Abstract:
A microstrip antenna includes at least one parasitic patch, located beside a central patch. The parasitic patch is electrically disconnected from the central patch, yet coupled to it, inductively or otherwise, to aid in transferring energy to/from the central patch.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to antennas, and more particularly to a micro-strip antenna having coupled patches, providing broad frequency response. 
       BACKGROUND OF THE INVENTION 
       [0002]    Radio receivers/transmitters require one or more antennas. Modern electronic devices, such as portable computing devices including laptops, tablet and cellular telephones, wireless network base stations, wireless network interfaces, and the like, all require inclusion of one more such antennas. As these devices have become smaller and more versatile, the size of these antennas has also needed to be reduced. 
         [0003]    One common type of antenna is a “patch” or mircostrip antenna. Patch antennas are often used in electronic devices such as cellular handsets, because they have a low profile, and can be mounted or formed on flat surfaces. Typically, a patch antenna is formed as a flat sheet of conductive material (usually metal), mounted over a metal sheet acting as ground plane, and separated by a dielectric. The two metal sheets on either side of the dielectric together form a resonant piece of microstrip transmission line that acts as the antenna. 
         [0004]    Reducing antenna size while providing adequate gain, over a desired frequency range and reception/transmission angles remains a challenge. 
         [0005]    Accordingly, there remains a need for small antennas capable of being contained in small packages, and that provide a desired gain across a frequency range. 
       SUMMARY OF THE INVENTION 
       [0006]    Exemplary of an embodiment of the present invention, a microstrip antenna comprises at least one parasitic patch, located beside a central patch, the parasitic patch electrically disconnected from the central patch, but inductively coupled thereto aid in transferring energy to and from the central patch. 
         [0007]    In accordance with an embodiment, an antenna includes a central patch dimensioned to transmit or receive a radio signal at a center frequency f c  having a corresponding wavelength λ, formed on a substrate; a feed line extending from said central patch; at least one parasitic patch, located beside the central patch; a ground plane formed on an opposite side of said substrate. The parasitic patch is electrically disconnected from the central patch, and located a lateral distance d≦λ/8 from the central patch. 
         [0008]    In accordance with another aspect of the present invention, there is provided a method of operating an antenna to radiate an electromagnetic field. The method comprises: providing a central patch dimensioned to emit a radio signal at a center frequency f c  having a corresponding wavelength λ, formed on one side of a substrate having a ground plane formed on an opposite side thereof; providing at least one parasitic patch, located beside the central patch; driving the central patch by current from a transmitter and thereby inducing current to the parasitic patch that contributes constructively in radiating the electromagnetic field. 
         [0009]    Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    In the figures which illustrate by way of example only, embodiments of the present invention, 
           [0011]      FIG. 1  is a perspective view of a conventional patch antenna; 
           [0012]      FIG. 2  is a plan view of a parasitic patch antenna, exemplary of an embodiment of the present invention; 
           [0013]      FIG. 3  is a cross-sectional view of  FIG. 2 , along lines 
           [0014]      FIG. 4  is a graph illustrating the reflection coefficient an antenna in the form of the antenna of  FIG. 2 ; 
           [0015]      FIG. 5  is a receive radiation pattern of an antenna in the form of the antenna of  FIG. 2 ; and 
           [0016]      FIG. 6  is a transmit radiation pattern an antenna in the form of the antenna of  FIG. 2 ; 
           [0017]      FIGS. 7A-7E  are plan views of alternate antennas exemplary of embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  is a perspective view of a conventional patch antenna  10 . Antenna  10  includes a rectangular patch  14 , formed of a conductive material formed on one side of substrate  12 . A feed line  16  extends from patch  14 . A conductive sheet  17  that covers some or all of the opposite side of substrate  12  forms a ground plane for antenna  10 . 
         [0019]    The gain and bandwidth of antenna  10  is controlled by the geometry of patch  14  (e.g. length and width), and physical characteristics of substrate  12  (e.g. height h, and dielectric constant, ∈ r ). 
         [0020]    Similarly, the matching impedance of antenna  10  is controlled by the geometry of feed line  16  and physical characteristics of substrate  12 . 
         [0021]    As for example detailed in EE144/245 Patch Antenna Design, Spring 2007, H. Miranda, Stanford University, the contents of which are hereby incorporated, the length and width of patch  14 , expressed as function of the desired reception/transmission (i.e. center) frequency of antenna  10 , may be calculated as, 
         [0000]    
       
         
           
             W 
             = 
             
               
                 
                   c 
                   
                     2 
                      
                     
                       f 
                       r 
                     
                   
                 
                  
                 
                   
                     ( 
                     
                       
                         
                           ɛ 
                           r 
                         
                         + 
                         1 
                       
                       2 
                     
                     ) 
                   
                   
                     
                       - 
                       1 
                     
                     / 
                     2 
                   
                 
               
               = 
               
                 
                   λ 
                    
                   
                     
                       1 
                       
                         2 
                          
                         
                           ( 
                           
                             
                               ɛ 
                               r 
                             
                             + 
                             1 
                           
                           ) 
                         
                       
                     
                   
                 
                 ≈ 
                 
                   λ 
                   / 
                   2 
                 
               
             
           
         
       
       
         
           and 
         
       
       
         
           
             L 
             = 
             
               
                 c 
                 
                   2 
                    
                   
                     f 
                     r 
                   
                    
                   
                     
                       ɛ 
                       
                         eff 
                          
                         
                             
                         
                       
                     
                   
                 
               
               - 
               
                 2 
                  
                 Δ 
                  
                 
                     
                 
                  
                 l 
               
             
           
         
       
       
         
           with 
         
       
       
         
           
             
               ɛ 
               eff 
             
             = 
             
               
                 
                   
                     ɛ 
                     r 
                   
                   + 
                   1 
                 
                 2 
               
               + 
               
                 
                   
                     
                       ɛ 
                       r 
                     
                     - 
                     1 
                   
                   2 
                 
                  
                 
                   
                     ( 
                     
                       1 
                       + 
                       
                         
                           12 
                            
                           h 
                         
                         W 
                       
                     
                     ) 
                   
                   
                     
                       - 
                       1 
                     
                     / 
                     2 
                   
                 
               
             
           
         
       
       
         
           and 
         
       
       
         
           
             
               Δ 
                
               
                   
               
                
               l 
             
             = 
             
               0.412 
                
               
                 h 
                  
                 
                   ( 
                   
                     
                       
                         ɛ 
                         eff 
                       
                       + 
                       0.3 
                     
                     
                       
                         ɛ 
                         eff 
                       
                       - 
                       0.258 
                     
                   
                   ) 
                 
               
                
               
                 ( 
                 
                   
                     
                       W 
                       / 
                       h 
                     
                     + 
                     0.264 
                   
                   
                     
                       W 
                       / 
                       h 
                     
                     + 
                     0.8 
                   
                 
                 ) 
               
             
           
         
       
     
         [0000]    where c is the speed of light, f r  is the center frequency of the antenna, ∈ r  is the relative permeability of the substrate  12 , h is the height of substrate  12 , and W is the width of the patch  14 . 
         [0022]    As will be appreciated, antenna  10  will radiate/absorb electro-magnetic waves in different planes with different efficiencies, in dependence on the geometry of antenna  10 . It may, for example, be shown that the beam width for antenna  10  is about 65° and the gain is between about 7 and 9 dBi. As will be appreciated, gain is linked to overall geometry of the patch. With a simple rectangular patch as in antenna  10 , geometric variations are limited. 
         [0023]    Exemplary of an embodiment of the present invention, the effective area of a patch antenna may be increased, by including one or more coupled (also referred to as parasitic) patches, as illustrated in  FIGS. 2 and 3 . 
         [0024]    As illustrated, an exemplary antenna  20  includes central patch  24 , interconnected with a feed  26 . Coupled parasitic patches  28   a  and  28   b  are located laterally on either side of patch  24  formed on a substrate  22 . The side of substrate  22  opposite patch  24  and patches  28   a  and  28   b  is conductively coated, to provide a ground plane  42 . 
         [0025]    In the depicted embodiment, antenna  20  central patch and parasitic patches  28   a  and  28   b  are generally rectangular. As illustrated, central patch  24  has a width W and a height L. Patches  28   a  and  28   b  are also each rectangular, with a width w, and height L, equal to the height of central patch  24 . Patches  28   a  and  28   b  are aligned vertically, with vertical center (C) of patch  24  and are aligned with the vertical center (C a , C b ) of each of patches  28   a  and  28   b . As heights of patches  28   a ,  28   b  and  24  are equal, tops and bottoms of patches  28   a ,  28   b  and  24  are also aligned. 
         [0026]    Patches  24  and  28   a  and  28   b  are electrically isolated (i.e. not conductively interconnected) from each other. Rather, patches  28   a  and  28   b  are coupled to central patch  24 . For a transmitting antenna, patch  24  can thus be thought of the driven patch, driven by current from then transmitter. Current is induced to parasitic patches  28   a ,  28   b  and contributes constructively in radiating electromagnetic fields. For a receiving antenna, patch  24  may be considered a driving patch that drives the receiver. Again, current is induced to parasitic patches  28   a ,  28   b  and contributes constructively in receiving radiated electromagnetic fields. In order to be coupled to patch  24 , patches  28   a  and  28   b  are in sufficiently proximity to central patch  24 . In particular, patches  28   a  and  28   b  are spaced at a distance d from central patch  24 . In the depicted embodiment, distance d is chosen to be less than λ/8. Without wishing to be bound a particular theory, it is believed that d is chosen to arrange patches  28   a  and  28   b  sufficiently close to central patch  24 , so that electromagnetic radiation emitted by central patch  24  is coupled, inductively or otherwise, to patches  28   a  and  28   b  to assist in transmission of a signal from antenna  20 ; likewise electromagnetic radiation received by patches  28   a  and  28   b  is coupled to patch  24  to assist in reception of a signal at antenna  20 . 
         [0027]    The presence of parasitic patches  28   a ,  28   b  thus increases the effective area of antenna, without significantly affecting the center frequency of patch  24 . In the depicted embodiment, the dimensions of patch  24  are chosen based on the desired center frequency f/ wavelength λ of antenna  20 . The area of patch  24  is also chosen to be less than or equal to the area of the patch  14  of a conventional patch antenna ( FIG. 1 ). 
         [0028]    That is L*W≦ 
         [0000]    
       
         
           
             
               λ 
               2 
             
              
             
               
                 ( 
                 
                   
                     
                       ɛ 
                       r 
                     
                     + 
                     1 
                   
                   2 
                 
                 ) 
               
               
                 
                   - 
                   1 
                 
                 / 
                 2 
               
             
             * 
             
               { 
               
                 
                   λ 
                   
                     2 
                      
                     
                       
                         ɛ 
                         
                           eff 
                            
                           
                               
                           
                         
                       
                     
                   
                 
                 - 
                 
                   2 
                    
                   Δ 
                    
                   
                       
                   
                    
                   l 
                 
               
               } 
             
           
         
       
       
         
           with 
         
       
       
         
           
             
               Δ 
                
               
                   
               
                
               l 
             
             = 
             
               0.412 
                
               
                 h 
                  
                 
                   ( 
                   
                     
                       
                         ɛ 
                         eff 
                       
                       + 
                       0.3 
                     
                     
                       
                         ɛ 
                         eff 
                       
                       - 
                       0.258 
                     
                   
                   ) 
                 
               
                
               
                 ( 
                 
                   
                     
                       W 
                       / 
                       h 
                     
                     + 
                     0.264 
                   
                   
                     
                       W 
                       / 
                       h 
                     
                     + 
                     0.8 
                   
                 
                 ) 
               
             
           
         
       
       
         
           and 
         
       
       
         
           
             
               ɛ 
               eff 
             
             = 
             
               
                 
                   
                     ɛ 
                     r 
                   
                   + 
                   1 
                 
                 2 
               
               + 
               
                 
                   
                     
                       ɛ 
                       r 
                     
                     - 
                     1 
                   
                   2 
                 
                  
                 
                   
                     ( 
                     
                       1 
                       + 
                       
                         
                           12 
                            
                           h 
                         
                         W 
                       
                     
                     ) 
                   
                   
                     
                       - 
                       1 
                     
                     / 
                     2 
                   
                 
               
             
           
         
       
     
         [0029]    As before, ∈ r  denotes the relative permittivity of substrate  22 , and h denotes its thickness. 
         [0030]    From the foregoing, it may be recognized that L*W≦λ 2 /4. Specifically, L*W≦0.55*0.4λ 2    
         [0031]    Now, w is chosen to be about ¼ of W, e.g. w=0.14λ, and d≦λ/8. 
         [0032]    For an antenna having a center frequency of about 60 GHz, on a substrate with ∈ r ˜3.5 and h˜125 μm, the size of the central patch  24  is L=1700×W=1240 μm 2 (0.55×0.4λ 2 ). Each parasitic patch  28   a ,  28   b  is w=420×W=1240 μm 2  (0.14×0.4λ 2 ). The space d between central patch  24  and patches  28   a ,  28   b  is 280 μm (0.09λ 2 ). 
         [0033]    As will become apparent, the presence of parasitic patches  28   a ,  28   b  couples energy at frequencies other than the center frequency f to central patch  24 . So as not to unduly attenuate or filter signal at these additional frequencies, a feed line  26  that passes a broad frequency of electromagnetic signals is provided. To this end, central patch  24  further includes a slot  40  from which feed line  26  extends. Slot  40  creates two equal smaller notches  44   a  and  44   b  between feed line  26 , and central patch  24 . In the depicted embodiment, slot  40  has a width of 400 μm and a depth of 250 μm, while notches  44   a  and  44   b  each have width of 125 μm. 
         [0034]    Feed line  26 , in turn, includes several tapered sections  30 ,  32  and  34 . The first tapered section  30  has a width of about 150 μm, a length, I 1  of about 950 μm (0.3λ), and an impedance of 70Ω; section  32  has a width of about 190 μm, a length of 500 μm (0.16λ) and an impedance of 60Ω; section  34  has a width of 275 μm. 
         [0035]    The feed line sections  30 ,  32  and  34  of differing widths, allow feed line  26  to guide signal of a broader bandwidth than a single width feedline, allowing energy at frequencies outside the center frequency of central patch  24  to be coupled between parasitic patches  28   a ,  28   b  and central patch  24 . 
         [0036]    Conveniently, a receiver/transmitter  50  may be formed on substrate  22 , along with antenna  20 . A bend  36  may interconnect section  34  to a terminating section  38 , also having a width of 275 μm, which in turn may interconnect antenna  20  to receiver/transmitter  50 . 
         [0037]    Antenna  20  may be etched or plated using traditional techniques. The thickness of the conductive material forming antenna  20  does not materially impact the operation/effectiveness of antenna  20 . Antenna  20  may thus be etched or plated using conventional copper, aluminium, silver, gold or other conductive material. 
         [0038]    Antenna  20  may be a transmit antenna; a receive antenna; or a combined transmit/receice antenna. 
         [0039]    A graph illustrating (simulated) reflected power (antenna parameter S 11 ) against frequency for antenna  20  is illustrated in  FIG. 4 . As illustrated, reflection of antenna  20 , is at a minimum (and thus maximum coupled energy) at f c =57.4 MHz. Interestingly, reflection of antenna  20 , is at a further local minimum (and thus maximum coupled energy) at f=66 MHz 
         [0040]      FIG. 5  is a simulated radiation pattern of a received signal received at an antenna of the form of antenna of  FIG. 2 , at various angels.  FIG. 6  is a simulated receive radiation pattern for this antenna. 
         [0041]    The performance of each antenna for various radio transmission channels may be further characterized by 5 parameters: Coverage, Max Gain, HPBW (deg), H0dB-Beam 1, E0dB-Beam 2 which are defined as follows. 
         [0042]    Coverage represents the portion of the upper hemisphere where the realized gain is above 0 dBi. 
         [0043]    Maximum Gain is the maximum realized gain at the centre frequency of the channel. Realized gain includes the antenna mismatch effects and is always smaller than (or equal to) the antenna gain. 
         [0044]    0-dB Beamwidth is the angular separation between two points on opposite sides of the maximum of the antenna radiation pattern where the sign of the radiation gain in dB changes. 
         [0045]    The above criteria help provide a better understanding of antenna coverage, because anywhere within the O-dB beamwidth the antenna is focusing the transmitted/received energy. 
         [0046]    H0dB-Beam: The angular separation between two points on opposite sides of the pattern maximum in H-plane, where the sign of the radiation gain in dB changes. E0dB-Beam: The angular separation between two points on opposite sides of the pattern maximum in E-plane, where the sign of the radiation gain in dB changes. 
         [0047]    The frequency separation between two points on opposite sides of the resonance frequency in S 11  or S 22  curves where the absolute value of the reflection coefficient is larger than or equal to 10 dB (or 8 dB). 
         [0048]    For an example antenna of the form of antenna  20  of  FIG. 2 , having a center frequency of about 60.48 GHZ, characteristics of the following channels were assessed: 
         [0000]                                                                      Start (GHz)   Stop (GHz)   Center (GHz)                                        Channel 1   57.24   59.4   58.32           Channel 2   59.4   61.56   60.48           Channel 3   61.56   63.72   62.64           Channel 4   63.72   65.88   64.8                        
As depicted below, the RX/TX characteristics of the antenna at these channels were measured:
 
         [0000]    
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 Max 
                   
                 0 dB- 
                 0 dB- 
               
               
                   
                   
                 Coverage 
                 Gain 
                 HPBW 
                 Beam 1 
                 Beam 2 
               
               
                 RX/TX 
                 Channel 
                 (%) 
                 (dBi) 
                 (deg) 
                 (deg) 
                 (deg) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 RX 
                 1 
                 60.3 
                 7.2 
                 76 
                 98 
                 132 
               
               
                 RX 
                 2 
                 64.5 
                 8.7 
                 87 
                 96 
                 141 
               
               
                 RX 
                 3 
                 60.1 
                 8.2 
                 108 
                 69 
                 150 
               
               
                 TX 
                 1 
                 61.6 
                 8.1 
                 48 
                 96 
                 131 
               
               
                 TX 
                 2 
                 65.9 
                 9.1 
                 75 
                 98 
                 143 
               
               
                 TX 
                 3 
                 60 
                 8.1 
                 107 
                 73 
                 149 
               
               
                   
               
             
          
         
       
     
         [0049]    As can be appreciated, the above table and  FIGS. 5-6  illustrate that antenna  20  provides moderate gain, with a broader coverage (i.e. beam width) than antenna  10 , and a relatively broad bandwidth (about 8 MHz). 
         [0050]    As should now be appreciated, antenna  20  is only a single possible embodiment of the present invention. Many other geometries of an antenna exemplary of the present invention are possible. For example, antennas exemplary of embodiments of the present invention as illustrated in  FIGS. 7A to 7E  may be formed. 
         [0051]    As depicted in plan view in  FIG. 7A , multiple parasitic patches may be formed on each side of the central patch, with several parasitic patches on each side. In the depicted embodiment of  FIG. 7A  two parasitic patches to be coupled to each other, and to the central patch. Again, each parasitic patch may be spaced by a distance less than λ/8, from an adjacent patch, allowing the multiple parasitic patches on each side of central patch are on either side of the central patch. More are of course possible. Parasitic patches need not be the same height, or shape or centered with the central patch Again, a feed line having sections of differing widths, that passes a broader range of frequencies from the central patch may be used. 
         [0052]    In other geometries, as illustrated in  FIG. 7B , the central patch may be circular or oval. In the even the central patch is circular/oval, parasitic patches may be crescent shaped, or may take the forme of a circular or elliptical segment (not shown). 
         [0053]    In yet other geometries, as illustrated in  FIGS. 7C and 7D , the central patch need not be square, but may instead be hexagonal ( FIG. 7C ) or octagonal ( FIG. 7D ) (with each corner of a rectangular patch eliminated). Again, the parasitic patches may be rectangular. 
         [0054]    In yet further geometries, the central patch need not be continuous, and may have areas where the substrate is exposed, as for example in  FIG. 7E . 
         [0055]    The choice of size/geometry of central patch will be dependent on the desired center frequency of the antenna, determined as understood by those of ordinary skill. Similarly, the exact ideal shape or number of one or more parasitic patches may be experimentally determined. Again parasitic patches may be spaced suitably close to central patch (e.g. λ/8). Likewise, a suitable feedline may extend from various portions/locations of the driving/driven patch. 
         [0056]    Of course, the above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments of carrying out the invention are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.