Patent Publication Number: US-8970444-B2

Title: Polarization dependent beamwidth adjuster

Description:
TECHNICAL FIELD 
     The invention relates to the technical field of antennas used in wireless communication systems. 
     BACKGROUND 
     The beamwidth of antenna elements located near groundplanes is traditionally adjusted by changing the antenna element dimensions and the groundplane extension. 
     Base station antennas frequently operate with two orthogonal linear polarizations for diversity (polarization diversity). For GSM (Global System for Mobile Communication) and WCDMA (Wideband Code Division Multiple Access) it is common to use slant linear polarizations, oriented +/−45 degrees with respect the vertical plane. An attractive alternative is to use vertical and horizontal polarization, i.e. 0 and 90 degrees polarization. When using antennas with dual polarization (e.g. vertical and horizontal polarization) on the same mechanical structure, it can be quite complicated to make a design that gives the desired horizontal beamwidth for both polarizations simultaneously. Thus it is beneficial with a design that contains design parameters that controls the horizontal beamwidth for each polarization individually. 
     SUMMARY 
     The object of the invention is to provide a dual polarized antenna or antenna array with a first and second radiation pattern having a first and second polarization, a method for adjustment of said antenna or antenna array and a wireless communication system comprising said antenna or antenna array which can solve the problem to obtain a desired horizontal beamwidth simultaneously for the first radiation pattern with a first polarization and the second radiation pattern with the second polarization. The antenna or antenna array comprises a main radiating antenna element, or array of main radiating antenna elements, having a main extension in an extension plane and a longitudinal extension. The main radiating antenna element or array of main radiating antenna elements is arranged above a conductive frame, the perpendicular projection of the main radiating antenna element or array of main radiating antenna elements towards a frame surface falling within an area of the frame surface. 
     This object is achieved by:
         an antenna or antenna array wherein a combination of conductive parasitic strips and chokes is arranged in association with the main radiating antenna element, or array of main radiating antenna elements, to achieve means for independently controlling beamwidths of the first and second radiation pattern in a plane substantially perpendicular to the longitudinal extension of the antenna or antenna array   a method for adjustment to achieve a desired beamwidth in a plane substantially perpendicular to the longitudinal extension for each polarization, wherein the beamwidth adjustment for the first and the second radiation pattern is made independently of each other and comprising the steps of:
           arranging conductive parasitic strips in association with a main radiating antenna element or an array of main radiating antenna elements to control the beam width of the first polarization and arranging at least two chokes in association with the main radiating antenna element or array of main radiating antenna elements to control the beamwidth of the second polarization.   
           a wireless communication system including base stations equipped with a dual polarized antenna or antenna array according to the invention.       

     A radiation pattern in a plane substantially perpendicular to the longitudinal extension of the antenna or antenna array is henceforth in the description called the horizontal radiation pattern. 
     Polarization substantially parallel to the extension plane and the longitudinal extension of the antenna or antenna array is henceforth in the description called the vertical polarization. 
     Polarization substantially parallel to the extension plane and perpendicular to the longitudinal extension of the antenna or antenna array is henceforth in the description called the horizontal polarization. 
     The invention makes it possible to individually tune the beamwidth for vertical and horizontal polarization and when desired, tune such as to obtain equal beamwidths for both polarizations. The invention also makes it possible to accomplish equal horizontal beamwidth and horizontal beam pointing for any other dual polarization (e.g. +/−45° since any polarization can be decomposed into one vertically polarized component and one horizontally polarized component and thus having equal radiation patterns for vertical and horizontal polarization will give equal patterns for any other pair of polarization. The implementation of the tuning is simple to achieve, the conductive parasitic strips can in one embodiment be etched on a substrate common with the antenna. The mechanical implementation of the choke is simple and can be realized with traditional die-casting or extrusion. 
     The conductive parasitic strips and chokes are located with reference to the main radiating antenna element, such as a patch antenna. The main radiating antenna element can also be of other types, such as dual polarized dipoles, slots, stacked patches, etc. The main radiating antenna element is henceforth in the description exemplified with a patch element. 
     When exciting the patch with vertical polarization (normal to the plane of  FIG. 1 ), the fields will be short circuited by the conductive parasitic strips since the field is parallel to the conductive parasitic strips, i.e. the conductive parasitic strips will act as a broadening of the ground plane. By choosing the position and the width of the conductive parasitic strips, the beamwidth for the vertical polarization can hence be adjusted. There can also be two or more conductive parasitic strips on each side. The choke will have negligible influence on the field as long as the width is small in terms of the wavelength; since the field in this case is oriented parallel to the choke (i.e. the chokes are almost invisible to the E-field parallel to the choke). 
     When exciting the patch with horizontal polarization, the field will cross the conductive parasitic strips perpendicular to the conductive parasitic strips and as long as the width of the conductive parasitic strips is small with respect to the wavelength the field is almost unaffected (i.e. the conductive parasitic strips are almost invisible to the E-field perpendicular to the conductive parasitic strips). However, choosing the position and the depth of the chokes will affect the beamwidth of the horizontal polarization since the current flow at the choke entrance will be affected by the choke impedance. Thus the position, dimensions and orientation of chokes can be used to control the horizontal radiation pattern for the horizontal polarization with a minor impact on the radiation pattern for the vertical polarization. 
     Further advantages can be obtained by implementing features of the dependent claims covering different embodiments of the antenna or antenna array with variations regarding the position of the conductive parasitic strips in relation to the main radiating antenna element, number and shape of conductive parasitic strips, an angle of the conductive parasitic strips in relation to the frame surface and the relative position between the conductive parasitic strips. The conductive parasitic strips can also be realized as wires, rods or tubes. Variations regarding the position of the chokes in relation to the main radiating antenna element, number of chokes, as well as alignment of the chokes in relation to the frame surface are also within the scope of the invention and covered in the dependent claims. 
     The chokes can be aligned parallel to the extension plane of the antenna or antenna array and extending in the longitudinal extension of the antenna or antenna array. This is henceforth in the description called the extension plane alignment. 
     The chokes can also be aligned in a normal plane, perpendicular to the extension plane of the antenna or antenna array and extending in the longitudinal extension of the antenna or antenna array. This is henceforth in the description called the normal plane alignment. 
     Additional advantages are obtained if features of the dependent claims for the adjustment method are implemented. An adjustment method of the first polarization can be performed by optimizing certain parameters regarding the conductive parasitic strips such as the position of the strips in relation the main radiating antenna element, number of conductive parasitic strips, and angle of the conductive parasitic strips in relation to the frame surface. Other optimizing parameters can be the width of the conductive parasitic strip. The conductive parasitic strips can also e.g. be realized as wires. 
     An adjustment method of the second polarization can be performed by optimizing a number of choke parameters, practically independent of the adjustment parameters of the first polarization. These choke parameters comprise the position of the chokes in relation to the main radiating antenna element, number of chokes and alignment of the chokes in relation to the frame surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows a cross section of an antenna structure where the conductive parasitic strips are located in the same plane as the substrate, and the chokes have extension plane alignment. 
         FIG. 2  schematically shows a perspective view of an array of patches. 
         FIG. 3  schematically shows a cross section of an antenna structure where the conductive parasitic strips are angled with reference to the substrate plane, and the chokes have extension plane alignment. 
         FIG. 4  schematically shows a cross section of an antenna structure where the conductive parasitic strips are realized as wires, rods or tubes and the chokes have extension plane alignment. 
         FIG. 5  schematically shows a cross section of an antenna structure where the conductive parasitic strips are realized as several wires, rods or tubes and the chokes have extension plane alignment. 
         FIG. 6  schematically shows a cross section of an antenna structure where the conductive parasitic strips are aligned with the substrate, and the chokes have normal plane alignment. 
         FIG. 7  schematically shows a cross section of an antenna structure where the conductive parasitic strips are realized as two wires, rods or tubes and two chokes with extension plane alignment. 
         FIG. 8  schematically shows a cross section of an antenna structure where the conductive parasitic strips are non planar, and the chokes have extension plane alignment. 
         FIG. 9  schematically shows a cross section of an antenna structure with several conductive parasitic strips that can be non planar and chokes that have extension plane alignment. 
         FIG. 10  schematically shows a cross section of an antenna structure with the conductive parasitic strips attached to the conductive frame by a support structure. 
         FIGS. 11   a  and  11   b  shows beam width diagrams as a function of frequency for vertical and horizontal polarization for an antenna structure according to the invention but without chokes. 
         FIGS. 12   a  and  12   b  shows beam width diagrams as a function of frequency for vertical and horizontal polarization for an antenna structure according to the invention. 
         FIG. 13  is a block diagram illustrating the method for adjusting the beamwidths of the two polarizations. 
         FIG. 14  schematically shows a wireless communication system. 
     
    
    
     DETAILED DESCRIPTION 
     The invention will now be described in detail with reference to the drawings and some examples on how to implement the invention. Other implementations are possible within the scope of the invention. 
     A first implementation example of an antenna or antenna array having a main extension in a plane parallel to an x/z-plane as defined by coordinate symbol  112  is shown in  FIG. 1 . This is henceforth in the description called the extension plane of the antenna or antenna array. A plane parallel to the y/z-plane is defined as a normal plane of the antenna or antenna array. The antenna or antenna array also has an extension direction in the z-direction defined as a longitudinal extension, henceforth in the description called the longitudinal extension. In  FIG. 1  conductive parasitic strips are located in the same plane as a substrate, and chokes are aligned parallel to the extension plane, i.e. they have an extension plane alignment. The antenna structure comprises a substrate  103  mounted on a conductive frame  101 , serving as a ground plane and having a frame surface  111  facing a main radiating antenna element  102 . The substrate extends outside the frame on two opposite sides by a distance  106 . Conductive parasitic strips  104  with a width  107  are applied on the surface of the parts of the substrate extending outside the frame. A gap  108  between the conductive parasitic strips and the frame is defined as the difference between the distances  106  and  107 . The conductive main radiating antenna element  102 , here exemplified with a patch, is arranged above and substantially parallel with the substrate with a perpendicular projection towards the frame surface  111  being within the surface area of the frame and at a distance  109  from the longitudinal side edges of the frame. A choke  105  realized as a notch, having an extension plane alignment, with a depth  110  extends along two opposite longitudinal sides of the frame and in the same direction as the conductive parasitic strips. For vertical polarization, i.e. when the electrical field is perpendicular to the plane of the figure, the fields will be short circuited by the conductive parasitic strips since the E-field is parallel to the conductive parasitic strips. This has the effect that the conductive parasitic strips will act as broadening of the ground plane. By choosing the position and the width of the conductive parasitic strips, the beamwidth for the vertical polarization can hence be adjusted. In the example of  FIG. 1  there is one conductive parasitic strip at each opposite longitudinal side of the frame. There can also be two or more conductive parasitic strips at each side. The choke will have a negligible influence on the field as long as the notch width is small in terms of wavelength, since the field in this case is oriented parallel to the choke (i.e. the chokes are almost invisible to the E-field parallel to the choke). In order for the conductive parasitic strips to have the broadening effect the gap  108  as defined above has to be less than roughly ¼-½ wavelength. 
     The patch can e.g. be arranged above the substrate and the frame by plastic supports (not shown in the figure) provided at each corner of the patch and attached to the substrate. In a further embodiment the patch can be attached directly to the substrate, i.e. both the patch and the conductive parasitic strips are attached to the substrate. 
     When exciting the patch with horizontal polarization, i.e. in the plane of the figure, the field will cross the conductive parasitic strips perpendicular to the conductive parasitic strips and as long as the width of the conductive parasitic strips is small with respect to the wavelength the field is almost unaffected (i.e. the conductive parasitic strips are almost invisible to the E-field perpendicular to the conductive parasitic strips). However, choosing the position and the depth of the chokes will affect the beamwidth of the horizontal polarization since the current flow at the choke entrance will be affected by the choke impedance. Thus the position, dimensions and orientation of the chokes can be used to control the horizontal radiation pattern, i.e. the radiation in a plane substantially perpendicular to the longitudinal extension of the antenna or antenna array, for the horizontal polarization with a negligible impact on the radiation pattern for the vertical polarization. The most sensitive tuning parameter is the depth of the choke notch. 
     The dual polarization feeding of the patch can be arranged in any conventional way well known to the skilled person. A typical feeding solution is to use a multilayer Printed Circuit Board (PCB) as the substrate and integrate a crossed slot in a metallized bottom layer of the PCB, the feeding of each slot in a second layer and the conductive parasitic strips in a third top layer. The patches can also be arranged in this third, top layer or above the substrate on plastic supports attached to the substrate and each corner of the patches. 
     The antenna structure can include one patch or a number of patches arranged in a linear array. A linear array with the longitudinal extension  207  is shown in  FIG. 2  with a substrate  202  mounted on a frame  201 , usually referred to as the ground plane. Chokes  204  with extension plane alignment are arranged on opposite longitudinal sides of the frame. Conductive parasitic strips  203  are applied to opposite longitudinal sides of the substrate and one column  208  of patches  205  are mounted on supports  206  attached to the substrate and each corner of the patch. The number of patches is depending on the actual application but is typically around 4-20 for base station applications, but other numbers are also possible within the scope of the invention. For certain application it can also be suitable to use two or more columns  208  of patches mounted in parallel. The extension plane of the antenna array, as defined above, is the x/z-plane. The normal plane is a plane parallel to the y/z-plane. 
     A second implementation is shown in  FIG. 3  where conductive parasitic strips  301  are angled with reference to the substrate plane, and the chokes with extension plane alignment as in  FIG. 1 . The example according to  FIG. 3  has the same structure as the example of  FIG. 1  except that the conductive parasitic strips  301  are now arranged at two opposite side edges with an angle  302  between the conductive parasitic strips and the substrate. The arrangement of the conductive parasitic strips can be made by any suitable mechanical means. This example adds an additional parameter, the angle  302 , to be used for fine tuning and optimizing the beam width for vertical polarization. 
     A third implementation is shown in  FIG. 4  where conductive parasitic strips are realized as wires, rods or tubes  401 , and the chokes have extension plane alignment. Henceforth in the description the realization of strips as wires, rods or tubes are exemplified by wires. The antenna structure has the same basic structure as in  FIG. 1  except that a substrate  402  now has the same dimensions as the frame, and thus not extending outside the frame as described in association with  FIG. 1 , and that the conductive parasitic strips now are realized as the wires  401 . The wires are aligned along two opposite sides of the substrate at a constant distance  403  from the substrate and extending in the same direction as the chokes. The distance  403  has to be less than ¼-½ wavelength in order to obtain the effect of serving as broadening of the ground plane for the vertical polarization. Spacers, between wire and substrate, can be used to align the wires along the sides of the substrate (not shown in the figure). 
     A fourth implementation is shown in  FIG. 5  where the conductive parasitic strips are realized as several wires, and the chokes have extension plane alignment. This embodiment differs from the alternative in  FIG. 4  only by the addition of a further wire  501  on each side of the frame. Three or more wires can also be used at each side. This example adds additional parameters, the number of wires and distances between wires, to be used for fine tuning and optimizing the beam width for vertical polarization. 
     A fifth implementation is shown in  FIG. 6  where the conductive parasitic strips are aligned with the substrate, and the chokes have normal plane alignment. This means that the angle  602  between the extension plane and the alignment of the notch of the choke is 90°. This embodiment differs from the alternative according to  FIG. 1  by replacing the chokes with an extension plane alignment, by chokes  601  having normal plane alignment. The angle  602  can also have any value between 0-180°. This is an alternative mechanical embodiment to the embodiment of  FIG. 1  illustrating that the orientation of the choke is not critical for the optimization of the beamwidth for the horizontal polarization. The chokes can also have an angle between 0-90 degrees to the y/z-plane, 90 degrees being the extension plane alignment of the choke. The orientation of the chokes adds additional possibilities for tuning the beamwidth for the horizontal polarization. 
     A sixth implementation is shown in  FIG. 7  where the conductive parasitic strips are realized as several wires, and several chokes have extension plane alignment. This embodiment differs from the embodiment of  FIG. 5  by adding an additional choke  701  having extension plane alignment at each side of the frame. This adds additional parameters, the number of chokes and distance between chokes, to be used for fine tuning and optimizing the beam width for horizontal polarization. Further chokes can be added at each side of the frame. 
     A seventh implementation is shown in  FIG. 8  where the conductive parasitic strips are non-planar, and the chokes have extension plane alignment. This embodiment differs from the embodiment of  FIG. 1  by adding a flange  801  to the conductive parasitic strip  802 . There is an angle  803  between the conductive parasitic strip and the flange. In the embodiment of  FIG. 8  the angle  803  is 90°. The angle can however assume any value between 0-360°. The height and angle of the flange adds additional possibilities for tuning the beamwidth for the vertical polarization. 
     An eighth implementation is shown in  FIG. 9  where several non-planar conductive parasitic strips and chokes with extension plane alignment are used. This embodiment differs from the embodiment of  FIG. 1  in that conductive parasitic strips  902  attached to the dielectric substrate has a distance  904  to the longitudinal sides of the dielectric substrate and that additional conductive parasitic strips  901  are added and attached to the opposite longitudinal side edges of the dielectric substrate with an angle  903  between the dielectric substrate and the conductive parasitic strips. The angle can however assume any value between 0-360°. The conductive parasitic strip  901  can be planar or curved. Additional planar and curved conductive parasitic strips can be added. 
     In the examples described the frame surface  111  is planar. In other embodiments the frame surface can also be curved. 
       FIG. 10  shows an embodiment without the dielectric substrate. The conductive parasitic strips are here attached to the conductive frame by a support structure  1001 , here realized as support pins. 
     Farfield radiation measurements have been performed on an antenna with different polarizations (e.g. vertical and horizontal polarization) on the same mechanical structure. An implementation example with and without chokes in the structure has been examined. Position and configuration of the conductive parasitic strips, choke position and depth have been tuned to obtain the optimum beamwidth for the two polarizations.  FIGS. 11 and 12  show beamwidth versus frequency for vertical and horizontal polarization.  FIG. 11  shows beamwidths without chokes and  FIG. 12  shows the same, but with chokes implemented. 
       FIGS. 11 and 12  have 3 dB beamwidth values in degrees on the vertical axis and frequency in MHz on the horizontal axis.  FIGS. 11   a  and  12   a  show beamwidths for vertical polarization and  FIGS. 11   b  and  12   b  show beamwidths for horizontal polarization.  FIG. 11   b  shows very large variations in beamwidth when chokes are not used.  FIG. 12   b  shows the result when chokes are implemented; the horizontal beamwidth becomes very stable within the frequency range.  FIG. 12   a  shows the result for the vertical polarization when configuration and position of the conductive parasitic strips have been tuned to optimize the beamwidth for the vertical polarization. In summary, the vertical polarization is tuned with varying conductive parasitic strip parameters and the horizontal polarisation by tuning depth and position of the chokes. The tuning procedures for the beamwidth of the polarizations are almost independent of each other, i.e. when tuning the beamwidth of the vertical polarization by changing conductive parasitic strip parameters it does not affect the beamwidth of the horizontal polarization. 
     The basic method for adjusting the beamwidth is described in  FIG. 13 . The beamwidth adjustment for first and second radiation pattern is made by arranging parasitic elements in association with the main radiating element to control the beam width of the first polarization  1301  and by arranging chokes in association with the main radiating element to control the beamwidth of the second polarization  1302 . In  FIG. 13  the first polarization is exemplified with vertical polarization (V) and the second polarization by horizontal polarization (H). 
     The beamwidth of the vertical polarization can then be further adjusted and optimized by:
         locating the conductive parasitic strips at certain positions in relation to the main radiating element   modifying the shape and/or number of the conductive parasitic strips   changing the relative position between the conductive parasitic strips.       

     The beamwidth of the horizontal polarization can then also be further adjusted and optimized by:
         locating the at least two chokes at certain positions in relation to the main radiating element   modifying the shape, depth and/or number of chokes   modifying the relative position between the chokes   varying the alignment of the chokes.       

     A wireless communication system comprising a base station  1401  connected to a communications network  1402  and to mobile units  1403  via an air interface  1404  is shown in  FIG. 14 . Examples of such systems are networks for GSM (Global System for Mobile Communication) and various 3G (third generation) systems for mobile communication. The invention also covers such wireless communication systems including base stations equipped with an antenna or antenna array according to the apparatus claims of the invention. 
     The invention is not limited to the embodiments above, but may vary freely within the scope of the appended claims.