF radiofrequency signals travel. 
         [0005]    The various filters  11  of the OMUX are conventionally fixed horizontally and in parallel to one another on a thermally conducting, and generally metallic, common support  12  in such a way that the longitudinal axis Z of each channel is substantially parallel to the plane of the support  12 . The longitudinal walls of each cavity are then in contact with the support  12 , either directly or by way of fixing brackets  7  thereby making it possible, by thermal conduction, to be able to remove the thermal energy dissipated by the cavities of the filter  11  to the support  12 . Conventionally, the thermal flux crosses the support  12  perpendicularly to the filter  11  toward heat pipes disposed on a panel of the satellite. 
         [0006]    In the nominal operating mode corresponding to operation of the filter in the frequency band for which it is dimensioned, this thermal energy is essentially due to losses by skin effects due to a Joule effect in the walls of the filter, these losses being dissipated by conduction from the interior to the exterior of the filter. In an operating mode called “off-band” corresponding to an anomaly in the transmission frequency around a filter of the OMUX, the filter operates outside of the frequency band for which it is dimensioned. In this off-band operating mode, the filter absorbs and dissipates a large part of the energy of the signal. The power dissipated by the filter in the off-band operating mode is of the order of three higher than in the nominal operating mode. In the case where the OMUX is of the thermocompensated type and where each filter comprises a flexible membrane making it possible to control the volume of the cavity and thus to adjust the operating frequency as a function of temperature, this large power dissipation can have a penalizing effect on the flexible membrane since this part is highly resistive and generates strong temperature gradients. 
         [0007]    The channels of the filters of an OMUX are therefore always dimensioned thermally with respect to the off-band mode. 
         [0008]    A horizontal architecture of the OMUX is very suitable for the control of the thermal gradients of the channels, but remains limiting for meeting the new requirements encountered within the framework of space applications since, on the one hand, in the case of an application requiring very large powers, greater than or equal to 500 W, this architecture generates significant thermal flux densities at the interfaces of the off-band channel on the heat pipes of the panel of the satellite, which means there is a risk of these heat pipes drying out; on the other hand, this architecture requires a large installation footprint in the plane of the support, this being penalizing in the case of an arrangement of payloads in a very limited bulk. 
         [0009]    To solve the problem of the flux density constraints on the heat pipes, it is conventional to develop overdimensioned heat pipes, thereby penalizing the arrangement of the payload of the satellite. 
         [0010]    To solve the problem of the OMUX bulk and to optimize its installation, the vertical architecture may be preferred to the horizontal architecture, but it causes much more significant thermal gradients than those obtained with a horizontal architecture. Currently, a known solution for solving this thermal gradient problem consists in increasing the conductive cross section of each channel by increasing the thickness of the walls of each filter. However this requires consequent additional material which significantly increases the mass of the OMUX, this being penalizing, or indeed prohibitive, for space applications. 
         [0011]    The aim of the invention is to produce a microwave channel multiplexing device optimized in mass making it possible to decrease the thermal flux density at the interface of the off-band channel, notably in the case of an application requiring very large powers. 
       SUMMARY OF THE INVENTION 
       [0012]    For this purpose, the invention relates to a microwave channel multiplexing device comprising several elementary filters connected in parallel with a common output port by way of a transverse waveguide, each filter comprising a lower end fixed to a support common to all the filters and an upper end away from the support, an external peripheral wall, at least one internal cavity defining an internal channel, a signal input connected to the internal cavity and a signal output connected to the transverse waveguide, characterized in that it furthermore comprises a conducto-radiative device coupled mechanically and thermally to at least two filters, the conducto-radiative device comprising at least one thermally conducting plate and linked to the external peripheral walls of each of said at least two filters, the plate being fixed at the level of the upper end of the filters. 
         [0013]    Advantageously, the plate comprises recesses cooperating with the external peripheral walls of said at least two filters in such a way that the external peripheral walls of said filters fit within a corresponding recess of the plate. 
         [0014]    Preferably, each filter comprises an external annular collar secured to the external peripheral wall and the plate is mounted and fixed to the collars of said at least two filters. 
         [0015]    According to one embodiment, the upper end of each filter comprises a lid for closing the longitudinal channel and the plate is fixed between the annular collar and the lid of said at least two filters. 
         [0016]    Advantageously, the plate may be equipped with mini-heat pipes comprising a conducting material wall furnished with a circuit for circulating a heat-carrying fluid. 
         [0017]    According to one embodiment, the plate can comprise two distinct walls, respectively lower and upper, and mini-heat pipes fixed between the two walls. 
         [0018]    Advantageously, the plate is made of a thermal conducting material chosen from among metallic materials or composite materials with metallic matrix reinforced with conducting fibers. 
         [0019]    The conducto-radiative device can comprise a single thermally conducting plate, linked and fixed to the external peripheral walls of all the filters. 
         [0020]    Alternatively, the conducto-radiative device can comprise at least two thermally conducting plates linked respectively to the external peripheral walls of a first set of at least two filters and of a second set of at least two filters. In the case where the conducto-radiative device comprises two plates, the two plates may be mutually thermally coupled. 
         [0021]    According to one embodiment, the elementary filters are disposed in parallel on a common support and have their longitudinal axis perpendicular to the common support and the conducto-radiative device is coupled thermally to a single cavity of each channel of the filters. 
         [0022]    According to another embodiment, the elementary filters are disposed in parallel on a common support and have their longitudinal axis parallel to the common support and the conducto-radiative device is coupled thermally to all the cavities of each channel of the filters. 
         [0023]    The invention also relates to a signals repetition device comprising at least one such multiplexing device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    Other features and advantages of the invention will be clearly apparent in the subsequent description given by way of purely illustrative and nonlimiting example, with reference to the appended schematic drawings which represent: 
           [0025]      FIG. 1 : a basic diagram of an exemplary signals repetition device; 
           [0026]      FIG. 2 : a diagram of an exemplary microwave channel multiplexing device with horizontal architecture, according to the prior art; 
           [0027]      FIG. 3 : a diagram, during assembly, of an exemplary thermally optimized microwave channel multiplexing device with vertical architecture, according to the invention; 
           [0028]      FIG. 4   a : a schematic cross-sectional view of an exemplary filter for an OMUX comprising two cavities, according to the invention; 
           [0029]      FIGS. 4   b  and  4   c : two schematic profile views of an exemplary filter for an OMUX, according to the invention; 
           [0030]      FIG. 5 : a detailed diagram from above of an OMUX with vertical architecture furnished with a conducto-radiative plate, according to the invention; 
           [0031]      FIGS. 6   a ,  6   b : two diagrams, during assembly and assembled, of a variant embodiment of the thermally optimized microwave channel multiplexing device with vertical architecture, according to the invention; 
           [0032]      FIGS. 7   a ,  7   b : two schematic detail views in perspective and in transverse section, of a variant embodiment of a conducto-radiative plate, according to the invention; 
           [0033]      FIG. 8 : a diagram of an exemplary thermally optimized microwave channel multiplexing device with horizontal architecture, according to the invention; 
           [0034]      FIG. 9 : a diagram of a variant embodiment of the thermally optimized microwave channel multiplexing device with vertical architecture comprising two conducto-radiative plates, according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    The microwave channel multiplexing device, called an OMUX, represented in the example of  FIG. 3  comprises a set of five filters  11  disposed according to a vertical architecture of the channels. Each filter  11  represented in detailed view in  FIGS. 4   a ,  4   b  and  4   c  comprises, according to a longitudinal axis Z, an external peripheral wall  30 , a lower end  31  positioned in a plinth  32 , an upper end  33  comprising an upper closure lid  34 , the lid  34  possibly being furnished with a flexible and deformable part and with a fixing collar, and at least one internal cavity  35 ,  36  disposed between the two ends  31 ,  33 . In the nonlimiting example of  FIG. 4   a , the filter represented comprises two internal cavities  35 ,  36  superposed along the Z axis. On variants of filter topologies, the number and the geometry of the cavities may be different. It is for example possible to use a filter with three cavities, two of which are aligned along the Z axis and the third coupled on one side, orthogonally to the Z axis. The two internal cavities are coupled together electrically by irises, not represented. The filter  11  comprises an input interface  13  for an RF radiofrequency signal linked to the upper cavity  36  and an output interface  14  for an RF radiofrequency signal connected to the lower cavity  35 . The plinths  32  of each filter  11  of the OMUX are fixed to a common support  12  in such a way that the longitudinal axis of each filter is substantially perpendicular to the support. Each filter operates on a predefined central frequency, differing from one filter to another of the OMUX. According to the type of technology chosen, the filter may be made of a material with a low thermal expansion rate such as Invar, or the filter can optionally be temperature compensated, and/or optionally comprise a dielectric resonator. In the example of  FIGS. 4   b  and  4   c , the filter represented is thermo-compensated, the lid  34  of each filter  11  comprising a temperature compensation device  44  making it possible to automatically modify the volume of the internal cavities  35 ,  36  of the filter  11  as a function of the temperature so as to stabilize the operating frequency of the filter. 
         [0036]    This vertical architecture exhibits the advantage of being more compact from the standpoint of the support  12  than a horizontal architecture but comprises the drawback, however, in the case where the number of cavities of each filter is greater than one, of having only the lower cavity  35  in contact with the support  12  and it is difficult to remove the heat of the parts furthest from the support  12 . Indeed, the thermal flux arising from the energy dissipation in the upper cavity  36  must travel through the lower cavity  35  before being removed in the support  12 . The lower cavity  35  in contact with the support  12  must therefore absorb its own thermal flux and the thermal flux dissipated by the upper cavity  36 , thereby generating heavy constraints from the standpoint of the thermal control of the channel. This vertical architecture therefore exhibits significant thermal gradients which take a considerably augmented magnitude when one of the filters is situated in an off-band operating mode. In this case, the high parts of the off-band channel reach very high temperatures while the channels adjacent to this off-band channel, operating in a nominal mode, remain at much lower temperatures. 
         [0037]    To improve the diffusion of the thermal fluxes and decrease the thermal gradients in the OMUXs in the off-band mode, the invention consists in mechanically and thermally coupling the channels together, preferably at the level of their hottest part, and in increasing the radiative exchanges to the environment outside the OMUX. The exemplary embodiment represented in  FIG. 3  relates to the most critical case of a vertical architecture of the channels, but the invention can also apply to a horizontal architecture in the case of an application requiring very large powers, as represented in the example of  FIG. 8 . 
         [0038]    In the example of  FIG. 3 , the hottest part is the upper part of the channels at the level of the lid  34  closing the upper cavity  36  of each filter  11 . The invention consists in fixing a conducto-radiative device comprising at least one thermally conducting plate  38  on the external peripheral walls  30  of the filters. According to the embodiment represented in  FIG. 3 , the plate  38 , called a conducto-radiative plate, comprises recesses  39  passing through the whole of its thickness, the recesses cooperating with the external peripheral walls of each filter  11  in such a way that the external peripheral walls  30  of each filter  11  fit within a corresponding recess  39  of the plate  38 . Advantageously, an external annular collar  40  is arranged on the external peripheral walls of each filter, for example at the upper end  33  of the channel of each filter  11 , the collars  40  of all the filters being located in one and the same plane substantially parallel to the plane of the support  12 , and the plate  38  is assembled onto and fixed to the collars  40 . The plate  38  then covers all the collars  40  of the filters  11  of the OMUX as represented in  FIG. 5  and is thus in contact with the peripheral walls of each filter. The conducto-radiative plate  38  is made of a thermal conducting material, metallic or composite, such as for example aluminum which exhibits the advantage of low density associated with good thermal conductivity relative to other metallic materials, or a composite material with metallic matrix reinforced with highly conducting fibers. The conducto-radiative plate  38  comprises recesses  39  disposed opposite the channels of each filter  11 , the recesses  39  being of slightly greater dimensions than the diameter of each channel so that the plate  38  fits around the walls  30  of the channels and rests on each collar  40 . The fixing of the conducto-radiative plate  38  onto the collars  40  may be carried out with any fixing means such as for example with screws. The fixing of the lids  34  and of the optional temperature compensation devices  44  is thereafter carried out at the end of each channel, above the conducto-radiative plate  38 . In this configuration, a single cavity  36  of each filter  11 , corresponding to the input cavity of the radiofrequency signals, is linked to the conducto-radiative plate  38  and coupled thermally to this plate  38 . The plate  38  being in contact with the external peripheral walls  30  of all the channels on the upper part, this makes it possible to thermally couple all the channels together on their hottest part and to direct, by thermal conduction in the peripheral walls  30  of the filters, the thermal flux of a channel which operates in off-band mode toward the much colder channels which operate in nominal mode and then act as thermal sinks. The conducto-radiative plate  38  having a larger external surface area than the area occupied by the aggregated upper part of all the channels, also makes it possible to increase the radiative area of the various channels of the OMUX  10  and to increase the share of the overall radiative thermal flux of the OMUX  10  to its environment. To increase the exchanges by conduction and radiation and to diffuse the thermal flux in a homogeneous manner throughout the plate  38 , the conducto-radiative plate  38  can comprise heat pipes  41  brazed or glued onto its exterior surface as represented in  FIGS. 6   a  and  6   b . Alternatively, as represented in  FIGS. 7   a  and  7   b , the conducto-radiative plate  38  can comprise two distinct walls  42 ,  43 , respectively lower and upper, substantially mutually parallel and the heat pipes  41  be fixed between the two walls  42 ,  43  of the plate  38 . The heat pipes  41  are preferably chosen from among micro-heat pipes or mini-heat pipes comprising a conducting material wall furnished with a circuit for circulating a heat-carrying fluid. For example the pair of materials constituting the wall and the fluid of the heat pipe may be chosen from among the pair copper and water, or the pair aluminum and ethanol, or the pair aluminum and methanol. The mini-heat pipes and the micro-heat pipes made with these pairs of materials exhibit the advantage of being very insensitive to gravity and of being able to operate in any position and in particular in the vertical position notably for ground tests. 
         [0039]    In the exemplary embodiment represented in  FIG. 8 , the various filters  11  of the OMUX  10  are fixed horizontally and in parallel with one another on a common support  12  in such a way that the longitudinal axis Z of each filter is substantially parallel to the plane of the support  12 , the support constituting the lower part of the OMUX. A conducto-radiative plate  38  is assembled on and fixed to the longitudinal walls of the filters  11  so as to be substantially parallel to the plane of the support  12 , on the upper part of the OMUX away from the support  12 . The filters of the OMUX are then disposed between the support  12  and the conducto-radiative plate  38 . The conducto-radiative plate  38  comprises recesses which hug the walls of the input orifices  13  and output orifices  14  of each filter  11 . In this configuration, the two cavities  35 ,  36  of each filter  11  are linked to the conducto-radiative plate  38  and are therefore mutually thermally coupled. 
         [0040]    In the preferred embodiment of the invention, the conducto-radiative device comprises a single conducto-radiative plate  38  coupled to all the filters of the OMUX, but notably in the case of an application to an OMUX comprising substantially different length filters as represented in  FIG. 9 , it is also possible to use a conducto-radiative device comprising several conducto-radiative plates coupled respectively to a first set and to a second set of at least two filters of the OMUX. When the OMUX comprises several conducto-radiative plates  38 , the various plates may be mutually thermally coupled or independent. 
         [0041]    Although the invention has been described in conjunction with particular embodiments, it is very obvious that it is in no way limited thereto and that it comprises all the technical equivalents of the means described as well as their combinations if the latter enter within the framework of the invention.