DECOUPLING ARRAYS OF RADlATING ELEMENTS OF AN ANTENNA CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on French Patent Application No. FR 0854140 filed October 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 Field of the invention

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. Description of the prior art

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 cailed "patch" antennas, are widely used at present because of their very small overall size, their extrerneiy simple fabrication technology, and their moderate cost, because they are mass produced.

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.

In order to preserve uniformity of radiation, it is necessary to maintain decoupling of good quality between the arrays of dipoies. 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.

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,

This embodiment applies only to elements contained entirely within a piane 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.

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

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 criss-cross metal filaments forming a grid and adapted always to be placed between two radiating elements.

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. In one embodiment of the invention, the criss-cross 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. The width of the fiiarneπts 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.

The meta! 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.

An additional advantage of the present invention is that it contributes to the mechanical stiffness of the antenna. The present invention applies to base station antennas for mobile telephony in general, and in particular WiMax (Worldwide lnteroperability for Microwave Access) applications.

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 nonlirniting example, of course, and from the appended drawings, BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a diagrammatic representation of a radiating element in a confined environment.

Figure 2 is a diagram showing an antenna of the invention. Figures 3A and 3B represent one embodiment of the invention.

Figures 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 piotted on the abscissa axis. DETAILED DESCRIPTIQN OF THE PREFERRED EMBODIMENT

Figure 1 represents a unit dipole 1 fixed to the bottom 2 of the casing 3 of an antenna and surrounded by metai screens 4. The arrows 5 symbolize the multiple reflections that occur at the screens 4 because of their proximity.

Figure 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 waifs 25 so as Io be positioned over areas in which there is no radiating element 23, in order not to disturb the SWR. Sn 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.

Sn the embodiment of the invention shown in figures 3A and 38, 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- waveSength. To reduce the coupling, the arrays 32 are such that the radiating elements 33 are offset relative to each other by one haff-wavelength.

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.

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 meta! 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 .

Figures 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. 2O dB. The curves 51- 56 on the one hand and 81-68 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 iess than the curves 51-56, reflecting an improvement in the decoupling between fhe arrays, 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.