Patent Publication Number: US-7719381-B2

Title: Transmission line balun for broadband combiners, splitters and transformers

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   The United States Government has certain rights in and to this invention pursuant to Government Contract No. FA8709-04-C-0010. 

   BACKGROUND OF THE INVENTION 
   1. Field of The Invention 
   This invention relates to transmission line transformers. More particularly, this invention relates to broadband transmission line baluns. 
   2. Description of the Background Art 
   A balanced to unbalanced transformer, also called by its contraction “balun”, essentially provides the same characteristics as an isolation transformer that transmits the energy from input to output by a transmission line mode instead of by flux linkages as in the case of conventional transformers. 
   Baluns allow a grounded source to drive an ungrounded load or a balanced load where the midpoint is grounded. Further, baluns allow phase reversal of the signal. Still further, if a positive signal is applied to its input, the positive output lead could be grounded. If the return current is forced to flow in the shield, then there is no radiated field from the transmission line because, from a point outside the line, both currents are located at the center of the line and cancel each other out. However, a field still exists between the center conductor and the shield. 
   Baluns in the form of a conductive sleeve, also known as bazooka baluns, are commonly used to match balanced antennas to unbalanced coax feeders. A prior art bazooka balun  10  is shown in  FIG. 1  as comprising a conductive sleeve  12  of a quarter wavelength long that is positioned over the end  16  of a 75Ω cable  14  and shorted to the shield  18  of the cable  14  (i.e., its unbalanced line  20 ) only at its proximal end  22  to reflect an open circuit at its feed point  24  of its inner conductor  26 . The balanced line  28  constituting the load Z may comprise, as shown, a half-wave dipole antenna  30 . The conductive sleeve  12  may fit tightly around the existing outer plastic mantle  32 . 
   Notably, the bazooka balun&#39;s sleeve  12  shorted to the shield  18  forms a second coaxial transmission line (i.e., the sleeve being the “shield” of the new line and the original “shield” being the inner conductor). In theory, a perfect lossless quarter wave transmission line would present an infinite impedance at the frequency where it is a quarter wavelength long. In practice, however, using a small-diameter sleeve positioned tightly over only the cable&#39;s existing outer plastic mantle as the dielectric spacer presents a low impedance. In contrast, a significantly higher impedance can be attained by using a dielectric tubular cylinder installed over the coaxial cable to which a larger-diameter sleeve is then installed. A representative patent showing a larger-diameter sleeve includes U.S. Pat. No. 4,737,797, the disclosure of which is hereby incorporated by reference herein. U.S. Pat. No. 6,552,689, the disclosure of which is hereby incorporated by reference herein, discloses a balun in which a conductive sleeve and a dielectric cylinder of the same diameter are stacked and then positioned within a larger-diameter outer dielectric cylinder. 
   The impedance and the lower frequency end of the bandwidth may be increased through the use of baluns that employ ferrites in lieu of sleeves. Without the use of ferrite, the inductance of the shield to ground limits the low frequency end. 
   Prior art ferrite baluns employing ferrite beads have often comprised a plurality of ferrite beads positioned over the cable close to the feed point to maintain a high impedance across the frequency band. See for example, U.S. Pat. No. 4,962,359, the disclosure of which is hereby incorporated by reference herein. Other prior art ferrite baluns have employed ferrite sleeves. See for example, U.S. Pat. No. 4,719,699, the disclosure of which is hereby incorporated by reference herein. Finally, still other ferrite baluns have been incorporated into the frame of a Yagi antenna. See for example, U.S. Pat. No. 4,028,709, the disclosure of which is hereby incorporated by reference herein. 
   Unfortunately, when using ferrite baluns, a parasitic line is formed by the shield to ground. The parasitic effects of employing ferrite at high frequencies is well documented. U.S. Pat. No. 5,296,823, the disclosure of which is hereby incorporated by reference herein, acknowledges such parasitic effects and teaches specific printed circuit and shielded structures for parasitic mode suppression. Specifically, as illustrated in  FIG. 11  thereof, one shielded structure includes the side-by-side positioning of a ferrite core having a central passage containing a two-conductor transmission line and a dielectric having a central passage containing a single-conductor transmission line, into an enlarged shielding tubular signal ground conductor. According to its teachings, the shielded structure confines signal energy and prevents coupling to other outside circuits and structures. 
   There presently exists a need for an improved balun that isolates the balun from the effects of ferrite at low frequencies, thereby significantly reducing losses and improving voltage standing wave ratio (VSWR). 
   Therefore, it is an object of this invention to provide an improvement which overcomes the aforementioned inadequacies of the prior art devices and provides an improvement which is a significant contribution to the advancement of the balun art. 
   Another object of this invention is to provide a balun that employs a ferrite to increase the impedance and the lower frequency end of the bandwidth while isolating the balun from the parasitic effects of ferrite at low frequency and thereby significantly reducing losses and improving VSWR. 
   The foregoing has outlined some of the pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings. 
   SUMMARY OF THE INVENTION 
   For the purpose of summarizing the invention, the invention comprises a transmission line balun for broadband combiners, splitters and transformers. In general, the balun of the invention comprises a tubular ferrite having a longitudinal passageway. A conductive sleeve is positioned concentrically within the passageway of the tubular ferrite. The length of the tubular ferrite containing the conductive sleeve and the length of the conductive sleeve are both preferably one-quarter of the wavelength. The distal end of the transmission line is positioned into the conductive sleeve and the sleeve is grounded at its proximal end to the shield of the coax. The resulting assembly comprises the coax, the conductive sleeve and the tubular ferrite positioned concentrically with respect to each other at the proximal end of the coax. 
   More specifically, the balun of the invention comprises a coax cable, with a conducting sleeve surrounding it, and a ferrite sleeve over the conducting sleeve. The sleeves are separated by insulating material. The conducting sleeve is connected to the transmission line at one end. 
   In order to the test the concept of the invention, an implementation of the invention, as represented in the data hereinafter set forth, was constructed comprising two 50 ohm unbalanced to balanced 50 ohm units connected back-to-back. The test implementation used a 25 ohm semi rigid 0.07 diameter coax connected in series at the input and output of the back-to-back assembly. The conductive sleeve was positioned on the outside shield of a semi-rigid 50 ohm coax. Two ferrite cores with a permeability (μ) of 40, each having an inside diameter of 0.281 inches and an outside diameter of 0.50 inches, were placed over the shield. 
   It is noted that if the coax length were a half wavelength long, the short at the source end appears at the load end and would destroy the balun operation. Therefore, the length of the balun should normally be under a half wavelength long. However, disadvantageously, this requirement would severely limit the low frequency response. Thus, in order to expand the low frequency operation, the ferrite according to the invention was used around the coax. 
   The use of the ferrite according to the invention results in the parasitic line being electrically longer than the coax because the transmission medium for the parasitic line is through the magnetic material which has a high permeability and dielectric constant. Use of the conducting sleeve of the invention significantly diminishes this effect. 
   It is noted that further testing indicated that if the sleeve of the invention was not employed, large dips in its response were suffered. It is believed that the large dips in the response in the assembly without the sleeve were due to the parasitic line formed by the shield of the coax and ground using the ferrite core as part of the medium for the parasitic line. A representative VSWR without the use of the sleeve of the invention is shown in  FIG. 2 . 
   In summary, according to the present invention, the use of the conductive sleeve encompassing a tubular ferrite isolates the balun from the effects of ferrite at low frequency and thereby significantly reduces losses and improves VSWR. Many of the disadvantages associated with prior art ferrite baluns are therefore overcome. 
   The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood so that the present contribution to the art can be more fully appreciated. Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other circuits and assemblies for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent circuits and assemblies do not depart from the spirit and scope of the invention as set forth in the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description of the preferred embodiment taken in connection with the accompanying drawings in which: 
       FIG. 1  is a longitudinal cross-sectional view of a prior art bazooka balun employing a sleeve; 
       FIG. 2  is a VSWR plot of a typical prior art ferrite balun; 
       FIG. 3  is a longitudinal cross-sectional view of a balun according to the present invention that employs a conductive sleeve encompassing a tubular ferrite installed over the terminal end of a coax feeder; and 
       FIG. 4  is a VSWR plot of the balun of  FIG. 3 . 
   

   Similar reference characters refer to similar parts throughout the several views of the drawings. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 3 , the preferred embodiment of the balun  50  of the invention comprises a tubular ferrite  52  having a longitudinal passageway  54  concentrically positioned over a conductive sleeve  56  having a longitudinal  58  passageway. The proximal end  60  of the conductive sleeve  56  is preferably inwardly turned. Preferably, the tubular ferrite  52  and the conductive sleeve  56 , along with their respective passageways  54  and  58 , comprise circular cylindrical configurations. 
   In one application, the balun  50  of the invention is intended to be coupled over the end  16  of a 75Ω cable  14  and the conductive sleeve  56  shorted to the shield  18  of the cable  14  (i.e., its unbalanced line  20 ) only at its proximal end  60 , such as by soldering, to reflect an open circuit at its feed point  24  of the cable&#39;s inner conductor  26 . 
   The coupled conductive sleeve  56  forms a parasitic line from the shield  18  of the coax cable  14 . The tubular ferrite  52  coupled to the sleeve  18  forms an additional parasitic line between the sleeve  56  and ground. The effect of the parasitic line which uses the ferrite  52  for its medium is buffered by the sleeve  56 . When the parasitic line formed by the sleeve  56  and the shield  18  is a quarter wavelength, the impedance between the unbalanced line  20  and the end of the sleeve  56  is very high, thereby reducing the effect of the impedance from the end of the sleeve to ground. 
   The longitudinal passageway  58  of the conductive sleeve  56  may be dimensioned to fit tightly around the existing outer plastic mantle  32  of the cable  14 . Alternatively, the mantle  32  may be removed and the longitudinal passageway  58  of the conductive sleeve  56  may be dimensioned to fit tightly around the shield  18  of the cable  14 . Still alternatively, the mantle  32  may be replaced by or supplemented with a thicker dielectric spacer, with the diameter of the longitudinal passageway  58  of the conductive sleeve  56 , and of the tubular ferrite  52 , correspondingly increased. 
   The tubular ferrite  52  may dimensioned such that its longitudinal passageway  54  fits tightly around the conductive sleeve  56 . Alternatively, the diameter of the longitudinal passageway  54  of the tubular ferrite  53  may be greater than the outer diameter of the conductive sleeve  56  to form a space therebetween, that itself may be filled with a dielectric spacer. 
   The wall thickness of the ferrite  52  and its permeability and the space between the ferrite  52  should be selected to optimize the performance of the balun. The following chart sets forth an example of an actual prototype: 
   
     
       
         
             
             
             
           
             
                 
                 
             
           
          
             
                 
               Tubular Ferrite Material 
               Ferronic-P 
             
             
                 
                 
               Part Number 11-761P 
             
             
                 
               Permeability of Tubular 
               40 
             
             
                 
               Ferrite 
             
             
                 
               Diameter of Longitudinal 
               0.281 inch 
             
             
                 
               Passageway of Tubular 
             
             
                 
               Ferrite (Inside Diameter of 
             
             
                 
               Core) 
             
             
                 
               Wall Thickness of Tubular 
               0.105 inch 
             
             
                 
               Ferrite 
             
             
                 
               Outside Diameter of 
                0.50 inch 
             
             
                 
               Tubular Ferrite 
             
             
                 
               Spacing Between Outer 
               0.065 inch 
             
             
                 
               Diameter of Sleeve and 
             
             
                 
               Diameter of Longitudinal 
             
             
                 
               Passageway of Tubular 
             
             
                 
               Ferrite (Inside Diameter of 
             
             
                 
               Core) 
             
             
                 
                 
             
          
         
       
     
   
   The balun  50  of the invention is principally intended to function as a balanced/unbalanced transformer for a load Z. For example, the balanced line  28  constituting the balun&#39;s load Z may comprise, as shown in  FIG. 3 , a half-wave dipole antenna  30 . Alternatively, without departing from the spirit and scope of the invention, the balun  50  may be configured as a splitter or a combiner. 
   The present disclosure includes that contained in the appended claims, as well as that of the foregoing description. Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention. 
   Now that the invention has been described,