RELATED ART AND BACKGROUND OF THE INVENTION Basically, the radiating antenna elements, e. g. in the form of radiating apertures, may be formed in a planar portion of the reflector itself, in which case the PCB is provided with a conductive layer on one side only (and the reflector serves as a ground plane on the front side), or on the PCB, in which case the PCB normally has two conductive layers, one on each side thereof.

In the former case, there are problems with mechanical tolerances, since radiating apertures have to be formed in the rigid metal sheet constituting the reflector, e. g. by a punching process, which involves a slight deformation of the metal sheet forming the ground plane for the microstrip lines in the feeding network.

In the latter case, on the other hand, see e. g. WO 97/43799 (Allgon), the ground plane layer of the PCB, which is located on the front side of the reflector, has to be RF-coupled to the reflector, which leads to structural complications.

Alternatively, see e. g. US 5.896,107 (Allen Telecom), the PCB is held in place in longitudinal side walls being unitary with the reflector and extending therealong so as to hold the PCB

in front of the reflector. This structure will generally reduce the possibilities of shaping the reflector in an optimal way for achieving a desired radiation pattern.

Reference is also made to the two Swedish applications 9904369-7 (Allgon) and 9904370-5 (Allgon), from which priority is claimed. The contents of these two applications are included herein by reference.

SUMMARY OF THE INVENTION Against this background, the main object of the invention is to provide an antenna assembly of the kind discussed above which permits the production of a well-defined structure with high precision of the critical dimensions of the assembly and which enables an easy and inexpensive mounting method.

This object is achieved for an antenna assembly having the features stated in claim 1. Accordingly, a ground plane layer is located on the front side of the PCB, and each radiating element thereon is confined within a corresponding opening in a planar portion of the reflector. Peripheral portions of the ground plane layer on the PCB are held in close proximity to the reflector around the contour of each opening so as to provide a capacitive or conductive coupling therebetween.

In this way, the radiating elements on the PCB can be formed by known techniques, e. g. by etching so as to provide precise dimensions of these elements. Furthermore, the reflector will form an extension, both geometrically and electrically, of the ground plane on the front side of the PCB. The reflector portions surrounding each opening is positioned exactly in direct contact with or at a well-defined distance (defined by the thickness of a thin intermediary layer such as an adhesive tape) from the peripheral portions of the ground plane on the

front side of the PCB. Therefore, the radiating elements and the adjoining portions of the reflector can be configured with great precision and with a shape adapted to the desired radiation pattern.

There are many practical embodiments of the antenna assembly according to the invention, as will be apparent from the detailed description below. Thus, the invention will now be explained in more detail with reference to the appended drawings illustrating some preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates schematically, in an exploded, perspective view, various parts of the antenna assembly before being mounted on a fixture at the bottom of the figure; Fig. 2 shows a cross-section through a first embodiment of the antenna assembly according to the invention; Fig. 3 shows, in a cross-sectional partial view, a modified version of the first embodiment shown in figs. 1 and 2; Fig. 4 shows, in a perspective, partially cross-sectional view, a second embodiment of the antenna assembly according to the invention; Figs. 5 and 6 show, in cross-sectional views, two modified versions of the second embodiment shown in fig. 4; and Figs. 7-10 show various embodiments of the radiating elements on the front side of the PCB.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In fig. 1 there is illustrated, at the bottom, a fixture F for mounting an antenna assembly 1, the major parts 10,20,30,40

of which are shown above the fixture F. The antenna assmbly is suitable for transmitting and/or receiving RF electromagnetic waves, e. g. at a base station in a cellular mobile telephone system.

Basically, the antenna assembly 1 comprises a relatively flexible printed circuit board 10 (PCB) of a dielectric material with electrically conductive layers on both sides thereof, a reflector body 20 of a rigid metal sheet having a central, substantially planar portion 21, a number of radiating patches 30 of an electrically conductive material, and an external casing or radome 40 of a dielectric material.

In the preferred embodiments shown in the drawings, the basic parts 10,20 and 40 are elongated in a longitudinal direction, as will be apparent from fig. 1, and contain a linear row of antenna elements.

As shown also in fig. 2, the PCB 10 has on its front side (the upper side in fig. 1) a thin metal coating constituting a ground plane layer 11. This ground plane layer 11 is provided with a number of apertures 12 in the form of mutually crossing slots 12a, 12b serving as radiating elements in order to couple high frequency electromagnetic power between a feed network on the rear side of the PCB and the radiating patches 30. The general structure of such an antenna assembly is described in more detail in the above-mentioned document WO 97/43977.

The feed network on the rear side of the PCB includes microstrips 13 and associated feed elements (not shown) in registry with the respective radiating aperture 12, e. g. of the kind described in the document W098/33234 (Allgon). Thus, two orthogonally polarised microwave carriers, with mobile telephone signals superposed thereon, are transferred between each pair of feed elements and the respective radiating patch

30. In order to prevent backward radiation and the propagation of electromagnetic radiation in the longitudinal direction on the rear side of the PCB, a number of shielding boxes 14 of a metal material are secured to the PCB behind each radiating aperture 12. The boxes 14 are secured mechanically and electrically to the upper ground layer 11 by means of closely distributed soldered pins 15.

The elongated PCB, with the ground plane layer 11 and the radiating apertures 12 on the front side, and with the microstrips 13 and the shielding boxes 14 on the rear side, is first made as a unit 10, which is positioned onto the fixture F with the boxes 14 fitting into corresponding recesses R in the fixture. Then, in accordance with the present invention, the reflector 20 is secured to the PCB by securing the peripheral portions of the ground plane layer 11, outside the respective region containing the apertures 12 (or corresponding radiating elements), to the substantially flat or planar portion 21 of the reflector. For this purpose, and in accordance with the present invention, the reflector 20 is provided with a number of openings 22 in the planar portion 21. Each such opening is symmetric in relation to the centre of the associated radiating aperture 12. It is important that the reflector is securely fastened to the PCB along the contour of the respective opening 22, i. e. along the edges 22a of the openings 22 which are generally rectangular or square in the embodiment shown in figs. 1 and 2.

According to a preferred method, the reflector 20 is secured to the PCB by means of an adhesive tape. Such a tape, having an adhesive layer on each side thereof, is applied to lower side of the planar portion 21 of the reflector body 20. The adhesive tape should cover the regions surrounding the

openings 22, but not the openings as such. Therefore, the adhesive tape is preferably applied before the openings are punched out of the reflector body (and out of the tape). Next, the reflector body 20, with the adhesive tape 16 (fig. 2), is lowered onto the PCB, so that the planar portion 21 is fitted and securely fastened to the PCB.

Thereupon, the patches are mounted at a distance from the reflector, e. g. by means of plastic legs with snap fittings, and the radome 40 is finally fitted onto the reflector to complete the assembly. Of course, there are several mechanical and electrical parts, which have to be fitted into the total structure. For clarity, these parts are not shown in the drawings.

It is essential that the reflector body 20 is secured in a well-defined position in relation to the ground plane layer 11, so that a good electrical coupling is achieved, in this case in the form of a capacitive coupling. Also, it is important to establish a well-defined mechanical bond, so that the radiation parameters are obtained as desired and calculated in advance. The radiating aperture 12 is formed with high precision, e. g. by etching of the conductive layer 11, and the planar portion 21 of the reflector will form a well-defined mechanical and electrical extension of the ground plane layer 11. The opening 22 should circumscribe the radiating aperture 12 and leave that region of the ground plane totally free, so that the electromagnetic radiation can propagate freely therethrough. However, still, the other portions of the reflector 20 may be configured at will in order to obtain the desired radiation pattern.

In the example shown in figs. 1 and 2, the reflector 20 is provided with side portions 23, which extend generally obliquely backwards towards the rear of the assembly.

As shown in fig. 3, the reflector may alternatively be secured to the edge portions of the PCB by means of mechanical fasteners 16a, such as screw fasteners or rivets (not shown).

It is also possible to make soldered connections by applying a soldering paste and processing the assembly in an oven. In these alternative fastening methods, there will be a direct electrical contact between the metal material of the reflector 20 (the planar portion 21 therof) and the ground plane layer 11 (as illustrated in fig. 3).

Possible other embodiments are illustrated in figs. 4-10. In fig 4, there is shown a second embodiment of the antenna assembly 1', where the reflector 20'is formed as a closed housing with a bottom wall 20'a, outer side walls 20'b, upper corrugated wall portions 20'c (serving as cooling flanges), inner longitudinal side walls 20'd, which diverge away from each other obliquely upwards (towards the front side of the assembly, in the forward direction of the radiated power), a central, horizontal or planar wall portion 20'e and inner wall portions 20'f defining a shielding box. As in the previous embodiment, there is a rectangular (or square) opening 22'in the central, planar wall portion of the reflector which permits a free propagation of radiated microwave power from a radiating aperture 12' (fed by a feeding element 13') in the upper ground plane layer 11'on the front side of the PCB 10'.

In this embodiment, the inner wall portions 20'f divide the PCB 10'into a central portion and two longitudinal outer portions.

These three portions of the PCB can be provided with mutually separated circuitry, e. g. transceiver circuitry included in a transmitter/receiver forming parts of a cellular base station in a mobile telephone network, and the radiating elements 12', respectively. In the upper part of the flared opening defined by the opening 22'and the diverging inner side walls 20'd, there is a radiating patch 30'operating as the effective radiating element of the antenna assembly 1'.

In fig. 5 there is shown a modified version of the second embodiment, where corresponding parts are provided with numerals having a bis notation ("). The circuitry on the longitudinal, outer portions of the PCB are denoted 17".

Instead of a separate shielded box, there is a shielding box 14"enclosing a central portion of the PCB, centered around the radiating aperture 12", and containing a central strip line 13" (instead of the microstrip 13 in the first embodiment).

Fig. 6 shows another modified version of the second embodiment, where corresponding parts are provided with numerals having a ter notation ("'). In this case the PCB 10"' is secured to the planar portion 21"'by means of screws 16"'a extending through the bottom wall 20"'of the reflector housing, spacer elements 18"', the PCB 10"'and the central planar portion 21"'of the reflector.

In figs. 7-10 there are illustrated various forms of radiating elements on the front side of the PCB, inside the respective opening 22,22'or 22". In fig. 7, there is a single slot 12' in the ground plane layer 11', fed by a lower feeding element 13'in the form of a microstrip line, as in the embodiment shown in fig. 4.

In fig. 8, there is shown a similar slot 12'a in a ground plane layer 11'a, fed by a feeding element 13'a for feeding two different patches 30'and 31'operating in two different frequency bands or in a single, relatively wide band.

Fig. 9 illustrates a ground plane layer llb, in which there is formed a dipole arm 12b co-operating with a supplementary dipole arm of a feeding element 13b, so as to form a dipole- type radiating element 12b, 13b.

In fig. 10, finally, there is shown a ground plane layer lie being cut out by etching into a spiral-like configuration 12c, which is connected (through the PCB) with a feeding element so as to form a radiating element.

In this disclosure, it is to be understood that each radiating element is operable to transmit and/or receive RF signals.

Thus, the antenna assembly can be used in combination with a transmitter and/or a receiver.