Abstract:
A broadband antenna element configuration having a radiation pattern useful in an antenna array containing a plurality of driven radiating elements that are spatially arranged is disclosed. The antenna element is coplanarly disposed on a suitable planar substrate of dielectric material. The antenna element utilizes a pair of balanced dipole arm elements symmetrically disposed about the centerline of a balanced feed network. Balanced feed network elements are disposed in a broadside symmetrical configuration on first plane and second plane on each side of the aforementioned dielectric. Disposed proximate to each dipole arm element are partially overlapping, parallel planar, frequency bandwidth expanding microstrip lines. The combination of dipole arms and parasitically coupled microstrip lines provides a broad bandwidth radiating element suitable for use in antenna arrays.

Description:
RELATED APPLICATION INFORMATION 
     The present application claims priority under 35 USC section 119(e) to U.S. provisional patent application Ser. No. 60/994,557 filed Sep. 20, 2007, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to radio communication systems and components. More particularly the invention is directed to antenna elements and antenna arrays for radio communication systems. 
     2. Description of the Prior Art and Related Background Information 
     Modern wireless antenna implementations generally include a plurality of radiating elements that may be arranged to provide a desired radiated (and received) signal beamwidth and azimuth scan angle. For a wide beamwidth antenna it is desirable to achieve a near uniform beamwidth that exhibits a minimum variation over the desired azimuthal as degrees of coverage. Such antennas provide equal signal coverage over a wide area which is useful in certain wireless applications. In modern applications, it is also necessary to provide a consistent beamwidth over a wide frequency bandwidth. 
     Consequently, there is a need to provide an improved broadband antenna structure with desired beamwidth. Furthermore, it is desirable to provide such an antenna in a relatively compact and low cost construction suitable for use in antenna arrays. 
     SUMMARY OF THE INVENTION 
     In a first aspect the present invention provides an antenna radiating structure comprising a generally planar dielectric support structure, a first generally planar radiating element configured on one side of the dielectric support structure, a second generally planar radiating element configured on an opposite side of the dielectric support structure and configured in a generally parallel plane with the first generally planar radiating element, and means for expanding the bandwidth of the antenna radiating structure configured on the dielectric support structure and spaced apart from the radiating elements. 
     In a preferred embodiment of the antenna radiating structure the means for expanding the bandwidth of the antenna radiating structure comprises first and second conductive elements formed on opposite sides of the dielectric support structure. The first and second planar radiating elements preferably comprise elongated conductive strips and the first and second conductive elements preferably comprise planar strips parallel to and spaced apart from the elongated conductive strips of the first and second planar radiating elements. The first and second conductive elements preferably have a partial overlap and the amount of overlap controls the amount of beamwidth expansion. The strips comprising the first and second conductive elements are preferably shorter than the elongated conductive strips of the first and second planar radiating elements. The strips comprising the first and second conductive elements are preferably wider than the elongated conductive strips of the first and second planar radiating elements. In one example application the amount of overlap is between 240 and 270 mils. For example, in this application the antenna radiating structure operational radio frequency (RF) may be approximately 3.15 GHz to 3.80 GHz. In such example the planar strips are preferably spaced apart from the elongated conductive strips of said first and second planar radiating elements by about 180 to 210 mils. 
     In another aspect the present invention provides an antenna radiating structure, comprising a planar dielectric substrate, first and second ┌-shaped dipole radiating elements formed on opposite sides of the dielectric substrate, first and second bandwidth enhancement elements formed on opposite sides of the dielectric substrate proximate to respective dipole radiating elements, and a balanced RF feed network feeding the dipole radiating elements. 
     In a preferred embodiment of the antenna radiating structure the shape of the dipole radiating elements is mirror symmetric and the overall structure, including the feed network, has a T-shape. The dipole radiating elements preferably comprise microstrip dipole arms on respective sides of the dielectric substrate, and the bandwidth enhancement elements preferably comprise planar microstrips which are parallel to each dipole arm and at least partially overlapping each other. When an x-y coordinate system is defined so that the origin is set at the bottom end of the T shaped structure of the antenna element, the y-axis is the symmetric vertical line of the T shape, and the x-axis is parallel to the top of the T shape and perpendicular to the y-axis, the balanced feed network center line is in the longitudinal direction of the y-axis before transitioning to each planar dipole arm which extend parallel to the x-axis along a centerline axis CL 1 , but in opposite directions relative to the balanced feed network center line. The bandwidth enhancement microstrips preferably extend parallel to the x-axis along a centerline axis CL 2  separated by a distance s 1  from centerline axis CL 1 . The microstrip dipole arms have a width w 1  and the bandwidth enhancement microstrips preferably have a defined width w 2  greater than w 1 . The bandwidth enhancement microstrips preferably share broadside overlap dimension o 1  over each other and the amount of overlap provides control over useful frequency bandwidth. The two dipole arms are preferably identical in width w 1  and length L 1 . The bandwidth enhancement microstrips preferably are identical in width w 2  and length L 2 . 
     In another aspect the present invention provides an antenna array, comprising a ground plane and a plurality of radiating structures configured on the ground plane, each comprising a planar dielectric substrate extending perpendicularly to said ground plane, a balanced RF feed network formed on the substrate, a pair of balanced dipole radiating elements including a pair of dipole arm elements symmetrically disposed about the centerline of said balanced feed network, and partially overlapping, planar, frequency bandwidth expanding microstrip lines disposed proximate to the dipole arm elements. 
     In a preferred embodiment of the antenna array the balanced RF feed network comprises balanced feed network elements disposed in a symmetrical configuration on a first plane and second plane on each side of the dielectric substrate. 
     Further features and advantages of the present invention will be appreciated from the following detailed description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view and selected planar cross-sections of an antenna element in accordance with a preferred embodiment of the invention. 
         FIG. 2  is an isometric view of an antenna element in accordance with a preferred embodiment of the invention mounted on a ground plane. 
         FIG. 3  is a graph showing simulated input return loss over frequency for various overlap (o 1 ) dimensions. 
         FIG. 4  is a graph showing simulated azimuth and elevation radiation plots of an exemplary antenna element in accordance with the invention. 
         FIG. 5  is a graph showing simulated return loss vs. bandwidth for various lengths (L 2 ) of bandwidth expanding microstrip lines. 
         FIG. 6  is a graph showing simulated return loss vs. bandwidth for various lengths (L 1 ) of dipole arms. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will be made to the accompanying drawings, which assist in illustrating the various pertinent features of the present invention. Some of the components represented in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. In certain instances herein chosen for illustrating the invention, certain terminology is used which will be recognized as being employed for convenience and having no limiting significance. For example, the terms “horizontal”, “vertical”, “upper”, “lower”, “bottom” and “top” refer to the illustrated embodiment in its normal position of use. 
     One object of the present invention is to provide a dielectric based coplanar antenna element which has broad frequency bandwidth, is easy to fabricate using conventional PCB processes, and has a low profile. In carrying out these and other objectives, features, and advantages of the present invention, a broad bandwidth antenna element is provided for use in a wireless network system. 
       FIG. 1  shows a top view of a coplanar antenna element,  10 , according to an exemplary implementation, which utilizes a substantially planar dielectric material  12 . Radiating element  10  may be of any suitable construction preferably employing a method which prints or attaches metal conductors directly on top and bottom  12   b  sides of a dielectric substrate  12  such as a PCB (printed circuit board). The square dielectric plane  12  is dimensioned to fit all necessary conductors in a manner which is not only compact but which provides radiation pattern, frequency response and bandwidth over the desired frequency. In one embodiment the desired radio frequency (RF) may be approximately 3.15 GHz to 3.80 GHz and the antenna element is constructed utilizing a commercially available PCB material, such as manufactured by Taconic, specifically Taconic RF-35, ∈ r =3.5 and thickness=30 mils. Alternative dielectric substrates (PCB material)  12  are possible provided that properties of such substrate are chosen in a manner to be compatible with commonly available PCB processes. Alternatively metal conductor attachment to alternative dielectric substrates can be achieved through various means known to those skilled in the art. 
     As shown, antenna element  10  is provided with an upper dielectric side RF input-output port  14 . The input RF signal is further coupled over a balun structure comprising top coplanar microstrip element  16  and bottom microstrip element  18 . A balun is an electromagnetic structure for interfacing a balanced impedance device or circuit, such as an antenna, with an unbalanced impedance, such as a coaxial cable or microstrip line. In its common use a balanced signal comprises a pair of symmetrical signals, which are equal in magnitude and opposite in phase (180 degrees). In contrast, an unbalanced impedance may be characterized by a single conductor for supporting the propagation of unbalanced (i.e., asymmetrical) signals relative to a second conductor (i.e., ground). Numerous balun structures are known to those skilled in the art for converting unbalanced to balanced signals and vice versa. 
     Thereafter, balanced RF signals are coupled into a multi section impedance transformer. A multi-section impedance transformer is employed to match balun impedance to a dipole feed point impedance without reducing useful frequency bandwidth. In this manner a first transformer section is comprised of a top microstrip line  20  and a bottom microstrip line  34 . The first transformer section has a length L 4  which is optimized along with other dimensions for the target operating frequency range. Output of the first transformer section is coupled to a second transformer section which is further comprised of a top microstrip line  22  and a bottom microstrip line  32 . Output of the second transformer top microstrip line  22  is coupled to the top side dipole  24  element and bottom microstrip line  32  is coupled to the bottom dipole  26  element. The second transformer section has a length L 3  which is also optimized along with other dimensions for the target operating frequency range. 
     Radiating element  10  is comprised of top sided dipole element  24  having its longitudinal center axis CL 1  perpendicular to the y axis and traversing away from the y-axis in a negative x dimension direction, and bottom dipole element  26  having its longitudinal center axis CL 1  perpendicular to the y axis and traversing away from the y-axis in a positive x dimension direction. The two dipole arms  24 ,  26  are symmetrical about the y-axis, and disposed on the opposite sides of the planar dielectric  12 . The two dipole arms  24 ,  26  are preferably identical in width w 1  and length L 1 . Alternative implementations using an asymmetric dipole structure can be devised, but such configuration may introduce unbalancing effects on a balanced feed network and thus may not be preferred. 
     In further reference to  FIG. 1 , disposed proximate to dipole arms  24 ,  26  (on a corresponding side of dielectric substrate  12 ,  12   b ) are bandwidth expanding microstrip elements  28 ,  30  separated by distance s 1  between corresponding centerline axis CL 1  and CL 2 . The bandwidth expanding microstrip elements  28 ,  30  have a defined width w 2 , and longitudinal center axis aligned with the CL 2  axis which is also perpendicular to the y axis. Microstrip elements  28 ,  30  share broadside overlap dimension o 1  over each other and the amount of overlap provides control means over useful frequency bandwidth. It will be apparent to those skilled in the art that antenna radiating structure  10  may include an additional number of bandwidth expanding microstrip element pairs (i.e., one or more) implemented in accordance with the present invention to augment the radiation pattern as desired. 
     Referring to  FIG. 2 , an embodiment of the invention with plural antenna radiating structures  10  mounted on a ground plane  200  to form an antenna array is illustrated. Each of the structures  10  correspond to that of  FIG. 1  and need not be further described. The RF input/output ports of antenna radiating structures  10  are coupled to feed lines  214  which may be microstrip lines formed on a dielectric and coupled to the RF sources. Although two antenna radiating structures  10  are shown it will be appreciated that additional antenna radiating structures  10  can be mounted on ground plane  200  to form the antenna array. Further it will be appreciated by those skilled in the art that antenna radiating structures  10  can be arranged in various configurations, including plural rows and columns. Therefore, although two structures  10  are shown for ease of illustration, such embodiments with additional numbers and configurations of antenna radiating structures  10  are equally implied herein. 
     Referring to  FIGS. 3-6  simulated antenna performance including the effects of variation of the above noted parameters on antenna performance are illustrated.  FIG. 4  is a graph showing simulated azimuth and elevation radiation plots of an exemplary antenna element in accordance with the invention. The simulated bandwidth variation vs. overlap distance o 1  of the microstrip lines  28 ,  30  is presented in  FIG. 3 .  FIG. 5  is a graph showing simulated return loss vs. bandwidth for various lengths (L 2 ) of bandwidth expanding microstrip lines  28 ,  30 .  FIG. 6  is a graph showing simulated return loss vs. bandwidth for various lengths (L 1 ) of dipole arms  24 ,  26 . 
     Preferred dimensions for a 3.15 GHz to 3.80 GHz embodiment with 50 impedance source  14  are shown in the following table. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Reference 
                 Min (mils) 
                 Max (mils) 
                 Typical (mils) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 L1 
                 670 
                 700 
                 684 
               
               
                   
                 L2 
                 560 
                 590 
                 576 
               
               
                   
                 L3 
                 481 
                 520 
                 496 
               
               
                   
                 W3 
                   
                   
                 62.8 Ω 
               
               
                   
                 L4 
                 475 
                 510 
                 491 
               
               
                   
                 W4 
                   
                   
                 54.8 Ω 
               
               
                   
                 L5 
                 180 
                 310 
                 195 
               
               
                   
                 o1 
                 240 
                 270 
                 258 
               
               
                   
                 w1 
                 80 
                 95 
                  88 
               
               
                   
                 w2 
                 100 
                 130 
                 112 
               
               
                   
                 s1 
                 180 
                 210 
                 192 
               
               
                   
                   
               
             
          
         
       
     
     It will be appreciated that antennas operating at alternative frequency ranges may employ the teachings of the present invention and the above parameters may be varied for such applications. 
     The present invention has been described in a preferred embodiment but the description is not intended to limit the invention to the form disclosed herein. Accordingly, variants and modifications consistent with the following teachings, and skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described herein are further intended to explain modes known for practicing the invention disclosed herewith and to enable others skilled in the art to utilize the invention in equivalent, or alternative embodiments and with various modifications considered necessary by the particular application(s) or use(s) of the present invention.