Patent Publication Number: US-7592965-B2

Title: Decoupling arrays of radiating elements of an antenna

Description:
The present invention relates to a telecommunications antenna as used in particular for cellular telephony, such as, for example, an adaptive array system (AAS) for the WiMax application (Worldwide Interoperability for Microwave Access). 
   An antenna of this type is made up of closely-spaced arrays of radiating elements obtained by the printed circuit technique. Such an antenna is made up of parallel arrays of dipoles placed in a housing that acts as a reflector. These antennas, often referred to as “patch” antennas, are presently in widespread use because of their very small size, of their manufacturing technology that is extremely simple, and of their cost that is moderate since they are mass-produced. 
   Nevertheless, such antennas present difficulties in manufacture because of conflicts that exist between different design criteria. In particular, the mutual coupling that can occur between individual radiating elements when they are close together, even though it can improve the performance of the antenna, nevertheless also presents certain negative effects, such as distorting the spectrum of the antenna or modifying the input impedance of the elements at a given frequency. It is therefore appropriate to limit such coupling without significantly increasing the weight or the size of the antenna. 
   In order to preserve uniform radiation, it is necessary to maintain good quality decoupling between the arrays of dipoles. Usually, the arrays of dipoles are generally isolated from one another by simple screen-forming metal walls. In order to obtain better decoupling, one solution is to increase the height of the screen so as to block electromagnetic transmission between the elements. However, when the walls are very close together, the radiating elements become confined in a small space by the screens on which multiple reflections occur, thereby reducing bandwidth. The performance of the antenna, and in particular its standing wave ratio (SWR), is thus degraded which leads to a mismatch between the input impedance of the antenna and the impedance of the transmitter (when transmission is involved). It is associated with the modulus of the reflection coefficient of the antenna. 
   To solve this problem, proposals have been made to place 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 these radiating elements. The radiating elements and the metal line can be made in particular by etching a layer of copper that covers a dielectric layer. 
   That embodiment is applicable only to elements that are contained completely within a plane parallel to that of the reflector. That solution is not applicable to radiating elements that occupy a plane perpendicular to the reflector, as applies to dipoles. The mechanical structure to be implemented under such circumstances is complex and onerous. 
   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 and the radiating elements, while conserving a high level of decoupling. 
   The present invention provides an antenna comprising at least two arrays comprising respective pluralities of radiating elements in alignment, disposed in parallel planes, and metal screens interposed between the arrays, the antenna being characterized in that each metal screen comprises a bottom portion facing non-radiating portions of the radiating elements and comprising respective plane sheets disposed in planes parallel to the planes of the arrays, and top portions facing the radiating portions of the radiating elements and comprising panels, each forming an angle with the planes of the bottom portions. 
   Advantageously, this angle is no greater than 45°, and preferably lies in the range 25° to 45° so as to deflect reflections, thus preventing them from reaching the dipoles. 
   The total length of a panel preferably lies in the range λ/2 to λ/4, where λ is the wavelength of the center frequency of antenna operation. 
   Each panel is oriented in alternation in one direction and in the other, on either side of the plane of the bottom portion. 
   In a preferred embodiment of the invention, each panel comprises two wings connected to a central zone attached to the bottom portion of the screen. 
   Preferably, the central zones of two panels oriented in the same direction are separated by a distance lying in the range 0.7λto 0.9λ. 
   Preferably, each wing has a height lying in the range λ/7 to λ/11. 
   Also preferably, each wing has a length lying in the range λ/7 to λ/11. 
   The present invention also provides a method of manufacturing an antenna comprising at least two arrays of radiating elements in alignment disposed in parallel planes, and metal screens interposed between the arrays comprising a bottom portion facing non-radiating portions of the radiating elements and comprising respective plane sheets disposed in planes parallel to the planes of the arrays, and top portions facing the radiating portions of the radiating elements and comprising panels, each forming an angle with the planes of the bottom portions. According to the invention, the method of making a screen comprises the following steps in particular:
         cutting at least two spaced-apart vertical slots in a fraction of the height of a plane plate;   extending the slots horizontally towards each other in incomplete manner so as to retain a central zone attached to the non-cut portion of the plate and form wings on either side of the central zone; and   folding the resulting wings on either side of the central zone in alternation in one direction and in the other direction.       

   
     Other characteristics and advantages of the present invention appear on reading the following description of an embodiment, given naturally by way of non-limiting illustration, and from the accompanying drawings, in which: 
       FIG. 1  is a diagrammatic view of a radiating element in a confined environment; 
       FIG. 2  is a diagrammatic plan view of a portion of the antenna of the invention; 
       FIG. 3  is a diagrammatic side view of a screen of the  FIG. 2  antenna; 
       FIG. 4  is a perspective view of an embodiment of an antenna of the invention; 
       FIG. 5  is a detailed view of a portion of the  FIG. 4  antenna; 
       FIG. 6  is a fragmentary perspective view of a screen of the  FIG. 4  antenna; and 
       FIG. 7  is a fragmentary plan view of a screen of the  FIG. 4  antenna. 
   

     FIG. 1  shows a single dipole  1  secured to the bottom  2  of an antenna housing  3  and surrounded by metal screens  4 . The dipole is made up of a non-radiating bottom portion  5  and a radiating top portion  6 . Arrows  7  represent multiple reflections that occur on the screens  4  because of their proximity. 
   A portion of an antenna of the present invention is shown diagrammatically in  FIG. 2 . The antenna comprises two arrays  21  forming parallel rows, made up of individual radiating elements  22 . In order to reduce coupling, the arrays  21  are implanted in such a manner that the radiating elements  22  are offset relative to one another. The arrays  21  are separated by screens  23  which are parallel thereto along the longitudinal axis Z-Z of the antenna.  FIG. 3  is a side view of a screen  23  looking in the transverse direction X-X of the antenna. The bottom portion  24  of the screen  23  is constituted by a plane metal sheet of height that is of the same order as the height of the non-radiating bottom portion of the individual radiating elements  22 . The screen  23  also has a high portion  25  made up of panels  26  following one another in the longitudinal direction Z-Z. Each panel  26  is made up of a central zone  27  extending the bottom portion, and lying between two wings  28 . The panels  26  are inclined so as to make an angle with the longitudinal axis Z-Z of the antenna, alternating in one direction and in the opposite direction relative to the axis Z-Z, thus forming a substantially zigzag line  29 . Contiguous screens  23  are installed in such a manner that the concave and convex direction changes of the zigzag line  29  correspond so as to form a deflector around each radiating element  22  as represented by arrows  30  for the purpose of deflecting reflections and thus preventing them from returning to the radiating elements  22 . 
   In the embodiment shown in  FIGS. 4 and 5 , there can be seen an antenna  41  of the invention comprising four arrays  42  of individual radiating elements or dipoles  43  in alignment, forming parallel plane rows disposed perpendicularly to the base  44  of the antenna  41  on which they are secured. The dipoles  43  are made on a printed circuit. For reasons of radio-frequency performance, the distance between the arrays  42  is half a wavelength λ, where λ is the wavelength at the center frequency of antenna operation. In order to reduce coupling, the arrays  42  are implanted so that the dipoles  43  are offset relative to one another by one half-wavelength. The arrays  42  are secured to the base  44  of the antenna  41  that has four inlet connectors  45  each corresponding to a respective one of the four arrays  42  of radiating elements  43  as shown. Each dipole  43  has a non-radiating bottom portion  46  surmounted by a radiating top portion  47 . Between the arrays  42 , there are placed parallel metal screens  48  presenting total height of substantially the same order as the height of the arrays  42 . Each screen  48  has a plane bottom portion  49  and a top portion  50  presenting panels  51  extending in a plane different from that of the plane bottom portion  49 . The plane of the bottom portion  49  is also the mean plane P-P of the screen  48 . Each panel  51  has a central zone  52  extending the bottom portion  49  and wings  53  cut out on either side of the central zone  52 . 
   A screen  48  of the invention is shown in an enlarged view in  FIGS. 6 and 7 . The bottom portion  49  is of a height that corresponds to the height of the non-radiating portions  46  of the facing dipoles  43 . The top portion  50  extends the bottom portion  49  and faces the radiating portions  47  of the dipoles  43 . The top portion  50  presents cuts in the form of regularly-spaced vertical slots that are extended on either side by incomplete horizontal cuts leaving between each pair of slots a central zone  52  that remains attached to the bottom portion  49 . On either side of each central zone  52  there are two wings  53 , folded towards opposite sides of the mean plane P-P of the screen  48  containing the central zone  52 , in such a manner as to form an angle  54  with the mean plane P-P lying in the range 25° to 45°. The total length L of a panel  51  lies in the range λ/2 to λ/4. Adjacent wings  53  connected to different central zones  52  are folded symmetrically to the same side, such that two successive panels  51  are both oriented in respective different directions symmetrically relative to the mean plane P-P of the screen  48 . The distance D between two central zones  52  included in panels  51  that are oriented in the same direction lies in the range 0.7λ and 0.9λ. In the embodiment shown, the wings  53  are preferably of a height h lying in the range λ/7 and λ/11, and of a length d lying in the range λ/7 and λ/11.