Abstract:
Embodiments in accordance with the invention include a linearly polarized dipole antenna with an unbalanced microstrip feed line. More specifically, embodiments in accordance with the invention utilize a fed dipole connected to a microstrip feed line, and separated a gap distance from a parasitic dipole not connected to the microstrip feed line. When an electrical signal is input to the microstrip feed line, the microstrip feed line creates a current flow in the fed dipole which induces a nearly equal current on the parasitic dipole that is out of phase. The result is a current flow, I, in the same direction in both fed and parasitic dipoles allowing for efficient radiation of the linearly polarized dipole antenna. Embodiments in accordance with the invention eliminate the need for a balun circuit thereby reducing the size, complexity and feed loss of the feed circuit. Embodiments in accordance with the invention are effective for dipoles with relatively small ground planes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/599,308 filed Feb. 15, 2012, which is hereby incorporated in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to dipole antennas. 
     2. Description of the Related Art 
     Linearly polarized dipole antennas are commonly used in communication and radar applications. Dipoles can be used individually or as elements in an array antenna. Often a dipole antenna is fed using a microstrip feed line. Microstrip feed lines are utilized in many applications because the devices connected to the dipole are often printed on microstrip boards. 
     Prior art dipole structures utilized a balanced structure that generally required a balanced-to-unbalanced circuit, also termed a balun circuit, or simply a balun, when fed by a microstrip line. Depending on how the balun circuit was implemented, it had the undesired effect of increasing either the depth or area of the assembly. FIGS.  1 A/ 1 B and  2 A/ 2 B show instances of prior art dipole antennas utilizing balun circuits. In FIGS.  1 A/ 1 B and  2 A/ 2 B, the balun circuit is used to split a single input line into a two wire line for connecting to the dipole antenna. FIGS.  1 A/ 1 B illustrate how the prior art vertical implementation of a balun circuit increased the depth of the assembly by utilizing area on the vertical ground plane of the assembly. FIGS.  2 A/ 2 B illustrate how the prior art horizontal implementation of the balun circuit increased the area of the assembly by utilizing area on the horizontal ground plane of the assembly. 
     SUMMARY OF THE INVENTION 
     Embodiments in accordance with the invention include a linearly polarized dipole antenna with an unbalanced microstrip feed line which eliminate the need for a balun circuit thereby reducing the size, complexity and feed loss of the feed circuit. Embodiments in accordance with the invention are effective for dipoles with relatively small ground planes. In accordance with one embodiment, a linearly polarized dipole antenna with an unbalanced microstrip feed including: a substrate having a top surface and a back surface; a microstrip feed line in contact with the back surface of said substrate, the microstrip feed line having an input end for accepting an electrical signal; a ground plane having a top surface and a back surface, wherein the back surface of said ground plane is in contact with at least a portion of the top surface of the substrate, the ground plane further having a ground plane aperture in the top surface of the ground plane that exposes at least a portion of the substrate; a first conductive element having a first vertical stem and a first horizontal arm, wherein a first end of the first conductive element extends through said ground plane aperture through the substrate and contacts the microstrip feed line; and a second conductive element having a second vertical stem and a second horizontal arm, wherein the second vertical stem is spaced a gap distance, g, apart from the first vertical stem, and further wherein a first end of the second conductive element extends through the ground plane aperture through the substrate and is connected to the back surface of said substrate. 
     Embodiments in accordance with the invention are best understood by reference to the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a front side view of a prior art vertical implementation of a balun circuit. 
         FIG. 1B  illustrates a back side view of a prior art vertical implementation of a balun circuit. 
         FIG. 2A  illustrates a bottom side view of a prior art horizontal implementation of a balun circuit. 
         FIG. 2B  illustrates a top side view of a prior art horizontal implementation of a balun circuit. 
         FIG. 3A  illustrates a transverse top view of a linearly polarized dipole assembly with unbalanced microstrip feed in accordance with one embodiment. 
         FIG. 3B  illustrates a transverse bottom view of the linearly polarized dipole assembly of  FIG. 3A  in accordance with one embodiment. 
         FIG. 3C  illustrates a cross-sectional side view of the linearly polarized dipole assembly of  FIG. 3A  in accordance with one embodiment. 
         FIG. 4A  illustrates a transverse top view of a linearly polarized dipole assembly with unbalanced microstrip feed of  FIG. 3A  including a support medium in accordance with another embodiment. 
         FIG. 4B  illustrates a transverse bottom view of the linearly polarized dipole assembly of  FIG. 4A  in accordance with another embodiment. 
         FIG. 4C  illustrates a cross-sectional side view of the linearly polarized dipole assembly of  FIG. 4A  in accordance with another embodiment. 
         FIG. 5  illustrates a return loss plot (in dB) showing good match to a 50 ohm microstrip line at 10 GHz for a sample design of a dipole with unbalanced microstrip feed in accordance with one embodiment. 
         FIG. 6  illustrates an H-plane radiation pattern for the sample design in accordance with one embodiment. 
         FIG. 7  illustrates an E-plane radiation pattern for the sample design in accordance with one embodiment. 
     
    
    
     Embodiments in accordance with the invention are further described herein with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 3A and 3B  and  3 C illustrate a linearly polarized dipole antenna assembly with unbalanced microstrip feed line in accordance with one embodiment.  FIG. 3A  illustrates a transverse top view of a linearly polarized dipole assembly  300  with unbalanced microstrip feed line in accordance with one embodiment.  FIG. 3B  illustrates a transverse bottom view of the linearly polarized dipole assembly of  FIG. 3A  in accordance with one embodiment.  FIG. 3C  illustrates a side view of the linearly polarized dipole assembly of  FIG. 3A  in accordance with one embodiment. 
     Referring to  FIGS. 3A ,  3 B, and  3 C, in one embodiment, linearly polarized dipole antenna assembly  300  includes two elements: a conductive first dipole element  302 , and a conductive second dipole element  304 . In one embodiment, a first end of first dipole element  302  extends through a ground plane aperture  306  in a top surface of a ground plane  308  through a substrate  310  and is attached to a microstrip feed line  312  at a point A at a back surface of substrate  310 . A first end of second dipole element  304  extends through ground plane aperture  306  through substrate  310  and is attached at a point B to the back surface of substrate  310 . This arrangement of first dipole element  302  and second dipole element  304  can also be generally described as a bent monopole with a parasitic element. The remaining portions of first dipole element  302  and second dipole element  304  extend above the top surface of ground plane  308 . In one embodiment, first dipole element  302  and second dipole element  304  are positioned apart from each other at a gap distance  316 , g. 
     In one embodiment, first dipole element  302  includes a first horizontal arm  318  of first length, L1, and a first vertical stem  320  of height, h, where height, h, is measured from the top surface of ground plane  308  to first horizontal arm  318 . Second dipole element  304  includes a second horizontal arm  322  of second length, L2, and a second vertical stem  324  of height, h. In one embodiment, first dipole element  302  is formed of a conductive material, such as a metal wire of radius, r, and second dipole element  304  is formed of a conductive material, such as a metal wire of radius, r. In one embodiment, first dipole element  302  and second dipole element  304  are formed of the same conductive material, such as a metal wire having the same radius, r. In one embodiment, the metal wire can be copper. In other embodiments, other conductive metals or combination of metals can be used, dependent upon the application. In alternate embodiments, first dipole element  302  and second dipole element  304 , can be formed of different conductive materials and/or have different radii, with resultant changes in radiation patterns. 
     In one embodiment, the overall length of the dipole assembly, L, is defined as L1+L2+g, i.e., the first length plus the second length plus the gap distance  316 . In one embodiment, the first length, L1, is equal to second length, L2, and each of L1 and L2 is equal to L/2−g/2, i.e., the overall length divided by two minus the gap distance, g, divided by two. 
     In one embodiment, a back surface of ground plane  308  is attached to, or formed on, a top surface of substrate  310 . Ground plane aperture  306  is formed in ground plane  308  exposing substrate  310  and allowing first dipole element  302  and second dipole element  304  to be extended through the top surface of substrate  310  to the back surface of substrate  310 . On the back surface of substrate  310 , a first end of first element  302  is attached to microstrip feed line  312  at a point A. Microstrip feed line  312  is formed on, or attached to, the back surface of substrate  310 . A first end of second element  304  is attached to the back surface of substrate  310  at a point B, but second element  304  is not connected to microstrip feed line  312 . 
     In one embodiment, ground plane  308  is formed of a conductive metal and has a thickness, m. In one embodiment, the conductive metal can be copper. In other embodiments, other conductive metals or combination of metals can be used, dependent upon the application. In one embodiment, ground plane  308  serves as the ground plane for the dipole assembly, i.e., first dipole element  302  and second dipole element  304 , as well as the ground plane for microstrip feed line  312 . In one embodiment ground plane  308  has dimensions width, X, by length, Y. In various embodiments, the dimensions X and Y of ground plane  308  can be varied, depending on the requirements of the application. In one embodiment, substrate  310  is formed of a dielectric material of thickness, t, and relative dielectric constant, E. In one embodiment, microstrip feed line  312  is formed having a line width, d, and length, z. 
     As described above, first dipole element  302  is attached to a first end of microstrip feed line  312  at a point A. A second end of micro strip feed line  312 , also termed the input end of microstrip feed line  312 , is available for input of an electrical signal from a signal source (not shown) and powering of dipole assembly  300 . The signal source can be attached to microstrip feed line  312 , or other devices and circuit elements can be mounted directly on microstrip feed line  312 . 
     In an alternate embodiment, a support medium is placed on the top surface of ground plane  308  through which first vertical stem  320  and second vertical stem  324  extend allowing first horizontal arm  318  and second horizontal arm  322  to rest on a top surface of the support medium. In this way the support medium provides structural support and protection to first dipole element  302  and second dipole element  304 . 
       FIGS. 4A and 4B  and  4 C illustrate a linearly polarized dipole antenna assembly  400  with an unbalanced microstrip feed including a support medium  426  in accordance with another embodiment.  FIG. 4A  illustrates a transverse top view of a linearly polarized dipole assembly with unbalanced microstrip feed of  FIG. 3A  including a support medium  426  in accordance with another embodiment.  FIG. 4B  illustrates a transverse bottom view of the linearly polarized dipole assembly of  FIG. 4A  in accordance with another embodiment.  FIG. 4C  illustrates a cross-sectional side view of the linearly polarized dipole assembly of  FIG. 4A  in accordance with another embodiment. 
     In one embodiment, support medium  426  is placed on the top surface of ground plane  308  through which first vertical stem  320  and second vertical stem  324  extend allowing first horizontal arm  318  and second horizontal arm  322  to rest on a top surface of support medium  426 . In this way support medium  426  provides structural support and protection to first dipole element  302  and second dipole element  304 . In some embodiments, support medium  426  is a foam block spacer of thickness, h, which is placed over the top surface of ground plane  308 . In other embodiments, different materials can be utilized for support medium  426 . 
     When a signal source (not shown) is connected to microstrip feed line  312 , due to the small gap distance  316 , g, current flows in first dipole element  302  and induces a nearly equal current on the open arm of parasitic second dipole element  304  that is out of phase according to Lentz&#39;s law. The result is a current flow, I, in the same direction in first dipole element  302  and second dipole element  304 , allowing for efficient radiation. The resulting radiation pattern is nearly identical to that of a conventionally fed dipole for small ground plane sizes. 
     As the size of ground plane  308  becomes larger, the radiation pattern becomes more asymmetrical, which is undesirable for many applications. This occurs because currents induced on ground plane  308  and first vertical stem  318  of first dipole element  302  are not equal to those induced on parasitic second dipole element  304 . However, the impedance match is relatively unaffected by the size of ground plane  308  and in many applications the variation in the gain inside of the half power beam widths, i.e., gain ripple, is not a problem as long as the gain is above a specified minimum value. 
     In one embodiment, the physical dimensions of the dipole, i.e., first dipole element  302  and second dipole element  304 , and ground plane  308  are selected so that first dipole element  302  and second dipole element  304  are “tuned” (resonant) and the impedance is matched to that of microstrip feed line  312 , for example, in one embodiment, 50 ohms. The tuning can be achieved using standard antenna design techniques and commercially available computer software. Parameters that can be adjusted include the dipole radius, r, dipole height above the ground plane, h, dipole length from end to end L, the area (a by b) of the ground plane aperture  306 , the dimensions, X by Y of ground plane  308 , and the distance of the gap distance  316 , g, between first dipole element  302  and second dipole element  304 . The thickness, t, of substrate  310 , the width, d, of microstrip feed line  312 , and relative dielectric constant, &amp;, determine microstrip feed line  312  characteristic impedance. Widely available and well-known formulas exist for calculating the impedance as a function of these parameters. 
     Table 1 illustrates parameters of a sample design of a dipole assembly, such as shown in  FIGS. 4A ,  4 B, and  4 C with an unbalanced microstrip feed in accordance with one embodiment. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Parameter 
                 Variable 
                 Value 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Dipole length 
                 L 
                 25  
                 mm 
               
               
                   
                 Dipole radius 
                 r 
                 0.25  
                 mm 
               
               
                   
                 Gap 
                 g 
                 1.5  
                 mm 
               
               
                   
                 Dipole height 
                 h 
                 9  
                 mm 
               
               
                   
                 Microstrip line width (Z o  = 50) 
                 d 
                 1.6829 
                 mm 
               
               
                   
                 Ground plane length in x 
                 X 
                 35  
                 mm 
               
               
                   
                 Ground plane length in y 
                 Y 
                 35  
                 mm 
               
               
                   
                 Ground plane aperture length in x 
                 a 
                 2.5  
                 mm 
               
               
                   
                 Ground plane aperture length in y 
                 b 
                 3.5  
                 mm 
               
               
                   
                 Substrate thickness 
                 t 
                 0.508  
                 mm 
               
               
                   
                 Substrate relative dielectric 
                 ε r     m     
                 1.96 
                   
               
               
                   
                 constant 
                   
                   
                   
               
               
                   
                 Support medium thickness 
                 h 
                 9  
                 mm 
               
               
                   
                 Support medium relative dielectric 
                 ε r     s     
                 1.04 
                   
               
               
                   
                 constant 
                   
                   
                   
               
               
                   
                   
               
             
          
         
       
     
       FIG. 5  illustrates a return loss plot  500  (in dB) showing good match to a 50 ohm microstrip line at 10 GHz for the sample design of Table 1.  FIG. 6  illustrates an H-plane radiation pattern  600  for the sample design of Table 1.  FIG. 7  illustrates an E-plane radiation pattern  700  for the sample design of Table 1. 
     As earlier discussed, when receiving an input signal, the resulting radiation pattern of dipole assembly  300 / 400  is nearly identical to that of a conventionally fed dipole for small ground plane sizes. As the size of ground plane  308  becomes larger, the radiation pattern becomes more asymmetrical, which is undesirable for most applications. This occurs because currents induced on ground plane  308  and first vertical stem  318  of first dipole element  302  are not equal to those induced on parasitic second dipole element  304 . In some embodiments, it is possible in some cases to restore the symmetry in the radiation pattern by introducing asymmetry into the geometry of the dipole assembly. In various embodiments, the height, h, or length, L2, of parasitic second dipole element  304 , can be made different than the height, or length, L1, of the fed first dipole element  302 . 
     Also, the length of ground plane  308  on the side on which first element  302  is located (relative to a center line of length, Y) can be made different from the length of ground plane  308  on the side on which second element  304  is located. In this embodiment, ground plane aperture  306  would not be centered at Y/2 on ground plane  308  and would be offset. 
     Embodiments in accordance with invention described herein can be made using conventional fabrication techniques. For example, substrate  310 , ground plane  308 , ground plane aperture  306 , and microstrip feed line  312  can be manufactured using conventional fabrication techniques, for example, conventional techniques of deposition, patterning, and/or etching. In some embodiments, some or all of the above elements can be separately manufactured using conventional fabrication techniques and then assembled. First dipole element  302  and second dipole element  302  can be fabricated using conventional dipole manufacturing techniques, such as manufacturing and shaping a selected wire having a selected radius, r. In various embodiments, vias can be formed in substrate  310  to permit the first end of first dipole element  302  and the first end of second dipole element  304  to pass through substrate  310  and allow attachment at points A and B, respectively. In one embodiment, the vias can be formed mechanically, while in other embodiments, the vias can be formed using conventional fabrication techniques of deposition, patterning, and/or etching. In one embodiment support medium  426  can be a foam block as earlier described, which is shaped in accordance with the size parameters of ground plane  308  and height, h. During assembly support medium  426  is placed over ground plane  308  and first dipole element  302  and second dipole element  404  are inserted through support medium  426 , through ground plane aperture  306  and substrate  310  and attached respectively at points A and B. 
     Embodiments in accordance with the linearly polarized unbalanced microstrip fed dipole assembly described herein are simple, low loss and compact. Embodiments in accordance with the invention are easily integrated into micro strip structures. Embodiments in accordance with the invention have applicability as a “rectenna”, also termed a rectifying antenna. Rectennas are used in converting microwave signals to direct current for wireless power and energy harvesting applications. Important civilian and military applications are for powering small UAVs, satellite station keeping, and wireless battery charging. Most prior art rectenna structures can only provide half wave rectification, however, embodiments in accordance with the invention can easily be extended to provide full-wave rectification with resultant efficiencies. 
     This disclosure provides exemplary embodiments of the present invention. The scope of the present invention is not limited by these exemplary embodiments. Numerous variations, whether explicitly provided for by the specification or implied by the specification or not, may be implemented by one of skill in the art in view of this disclosure.