Abstract:
A space-borne antenna system includes a number of panels being moveable to each other and having a gap in between them when the panels are arranged in an operation condition. The system also includes an RF distribution network for providing transmit signals to the number of panels and combining received signals from the number of panels. The system further includes a set of choke flange assemblies that allow a contactless inter-panel signal transmission across a dedicated gap. A respective choke flange assembly is arranged on the far side of a radiating surface of the dedicated adjacent panels. The system also includes an RF seal assembly for suppressing a signal coupling of signals radiated from the number of panels to the set of choke flange assemblies by sealing the gap.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 to European patent application number 13 004 944.8-1812, filed Oct. 16, 2013, the entire disclosure of which is herein expressly incorporated by reference. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Exemplary embodiments of the invention relate to a space-borne antenna system, comprising a number or panels being moveable to each other and having a gap in between them when the panels are arranged in an operation condition. The antenna system further comprises a RF distribution network for providing transmit signals to the number of panels and combining received signals from the number of panels and a set of choke flange assemblies which allow a contactless inter-panel signal transmission across a dedicated gap, wherein a respective choke flange assembly is arranged on the far side of a radiating surface of the dedicated adjacent panels. 
     Antenna systems for space applications are deployed in space while they are folded for transportation. After having deployed the antenna system it is necessary to couple adjacent panels of the antenna system for signal transmission. 
     An antenna system of the type above is, for example, the Sentinel-1 SAR Antenna Subsystem (SAS) for the Sentinel-1 mission. This antenna system is a deployable planar active phased array antenna working in C-band (5.405 GHz) with a frequency bandwidth of 100 MHz. The antenna has an overall size of 12.3 m×0.84 m and is formed by a central panel mounted on top of the spacecraft and two antenna side wings at the two adjacent sides of the spacecraft. The central panel is equipped with two SAS tiles, whereas the two panels of each side wing carry three SAS tiles each. This leads to an overall number of 14 identical tiles: 6 (SAS right wing)+2 (SAS central panel)+6 (SAS left wing). Each SAS tile possesses all the functions needed to allow for beam shaping and steering. 
     Generally, the SAS encompasses the following principal functionalities: signal radiation and reception (WG-Assy); distributed transmit signal high power amplification (EFEs, TAAs); distributed receive signal low noise amplification with LNA protection (EFEs); signal and power distribution (corporate feed, power converter) (RFDN); phase and amplitude control including temperature compensation (EFEs via TCU); internal calibration loop; deployment mechanisms including hold down and release; and antenna mechanical structure. 
     Regarding the RF-signal power distribution, on panel level the Sentinel-1 SAR Instrument RF-Distribution Network (RFDN) distributes in TX the signals from the SAR Electronic Subsystem (SES) to the antenna tiles (i.e. to the input port of the Tile Amplifier Assembly (TAA)) with a good phase match. On SAS tile level the RFDN distributes the TX signals from the output of the tile amplifier assembly to the Electronic Front End (EFE) modules with a good phase match. For RX the RFDN combines the received signal in the reverse direction. 
     The RF-Distribution Network is made up of the following elements:
         the Azimuth Plane Distribution Network (APDN), for panel level signal distribution   the Elevation Plane Distribution Network (EPDN), for SAS tile level signal distribution   the RF harness       

     In summary, the RFDN possesses the following major functions:
         For TX: Distribute the TX signal from the SES via the tile amplifiers to the EFEs with a small phase variation between the output ports.   For RX: Combine the received signal from the EFEs via the tile amplifiers towards the SES with a small phase variation between the different RX paths.   Band pass filtering in the TX and RX path.       

     On tile level, the EPDN of the RFDN consists of coaxial cables and power dividers/combiners. On panel level, the APDN encompasses coaxial cables and power divider/combiner composite as well. For the Inter-Panel RF Harness routing, connection of the three RF harness branches (TX, RX-V and RX-H) from panel to panel after deployment is achieved by a set of dedicated choke flange connections, which allow a contactless inter-panel signal transmission. The choke flange assemblies are located in the center of the Antenna Panel Frame (APF) transverse beam. 
     It has been found in tests that a high amplitude ripple in transmit calibration mode (TX Cal) occurs for horizontal polarized signals. This makes it difficult to conduct an internal calibration. 
     Accordingly, exemplary embodiments of the present invention are directed to an antenna system in which an internal calibration can be made easier and more reliable. 
     In order to improve internal calibration, a space-borne antenna system is disclosed, which comprises a number or panels being moveable to each other and having a gap in between them when the panels are arranged in an operation condition; an RF distribution network for providing transmit signals to the number of panels and combining received signals from the number of panels; and a set of choke flange assemblies which allow a contactless inter-panel signal transmission across a dedicated gap, wherein a respective choke flange assembly is arranged on the far side of a radiating surface of the dedicated adjacent panels. Furthermore, the antenna system comprises an RF (radio frequency) seal assembly for suppressing a signal coupling of signals radiated from the number of panels to the set of choke flange assemblies by sealing the gap. 
     The invention is based on the consideration that the high amplitude ripple in transmit mode that occurs for horizontal polarized signals is a result of coupling from the antenna waveguide radiators to the choke flange assembly between two panels. An RF seal is added to the junction between two adjacent panels to minimize the coupling from waveguide radiators to the choke flange assembly. The added seal is made such that it does not counter-act to a panel latching mechanism. Hence, the RF seal is provided in a way to not exert excessive additional mechanical force while it does not require mechanical contact between the panels. As a result, the RF seal assembly closes the gaps between panels, specifically tiles between two adjacent panels. 
     According to a further embodiment a respective RF seal assembly is dedicated to a gap between two adjacent panels of the number of panels. 
     A respective RF seal assembly may comprise a first and a second seal profile that are affixed in opposing pairs in the gap between two adjacent panels of the number of panels. The profiles enable closing the gap between the panels, specifically tiles within the panels. 
     The first and the second seal profile may have an L-shaped cross-section, in a side view in a longitudinal section through the antenna system. First portions of the first and the second seal profile extend in a plane of the number of panels, when the panels are arranged in an operation condition, and are attached to the dedicated adjacent panel. Second portions of the first and the second seal profile extend in a direction of radiation of signals such that they are opposing and having a gap in between them. This shape, on the one hand, enables closing the gap between the panels. On the other hand, it does not count to a panel latching mechanism. 
     In one embodiment, the gap between the second portions of the first and the second seal profile has a constant width in a direction of radiation of signals. In this configuration, the second seal profiles are perpendicular to the plane of the panels, when the panels are arranged in an operation condition, i.e., the angle between the first and the second portion of a respective profile is 90°. 
     In an alternative embodiment, the gap between the second portions of the first and the second seal profile has a widening or a narrowing width in a direction of radiation of signals, resulting in an angle between the first and the second portion of a respective profile which is less or r more than 90°. 
     It is preferred that the RF seal assembly is made from the material of radiating waveguides of the set of panels. This ensures that the RF seal assembly and the waveguides have the same coefficients of thermal expansion resulting in minimized therm-mechanical stress. The profiles of the RF seal assembly may be made from CFRP, in particular metallized CFRP. CFRP is a Carbon fiber reinforced plastic. This allows manufacturing the profiles from left-over antenna waveguides. Alternatively, the RF seal assembly may be made from a metal, e.g. aluminum. 
     In a further preferred embodiment, the RF seal assembly is mechanically attached to the adjacent panels via at least one adhesive tape, in particular a high adhesive double sided tape. As one of the adhesive tapes, for example, 3M #Y966 tape may be used. Such kind of tape is used for heavy duty hold down applications where a high level of adhesion is required. 
     In a further preferred embodiment, the RF seal assembly is electrically coupled to the adjacent panels via a metal adhesive tape. The metal adhesive tape may be, for example, a Cho-foil, which has good shielding and conductivity properties with respect to EMI (Electro-magnetic Interference). This assists suppressing the signal coupling of signals radiated form the number of panels to the set of choke flange assemblies 
     According to a further preferred embodiment, the RF seal assembly is arranged at a hinge line of the antenna system. 
     The RF seal assembly can be regarded as a choke configuration that is used to close the gaps between panels, i.e. tiles within the panels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       More details and advantages of the invention will be described by reference to the accompanying figures. 
         FIG. 1  shows a first embodiment of an RF seal assembly for use in a space-borne antenna system according to the invention. 
         FIG. 2  shows a second embodiment of an RF seal assembly for use in a space-borne antenna system according to the invention. 
         FIG. 3  shows a plan view of an RF seal assembly for use in a space-borne antenna system according to the invention. 
     
    
    
     In the figures, like elements are depicted with like reference numerals. It is to be noted that the embodiments shown in the figures are not drawn to scale and are used to illustrate the basic concept of the invention. 
     DETAILED DESCRIPTION 
     An RF seal assembly, as described below, is intended to be used in an antenna system for space-borne applications, for example the Sentinel-1 SAR Antenna Subsystem (SAS) for the Sentinel-1 mission. This antenna system is, as known to a skilled person, a deployable planar active phased array antenna working in C-band (5.405 GHz) with a frequency bandwidth of 100 MHz. The antenna is formed by a central panel mounted on top of the spacecraft and two antenna side wings at the two adjacent sides of the spacecraft. The central panel is equipped with two SAS tiles, whereas the two panels of each side wing carry three SAS tiles each. This leads to an overall number of 14 identical tiles. Each SAS tile possesses all the functions needed to allow for beam shaping and steering. 
     Each of the number of panels is movable to each other. During transport of the antenna system to space, the panels are folded by means of hinges, due to space reasons. In orbit, they are deployed. As shown in  FIG. 3 , the connection of two adjacent panels  10 ,  20  by means of a hinge  70  results in a small gap between the adjacent panels when the panels are arranged in an operation condition, i.e. when all of the panels are arranged in a common plane. A signal transmission coupling between two adjacent panels is realized by means of a choke flange assembly consisting of a first waveguide in one of the panels and a second waveguide in one of the other panels. The first and the second waveguide are affixed in opposing pairs to enable contactless signal transmission over the gap. 
     The detailed composition of this type of antenna system is known to the person skilled in the art, such as from the above mentioned Sentinel-1 SAR antenna, so that further explanations with respect to details of the antenna system will be omitted. 
     Referring now to  FIG. 1 , a part of an antenna system  1  of the type described above is illustrated in the region of two neighboring panels, a first of which is depicted with  10  and a second of which is depicted with  20 . As noted above, each of the panels  10 ,  20  consists of a number of tiles. A tile of the first panel  10  is depicted with  11 , a the of the second panel is depicted with  21 . The tiles  11 ,  21  are located adjacent to each other. A gap between the first panel  10  and the second panel  20  and the first tile  11  and the second tile  21 , respectively, is depicted with  60 . The gap  60  has a length  64  which typically is around 5 mm. In the figure, radiating surfaces  12 ,  22  of the first and second panel and tile  11 , respectively, are directed downwards in the plane of drawing. 
     To enable contactless inter-panel communication, a choke flange assembly  30  is arranged on the far side of the radiating surfaces of the dedicated adjacent panels  10 ,  20 . The choke flange assembly  30  consists of a first waveguide  31  which is embedded in a (not shown) housing of the first panel  10  and a second waveguide  32  which is embedded in a (not shown) housing of the second panel  20 . In between the first and the second waveguides  31 ,  32 , there is a gap  33 . Flanges  34 ,  35  of the first and the second waveguide  31 ,  32  are located (at least partly) within the gap  60 . 
     To suppress signal coupling of signals radiated from the panels  10 ,  20  and their tiles  11 ,  21 , respectively, an RF seal assembly  40  is provided within the gap  60 . The RF seal assembly  40  consists of a first seal profile  41  attached to the first panel  10  and a second seal profile  51  attached to the second panel  20 . The RF seal assembly  40  is provided to seal the gap  60  at least partly. 
     In a cross-section, i.e. in a side view in a longitudinal section through the antenna system  1 , the first and the second seal profile  41 ,  51  have the shape of an “L”. A respective first portion  45 ,  55  of the first and second seal profile  41 ,  51  extends in the plane of the panels  10 ,  20  (i.e. in a direction perpendicular to the plane of drawing from the left side to the right side) into the gap  60 . A respective second portion  46 ,  56  of the first and second seal profile  41 ,  51  extends in a direction of radiation of signals radiated from the panels  10 ,  20  (i.e. in a direction perpendicular to the plane of drawing top down). The length of the second portions  46 ,  56  is a quarter of the wavelength of the signals radiated from the panels  10 ,  20 . 
     A respective first portion  45 ,  55  of the first and second seal profile  41 ,  51  is attached to the dedicated panel  10 ,  20  by means of adhesive tape  43 , and  53 . The attachment of a respective first portion  45 ,  55  of the first and second seal profile  41 ,  51  to the dedicated panel  10 ,  20  may be made by an adhesive tape and/or epoxy glue. Moreover, the seal profiles  41 ,  51  are electrically coupled to the dedicated panel  10 ,  20  by means of a conductive foil  42 ,  52 , such as an so-called cho-foil, which is known from prior art as well. 
     The first and the second seal profile  41 ,  51  are arranged in opposing pairs in the gap  60  to seal the gap at least partly. In the plane of the first portions  45 ,  55  of the first and second seal profiles  41 ,  51 , there is a gap  61  having a first length between the seal profiles  41 ,  51 . At the outside ends of the second portions  46 ,  56 , directed to the radiating surfaces  12 ,  22 , there is a gap  62  having a second length between the seal profiles  41 ,  51 . In the first embodiment, shown in  FIG. 1 , the first length of gap  61  corresponds to the second length of the gap  62 . That means the second portions  46 ,  56  are parallel to each other. The length of the first and the second gap  61 ,  62  may be around 0.8 mm to 1 mm. 
     In the second embodiment, shown in  FIG. 2 , the first length of the gap  61  is smaller than the second length of the gap  62 . As a result, the gap between the second portions has a widening width in a direction of radiation of signals, i.e. the angle between the first and the second portions  45 ,  46 ;  55 ,  56  of a respective seal profile  41 ,  51  is less than 90°. The length of the gap  61  may be around 0.8 mm. The length of the gap  62  may be around 1.2 mm. The remainder of the configuration of the second embodiment, shown in  FIG. 2 , corresponds to the first embodiment, shown in  FIG. 1 . However, in an alternative embodiment the angle between the first and the second portion  45 ,  46 ;  55 ,  56  may be greater than 90°. 
     The first and second seal profiles  41 ,  51  may be made from the material of the radiating waveguides of the panels  10 ,  20 . This ensures that the RF seal assembly and the waveguides have same coefficients of thermal expansion and minimizes thereto-mechanical stress. Hence, the first and the second seal profiles may be made from CFRP (carbon fiber reinforced plastic), which has a metallization on its surface. For example, the first and the second seal profiles  41 ,  51  made from CFRP may be copper plated. This allows manufacturing the profiles from left-over waveguides. Alternatively, the seal profiles  41 ,  51  of the RF seal assembly  40  may be made from a metal, e.g. aluminum. 
     As shown in  FIG. 3 , the RF seal assembly may be attached to the panel-to-panel junctions at the hinge line  71 . 
     The effect of the RF seal assembly, i.e. a significant suppression of signal coupling of signals radiated from the panels  10 ,  20  to the choke flange  30 , has been verified with an S-parameter test. 
     As will be realized by a skilled person, the RF seal assembly  40  is contactless in the sense that the first and the second seal profile  41 ,  51  do not have any mechanical contact to each other. The configuration of the first and the second seal profile  41 ,  51  is such that it does not counter-act to the panel latching mechanism, i.e. no excessive additional mechanical force is exerted. 
     As a further advantage the RF seal assembly does not a mechanical contact between the panels  10 ,  20 . 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. 
     LIST OF REFERENCE SIGNS 
     
         
           1  antenna system 
           10  first panel 
           11  tile of first panel 
           12  radiating surface of tile  11   
           20  second panel 
           21  tile of second panel 
           22  radiating surface of tile  21   
           30  choke flange assembly 
           31  first waveguide 
           32  second waveguide 
           33  gap between first and second waveguide 
           34  flange of the first waveguide  31   
           35  flange of the second waveguide  32   
           40  RF seal assembly 
           41  first seal profile 
           42  conductive foil 
           43  adhesive tape 
           45  first portion of first seal profile extending in a plane of the panel into the gap  60   
           46  second portion of first seal profile extending in a direction of radiation of signals 
           51  second seal profile 
           52  conductive foil 
           53  adhesive tape 
           55  first portion of second seal profile extending in a plane of the panel into the gap  60   
           56  second portion of second seal profile extending in a direction of radiation of signals 
           60  gap between first and second panel 
           61  gap between first and second seal profile 
           62  gap between first and second seal profile at outside ends of portions  45 ,  55   
           63  length of portions  45 ,  55  of first and second profile 
           64  length of gap  60  between first and second panel