Patent Publication Number: US-7586454-B2

Title: Method of assembling a radiocommunication antenna, radiocommunication antenna assembled by such a method, and device adapted to implement such an assembly method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based on French Patent Application No. FR 0553937 filed on Dec. 19, 2005, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. §119. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method of assembling a radiocommunication antenna, a radiocommunication antenna assembled by such a method, and a device adapted to implement such an assembly method. 
   2. Description of the Prior Art 
   A radiocommunication antenna  10  ( FIG. 1 ) may comprise a main reflector  12  the shape whereof, for example that of a circular symmetry paraboloid, focuses the received or transmitted electromagnetic waves at a subreflector  14 . 
   That subreflector  14  is connected to the feeder device  11  of the antenna  10  via a waveguide  16  of generally circular section. 
   Such a circular section waveguide  16  may have imperfections that cause a modification of the plane of propagation of a polarized electromagnetic field transmitted by the waveguide, as explained hereinafter with the assistance of  FIG. 2   a.    
     FIG. 2   a  is a front view of the waveguide  16  described above, this representation foregrounding the elliptical shape of certain sections of the waveguide  16 . 
   These elliptical sections cause an offset between the plane of propagation of a polarized electromagnetic field  17  entering the guide  16  and the plane of propagation of the electromagnetic field  18  leaving the guide  16 . 
   Such an offset between the planes of propagation of the electromagnetic fields entering and leaving the guide is undesirable since it may cause interference between adjacent antennas. In fact, each antenna is defined by an ‘ideal’ transmission plan along which the transmitted signals theoretically propagate, the use of this ideal plane enabling different adjacent antennas to be allocated separate propagation planes in order to limit the interference between antennas. 
   Consequently, the offset introduced by a waveguide in the plane of propagation of a polarized electromagnetic field can limit the number of antennas that may be placed in the same vicinity. 
   This is why an antenna manufacturer is obliged to limit the offset of the plane of propagation of the polarized electromagnetic field transmitted, which offset can be evaluated by a parameter such as the transverse discrimination of the antenna, also called the cross polar discrimination (XPD). 
   More precisely, the XPD of an antenna fed by a plane electromagnetic field corresponds to the ratio in dB between the power Pc transmitted by the antenna in the component coplanar with the electromagnetic field provided and the power Pt transmitted by the antenna in the component transverse to, i.e. at a right-angle or 90° to, the electromagnetic field supplied, in accordance with the following formula:
 
 G=− 10 log( Pc/Pt )
 
these powers being measured over a particular angular aperture as a function of the standards concerned.
 
   To limit the offset introduced by a waveguide, it is known to use precise, and therefore long and costly, machining techniques so that the imperfections of the waveguide are limited. 
   The present invention results from the observation that a circular section waveguide in practice, and despite its imperfections, exhibits circular symmetry about its longitudinal axis and, because of this, it is possible to fix the waveguide to a reflector at any position obtained by pivoting the cylindrical waveguide relative to its longitudinal axis. 
   The invention is also a result of the observation that, as described hereinafter with the assistance of  FIG. 2   b , the offset electromagnetic field  18  may be considered as the sum of an electromagnetic field  18   a  propagating in a plane coplanar with the incoming electromagnetic field and an electromagnetic field  18   b  propagating in a plane transverse to or perpendicular to that incoming electromagnetic field. 
   SUMMARY OF THE INVENTION 
   This is why the present invention relates to a method of assembling a radiocommunication antenna comprising a reflector connected to a subreflector via a circular section waveguide extending along a longitudinal axis, comprising the following steps:
         the step of pivoting the waveguide about its longitudinal axis to determine a position such that an offset of the plane of propagation of a polarized electromagnetic field transmitted by this guide is limited,   the step of marking this position on the waveguide, and   the step of fitting the waveguide to the reflector as a function of this mark.       

   Such a method optimizes the use of a circular section waveguide by enabling the fitting of the waveguide to a reflector at a position that minimizes the offset caused by the waveguide between the plane of propagation of the polarized electromagnetic field introduced into the guide and the plane of propagation of the polarized electromagnetic field leaving the guide. 
   This method is simple and quick to implement using a device of low cost. It enables the use of waveguides having a circular section with imperfections which, without this method, would introduce excessive offsets of the plane of propagation of the transmitted electromagnetic field and leading, for example, to an XPD incompatible with their application. Thus the cost of the waveguide and consequently of the antenna is reduced. 
   In one embodiment, the method further comprises the step of measuring a component of the electromagnetic field leaving the waveguide in a plane transverse to the plane of propagation of the polarized electromagnetic field entering the waveguide. It is therefore particularly simple to determine the offset caused by the waveguide. 
   In one embodiment, the method further comprises the step of measuring the offset caused by the waveguide by means of a transition guide between the circular section of the waveguide and a rectangular section. Such an embodiment enables the transition guide to be pivoted 90°, for example, to measure the transverse component of the electromagnetic field leaving the waveguide in one embodiment of the invention. 
   In one embodiment, the method further comprises the step of measuring the offset caused by the waveguide by comparing the power radiated at the exit from the waveguide in a plane with a power supplied to the entry of the waveguide. 
   The invention also relates to a radiocommunication antenna comprising a reflector connected to a subreflector via a waveguide having a circular section and extending along a longitudinal axis, and comprising a reference on the waveguide for determining a relative position of the waveguide vis-à-vis the reflector. 
   Such an antenna can limit the offset of the plane of propagation of an electromagnetic field when the latter is transmitted by the guide and the reference has been determined by one of the above embodiments of the method. 
   In one embodiment, the subreflector also comprises a mark for determining a position vis-à-vis the reflector for fitting the waveguide, thereby facilitating the fitting of the waveguide to the reflector. 
   The invention also relates to a device for assembling a radiocommunication antenna comprising a reflector connected to a subreflector via a circular section waveguide extending along a longitudinal axis, which device comprises:
         means for pivoting the waveguide about its longitudinal axis and means for determining a position such that the plane of propagation of a polarized electromagnetic field transmitted by the waveguide is offset in limited fashion, and   means for marking this position on the waveguide.       

   Such a device enables implementation of any of the above embodiments of the method. 
   In one embodiment, the device comprises means for measuring a component of the electromagnetic field leaving the waveguide in a plane transverse to the plane of propagation of the polarized electromagnetic wave entering the waveguide. 
   In one embodiment, the device comprises, at the exit of the waveguide, a transition guide between the circular section of the waveguide and a rectangular section. In this case, and in one embodiment, the device comprises means for pivoting the transition guide 90°. 
   In one embodiment, the device comprises means for comparing the power radiated at the exit of the waveguide, in a plane, with a power supplied at the entry of the waveguide. 
   Other features and advantages of the invention will become apparent in the light of the following description, given by way of illustrative and nonlimiting example, of embodiments of the invention referring to the appended figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1 , already described, is a diagram representative of the elements guiding the electromagnetic waves in an antenna. 
       FIGS. 2   a  and  2   b , already described, represent the offset introduced in the plane of propagation of a polarized electromagnetic field transmitted by a circular section waveguide. 
       FIG. 3  represents a device for implementing a method according to the invention. 
       FIGS. 4   a ,  4   b ,  4   c  and  4   d  represent various steps of a method according to the invention using the device described with reference to  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In the figures described hereinafter, elements of the same kind or having the same function are identified by the same reference. 
     FIG. 3  represents a device  30  for determining a position of a waveguide  32  that minimizes the offsetting of the plane of propagation of a polarized electromagnetic field supplied to the waveguide  32 , this position of the guide being determined relative to this entry propagation plane. 
   To this end, the device  30  comprises, in this embodiment, two guides  33  and  34  making the transition between a rectangular section and a circular section, these transition guides  33  and  34  being situated at each end of the circular waveguide  32 . 
   On the one hand, the transition guide  33  is used to supply the electromagnetic field E entering the waveguide  32  in a particular propagation plane. 
   On the other hand, the transition guide  34  is used to obtain only one component of the electromagnetic field E leaving the waveguide  32  in a detection plane determined by the orientation of the transition guide  34 . 
   To vary this detection plane, the device  30  also includes means  35 , such as U-shaped supports, for pivoting or turning the waveguide  32  relative to the axis of circular symmetry of the waveguide  32 , also referred to hereinafter as the longitudinal axis  36 . 
   These U-shaped supports  35  enable the waveguide  32  to be rotated without modifying the orientation of the transition guides  33  and  34 . They also enable the transition guide  34  to be pivoted with the waveguide  32  remaining fixed. 
   Finally, the device  30  comprises means  38 , such as a metal stylus, for making a mark on the waveguide  32 , this mark identifying the optimum position of the guide  32  relative to the transition guide  33  or its corollary, the plane of propagation of the polarized electromagnetic field introduced via this transition guide  33 . 
   According to the invention, this optimum position is determined by measuring the component of the electromagnetic field leaving the guide  32  that propagates in a plane transverse to, or perpendicular to, the plane of propagation of the electromagnetic field introduced into the guide  32 . 
   To this end, this transverse component is measured for different positions of the waveguide relative to the transition guide  32 , those positions being obtained by turning the latter about its longitudinal axis  36  as described hereinafter with the assistance of  FIGS. 4   a ,  4   b ,  4   c  and  4   d.    
   Those figures represent the waveguide  32 , its longitudinal axis  36  and the transition guides  33  and  34  situated at the entry and the exit of the waveguide  32 , respectively. 
   During the first step ( FIG. 4   a ), the device  30  measures the component of the electromagnetic field E leaving the guide  32  coplanar with the electromagnetic field E supplied to the waveguide  32 . 
   To this end, the transition guides  33  and  34  are symmetrical relative to the waveguide  32  and a probe  39  supplies a signal representative of the power of the radiation leaving the guide, which power can be compared via a comparator  40  with the power measured at the entry of the guide  32 . 
   The result of the comparison is displayed on a screen  42  representing, in dB, the result of this comparison along the ordinate axis  44 . 
   During a second step, the transition guide  34  is tilted 90° ( FIG. 4   b ) so that only the transverse component of the electromagnetic field E is transmitted by the transition guide  34 . 
   The comparator  40  then supplies a signal representative of the power associated with this transverse component of the electromagnetic field E leaving the waveguide. 
   Thanks to such a signal, it is possible to determine ( FIG. 4   c ) the position of the waveguide that minimizes this transverse component by pivoting the guide  32  relative to its axis  36  whilst at the same time observing on the screen  42  the power associated with the transverse component of the electromagnetic field leaving the guide  32 . 
   When the position of the guide minimizing the transverse field E has been identified, the waveguide is marked with a reference  46  for identifying the relative position on a reflector that the guide  32  should have vis-à-vis the plane of propagation of the incoming electromagnetic field. 
   In fact, the reference or mark  46  represents the optimum position of the guide  32  relative to the plane of propagation of the electromagnetic field E supplied to the guide so that this plane of propagation of the incoming electromagnetic field can also be identified on the reflector by a second reference or mark in order to enable the waveguide to be fitted to the reflector with the assistance of these two marks.