Abstract:
An HF feed bushing device is provided that is designed to connect an HF transmitter/receiver to an antenna via an external medium by means of a balanced two-wire line. The balanced two-wire line is at least partially encapsulated in a central part of a closed conduit including a wall isolating the balanced two-wire line from the external medium. Applications: Feeding of antennas via balanced two-wire lines.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International patent application PCT/EP2010/069956, filed on Dec. 16, 2010, which claims priority to foreign French patent application No. FR 0906180, filed on Dec. 18, 2009, the disclosures of which are incorporated by reference in their entirety. 
     FIELD OF THE INVENTION 
     The invention concerns the feeding of HF telecommunication antennas and in particular a feed bushing device for antennas. 
     BACKGROUND 
     Telecommunication systems usually include transmitters/receivers that have to be connected to antennas to radiate and/or to receive an electromagnetic signal. 
     The RF connection between the transmitter/receiver and the antenna is made by an RF feed line that may be either of balanced type (for example in the most general case a coaxial cable) or of balanced type (most often a balanced two-wire line). 
       FIG. 1  shows an example of a prior balanced two-wire line including two electrical conductors  10 ,  12  insulated by respective sheaths  14 ,  16 . The two electrical conductors are kept at a constant distance D from each other by an insulative wall  18  (or insulative spreaders) fastened to the sheaths of the conductors. The insulative wall  18  and the sheaths  14 ,  16  of the conductors are flexible so as to be able to bend the two-wire line during its installation. The various parameters of the line, notably the diameter of the conductors, the distance D between conductors, the thickness and the dielectric characteristics of the sheath and the spreaders, determine a given characteristic impedance Zc of the feed line. 
     Balanced two-wire lines have the advantage over unbalanced coaxial lines of having lower transmission losses per unit length and high characteristic impedances. Balanced two-wire lines have known and proven characteristics and properties in a gas, solid or hybrid medium the radio-frequency characteristics of which are very similar to those of air (relative dielectric and magnetic permitivity very close to 1), but nevertheless significantly modified by the presence of an environment near the line having a dielectric permitivity different from that of air, notably when that environment is moist or of a metallic nature. 
     In some configurations for feeding an HF antenna by a transmitter/receiver relatively far from the antenna the balanced two-wire line must be away from the ground to prevent modifications of its radio-frequency characteristics, which would lead to high losses through mismatching of the antenna with the transmitter or coupling with the ground. 
       FIG. 2  represents an HF transmitter/receiver feeding an antenna via an overhead balanced two-wire line. 
     In the  FIG. 2  configuration, a transmitter/receiver  22  is in a semi-enclosed place  24  feeding, via a two-wire line  26 , an antenna  28  outside that place and at a great distance from that place. The two-wire feed line  26  is held at a distance from the ground  30  by supports  32  fixed to the ground and at least the head of which supporting the two-wire line is in insulative material. 
     This type of overhead solution represented in  FIG. 2  is not desirable for reasons of discretion in the case of military installations. On the other hand, the two-wire line  26  in  FIG. 2  cannot be buried without seriously degrading its characteristics, leading to high HF signal transmission losses. This deterioration is mainly caused by the state and the nature of the backfilling soil which significantly modifies the characteristics of the line, possibly being moist and thus more or less conductive. The state of the soil may also vary as a function of prevailing climatic conditions. 
     SUMMARY OF THE INVENTION 
     To alleviate the drawbacks of the prior art balanced two-wire antenna feeds, the invention proposes an HF feed bushing device designed to connect an HF transmitter/receiver to an antenna via an external medium by means of a balanced two-wire line, the balanced two-wire line being at least partially encapsulated in a central part of a closed conduit including a wall isolating the balanced two-wire line from the external medium, the wall separating the external medium from an internal medium in the closed conduit having dielectric and magnetic permittivity close to that of air. 
     The balanced two-wire line is held in the central part of the conduit by insulative material spreaders disposed along the two-wire line, the distance of the two-wire line away from the wall of the conduit and the shape of the spreaders being determined on the basis of parameters such as the maximum voltage and current that the two-wire line can withstand, the maximum allowed variation of the characteristic impedance of said two-wire line, the mechanical stresses induced by the required curvature. 
     In one embodiment the closed conduit has two ends and at each of the ends an insulative material cap for closing the conduit to isolate the internal medium from the external medium. 
     In another embodiment the conduit is a flexible and watertight material pipe of circular section to provide flexibility and isolation between the external medium and the internal medium. 
     In another embodiment the spreaders are held in position in the flexible pipe by flexible insulative material separators around the balanced two-wire line, a separator being disposed between two consecutive spreaders to absorb the stresses on the spreaders on curvature of the pipe. 
     In another embodiment the pipe includes a metal reinforcing armature in its wall. 
     In another embodiment the metal reinforcing armature is a metal wire wound helically along the wall of the pipe to preserve the flexibility properties of the bushing. 
     In another embodiment the wall of the pipe is metallized in order to prevent any external radio-frequency influence on the two-wire line. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood with the aid of an embodiment of an antenna feed through-lead device of the invention described with reference to the appended drawings, in which: 
         FIG. 1 , already described, shows a prior art two-wire line, 
         FIG. 2  represents an HF transmitter/receiver feeding an antenna via an overhead balanced two-wire line, 
         FIG. 3   a  is a partial view of the feed bushing device of the invention, 
         FIG. 3   b  is a plan view of the feed bushing device from  FIG. 3   b,    
         FIG. 4   a  shows one of the ends of the feed bushing device from  FIGS. 3   a  and  3   b,    
         FIG. 4   b  is a plan view of the end of the feed bushing device from  FIG. 4   a,    
         FIG. 5  is a complete view of a variant of the feed bushing device of the invention from  FIGS. 3   a  and  4   a,    
         FIG. 6  is a diagram of a first application of the feed bushing device of the invention, and 
         FIG. 7  is a diagram of a second application of the feed bushing device of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3   a  is a partial view of the feed bushing device of the invention and  FIG. 3   b  is a plan view of the feed bushing device from  FIG. 3   b.    
       FIG. 3   a  is a partial view in longitudinal section between two planes P 1 , P 2  of a feed bushing device of the invention. In this embodiment, the bushing device includes a pipe  40  of circular section about an axis of revolution XX′ (in a straight portion of the pipe) and a balanced two-wire line  42  (as represented in  FIG. 1 ) encapsulated in a central part of the pipe  40  on the axis XX′. 
     The flexible material pipe  40  includes a separating wall  44  between a medium  46  external to the pipe and an interior medium  48  the dielectric and magnetic permitivity of which are close to that of air. This interior medium  48  is constituted for the most part of air in the present example but it may equally be constituted of any gas or solid or mixture thereof offering a dielectric and magnetic permitivity very close to that of air. 
     The two-wire line  42  is held in the central part of the pipe by insulative material spreaders  50  in the form of circular rings normal to the axis of the pipe, regularly distributed over all of the length of the pipe. The spreaders ensure a constant distance along the pipe between the two-wire line and the wall  44  of the pipe. 
     The physical dimensions of the spreaders (among others, the thickness of the spreaders) are chosen to comply with, on the one hand, mechanical constraints, by virtue of a choice between a maximum thickness to withstand the stresses of bending the feed bushing and a minimum thickness for the retention of the two-wire line at the center of the pipe and, on the other hand, the radio-frequency characteristics by virtue of a choice of a maximum thickness and a distribution of the spreaders along the pipe so as not to modify the properties of the two-wire line too significantly. 
     The shape of the spreaders is also defined on the basis of parameters such as the maximum voltage and current that the two-wire line can withstand, the breakdown voltage between the two-wire line and the wall of the pipe if it is of metal, or the nature of the exterior environment, the maximum allowed variation of the characteristic impedance of said two-wire line, the mechanical stresses induced by the required radius of curvature. 
     The spreaders  50  are held in place inside the pipe  40  by insulative material separators  54  around the two-wire line  42 . 
     The separators  54  absorb the stresses of movement of the spreaders  50  when the pipe  40  is strongly curved. The separators  54  represented in  FIG. 3   a  are helical insulative material spirals around the two-wire line  42 . The materials and the shape of the spirals are chosen to ensure, on the one hand, mechanical properties such as the elastic spring property when the feed bushing is curved and, on the other hand, the radio-frequency properties which, by virtue of their presence in the pipe  40 , produce only acceptable modifications of the characteristics of the two-wire line  42 . 
     The wall  44  of the pipe  40  may be metallized in order to guarantee any influence on the two-wire line  42  of an external medium of a metallic or moist kind and to isolate the line at radio frequencies. 
     The watertightness of the feed bushing device is completed by closure of the pipe  40  at each of its two ends by an insulative closure cap, for example, in composite material of epoxy type that ensures good resistance to mechanical and environment stresses. 
       FIG. 4   a  shows one of the ends of the feed bushing device from  FIGS. 3   a  and  3   b  including a cylindrical closure cap  60  including an opening  64  at its center for the watertight passage of the two-wire line  42  for the connection to other balanced two-wire lines or to HF equipment having balanced two-wire type ports. 
     The closure cap  60  closes the ends of the pipe  40  in watertight fashion to ensure total separation of the interior medium  46  of the feed bushing device relative to the exterior medium  48 . To this end, the cap is for example stuck to the end of the pipe and to the two-wire line where it passes through the opening  64  in the cap. 
       FIG. 4   b  is a plan view of the end of the feed bushing device from  FIG. 4   a.    
     In the usual conditions of use of the feed bushing device of the invention, for example in the case of use of the feed bushing device along a wall or simply on the ground, the crushing force exerted on the pipe remains low. In other cases of use, for example, if the feed bushing device must be buried or resist reduced pressure stresses caused by altitude, the pipe must have an increased resistance to crushing. 
       FIG. 5  is a complete view of a variant of the feed bushing device of the invention from  FIGS. 3   a  and  4   a.    
     In this variant from  FIG. 5 , the polyurethane pipe  40  includes a metal reinforcing armature in its wall  44 . This reinforcing armature may be produced, for example, by a flexible metal wire  65  wound helically along the wall  44  to reinforce its resistance to crushing. The metal wire  65  integrated into the wall of the pipe  40  enables the feed bushing device to be curved with smaller radii of curvature R without deforming the circular section of the pipe. 
     The feed bushing device from  FIG. 5  includes at each of its ends a cap  66 ,  67  for closing the pipe  40 . The balanced two-wire line  42  encapsulated in the pipe  40  projects from the caps  66 ,  67  by a certain length including at one of the ends female banana plugs  68  and at the other end male banana plugs  69  for the connection of the bushing device to other balanced two-wire lines or ports. 
       FIG. 6  is a diagram of a first application of the feed bushing device of the invention. 
     The feed to the antenna  28  from the transmitter/receiver  22  includes a feed bushing device  70  of the invention having a part  72  buried in the ground  30  and two ends  74 ,  76  emerging from the ground  30 . One of the ends of the two-wire line leaving the bushing device  70  is connected to a two-wire input  80  of the antenna  28  and the other end to a two-wire port  82  of the transmitter/receiver  22 . In contrast to the prior art embodiment from  FIG. 1 , no line support  32  is necessary, vegetation presenting no obstacle to the passage of the bushing device, which to a great extent remains invisible. 
       FIG. 7  is a diagram of a second application of the feed bushing device of the invention. 
       FIG. 7  represents the antenna  28  fed by the transmitter/receiver  22  in a place  90  in the vicinity of the antenna. The feed to the antenna  28  from the transmitter/receiver  22  includes a feed bushing device  92  of the invention having a first part  94  passing through a hole  96  in a wall  98  of the place  90  and a second part  100  pressed against and running along the wall to the antenna  28 . The balanced two-wire line of the feed bushing device  92  leaving its protection pipe in the place  90  is connected to the two-wire port  82  of the transmitter/receiver  22 , the other end of the two-wire line of the bushing device  92 , outside that place, is connected to the two-wire input  80  of the antenna  28 . 
     In this embodiment shown in  FIG. 7 , the feed bushing device of the invention enables the two-wire line to be passed through a wall. However, the line may also be passed through a door or a window of the place with a small radius of curvature R without modification of the radio-frequency characteristics of the feed two-wire line of the antenna. The place may be an enclosed and air-conditioned place. 
     The feed bushing device of the invention has the following advantages:
         a low weight, which facilitates its transportation,   good flexibility enabling adaptation to installation constraints, small radius of curvature R, high crushing stresses, adaptability to installation constraints,   a length of the feed bushing that may be adapted as required: from 10 centimeters to a few meters,   watertightness enabling its use in rainy conditions or in moist outdoor environments,   a small volume (transportability, discretion),   resistance to crushing and to reduced pressure stresses,   easy implementation and production,   non-degraded radio-frequency characteristics (characteristic impedance and losses per unit length) independent of the deformation of the bushing device (radius of curvature) and the surrounding medium.