Abstract:
The present invention consists in an antenna including at least two arrays of radiating elements disposed linearly and parallel, plane metal screens being interleaved between the arrays. According to the invention screening means, added above the radiating elements, comprise crisscross metal filaments forming a grid and adapted always to be placed between two radiating elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based on French Patent Application No. FR 0654140 filed Oct. 9 2006, 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  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a telecommunication antenna, used in particular for cellular telephony. This kind of antenna is formed of arrays of closely spaced radiating elements.  
         [0004]     2. Description of the Prior Art  
         [0005]     Antennas of this type are obtained by means of the printed circuit technique and consist of parallel arrays of dipoles that are placed in a casing serving as a reflector. These antennas, usually called “patch” antennas, are widely used at present because of their very small overall size, their extremely simple fabrication technology, and their moderate cost, because they are mass produced.  
         [0006]     These antennas nevertheless are subject to production difficulties because of conflicts that exist between the various design criteria. In particular, although the mutual coupling that can occur between the individual radiating elements when they are close together improves the performance of the antenna, it also has certain negative effects, such as distortion of the spectrum of the antenna or modification of the input impedance of the elements for a given frequency. It is therefore a question of limiting this coupling without significantly increasing either the weight or the overall size of the antenna.  
         [0007]     In order to preserve uniformity of radiation, it is necessary to maintain decoupling of good quality between the arrays of dipoles. The arrays of dipoles are usually isolated from each other by simple metal walls forming screens. One solution for obtaining improved decoupling is to increase the height of the screen to block electromagnetic transmission between the elements. However, if the walls are very close together, the radiating elements are confined within a small space created by the screens at which multiple reflections occur that reduce the bandwidth. This degrades the performance of the antenna, in particular the standing wave ratio (SWR), which is reflected in a mismatch between the input impedance of the antenna and that of the transmitter (in the case of transmission). It is linked to the modulus of the coefficient of reflection of the antenna.  
         [0008]     To solve this problem, it has been proposed to dispose radiating elements side by side on a reflector, for example. A conductive metal line placed in the same plane as the elements and connected to ground and to the reflector surrounds the radiating elements. The radiating elements and the metal line can in particular be produced by etching a layer of copper covering a dielectric layer.  
         [0009]     This embodiment applies only to elements contained entirely within a plane parallel to that of the reflector. This solution is not applicable to radiating elements that are in a plane perpendicular to the reflector, as is the case with dipoles. The mechanical structure to be used in this case is complex and costly.  
         [0010]     An object of the present invention is to eliminate the drawbacks of the prior art, and in particular to minimize the reflections that exist between the metal walls of the antennas and the radiating elements, at the same time as maintaining a high level of decoupling without reducing the frequency band.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention consists in an antenna including at least two arrays of radiating elements disposed linearly and parallel, plane metal screens being interleaved between the arrays. According to the invention, screening means are added above the radiating elements. These screening means comprise crisscross metal filaments forming a grid and adapted always to be placed between two radiating elements.  
         [0012]     The screening means are disposed in a plane perpendicular to that of the metal screens separating the arrays, and thus in a plane perpendicular to the arrays.  
         [0013]     In one embodiment of the invention, the crisscross filaments form a grid extending over the entire width of the antenna. They extend over the transverse dimension of the antenna so as to cross the screens and the parallel arrays. The number of filaments used depends on the level of isolation required. The filaments are advantageously fixed to the lateral walls.  
         [0014]     The width of the filaments is preferably from one fifteenth ( 1/15 th ) to one twenty fifth ( 1/25 th ) of the wavelength at the center frequency, and preferably of the order of one twentieth ( 1/20 th ) of the wavelength.  
         [0015]     The metal filaments have a negligible influence on the SWR but significantly improve the decoupling between the array elements with a gain that can be as high as 3 to 5 dB. In parallel with this, the height of the metal screens can be limited to the value sufficient to obtain a satisfactory SWR over the frequency band.  
         [0016]     An additional advantage of the present invention is that it contributes to the mechanical stiffness of the antenna.  
         [0017]     The present invention applies to base station antennas for mobile telephony in general, and in particular WiMax (Worldwide Interoperability for Microwave Access) applications.  
         [0018]     Other features and advantages of the present invention will become apparent on reading the following description of one embodiment, given by way of illustrative and nonlimiting example, of course, and from the appended drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a diagrammatic representation of a radiating element in a confined environment.  
         [0020]      FIG. 2  is a diagram showing an antenna of the invention.  
         [0021]      FIGS. 3A and 3B  represent one embodiment of the invention.  
         [0022]      FIGS. 4A and 4B  are curves showing the decoupling between the arrays of radiating elements respectively for a prior art antenna and for an antenna according to the invention; the amplitude A in decibels (dB) is plotted on the ordinate axis and the frequency F in gigahertz (GHz) is plotted on the abscissa axis. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]      FIG. 1  represents a unit dipole  1  fixed to the bottom  2  of the casing  3  of an antenna and surrounded by metal screens  4 . The arrows  5  symbolize the multiple reflections that occur at the screens  4  because of their proximity.  
         [0024]      FIG. 2  is a diagram of an antenna  21  according to the present invention. The antenna  21  comprises four arrays  22  made up of unit radiating elements  23 . The arrays  22  form parallel rows separated by screens  24  and framed by the lateral walls  25  of the casing of the antenna  21 . The screening means  26 , disposed above the arrays  22 , are here made up of filaments  27  fixed to the lateral walls  25  so as to be positioned over areas in which there is no radiating element  23 , in order not to disturb the SWR. In the present case in which the arrays  22  each include six dipoles  23 , it suffices to use four filaments  27  to achieve the required isolation performance.  
         [0025]     In the embodiment of the invention shown in  FIGS. 3A and 3B , there is represented an antenna  31  according to the invention comprising four arrays  32  of aligned individual radiating elements or dipoles  33 , forming plane and parallel rows. The radiating element  33  is produced on a printed circuit. For reasons of radio-frequency performance, the distance separating the arrays  32  is one half-wavelength. To reduce the coupling, the arrays  32  are such that the radiating elements  33  are offset relative to each other by one half-wavelength.  
         [0026]     Between and parallel to the arrays  32  are disposed metal screens  34  having a height of the same order as the height of the arrays. The antenna  31  includes a casing  35  forming a base  36  and laterals walls  37  for the arrays  32  and the screens  34 . The casing  35  carries four input connectors  38  each corresponding to one of the four arrays  32  of radiating elements  33  that are represented here.  
         [0027]     According to the invention, screening means  39  are further disposed above the elements  32  and the screens  34 . These means  39  are made up of criss-cross metal filaments  40  forming a grid. The filaments  40  extend the whole width of the antenna and are disposed between two radiating elements  33  so as not to disturb the SWR. In the present case these means  39  are in a plane perpendicular to the plane of the screens  34  and the arrays  32 , thus closing the casing  35 . The width of the filaments  40  is of the order of one twentieth ( 1/20 th ) of the wavelength.  
         [0028]      FIGS. 4A and 4B  respectively show the performance obtained with a prior art antenna and an antenna according to the invention. The line  50  represents the amplitude reference line of the applicable specifications, i.e. 20 dB. The curves  51 - 56  on the one hand and  61 - 66  on the other hand correspond to measurements effected at the input connectors of the antenna taken two by two. The curves  51 - 56  obtained with a prior art antenna must be compared one by one with the respective curves  61 - 66  obtained with an antenna according to the invention. It is found that the curves  61 - 66  have an amplitude less than the curves  51 - 56 , reflecting an improvement in the decoupling between the arrays.  
         [0029]     The present invention is not limited to the embodiments that have been described explicitly, but encompasses diverse variants and generalizations thereof that will be evident to the person skilled in the art. In particular, without departing from the scope of the invention, the screening means can be fastened to the radome that protects the radiating structure of the antenna, in particular in the form of strips of metal having the characteristics of the filaments described hereinabove that are fixed (stuck) to the internal face of the radome.