Abstract:
A low profile wideband multi-beam integrated dual polarization antenna array with compensated mutual coupling effect. Instead of suppressing mutual coupling with post-element-design techniques by attempting to block the reflections between elements, an element of the array is designed using its active impedance, i.e. its impedance with mutual coupling once the element is part of the array. The active impedance is determined using various simulation techniques and the element is then designed such that its impedance is shifted in order to modify its active impedance. This technique does not reduce the mutual coupling itself but instead, compensates for the mutual coupling effect and improves the return loss of the element.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is the first application filed for the present invention. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to the field of wireless communication systems and antenna arrays suitable for both transmission and reception of electromagnetic radiation. 
       BACKGROUND OF THE ART 
       [0003]    Certain designs for antenna arrays consist of closely spaced wideband antenna elements. In order to maintain a small overall size and required antenna performances, such as a wide beam width and a high cross-over point between three individual beams, the spacing between the antenna elements is kept to a minimum (i.e, less than or equal to half a wavelength of a center frequency point). However, the close proximity of the antenna elements causes significant mutual coupling effects, thereby affecting the overall performance of the antenna array. 
         [0004]    It is well-known to reduce mutual coupling effects by putting isolators, such as electromagnetic bandgaps (EBGs), between element patches, or to add some slots to the element grounding plane. For applications requiring small spacing between the elements, these techniques do not work well. This is particularly the case for low profile wideband multi-beam integrated dual polarization antenna arrays. 
         [0005]    Therefore, there is a need to provide an alternative method of reducing mutual coupling effects for antenna arrays requiring closely spaced wideband antenna elements. 
       SUMMARY 
       [0006]    There is described herein a low profile wideband multi-beam integrated dual polarization antenna array with compensated mutual coupling effect. Instead of suppressing mutual coupling with post-element-design techniques by attempting to block the reflections between elements, an element of the array is designed using its active impedance, i.e. its impedance with mutual coupling once the element is part of the array. The active impedance is determined using various simulation techniques and the element is then designed such that its impedance is shifted in order to modify its active impedance. This technique does not reduce the mutual coupling itself but instead, compensates for the mutual coupling effect and improves the return loss of the element. 
         [0007]    In accordance with a first broad aspect, there is provided a method for designing an antenna element for an array of antenna elements, the method comprising: identifying a desired impedance for the antenna element within a required frequency band; determining an active impedance based on the desired impedance of the antenna element and mutual coupling with neighboring elements of the array; selecting an optimal impedance for the antenna element to cause the active impedance to substantially correspond to the desired impedance; and designing the antenna element with the optimal impedance, whereby the optimal impedance does not correspond to the desired impedance but the active impedance based on the optimal impedance does. 
         [0008]    In accordance with another broad aspect, there is provided a wideband multi-beam integrated dual polarization antenna array for at least one of transmission and reception of electromagnetic radiation, the array comprising: at least two wideband beam forming networks each having at least three inputs; at least four wideband sub-arrays of antenna elements connected between the at least two wideband beam forming networks; and at least two antenna elements in each of the sub-arrays of antenna elements, each of the at least two antenna elements having an actual impedance, and at least one of the at least two antenna elements having an active impedance that corresponds to a desired impedance for the at least one antenna element individually while the actual impedance does not. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
           [0010]      FIG. 1   a  is an example of a planar array (4×4 array) with azimuth (AZ) beam forming networks (BFN) located between the rows of elements and an elevation (EL) BFN; 
           [0011]      FIG. 1   b  is an example of planar array (4×4 array) with EL BFNs located between the columns of elements and an AZ BFN; 
           [0012]      FIG. 2  is an exemplary wideband multibeam integrated dual polarization antenna array; 
           [0013]      FIG. 3   a  is an exemplary wideband 1×2 sub-array from  FIG. 2 ; 
           [0014]      FIG. 3   b  is an exemplary layout schematic of the wideband 1×2 sub-array; 
           [0015]      FIG. 4   a  is an exemplary 3×4 Butler matrix from  FIG. 2 ; 
           [0016]      FIG. 4   b  is an exemplary layout schematic of the 3×4 Butler matrix; 
           [0017]      FIG. 5  is a cross-sectional view of an exemplary layout of a low profile wideband multibeam integrated dual polarization antenna array; 
           [0018]      FIG. 6  is an exemplary layout schematic of the low profile wideband multibeam integrated dual polarization antenna array; 
           [0019]      FIG. 7   a  is a schematic illustration of two antenna elements and their corresponding signals; 
           [0020]      FIG. 7   b  is a Smith Chart showing S 11  of the antenna element before/after tuning; 
           [0021]      FIG. 7   c  is a Smith Chart showing S active  of the antenna element before/after tuning; 
           [0022]      FIG. 8  is a graph of the measured return loss of the central beam port (B+45) of the C band 2×4 array; and 
           [0023]      FIG. 9  illustrates an exemplary embodiment for a support. 
       
    
    
       [0024]    It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
       DETAILED DESCRIPTION 
       [0025]      FIGS. 1A and 1B  are example architectures of a planar array with M rows and N columns consisting of three main parts: the antenna elements  102 , azimuth beam forming networks (AZ BFN)  104 , and an elevation beam forming network (EL BFN)  106 . There are two basic structures as shown.  FIG. 1A  is an example of the planar array (M=4 Rows and N=4 columns) with AZ BFN  104  located between the antenna elements and EL BFN  106 , and  FIG. 1B  is an example of the planar array (M=4 Row and N=4 column) with EL BFN  106  located between the antenna elements and AZ BFN  104 . For some arrays with simple functions such as single beam or fixed tilted arrays, the BFN may be as simple as a T-splitter network. For other arrays with more complex functions such as multibeam or variable tilted arrays, the BFN may be a Butler Matrix or a phase shifter. The corresponding architecture is determined based on the functions of the required planar array. In some embodiments,  FIG. 1A  is used for a variable-tilted array and  FIG. 1B  is used for a fixed-tilted array. 
         [0026]    For dual polarization three-beam arrays (total six beams: L+45, B+45, R+45; L−45, B−45, and R−45), a 2×4 planar array (M=2 and N=4) meets the basic beam requirements such as gain and beam width. In the case of a fixed-tilted multibeam array, the AZ BFN  104  is much more complex than the EL BFN  106 . Therefore, because the number M (=2) of rows of the array is less than the number N (=4) of columns of the array, in order to reduce the number of AZ BFN  104 ,  FIG. 1B  may be used for a C-band multibeam array, where only two AZ BFN are required. 
         [0027]      FIG. 2  is an exemplary block diagram of a wideband multibeam integrated dual polarization array antenna (M=2 and N=4). Two wideband 3×4 Butler BFN  202 ,  204  are provided in order to achieve the dual polarization and multibeam features. In the illustrated case of a Butler BFN, the inputs are isolated from each other and the phases of the outputs are linear with respect to position of the antenna element, so the left and right beams are tilted off the main axis. A set of wideband 1×2 sub-arrays  206 ,  208 ,  210 ,  212  are connected between the BFNs  202 ,  204 . 
         [0028]    An exemplary embodiment for one of the wideband 1×2 sub-arrays  206 ,  208 ,  210 ,  212  is illustrated in  FIGS. 3   a  and  3   b . As per the block diagram of  FIG. 3   a , a pair of wideband 1×2 T-splitter power dividers  302 ,  304  are connected between a pair of wideband elements  306 ,  308 .  FIG. 3   b  illustrates an exemplary layout schematic (top-view) for the wideband dual polarization 1×2 sub-array  206 , in which dual polarization slot coupled patch elements  306 ,  308  are used for easy integration. Due to the multilayer nature, vias  310  and  312  are used between different layers for cross-over, and a metal cavity is used for a reduced surface wave coupling. For the slot-coupled patch elements  306 ,  308 , two stack patches  314  and  316  are used for wideband operation. A slot  318  is used in a ground layer and a feeder stub  320  is used in a signal feeder layer. The size/position of the cavity vias  310 ,  312  are used for adjusting the impedance characteristics of the elements  306 ,  308 . 
         [0029]      FIGS. 4   a  and  4   b  illustrate an exemplary embodiment of one of the wideband 3×4 Butler BFNs  202 ,  204  from  FIG. 2 . As per the block diagram of  FIG. 4   a , a wideband 1×2 T-splitter power divider  402  receives the broadside beam (B-Beam) and divides the power into two wideband hybrid couplers  404 ,  408 . A third wideband hybrid coupler  406  receives the left-side beam and the right-side beam directly and feeds into wideband hybrid couplers  404  and  408 . Wideband hybrid coupler  404  will feed into wideband 1×2 sub-array  206  directly and into wideband 1×2 sub-arrays  210  and  212  through via cross  410 . Wideband hybrid coupler  408  will feed into wideband 1×2 sub-array  208  directly and into wideband 1×2 sub-arrays  210  and  212  through via cross  410 .  FIG. 4   b  illustrates an exemplary layout schematic for two wideband 3×4 Butler BFNs in a signal layer. In order to achieve the same beam pattern property for two polarizations (+45 and −45), two 3×4 Butler BFNs are identical and rotationally symmetrical. 
         [0030]      FIG. 5  is a cross-sectional view of an exemplary layout for the wideband multibeam integrated dual polarization antenna array. Five layers of Printed Circuit Board (PCB) are provided to account for a BFN layer  514 , a feed line layer  512 , a slot layer  510  (and slot  516 ), and two element layers  506 ,  508 . In one embodiment, the BFN layer  514  is composed of a six-layer PCB and both antenna element layers  506  and  508  are composed of double-layer PCBs. The two wideband 3×4 BFN  202 ,  204  may be realized on a single plane. 
         [0031]    An EBG  502  is provided at the end of each element layer  506 ,  508 . Another EBG  504  is provided at the end of the slot layer  510  and feed layer  512  for isolation between ground planes  502 . Any known EBG type, such as UCEBG (Uniplanar Compact EBG), SRR (Split Ring Resonator), and slot on the ground plane, may be used for the reduction of the mutual coupling between the elements. 
         [0032]    Vias (not shown) are provided between the feed layer  512  and the slot layer  510 , between the feed layer  512  and the BFN layer  514 , and between the two ground planes  520 . Patch/tracks  518  are also inserted between the layers where appropriate. Supports  522  are used between the two element layers  506  and  508 , and between element layer  508  and slot layer  510 . The supports  522  may be made of plastic or other alternative materials.  FIG. 6  is a layout schematic of an exemplary embodiment for the wideband multibeam integrated dual polarization antenna array, including the partial schematics shown in  FIGS. 3   b  and  4   b.    
         [0033]    The mutual coupling improvement obtained by putting EBG  502  and  504  between elements is very limited due to the narrow spacing of the array. In turn, it will degrade the return loss performance of the array, especially for the broadside beam (B+45 and B−45) ports. In order to improve the array performance, certain techniques to compensate the mutual coupling are used. 
         [0034]      FIG. 7A  is a schematic illustration of two antenna array elements, element x 1  and element x 2 . Signals a 1  and a 2  are incoming signals for elements x 1  and x 2 , respectively. Signals b 1  and b 2  are reflected signals for elements x 1  and x 2 , respectively. Elements x 1  and x 2  are separated by a distance d. The reflected signals b 1  and b 2  may be represented by the following scattering parameter (or S-parameter) equations: 
         [0000]    
       
      
       b 
       1 
       =S 
       11 
       a 
       1 
       +S 
       12 
       a 
       2  
      
     
         [0000]      − b   2   =S   21   a   1   +S   22   a   2  
 
         [0035]    where S 11  is the voltage reflection coefficient (or return loss in dB) of element x 1  (or the reflection from element x 1  by assuming a 2  equal to zero), S 22  is the voltage reflection coefficient (or return loss in dB) of element x 2  (or the reflection from element x 2  by assuming a 1  equal to zero), and S 21  and S 12  represent the mutual coupling between the element x 1  and the element x 2 . The active impedance S active  of an element may be defined as the total reflection felt at the element and may be represented (for element x 1 ) as follows: 
         [0000]    
       
         
           
             
               
                 S 
                 active 
               
               = 
               
                 
                   
                     b 
                     1 
                   
                   
                     a 
                     1 
                   
                 
                 = 
                 
                   
                     S 
                     11 
                   
                   + 
                   
                     
                       S 
                       12 
                     
                      
                     
                       
                         a 
                         2 
                       
                       
                         a 
                         1 
                       
                     
                   
                 
               
             
             ; 
           
         
       
     
         [0036]      FIG. 7B  is a schematic illustration of S ii  of the antenna array element. The curve  801  is perfectly located at the center of the Smith chart and the element is designed to match the 50 ohm impedance within the required frequency band.  FIG. 7C  is a schematic illustration of S active  of the antenna array element. Due to the impact of the mutual coupling, the curve  803  is located off the center of the Smith chart and the element and array have degraded impedance and pattern performances. In order to compensate for the mutual coupling effect of neighboring elements in an array, S ii  is shifted from its original value to a value that will provide a modified S active . In other words, when the impedance performance of the antenna element is shifted from the initial curve  801  to  802 , based on the above mentioned formula, the impedance of the element and array is improved from the curve  803  to the curve  804  located at the center of the Smith chart. 
         [0037]    Some of the techniques used to shift S 11  comprise changing the element&#39;s impedance value as follows: 
         [0000]    1. Adjusting the spacing between layers of the antenna element;
 
2. Adjusting the length and/or width of a feeder stub ( 312 );
 
3. Changing the length and/or width of the slot ( 311 ); and
 
4. Changing the placement and/or spacing and/or size of the plated through hole (PTH) between two grounding planes ( 312 ).
 
         [0038]    Other techniques known to those skilled in the art may also be used. The mutual coupling compensation technique described herein allows the antenna elements and a beam forming network to be integrated into a multi-layer structure using conventional multi-layer PCB technology. Other techniques may also be used in combination with the mutual coupling compensation technique to further improve the performance of the low profile wideband multibeam integrated dual polarization antenna array. 
         [0039]      FIG. 8  is the measured return loss of the central beam port (B+45) of the C band 2×4 array. The curve  901  is the return loss of the 2×4 array (&gt;12 dB) before the tuning and the curve  902  is the return loss of the 2×4 array (&gt;17 dB) after the tuning of the element using the above-described techniques. 
         [0040]    As per the above, one of the strategies used to shift the impedance of the element and thereby modify the active impedance of the element is to adjust the spacing between layers. Referring back to the embodiment illustrated in  FIG. 5 , the total thickness may be about 11 mm, or from about 9 mm to about 13 mm. Element layers  506  and  508  are both set to about 0.5 mm, and the supports  522  are each about 3.0 mm. The slot and feed layers  510  and  512  together with the patch/track  518  is about 0.8 mm. The PCB for the BFN  514  may have a thickness of about 0.8 mm while the EBGs  504  may be about 1.6 mm. When taking into account the additional space for the ground layers  520  and the patch/track  518 , the total thickness may be between about 10.2 mm and about 11.0 mm. The measurements included herein may be increased or decreased by about +/−20%. The supports  522 , the ground layers  520 , and the EBGs  504  may all be made thicker or thinner in order to shift the impedance of the element. 
         [0041]      FIG. 9  is an illustration of an exemplary embodiment for the supports  522  that may be used to shift the impedance. The length and/or width of the feed layer  510  may be adjusted, thereby causing a shift in impedance and a modified active impedance. The length and/or width of the slot  516  may be adjusted, thereby causing a shift in impedance and a modified active impedance. Changing the placement and/or spacing and/or size of the plated through hole (PTH) between two grounding planes, as shown in  FIG. 6 , may also be used to shift the impedance. 
         [0042]    The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.