Patent Publication Number: US-8125295-B2

Title: Orthogonal-mode coupler of the coaxial type having a branched central conductor

Description:
This is a non-provisional application claiming the benefit of International application number PCT/EP2007/061133 filed Oct. 18, 2007. 
     FIELD OF THE INVENTION 
     The present invention concerns an orthogonal-mode junction coupler with an ultrabroad operating bandwidth to separate dual-polarization bands propagating in a horn antenna, for example. 
     BACKGROUND OF THE INVENTION 
     In the field of radiofrequency antennas, it is well known to use orthogonal-mode junction couplers, or “ortho-mode junction” (OMJ) couplers, to separate dual-polarization bands. 
     For very broad operating frequency bands, these ortho-mode junction couplers are traditionally made up of a supply section of the waveguide in the shape of a cross comprising two central supply points, one supply point for each polarization, placed along the axis of the coupler, the points being offset along the axis of the coupler and ending with shield cavities in the rear. 
     This type of coaxial coupler has the drawback of being bulky and providing poor insulation between the two inlet ports of the supply points which are close to each other. 
     Furthermore, this type of coupler has an asymmetry which leads to degradation of the purity of the modal network due to the excitation of higher order modes. 
     SUMMARY OF THE INVENTION 
     One of the aims of the invention is therefore to resolve all of these drawbacks by proposing a particularly compact OMJ coupler with an ultrabroad bandwidth providing weak coupling between the input ports as well as a particularly stable ultrabroad bandwidth single-mode and bi-polarized excitation. 
     According to the invention, what is proposed is an orthogonal-mode junction coupler with an ultrabroad bandwidth for a wavelength waveguide which is remarkable in that the coupler comprises a so-called external conductor comprising a cavity in which a central conductor projects, the central conductor being electrically isolated at radiofrequencies with the external conductor, the central conductor being supplied by supply lines passing through the external conductor and emerging in the cavity of the external conductor. 
     The central conductor has a transverse cross-shaped section preferably having two orthogonal axes of symmetry. 
     Furthermore, the orthogonal-mode junction coupler according to the invention comprises four supply lines emerging in the cavity of the external conductor, each supply line being connected to a branch of the cross-shaped central conductor. 
     Preferably, each supply line is connected to a branch of the central conductor by an ohmic contact. 
     Two opposite branches the central conductor are supplied with radiofrequency signals by two opposite supply lines, respectively, to provide a given polarization. 
     To this end, the supply lines are connected to an external supply circuit determining the phase distribution of each signal sent by the supply lines. 
     According to one essential characteristic of the invention, the supply lines are connected to the central conductor in a same plane orthogonal to the axis of the central conductor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and characteristics will better emerge from the description which follows, of a single embodiment provided as a non-limiting example, of the orthogonal-mode junction coupler with an ultrabroad bandwidth, in particular a coaxial coupler, according to the invention, from the appended drawings in which: 
         FIG. 1  is a perspective view of the coaxial OMJ coupler with an ultrabroad bandwidth according to the invention coupled to a horn antenna, 
         FIG. 2  is a diagrammatic transverse cross-sectional view of the coaxial OMJ coupler with an ultrabroad bandwidth according to the invention, 
         FIG. 3  is a diagrammatic longitudinal cross-sectional view of the coaxial OMJ coupler with an ultrabroad bandwidth according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Below is an OMJ coupler with an ultrabroad bandwidth according to the invention in order to separate the orthogonal dual-polarization bands of a coaxial circular radiating horn filled with a dielectric cone; however, it is obvious that the coupler according to the invention may be used alone and/or in any other application well known by One Skilled in the Art. 
     In reference to  FIG. 1 , the OMJ coupler  1  with an ultrabroad bandwidth according to the invention is supported by a support frame  2  made up of two crowns, a lower crown  3  and an upper crown  4  connected by spacers  5  in the form of cylindrical columns, the upper crown  4  supporting a horn antenna  6 . The support frame  2  also supports an external supply circuit  7  of the coupler  1  which will be explained in detail later. 
     The coupler  1 , in reference to  FIGS. 2 and 3 , comprises a so-called external conductor  8  comprising a cavity  9  in which a central conductor  10  extends, the central conductor being electrically isolated at radiofrequencies with the external conductor  8 . It is to be noted that the central conductor is not electrically isolated at a discontinuous current. 
     The external conductor  8  includes a cylindrical tube having a coaxial cylindrical cavity  9 . 
     Furthermore, the central conductor  10  has a cross-shaped transverse section comprising two orthogonal axes of symmetry. Thus, the central conductor  10  comprises four branches  11 ,  12 ,  13  and  14 , opposite each other in pairs, as best shown in  FIG. 2 . 
     Furthermore, each branch  11 ,  12 ,  13  and  14  of the central conductor  10  is supplied by supply lines  15 ,  16 ,  17  and  18 , respectively, passing through the external conductor  8  by inlet ports  19 ,  20 ,  21  and  22  and emerging into the cavity  9  of the external conductor  8 , as best shown in  FIG. 2 . 
     Each supply line  15 ,  16 ,  17  and  18  is connected to a branch  11 ,  12 ,  13  and  14 , respectively, of the central conductor  10  by an ohmic contact  23 . The ohmic contact  23  will be obtained through any suitable means well known by One Skilled in the Art. 
     Particularly advantageously, and as shown in  FIG. 3 , the supply lines  15 ,  16  (not shown in  FIG. 3 ),  17  and  18  (not shown in  FIG. 3 ) are connected to each of the branches  11 ,  12  (not shown in  FIG. 3 ),  13  and  14  (not, shown in  FIG. 3 ), respectively, (through inlet ports  19 ,  20 ,  21 ,  22 ), of the central conductor  10  in a same plane orthogonal to the longitudinal axis of the central conductor  10 . 
     Furthermore, the supply lines  15 ,  16 ,  17  and  18  are connected to the external supply circuit  7  ( FIG. 1 ) determining the phase distribution of each signal sent by the supply lines  15 ,  16 ,  17  and  18 . The supply circuit  7  ( FIG. 1 ) supplies two opposite branches, for example (as shown in  FIG. 3 ) branches  11  and  13 , of the central conductor  10  with radiofrequency signals by the two respective opposite supply lines  15  and  17 , respectively, in order to provide a determined polarization. For example, the supply circuit  7  supplies the branches  11  and  13  with radiofrequency signals having the phase distributions (0,0°) and (0, 180°), respectively passing through inlet sorts  19 ,  21 , in order to provide a polarization of the branches  11  and  13  as diagrammatically illustrated by arrows in  FIG. 3 . 
     One will note that the electric symmetry of the coaxial OMJ coupler with an ultrabroad bandwidth according to the invention provides a stable single-mode and bi-polarized excitation with an ultrabroad bandwidth as well as weak coupling between the inlet ports of the supply lines. This weak coupling between the inlet ports makes it possible to do without an external compensating circuit. 
     Furthermore, the coaxial OMJ coupler with an ultrabroad bandwidth according to the invention is particularly compact given that the supply lines  15 ,  16 ,  17  and  18  are connected to each of the branches  11 ,  12 ,  13  and  14 , respectively, of the central conductor  10  in a same plane orthogonal to the longitudinal axis of the central conductor  10 . 
     It is quite clear that the coupler according to the invention may be obtained according to a precision trimming method well known by One Skilled in the Art or a method for manufacturing a multi-layer printed circuit, the multi-layer printed circuit being integrated into a waveguide, without, however, going beyond the scope of the invention. One will observe that, for a coupler obtained according to a printed circuit manufacturing method, the supply lines will be able to have an opposite direction. 
     Lastly, it goes without saying that the coaxial OMJ coupler with a medium or ultrabroad bandwidth according to the invention may be adapted for any other application well known by One Skilled in the Art and that the examples we have provided are in no way limiting as to the fields of application of the invention.