Patent Publication Number: US-11043747-B2

Title: Antenna with integrated balun

Description:
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     The United States Government has ownership rights in this invention. Licensing and technical inquiries may be directed to the Office of Research and Technical Applications, Naval Information Warfare Center Pacific, Code 72120, San Diego, Calif., 92152; voice (619) 553-5118; ssc_pac_t2@navy.mil. Reference Navy Case Number 111106. 
    
    
     BACKGROUND OF THE INVENTION 
     There is a constantly evolving need for better antennas for a variety of different applications. It can be a struggle to find an antenna that has the appropriate bandwidth, polarization, and/or power handling requirements for a given application in addition to meeting stringent size/weight requirements. For example, prior art attempts at providing broadband, high-power-handling, dual-polarized, directional antennas have employed multiple narrower band antennas to cover the same frequency range or have used multiple single polarization antennas. Significant drawbacks of the multiple antenna approach include high cost and large space requirements due to larger occupied volume. One prior art approach attempted to solve the above-identified drawbacks with a single reflector antenna with a crossed (quad) tapered slot antenna (TSA) with chamfered blade edges, but this approach did not yield satisfactory performance in that the chamfered edges caused the antenna inductance to increase and thus provide a poor impedance match to standard transmission lines. There is a need for an improved antenna. 
     SUMMARY 
     Disclosed herein is an antenna comprising first and second antenna elements, a center conductor, and a dielectric. The first antenna element is configured to be electrically connected to an outer conductor of a coaxial cable. The center conductor is electrically connected to the second antenna element and configured to be electrically connected to an inner conductor of the coaxial cable. The dielectric is disposed around the center conductor so as to separate the center conductor from the first antenna element. The first antenna element is shaped to gradually surround the dielectric and the center conductor over a length of the first antenna element so as to form a balun, integrated into the first antenna element, that is configured to gradually transform an unbalanced signal in the coaxial cable to a balanced signal that is characteristic of a two-conductor transmission line. 
     An embodiment of the antenna described herein may be described as a TSA comprising a dielectric bracket, first and second conductive blades, and a balun. In this embodiment, the first and second conductive blades are mounted to the dielectric bracket so as to define an air gap between edges of the first and second blades thereby forming a TSA. The balun is integrated into the first blade. The integrated balun comprises a center conductor and a dielectric. The center conductor is electrically connected to the second blade. The dielectric surrounds the center conductor and is disposed to electrically insulate the center conductor from the first blade. At a first location on the first blade, the dielectric abuts a bottom edge of the first blade. Over a length of the integrated balun, the first blade gradually surrounds more and more of the dielectric until, at a second location on the first blade, the dielectric is completely surrounded by the first blade so as to gradually transform an unbalanced signal at the second location to a balanced signal that is characteristic of a two-conductor transmission line at the first location. 
     Also described herein is a method for providing an integrated balun into an antenna comprising the following steps. One step provides for creating a cylindrical hole in a first antenna element from a back edge to a radiating edge. The next step provides for cutting away a portion of the first antenna element along a majority of the length of the hole along a downward-angled plane that is not parallel with the hole that intersects the radiating edge at approximately a top of the hole. The next step provides for inserting a center conductor surrounded by a dielectric through the hole such that the dielectric electrically insulates the center conductor from the first antenna element. The next step provides for electrically connecting the center conductor to a second antenna element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Throughout the several views, like elements are referenced using like references. The elements in the figures are not drawn to scale and some dimensions are exaggerated for clarity. 
         FIG. 1  is an illustration of an embodiment of an antenna with an integrated balun. 
         FIG. 2A  is a side-view illustration of an embodiment of a tapered slot antenna. 
         FIGS. 2B and 2C  are respectively bottom and side views of a blade of a tapered slot antenna. 
         FIG. 3  is a perspective view of an embodiment of a tapered slot antenna. 
         FIG. 4A  is a perspective view of an embodiment of a dual-polarization, quad, tapered slot antenna. 
         FIG. 4B  is a perspective view of a blade of an embodiment of a dual-polarization, quad, tapered slot antenna. 
         FIGS. 5A, 5B, and 5C  are respectively perspective, side, and top views of an embodiment of a dual-polarized, quad, tapered slot antenna. 
         FIG. 6A  is a close-up, perspective view of a bottom, center section of an embodiment of a dual-polarized, quad, tapered slot antenna. 
         FIGS. 6B and 6C  are cross-sectional views of the close-up, perspective view of the bottom, center section of the antenna shown in  FIG. 6A . 
         FIG. 7  is a perspective view of an embodiment of a prime focus fed parabolic antenna. 
         FIG. 8  is a side view of a cable insert of an integrated balun. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The disclosed antenna and method for providing an antenna below may be described generally, as well as in terms of specific examples and/or specific embodiments. For instances where references are made to detailed examples and/or embodiments, it should be appreciated that any of the underlying principles described are not to be limited to a single embodiment, but may be expanded for use with any of the other methods and systems described herein as will be understood by one of ordinary skill in the art unless otherwise stated specifically. 
       FIG. 1  is an illustration of an antenna  10  that comprises, consists of, or consists essentially of a first antenna element  12 , a second antenna element  14 , a center conductor  16 , and a dielectric  18 . The first antenna element  12  is shaped to gradually surround the dielectric  18  and the center conductor  16  over a length of the first antenna element  12  so as to form a tapered balun  19  that is integrated into the first antenna element  12 . The first and second antenna elements  12  and  14  may be made of any conductive material. A suitable example of material from which the first and second antenna elements  12  and  14  may be made is, but is not limited to, aluminum. 
     The first antenna element  12  is configured to be electrically connected to an outer conductor  20  of a coaxial cable  22 . The center conductor  16  is electrically connected to the second antenna element  14  and is configured to be electrically connected to an inner conductor of the coaxial cable  22 . In the embodiment of the antenna  10  shown in  FIG. 1 , the center conductor  16  is the same as the inner conductor of the coaxial cable  22 . The dielectric  18  is disposed around the center conductor  16  so as to separate the center conductor  16  from the first antenna element  12 . The balun  19  is configured to gradually transform an unbalanced signal in the coaxial cable  22  to a balanced signal that is characteristic of a two-conductor transmission line. The transition to a balanced, two-conductor transmission line reduces the reflections generated at the point where the inner conductor of the coaxial cable  22  connects to the second antenna element  14 . Further, the impedance transform allows for matching the impedance of the transmission line/balun and also allows the first and second antenna elements  12  and  14  to be spaced farther apart than similar prior art antennas that lack a tapered, coaxial balun that is integrated into an antenna element. 
       FIG. 2A  is a side-view illustration of a tapered slot antenna (TSA) embodiment of the antenna  10 . In this embodiment, the first and second antenna elements  12  and  14  are conductive blades of the TSA that are positioned with respect with each other so as to define an air gap or tapered slot  24  between radiating edges  26  and  28  of the first and second blades  12  and  14  respectively.  FIGS. 2B and 2C  are respectively bottom and side views of an embodiment of the first blade  12  of the TSA embodiment of the antenna  10  shown in  FIG. 2A . In this embodiment, the center conductor  16 , dielectric  18  and the first blade  12  form a tapered coaxial embodiment of the balun  19  that is integrated into the first blade  12 . The balun  19  has wide bandwidth potential with the lower frequency being limited by the length of the taper (˜¼ lambda) and the upper frequency being limited by the higher order modes of the coaxial cable  22 . To achieve a good match to a typical 50Ω coaxial transmission line (Z=50+0i), the air gap  24  between a single set of TSA elements, having a thickness of 2.54-5.08 millimeters (0.1-0.2 inches), at its narrowest should be approximately ¼ to ⅕ the thickness of the elements. For example, if the elements are 3.175 millimeters (0.125 inches) thick, the gap between elements at the intersection of the bottom edges and the radiating edges should be about 0.762 millimeters (0.03 inches). For thinner blades, the ratio between the thickness of the air gap and the blade thickness may be higher. For example, if the blade thickness is 0.762 millimeters (0.03 inches) the ratio between the narrowest part of the air gap and the blade thickness may be approximately 0.6. 
     In the TSA embodiment of the antenna  10 , the balun  19  enables efficient transition from the unbalanced structure of the coaxial cable  22  to the balanced structure of the first and second elements  12  and  14  without causing unwanted reflected energy in the coaxial cable  22 . The integrated balun  19  also transforms the impedance to a higher resistance. Both of these advantages allow the TSA embodiment of antenna  10  to handle higher power than similar prior art antennas that lack an integrated balun. The balun  19  may be formed by creating a cylindrical hole  30  from a back edge  31  to the radiating edge  26  of the first blade  12 . Then, a bottom edge  32  of the first antenna element  12  may be cut away at an angle a over a length L. In one embodiment the length L spans a majority of the length of the bottom edge  32  and the hole  30  is cut away along a downward-angled plane that is not parallel with the hole  30  and that intersects the radiating edge  26  at approximately a top of the hole  30 . In this embodiment, the degree to which the first antenna element  12  surrounds the dielectric  18  gradually transitions from a first location  34  near the intersection of the radiating edge  26  and the hole  30  of the first antenna element  12  where the dielectric  18  is in tangential contact with the first antenna element  12  to a second location  36  where the first antenna element  12  completely surrounds the dielectric  18 . The dielectric  18  is disposed to electrically insulate the center conductor  16  from the first blade  12 . The balun  19  serves to gradually transform an unbalanced signal at the second location  36  to a balanced signal that is characteristic of a two-conductor transmission line at the first location  34 . In one embodiment of the antenna  10 , the length L of the cut is approximately 56 millimeters (˜2.2 inches) and the angle a is approximately 3.5° such that at the radiating edge  26 , approximately 335° of an inner wall of the hole  30  is cut away, which corresponds to an impedance of ˜160Ω. In one example embodiment, the hole  30  has a diameter of 3.58 millimeters (0.141 inches). The integrated balun  19  may be optimized for different applications by varying the angle a and length L of the cut. 
     Also shown in  FIG. 2A  is a dielectric bracket  38  configured to hold the first and second blades  12  and  14  in position with respect to each other. The dotted-dashed line  39  represents a centerline that runs through the middle of the air gap  24 , or tapered slot, that separates the first and second radiating edges  26  and  28 . The dielectric bracket  38  may be constructed of any desired dielectric material. In one suitable example, the dielectric bracket is made of polyoxymethylene. The dielectric bracket  38  shown in  FIG. 2A  comprises an optional, dielectric support structure  40  that is disposed to maintain a positional relationship between the dielectric  18  and the first blade  12  and to counteract deflection of the dielectric  18  and the center conductor  16  due to gravity. In this embodiment, the dielectric  18  is cylindrical and the area of contact between the dielectric support structure  40  and the dielectric  18  does not exceed a circular, cross-sectional area of the dielectric  18 . It is to be understood that the dielectric  18  is not limited to cylindrical shapes, but may be any desired shape. The dielectric support structure  40  is placed a distance d away from the air gap  24 . In one embodiment, the distance d is at least 1/24 wavelength (at a lowest intended operating frequency of the antenna  10 ). In one embodiment, the distance d is at least 15 millimeters away from the air gap  24 . 
     In some embodiments of the antenna  10 , the center conductor  16  and the dielectric  18  may be the dielectric sheath and inner conductor of a semi-ridged coaxial cable. For example, with respect to the TSA embodiment of the antenna  10  shown in  FIG. 2A , the outer insulative layer and the outer conductor of a semi-rigid coaxial cable may be removed and the remaining dielectric sheath and inner conductor may be pressed into the hole  30 . The outer conductor of the coaxial cable would be electrically connected to the first blade  12  and the inner conductor would be electrically connected to the second blade  14 . 
       FIG. 3  is a perspective view of an embodiment of the antenna  10  where the first and second antenna elements  12  and  14  are blades of a TSA and where the antenna  10  further comprises a conductive element  42  electrically connected to both the first and second antenna elements  12  and  14 . The conductive element  42  is positioned such that the conductive element  42  spans the tapered slot/air gap  24 . Also shown in  FIG. 3  is a connector  41  for respectively attaching the outer and inner conductors of a coaxial cable to the first antenna element  12  and the center conductor  16 . 
       FIG. 4A  is a perspective view of a dual-polarization, quad, TSA embodiment of the antenna  10  that comprises two sets of first and second antenna elements  12  and  14  arranged with respect to each other to form the blades of two TSAs, one that is horizontally polarized and the other that is vertically polarized. The two TSAs are positioned with respect to one another such that the centerlines (such as the centerline  39  depicted in  FIG. 2A ) of their air gaps or tapered slots are aligned. 
       FIG. 4B  is a perspective view of one of the second blades  14  of the dual-polarized, quad TSA embodiment of the antenna  10  shown in  FIG. 4A . In this embodiment, a part of each of the radiating edges  26  and  28  (i.e., the tapered-slot-defining edges) of the vertically-polarized TSA and the horizontally-polarized TSA has a thinned-edge portion that has a thickness t that is non-tapered and stepwise-reduced from the thickness T of a non-thinned portion of the corresponding blade. In this embodiment, a section of the thinned-edge portion of antenna element  14  is non-tapered and stepwise-reduced for at least 12 millimeters (0.49 inches) from the radiating/slot-defining edge  28  as measured perpendicularly from the radiating edge  28 . In one example embodiment, t is 0.762 millimeters (0.03 inches) and T is 4.826 millimeters (0.19 inches). In this embodiment, the tapered slots or air gaps  24  have a narrowest dimension that is approximately 0.6 of the thickness t of the thinned edge portion of the first and second blades  12  and  14 . 
       FIG. 4B  also shows a receiving hole  44  in the radiating edge  28  of the second antenna element  14 , into which the center conductor  16  may be inserted. This embodiment of the antenna  10  further comprises a retainer  46  configured to hold the center conductor  16  in the receiving hole  44  so as to maintain electrical connectivity between the center conductor  16  and the second antenna element  14 . In one embodiment, the center of the receiving hole  44  is positioned 1.9 mm (0.075 inches) from the bottom  47  of the second antenna element  14 . Suitable examples of the retainer  46  include, but are not limited to, a spring, a retaining pin, and a set screw. In the embodiment of the second antenna element  14  shown in  FIG. 4B , the retainer  46  is an electrically conductive insert that is pressed into the receiving hole  44  and comprises a female feature for receiving, and maintaining electrical contact with, the center conductor  16 . Any portions of the retainer  46  that extend beyond the outer surfaces of the second antenna element  14 , such as surface  48  shown in  FIG. 4B , may be made flush with the second antenna element  14 . In some embodiments, the retainer  46  may be disposed in a recess in the second antenna element  14 , which recess may be subsequently filled with a conductive substance such as silver epoxy. The conductive substance filling the recess may be smoothed to conform with the outer surfaces of the second antenna element  14 , such as the side surface  48  and the radiating edge  28 . The smoothing of the conductive substance to conform with the outer surfaces of the second antenna element  14  had an unexpectedly large impact on the performance (specifically reducing the return loss) of a prototype embodiment of the antenna  10 . 
       FIGS. 5A, 5B, and 5C  are respectively perspective, side, and top views of a dual-polarized, quad TSA embodiment of the antenna  10 . In this embodiment, parts of the radiating edges  26  and  28  have thinned portions as described above with respect to  FIGS. 4A and 4B . Antenna  10  may be integrated into a high power broadband transceiver system. The dual-polarized TSA embodiment of the antenna  10  is capable of receiving or transmitting high power radio frequency energy on dual linear polarizations simultaneously (or other single polarizations such linear, right-hand circular, or left-hand circular) over a greater bandwidth than that which is achievable by prior art antennas. Thinning the blade, as depicted in  FIGS. 4A, 4B, 5A-5C , did not cause the inductance of the antenna  10  to increase the way a chamfer does and thus is able to provide a good impedance match to typical transmission lines over more broad frequency ranges than the prior art. A better impedance match has the advantage of higher power handling capability and higher overall antenna efficiency. 
       FIG. 6A  is a close-up, perspective view of a bottom, center section of the dual-polarized, quad TSA embodiment of the antenna  10 .  FIGS. 6B and 6C  are cross-sectional views of the close-up, perspective view of the bottom, center section of the dual-polarized, quad TSA embodiment of the antenna  10  shown in  FIG. 6A . 
       FIG. 7  is a perspective view of a prime focus fed parabolic (PFFP) antenna  50  that uses the dual-polarized, quad TSA embodiment of the antenna  10  shown in  FIG. 5A , as an antenna feed. In this embodiment, a reflector  52  is connected with feed support struts  54  to the dual-polarized, quad TSA embodiment of the antenna  10 . The PFFP antenna  50  shown in  FIG. 7  may be used with high power (i.e., &gt;200 W on each polarization) wideband (i.e., &gt;20:1 bandwidth) transceiver systems with the ability to transmit or receive vertical and horizontal polarizations simultaneously. With an additional feeding circuit, as is known in the art, antenna  10  can also produce any linear polarization angle, or right hand or left hand circular polarization. The overall gain performance of the PFFP antenna  50  may be determined by the performance of its feed along with how that feed&#39;s radiation pattern interacts with the parabolic reflector  52  and support structures  54 . The overall voltage standing wave ratio (VSWR) and power handling performance are almost entirely determined by the performance of the antenna&#39;s feed. As mentioned above, in this embodiment, the dual-polarized, quad TSA embodiment of the antenna  10  serves as the antenna feed for the PFFP antenna  50 . In this embodiment, the antenna feed (i.e., the dual-polarized, quad TSA embodiment of the antenna  10 ) meets the performance requirements shown in Table 1 below 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Performance Requirements met bv the dual-polarized, 
               
               
                 quad TSA embodiment of the antenna 10. 
               
            
           
           
               
               
            
               
                 Parameter 
                 Performance Requirement 
               
               
                   
               
               
                 Polarization 
                 Two port -vertical and horizontal elements 
               
               
                 Frequency Range 
                 &gt;20:1 bandwidth 
               
               
                 Power 
                 200 W continuous wave (CW) on each port 
               
               
                 Voltage Standing 
                 ≤2.3:1 
               
               
                 Wave Ratio (VSWR) 
               
               
                 Directivity 
                 Optimized for even dish illumination over 
               
               
                   
                 frequency range ~30deg BW 
               
               
                 Phase Center 
                 Remain relatively stable 
               
               
                   
                 across frequency range 
               
               
                   
               
            
           
         
       
     
     The PFFP antenna  50  may further comprise a conductive disk  56  placed approximately ¼ wavelength (at the lowest intended operating frequency) behind the quad-TSA  10 . The conductive disk  56  is electrically insulated from the antenna elements  12  and  14  and is mounted coaxially with the parabolic reflector such that the antenna elements  12  and  14  are disposed between the conductive disk  56  and the parabolic reflector  52 . The conductive disk  56  serves as a reflector element that improves the low frequency gain of the antenna  10 . In one embodiment, the conductive disk  56  is a flat 15.25-centimeter (6-inch) diameter disk located 7.62 centimeters (3 inches) behind the first and second antenna elements  12  and  14  that adds about 2 dB of forward gain at the lowest frequencies. The conductive disk  56  was found to have negligible effect on higher frequency performance of the PFFP antenna  50 . 
       FIG. 8  is a drawing of an embodiment of a cable insert  58  that may form part of the balun  19 . The cable insert  58  comprises the center conductor  16 , the dielectric  18 , and the connector  41 , which may be attached to a coaxial cable. The connector  41  may be attached to the back edge  31  of the first antenna element  12  such that the dielectric  18  and center conductor  16  are routed through the hole  30 . 
     From the above description of the antenna  10 , it is manifest that various techniques may be used for implementing the concepts of the antenna  10  without departing from the scope of the claims. The described embodiments are to be considered in all respects as illustrative and not restrictive. The method/apparatus disclosed herein may be practiced in the absence of any element that is not specifically claimed and/or disclosed herein. It should also be understood that the antenna  10  is not limited to the particular embodiments described herein, but is capable of many embodiments without departing from the scope of the claims.