Abstract:
A coaxial connecting element for the microwave range comprises a preferably bar-shaped inner conductor, which extends along an axis and is held concentrically in an outer conductor by means of at least one dielectric supporting element wherein the outer conductor has a plurality of radial slots extending in the axial direction distributed around the circumference in the region of the at least one supporting element. With such a connecting element, increased stability and precision as well as simplified production and fitting are achieved by virtue of the fact that the supporting element is designed to be integral.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the field of microwave engineering. The invention relates in particular to a coaxial connecting element for the microwave band according to the preamble of claim  1 , and to a method for producing a connecting element such as this. 
       PRIOR ART 
       [0002]    Coaxial components and connecting elements for the microwave band (from several GHz up to 250 GHz) generally have an outer conductor which extends along an axis and in which an inner conductor is arranged and held concentrically. One or more dielectric supporting element or elements is or are used for holding the inner conductor in the outer conductor. The configuration and the installation of the supporting elements influence not only the electrical but also the mechanical characteristics of the coaxial connecting element. On the one hand, the supporting elements should have as little influence as possible, in comparison with the air-cored line that is used as a reference, on wave propagation in the connecting element. On the other hand, the mechanical retention must be designed to be sufficiently permanent and precise that a large number of connecting cycles can be carried out with constant accuracy. 
         [0003]    Examples of supporting elements such as these are disclosed, for example, in the documents U.S. Pat. No. 4,718,864, U.S. Pat. No. 4,867,703 or U.S. Pat. No. 5,269,702. The technology of hermetically sealed glass bushings is also known, in which an inner conductor is held by means of glass in a concentric metal ring, with the glass filling the annular space between the metal ring and the inner conductor, and being introduced by means of a melting process. 
         [0004]    Another problem caused by the supporting elements is that higher transverse modes (in particular TE 11 ) are stimulated in the area of the supporting elements, which leads to distortion of the transmitted signals and limits the range of operation of the connecting element in the direction of lower frequencies. 
         [0005]    The article by R. Fuks, “New Dielectric Bead for Millimeter-wave Coaxial Components”, Microwave Journal No. 5, p. 318 ff (2001) explains this problem and, in order to solve it, proposes a novel type of supporting elements which is completely symmetrical on the radial plane and has inclined surfaces with respect to the axis. However, the supporting elements are produced as separate components and are then fitted, and this is at the expense of the precision of the arrangement. 
         [0006]    Another way to hold the inner conductor in the outer conductor has been proposed in the document U.S. Pat. No. 4,456,324. In this known solution, the inner conductor is centred by and held by four curved dielectric platelets, which are held in a slotted hollow cylinder of the outer conductor and form the configuration of a Maltese cross, through whose centre the inner conductor passes. This type of retention has the advantage that only a very small amount of dielectric material need be used for support purposes. However, one considerable disadvantage is the complicated and complex assembly of the arrangement, which leads to considerable difficulties, especially in the case of coaxial arrangements with diameters in the millimetre range. Furthermore, the desired accuracies can be achieved only with difficulty because it is assembled from a large number of individual parts. The slots in the outer conductor are used to hold the dielectric platelets. No other function is recognized for them. 
       DESCRIPTION OF THE INVENTION 
       [0007]    The object of the invention is to provide a coaxial connecting element for the microwave band, which avoids the disadvantages of known solutions and is distinguished, with high precision and good mechanical robustness at the same time, by simplified installation and excellent electrical characteristics, and to specify a method for producing a connecting element such as this. 
         [0008]    The object is achieved by the totality of the features of claims  1  and  17 . The essence of the invention is to form the supporting element integrally and, in particular, to mould it integrally or to spray it directly onto the outer conductor. This results in good mechanical robustness with high precision and simplified production and assembly at the same time, without having to accept any deterioration in the electrical characteristics. 
         [0009]    According to one preferred embodiment of the invention, the supporting element is formed integrally, in particular moulded, on the outer conductor, in which case preferably the supporting element is non-detachably connected to the outer conductor. 
         [0010]    Another embodiment is distinguished in that the outer conductor has a housing in which an essentially hollow-cylindrical insert is arranged and held concentrically, in that the supporting element is formed integrally on the insert and in that the slots are arranged in the insert, in which case, in particular, the supporting element fills the slots in the insert in the circumferential direction, in order to fix the supporting element on the insert. 
         [0011]    A further embodiment is characterized in that the insert has an annular groove on the outside, and in that the supporting element fills the annular groove in the axial direction, in order to fix the supporting element on the insert. 
         [0012]    The housing of the outer conductor preferably has two housing parts which are arranged one behind the other in the axial direction, can be screwed to one another, hold the insert with the integrally formed supporting element and fix it in the axial direction, in which case an axial through-hole is provided respectively in the two housing parts, through which the inner conductor runs, and in that both through-holes have a step on which the insert together with the integrally formed supporting element is supported in the axial direction. 
         [0013]    In particular, the insert has a third through-hole, and the internal diameters of the three through-holes are the same. 
         [0014]    According to another embodiment of the invention, the supporting element is arranged in the centre area of the connecting element seen in the axial direction. 
         [0015]    A further embodiment is characterized in that the supporting element has two hollow cylinders which are arranged concentrically in one another and are connected to one another preferably by radial webs, with the inner hollow cylinder holding the inner conductor and with the outer hollow cylinder being connected to the insert, and in that the preferably radial webs are formed by walls of blind holes which extend into the supporting element between the two hollow cylinders preferably in the axial direction and alternately from both sides. In particular, the blind holes taper towards the base, in which case in the area of the supporting element the inner conductor has a section which holds the inner hollow cylinder of the supporting element fixing it in the axial direction and/or securing it against rotation about the axis. This section may be knurled or may have a groove (round, triangular or polygonal), or some other shape which is suitable for fixing. 
         [0016]    According to a further embodiment, two or more slots ( 26 ) are provided, distributed uniformly over the circumference of the outer conductor of the insert. 
         [0017]    The supporting element ( 15 ) is preferably composed of a liquid-crystal polymer (LCP) or a thermoplastic PTFE. 
         [0018]    One preferred embodiment of the method according to the invention is characterized in that the outer conductor has a housing in which an essentially hollow cylindrical insert is arranged and held concentrically, in that the supporting element is moulded onto the insert and in that the insert, together with the moulded-on supporting element, is installed in the housing. 
     
    
     
       BRIEF EXPLANATION OF THE FIGURES 
         [0019]    The invention will be explained in more detail in the following text with reference to exemplary embodiments and in conjunction with the drawing, in which: 
           [0020]      FIG. 1  shows a longitudinal section through a coaxial connecting element according to one preferred exemplary embodiment of the invention; 
           [0021]      FIG. 2  shows a perspective side view of the connecting element of  FIG. 1 ; 
           [0022]      FIG. 3  shows a cross section through the connecting element from  FIG. 1 , on the plane III-III shown there; 
           [0023]      FIG. 4  shows a perspective side view of the insert with a moulded supporting element and inner conductor from  FIG. 1 ; 
           [0024]      FIG. 5  shows an exploded illustration of the individual elements of the configuration from  FIG. 4 ; 
           [0025]      FIG. 6  shows a perspective illustration of a longitudinal section through the supporting element from  FIG. 5 ; and 
           [0026]      FIG. 7  shows an illustration, comparable to  FIG. 5  of the individual elements, looking from the opposite direction. 
       
    
    
     APPROACHES TO IMPLEMENTATION OF THE INVENTION 
       [0027]      FIG. 1  shows a longitudinal section through a coaxial connecting element according to one preferred exemplary embodiment of the invention. The coaxial connecting element  10 , which may be a cable connector for example, has a housing which is composed of two cylindrical, metallic housing parts  11 ,  12 , extends along an axis  24  and, together with a cylindrical insert  14  accommodated in the housing, forms the outer conductor of the connecting element  10 . The two housing parts  11 ,  12 , can be screwed to one another by means of a screw thread  21 . The two housing parts  11 ,  12  and the metallic insert  14  each have a central through-hole  16 ,  17  and  17 ′, respectively, with the same internal diameter which, when the connecting element  10  has been assembled, complement one another to form one through-hole  16 ,  17 ,  17 ′ which passes through the entire connecting element  10 . However, it is also feasible for these through-holes  16 ,  17 ,  17 ′ to have different diameters and thus to provide matching between different connector types. The insert  14 , on which a dielectric supporting element  15  is integrally formed in order to hold a concentric inner conductor  13 , is housed between two steps  18  and  19  in the through-holes  16  and  17  in the two housing parts  11 ,  12 . The inner conductor  13  is mounted in the supporting element  15  and extends, at a sufficient distance from the wall, through the through-hole  16 ,  17 ,  17 ′. By way of example, it is in the form of a metallic pin which ends at one end in a slotted bush  22  and at the other end in a plug pin  23 . Other configurations of the inner conductor  13  are, however, also feasible within the scope of the invention. This also applies to the configuration of the housing  11 ,  12 . In the exemplary embodiment shown in  FIG. 1 , a union nut  20  is provided at the left-hand end, which can be rotated with respect to the housing  11 ,  12  and allows the connecting element to be detachably connected to an appropriately designed connecting part. The external view of the connecting element  10  from  FIG. 1  is reproduced in  FIG. 2  in the form of a cross-section on the plane III-III passing through the supporting element  15  in  FIG. 3 . 
         [0028]    The connecting element  10  is intended for use at frequencies in the two-digit to three-digit GHz range. On the one hand, the precision and mechanical robustness with which the inner conductor  13 , which has an external diameter of &lt;1 mm, is mounted in the outer conductor  11 ,  12 ,  14 , are therefore particularly important. On the other hand, the excitation of undesirable modes, in particular TE11 mode, in the area of the supporting element  15  must be suppressed as much as possible in order to ensure that the connecting element has optimum transmission characteristics. Finally, the production and assembly should be sufficiently simple that even relatively large quantities can be produced with high quality and at acceptable costs. 
         [0029]    This is achieved by integrally forming or moulding the supporting element  15  on the insert shown separately in  FIGS. 5 and 7 , and the arrangement of in particular radial slots  26  which preferably extend in the axial direction, on the circumference of the insert  14  in the area of the supporting element  15 . In the example shown in  FIGS. 1-7 , four slots  26  are provided, each offset by 90°. However, it is also possible to provide only two slots, offset by 180°, three slots or more than four slots. When the supporting element  15  is moulded on, the slots  26  can, at the same time, be used as channels to pass on the injected material, and can be filled with the material of the supporting element  15  during the moulding process (see for example  FIG. 4 ). The slots  26  interrupt the ring currents that occur in the TE11 mode on the outer conductor, and therefore prevent the excitation of this mode. 
         [0030]    In order to mould the supporting element  15 , the insert  14  is inserted into an appropriate mould, giving the supporting element  15  the shape illustrated in  FIG. 6 . The inner conductor  30  can also be included in the moulding process. However, it is also possible not to mount the inner conductor  13  in the supporting element  15  until later. The cylindrical supporting element  15  has an inner hollow cylinder  30  with a through-opening  31 , and an outer hollow cylinder  29 , arranged concentrically. The inner hollow cylinder  30  encloses the inner conductor  13 , such that it is fixed, in the area of one specific refinement, for example in the form of an annular groove  25  with a reduced external diameter. The outer hollow cylinder  29  rests on the inner wall of the through-hole  17 ′ of the insert  14 . The two hollow cylinders  29  and  30  are connected to one another by preferably radial webs  34 . The preferably radial webs  34  are formed by walls of blind holes  32 ,  33 , which extend into the supporting element  15  between the two hollow cylinders  29 ,  30 , preferably in the axial direction alternately from both sides. In order to allow them to be removed from the mould more easily after being moulded, the blind holes  32 ,  33  preferably taper towards the base. In the illustrated example, three blind holes, each offset through 120° are provided from each side, although other numbers and arrangements of blind holes are also feasible. In particular, it is also feasible to form all of the blind holes from one side. It is likewise also feasible to provide helical blind holes with corresponding helical webs. 
         [0031]    In the area of the slots  26 , the insert  14  has an annular bead  37  which is cut through by the slots  26 . An annular groove  27  with a reduced diameter is provided behind the annular bead  37 . The mould is designed such that the moulded supporting element  15  has a first ring  28  in front of the annular bead  37 , and the annular groove  27  is filled by a second ring  36 . The two rings  28 ,  36  are firmly connected to one another and to the outer hollow cylinder  29  by webs  35 , which are created by filling the slots  26 . The two rings  28 ,  36  with the annular bead  37  between them fix the supporting element in the axial direction, and the webs  35  fix the supporting element  15  in the circumferential direction. 
         [0032]    After the moulding of the supporting element  15 , the insert  14 , together with the moulded supporting element  15  and the inner conductor  13  held therein are inserted as far as the step  19  into one of the housing parts  12 , and the arrangement is then screwed to the other housing part  11 . 
         [0033]    The supporting element  15  is preferably composed of a liquid-crystal polymer (LCP) which has a relative dielectric constant ∈ r  of about 3. However, it is likewise also possible to use PTFE, which can be processed thermoplastically, or some other material. The supporting element  15  has an axial length in the region of about 2 to 3 mm for a connecting element  10  whose overall length is more than 7 times greater. 
         [0034]    Overall, the invention results in a coaxial connecting element which is suitable for frequencies up to 65 GHz and is distinguished by good electrical and transmission characteristics, with the capability to be produced easily and with good mechanical robustness, at the same time. It is self-evident that numerous modifications are possible within the scope of the invention and on the basis of the exemplary embodiment explained here. In particular, the frequency range can be extended up to 250 GHz or more by adaptation of the dimensions. 
       LIST OF REFERENCE SYMBOLS 
       [0000]    
       
           10  Coaxial connecting element 
           11 ,  12  Housing part (outer conductor) 
           13  Inner conductor 
           14  Insert 
           15  Supporting element 
           16 ,  17 ,  17 ′Through-hole 
           18 ,  19  Step 
           20  Union nut 
           21  Screw thread 
           22  Bush (slotted) 
           23  Plug pin 
           24  Axis 
           25  Annular groove 
           26  Slot 
           27  Annular groove 
           28  Ring 
           29 ,  30  Hollow cylinder 
           31  Through-opening 
           32 ,  33  Blind hole 
           34  Wall (web) 
           35  Web 
           36  Ring 
           37  Annular bead