Abstract:
The invention relates to a dual polarised microstrip patch antenna comprising at least one individual element, each individual element comprising at least one rectangular, preferably quadratic, patch arranged on the upper face of a printed circuit board, having a feed network on the upper side thereof and being metallized on the entire surface of the lower face thereof. The aim of the invention is to improve the polarization isolation, while simultaneously simplifying the feed network. To this end, the feed network is embodied in such a way that the feed is only fed on two corners of the patch, and the at least one patch is modified in such a way that the isolation is improved between the polarizations of at least one antenna element and a plurality of individual antenna elements in relation to a non-modified patch.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of International Application No. PCT/CH2003/000481 having an international filing date of Jul. 16, 2003, which designated the United States, the entirety of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of antenna technology. It relates in particular to a dual-polarized microstrip patch antenna. 
   BACKGROUND OF THE INVENTION 
   Network operators use the principle of polarization diversity in order to improve the transmission characteristics of a radio system. The conversion of linearly vertically polarized antennas to dual linearly polarized antennas took place several years ago in the GSM range (900 MHz and 1800 MHz). Only dual linearly polarized antennas have ever been used, from the start, in the UMTS range (2100 MHz). The requirement for dual linearly polarized antennas is now also being increasingly adopted in the WLAN range (2.4 GHz and 5.6 GHz). 
   Many of the dual linearly polarized antennas which have been proposed in the past are based on so-called SSFIP technology (SSFIP=Strip Slot Foam Inverted Patch), that is to say they relate to a slot-coupled patch antenna (see, for example, U.S. Pat. No. 5,355,143 (Zürcher et al.) or WO-A1-99/17403 (Sanzgiri et al.) or WO-Al-98/54785). One major disadvantage of these antennas is that the slot emits on both sides: on the one hand in the desired direction towards the patch and on the other hand in the opposite direction towards the reflector. This causes undesirable wave propagation, which leads to coupling between the polarizations of individual elements. Furthermore, undesirable coupling occurs between the individual antenna elements in an array of individual elements. In the past, it has been possible to suppress this coupling by suitable measures to such an extent that it was possible to achieve 30 dB isolation, which is the minimum requirement. As can easily be imagined, this disadvantage becomes more noticeable and limiting at higher frequencies. 
   The above disadvantage can be avoided by using a microstrip patch antenna. The isolation of a dual linearly polarized microstrip patch antenna is about 15 dB. The article by S. Assailly et al. “Some Results on Broad-Band Microstrip Antenna with Low Cross Polar and High Gain”, IEEE Trans. Antennas Propagat. Vol. 39, no. 3, p. 413-415 (March 1991), describes one option for improvement of the isolation. All 4 corners of the patch are fed, with the respectively opposite corner being fed with the phase shift of 180°. This results in very good isolation, although this solution has the disadvantage that it requires a relatively complicated feed network. 
   SUMMARY OF THE INVENTION 
   One object of the invention is thus to provide a dual polarized microstrip patch antenna, which requires only a simple feed network, that achieves considerably better isolation than SSFIP-based antennas. 
   This object is achieved by the dual polarized microstrip patch antenna of the present invention by having one or more individual elements with at least one rectangular patch and a feed network on the upper face of a printed circuit board with the lower face of the printed circuit board being completely metallized and a feed network feeding only two corners of the patch. The patch has modifications to improve isolation between the polarizations of at least one antenna element and a plurality of individual antenna elements. 
   In a first embodiment of the present invention, the modifications are arranged on the edges of the patch. These modifications may include two notches on opposite edges of the patch which, in particular, are rectangular and have a width of up to about 0.1λ and a depth of up to about 0.1λ, where λ is the wavelength at the operating frequency of the antenna. However, the modifications may also be encompassed by two lugs on opposite edges of the patch which, in particular, are rectangular and have a width of up to about 0.1λ and a depth of up to about 0.1 λ, where λ is the wavelength at the operating frequency of the antenna. Additionally, it is also feasible for the modifications to comprise cut-off corners at the corners of the patch, in which case, in particular, the cut-off corners are inclined at an angle of 45° with respect to the edges of the patch and have a length of up to about 0.1λ, where λ is the wavelength at the operating frequency of the antenna. 
   In a second embodiment of the present invention, the modifications are arranged in the center of the patch, with the modifications including a slot which runs parallel to the edges of the patch, is preferably rectangular and has a length of up to about 0.2λ, and a width of up to about 0.05λ, where λ is the wavelength at the operating frequency of the antenna. 
   Particularly advantageous isolation is obtained according to another embodiment of the invention in which a plurality of different modifications are combined with one another in the at least one patch. 
   The patch may be arranged with the edges parallel to the x axis and y axis of the antenna. However, it can also be arranged with the edges rotated through 45° with respect to the x axis and y axis of the antenna. 
   It is particularly advantageous for a plurality of patches to be arranged at a distance one above the other within the individual elements to increase bandwidth. In this case, it is advantageous for an individual element&#39;s plurality of patches to have at least one of a different group of modifications or a different orientation of the edges with respect to the x axis and y axis of the antenna. 
   In another embodiment of the present invention, a plurality of individual elements are arranged alongside one another in an array. In this case, it is particularly advantageous for the patches in the plurality of individual elements in an array to have at least one of different modifications or a different orientation with respect to the x axis or y axis of the antenna. 
   A particularly simple overall antenna design is obtained where a plurality of patches are arranged one above the other with the upper patches being mounted on the printed circuit board by means of spacers, and the printed circuit board mounted by means of spacers on a metal sheet, which can be inserted into a shroud which is open on one side. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in the following text using exemplary embodiments in conjunction with the drawings, in which: 
       FIG. 1  shows a perspective view of the shroud of a dual-polarized microstrip patch antenna according to one embodiment of the invention; 
       FIG. 2   a  shows a plan view from above of the supporting metal sheet, which can be inserted into the antenna shroud shown in  FIG. 1 , and  FIG. 2   b  shows a side view of the supporting metal sheet; 
       FIG. 3  shows a preferred printed circuit board with the feed network formed on the upper face and four patches, arranged in an array, as the basis of an individual element; 
       FIG. 4  shows a plan view of the patch, arranged above the printed circuit board, of an individual element of the antenna of one embodiment of the antenna; 
       FIG. 5  shows two orthogonal side views of a spacer for mounting the upper patches on the printed circuit board in one embodiment of the antenna; 
       FIGS. 6   a  and  6   b  show the feed points for two differently oriented patches, and  FIGS. 6   c  and  6   d  show two patches with modifications in the form of notches on the edges; 
       FIGS. 7   a  and  7   b  show two patches with modifications in the form of lugs on the edges,  FIGS. 7   c  and  7   d  show two patches with a central slot, and  FIG. 7   e  shows a patch which has been cut off at the corners; and 
       FIG. 8  shows a perspective view of a schematic design of the antenna with the stack comprising the metal sheet, printed circuit board and upper patches. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 8  shows a perspective illustration of a highly simplified form of a microstrip patch antenna according to one embodiment of the present invention. The antenna  43  essentially comprises a metal sheet  14  and four individual antenna elements EE 1 -EE 4  which are mounted on the metal sheet  14  in a square shaped pattern and at a distance above the upper face of the metal sheet. The shroud  10  of the microstrip patch antenna  43 , which is illustrated in  FIG. 1 , has been omitted in  FIG. 8  for clarity. The individual elements EE 1 -EE 4  are composed of a common printed circuit board (PCB)  19 , PCB surface mounted patches  20 - 23  and a feed network  44 , and at least one upper patch  29  arranged at a distance above the printed circuit board  19 . The upper patch/patches  29  govern increases in the bandwidth. 
     FIG. 3  depicts one embodiment of printed circuit board  19  in which a feed network  44  is formed on the upper face of printed circuit board  19 . The lower face of printed circuit board  19  is completely metallized. The feed network  44  has two branching conductor tracks,  24  and  25 , which are connected to the two adjacent corners of printed circuit board  19 . The upper face of printed circuit board  19  also includes surface patches  20 - 23 , which interface to and are fed by feed network  44 . As shown in  FIG. 2 , the conductor tracks  24  and  25  are connected to externally accessible connectors (not shown) on the lower transverse face of the printed circuit board  19 . These externally accessible connectors are mounted by means of holes  16  in an angled area (angle  15 ) of the metal sheet  14 . The connections between branching conductor tracks,  24  and  25  of feed network  44  and each of the surface patches  20 - 23  may be different. For example, as shown in  FIG. 3 , the lower left-hand corner of patch  20  is connected to the conductor track  25  and the lower right-hand corner is connected to the conductor track  24 . This connection orientation also applies to patch  22 . In contrast, the lower right-hand corners of patches  21  and  23  are connected to conductor track  24  and the upper right-hand corners are connected to the conductor track  25 . In addition, each of patches  20 - 23  has a rectangular notch  27  on or near the midpoint of each of its four faces and cut-off corners  28 . In the embodiment shown in  FIG. 3 , the cut-off corners  28  are angled at 45° and provide connection to conductive areas which are not connected to the conductor tracks  24  and  25 . The notches  27  and cut-off corners  28  modify the intrinsic characteristics of a square patch, thereby increasing the isolation between the polarizations. 
   Three patch mounting holes  26  are arranged in a triangular pattern within each of the mounting points for patches  20 - 23  on the upper face of the printed circuit board  19 , as shown in  FIG. 3 . Mounting hole spacers  33 , of the type illustrated in  FIG. 5 , can be inserted through the mounting holes to latch patches  20 - 23  to printed circuit board  19  and latch upper patches  29  to printed circuit board  19  at a distance above the printed circuit board&#39;s upper face (see also  FIG. 8 ). In addition, seven mounting holes  18 ′ are provided on the upper face of printed circuit board  19 . The printed circuit board&#39;s seven mounting holes  18 ′ are positioned identically to the mounting holes  18  on the metal sheet  14 . The mounting holes  18 ′ and mounting holes  18  are provided as a means to attach the printed circuit board  19  and metal sheet  14 . For example, mounting hole spacers  33  can be inserted through mounting holes  18  and  18 ′ and latch the printed circuit board  19  to the metal sheet  14  at a distance above the metal sheet (see  FIGS. 5 and 8 ). 
     FIG. 4  illustrates one example of an upper patch  29 , which is fixed at a distance above surface patches  20 - 23 . The patch  29  comprises a metal sheet having a thickness of, for example, 1 mm, which is similar to the thickness of the metal sheet  14 . Patch  29  has mounting holes  30 , whose number and arrangement are matched to the mounting holes  26  in patches  20 - 23 . The example of a patch  29 , depicted in  FIG. 4 , has two centrally arranged rectangular notches  31  on two opposite faces and cut-off corners  32  at all four corners. Again, the cut-off corners  32  and notches  31  are examples of modifications to the patch that improve the isolation between the polarizations of individual antenna elements. Additional suitable patch modifications are illustrated in  FIGS. 6 and 7 , and will be discussed further below. 
   The mechanical design of the antenna of a preferred embodiment is completed by a shroud  10 , as shown in  FIG. 1 . The shroud  10  is produced from a suitable plastic (for example Luran®) and is provided with internal bottom and side rails  12  and  13 , respectively, which guide the metal sheet  14  into the shroud  10  during the insertion into the shroud  10 . The shroud  10  has an insertion opening  11  on one transverse face. The insertion opening  11  is covered by an angle  15  on the angled metal sheet  14  when the metal sheet  14  has been inserted into the shroud  10 . The electrical part of the printed circuit board  19 , which is seated on the metal sheet  14 , is externally accessible through the connecting sockets which are inserted into the holes  16 . Support for the metal sheet  14  and shroud  10  is provided by a plurality of feet  17  that are stamped into the metal sheet  14 . 
   As has already been explained further above, the upper patches  29  are mounted by means of mounting hole spacers  33  at a distance above the printed circuit board  19 , and the printed circuit board  19  is mounted by means of spacers  33  at a distance above the metal sheet  14 . The spacers  33 , which are illustrated in the two side views of  FIG. 5 , are formed from plastic (for example polyamide) and, in a preferred embodiment, are designed for a distance of 5 mm between the patch  29  and the printed circuit board  19  and the metal sheet  14 . As shown in  FIG. 5 , spacers  33  have a spacer head in the form of a cup at the lower end and a rounded upper end and a center section with latching tongues  35  and  36  projecting from the sides of the center section. More specifically, latching tongues  35  are arranged a short distance behind the head  34  and latching tongues  36  are positioned behind a step which is located further upwards, closer to the upper end of spacer  33 . Functionally, the spacer  33  is pushed until the head of the spacer  33  is in contact with the surface of the printed circuit board  19  or upper patches  29 , and the latching tongues  35  and  36  clear the mounting hole and spring outwards, thereby latching the printed circuit board  19  and the metal sheet  14  or the printed circuit board  19  and the upper patches  29 . 
     FIGS. 6   a  and  6   b  show two ways patches  20 - 23  can be fed at the two adjacent corners. In  FIG. 6   a , the edges of the patch P 1  are parallel to the x axis and y axis (see the coordinates that are shown). The feed is provided at the feed points  37 ,  38 . Dual linear polarization is used, with a slant of ±45°. In  FIG. 6   b , the edges of the patch P 2  are rotated through 45° with respect to the x axis and y axis. The feed is once again provided at the corners (feed points  37 ,  38 ). Dual linear polarization is used, to be precise vertical and horizontal polarization. 
   As has already been mentioned in conjunction with  FIG. 4  for patch  29 , the patches can be changed by different modifications. In the case of the patches P 3  and P 4 , shown in  FIGS. 6   c  and  6   d , two rectangular notches  39  are provided in the center of two opposite edges as modifications. The dimensions of the notches  39  depend on the wavelength at the operating frequency of the antenna, λ, and are preferably up to about 0.1λ in width and up to about 0.1λ in length. The patches P 3  and P 4  may also be rotated through 45° with respect to the x axis and y axis. In the case of the patches P 5  and P 6  shown in  FIGS. 7   a  and  7   b , two rectangular lugs  40  are provided in the center of two opposite edges, as modifications. The dimensions of the lugs  40  are preferably up to about 0.1λ in width and up to about 0.1λ in length. The patches P 5  and P 6  may also be rotated through 45° with respect to the x axis and y axis. In  FIGS. 7   c  and  7   d , a rectangular slot  41  is provided in the center of patches P 7  and P 8 , respectively, as a modification. The dimensions of the slot  41  are preferably up to about 0.05λ in width and up to about 0.2λ in length. In this case as well, the patches P 7  and P 8  may be rotated through 45° with respect to the x axis and y axis. In the case of the patch P 9  shown in  FIG. 7   e , the modification comprises the corners being cut off. The cut-off corners  42  are inclined at 45°, and preferably have a length of up to about 0.1λ. In this case as well, the patch can be rotated through 45°, once again. 
   The described modifications to the patches  20 - 23  and  29  and P 3 -P 9  allow the isolation between the polarizations to be improved considerably. Very good isolation values are obtained by a suitable combination of these measures (for example notches and cut-off corners or the like). The described microstrip patch antenna  43  has a very narrow bandwidth. This bandwidth can be increased by the use of additional patches, which are placed on the already existing patches, at a distance from them. 
   The isolation can be improved further by a suitable combination of the patch modifications. In this case the modifications to a plurality of patches which are arranged one above the other (in a “stack”) may differ. For example, the lower patch has notches and the upper patch has lugs. The polarization is governed by the connections and feed of the lower patch. The upper patch can be rotated through 45° with respect to the lower. 
   The isolation in an array comprising a plurality of individual elements arranged alongside one another can be improved by the patches in the individual elements having different modifications. The antenna, which is shown as an exemplary embodiment in the figures, has external dimensions (of the shroud  10 ) of about 200 mm×200 mm×43 mm. The upper patches  29  have dimensions of 50 mm×50 mm×1 mm. This represents a 2×2 array with 4 individual elements, with each individual element having two patches  20 - 23  and  29 , which are arranged one above the other by means of spacers. 
   LIST OF REFERENCE SYMBOLS 
   
       
         10  Shroud 
         11  Insertion opening 
         12  Bottom rail 
         13  Side rail 
         14  Metal sheet 
         15  Angle 
         16  Hole 
         17  Foot 
         18 ,  18 ′ Mounting hole 
         19  Printed circuit board 
         20 , . . . ,  23  Patch (PCB) 
         24 ,  25  Conductor track 
         26  Mounting hole 
         27  Notch 
         28  Cut-off corner 
         29  Patch (metal sheet) 
         30  Mounting hole 
         31  Notch 
         32  Cut-off corner 
         33  Spacer 
         34  Head (in the form of a cup) 
         35 ,  36  Latching tongue 
         37 ,  38  Feed point 
         39  Notch 
         40  Lug 
         41  Slot 
         42  Cut-off corner 
         43  Microstrip patch antenna 
         44  Feed network 
       EE 1 , . . . , EE 4  Individual element 
       P 1 , . . . , P 9  Patch metal sheet