Abstract:
The present invention provides a compact slotline balun implemented on a 10-mil thick printed circuit card. The balun utilizes a transition region configuration of a six-port network to achieve a good impedance match and low insertion loss across a wide operating band. The balun is typically manufactured using standard printed circuit techniques which yield a thin, flexible, dimensionally stable device.

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Applications Serial No. 60/201,165, filed May 2, 2000 and Ser. No. 60/210,738, filed Jun. 12, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to baluns and, more specifically, to a compact, broadband, flexible printed circuit balun utilizing a six-port network. 
     BACKGROUND OF THE INVENTION 
     Baluns are used to provide an impedance-matched transition between a balanced and an unbalanced transmission line. They have been implemented in a variety of ways, such as with lumped constant reactive components. 
     At higher frequencies (e.g., microwave), the performance of a balun becomes critical from the point of view of both size and performance. For example, in a microwave antenna array, each antenna element may require an individual balun near the antenna element itself. This dictates a design that is compact and, due to the potentially large numbers needed in a large antenna array, low cost. 
     A typical prior art balun consists of a coaxial twin lead cable and a 180° hybrid transformer. This balun is both bulky and expensive. To overcome these difficulties, a slotline balun utilizing a metal (e.g., aluminum) substrate with the slotline etched in the aluminum was developed, as shown in FIG.  1 . In this design, a transition region was used to couple a coaxial connector to the slotline, which was subsequently coupled to a slot radiator. While this design exhibited improved electrical performance over other baluns of the prior art, it was still expensive and did not provide the broadband performance required for certain antenna-related applications. Because it was a rigid metal structure, it exhibited excessive stress concentration points that made it inherently unreliable. In addition, the manufacturing cost was objectionably high. 
     It is therefore an object of the invention to provide a compact slotline balun having a wide operating bandwidth. 
     It is a further object of the invention to provide a compact slotline balun having a wide operating bandwidth utilizing a six-port network. 
     It is an additional object of the invention to provide a compact slotline balun having a low insertion loss across a wide operating bandwidth. 
     It is another object of the invention to provide a compact slotline balun having a thin cross section for implementation on a printed circuit card. 
     It is a still further object of the invention to provide a compact slotline balun having a very low manufacturing cost. 
     It is yet another object of the invention to provide a compact slotline balun having good dimensional stability to ensure high performance reliability. 
     It is a still further object of the invention to provide a compact slotline balun which may be readily integrated with patch antenna elements formed using conventional printed circuit technology. 
     It is an additional object of the invention to provide a compact printed circuit slotline balun which is flexible. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a compact slotline balun implemented on a 10-mil thick printed circuit card. The inventive balun utilizes a transition region configuration of a six-port network to achieve a good impedance match and low insertion loss across a wide operating band. The balun is typically manufactured using standard printed circuit techniques which yield a thin, flexible, dimensionally stable device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent detailed description, in which: 
     FIG. 1 is a schematic, plan view of a rigid slotline balun assembly of the prior art; 
     FIG. 2 is a top plan view of the flexible, wideband stripline balun in accordance with the invention; 
     FIG. 3 is a bottom plan view of the flexible, wideband stripline balun of FIG. 2; 
     FIG. 4 a schematic view of a generalized six-port network; 
     FIG. 5 is a top plan view of an alternate embodiment of the flexible, wideband stripline balun of the invention; and 
     FIG. 6 is a bottom plan view of the flexible, stripline balun of FIG. 5 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention features a compact, wideband, printed circuit slotline balun. The inventive balun utilizes a transition region configuration of a six-port network to achieve a good impedance match and low insertion loss across a wide operating band. 
     Referring first to FIG. 1, there is shown a schematic plan view of a slotline balun of the prior art, generally at reference number  100 . Balun  100  is typically fabricated on an aluminum sheet. A coaxial connector  102  at the bottom edge of balun  100  allows the connection of an external, unbalanced transmission line, typically coaxial cable. The signal is carried from connector  102  along a coaxial cable  104  to a transition region  106 . A stub section  108  and a slotline open circuit  110 , both functionally connected at transition region  106 , are provided for impedance matching and broadbanding of the input/output signal. Slotline  112  carries the signal to a terminus  114  where the signal is coupled to a slot radiator or other antenna element (not shown). While a transmitted signal is described, it will be obvious that balun  100  may function in a receiving capacity as well. 
     The electrical performance of balun  100  has been found to be satisfactory. impedance matching and insertion loss are acceptable across the operating bandwidth of interest. However, the mechanical properties of balun  100  are less than satisfactory. Because balun  100  must be precision machined into the aluminum sheet, manufacturing costs are high. In addition, the rigid metal structure has multiple mechanical stress points. In the course of temperature cycling, these stress points may lead to mechanical failure of the balun. 
     Referring now to FIGS. 2 and 3, there are shown front and back plan views, respectively, of the inventive wideband, flexible balun, generally at reference number  200 . A thin substrate  202 , typically 10 mil FR 4  material, supports metallized patterns  204   a ,  204   b  disposed on both the front and back of substrate  202 , respectively. On the rear side of substrate  202  (FIG.  3 ), an input pad  206  allows for the connection of an external, unbalanced transmission line (not shown) to a micro stripline  208  which terminates at junction  210   a . An open circuit stub leg  212  and a short-circuited stub leg  214  are electrically connected to and radiate from junction  210   a.    
     On the front side of substrate  202 , a relatively large amount of metallized pattern  204   a , typically copper, remains. A slotline  216  etched in metallized pattern  204   a  extends from junction  210   b  to a terminus  218 . Typically, terminus  218  may be coupled to any type of balanced antenna (not shown) such as dipoles, slots, spirals, log-periodics, etc. In the example chosen for purposes of disclosure, terminus  218  would connect to a radiator slot (not shown), either directly or through a coupling dipole or similar coupling structure (not shown). 
     A short-circuited slotline branch  220  and an open-circuited slotline branch  224  are electrically connected to and radiate from junction  210   b . Open circuit slotline branch  224  is a meandering line which defines a relatively large, irregular space  226 . Junction points  210   a  and  210   b , located on opposite surfaces of substrate  202 , are placed directly over one another but are not directly electrically connected. 
     The combination of strip line  208  and slotline  216 , in cooperation with stubs  212 ,  214 ,  220 ,  224 , form the six-port network. 
     Referring now to FIG. 4, there is shown a schematic representation of a generalized six-port network, generally at reference number  400 . A stripline  402  feeds a signal from an input  404  to a junction  406 . A slotline  408  carries a balanced signal from junction  406  to a terminus  410 . Open and short circuit stripline branches  412  and  414 , respectively, are connected at junction  406 . Likewise, open and short circuit slotline branches  416  and  418 , respectively, are also connected to junction  406 . This combination forms a classic six-port network such as that utilized in the inventive balun. 
     Referring now again to FIGS. 2 and 3, in operation, an unbalanced RF signal is applied to the input  206  of the inventive balun  200 . The unbalanced RF signal is conducted to junction  210   a , where the interaction of balanced open and short-circuited stubs  212 ,  214 ,  220 ,  224  interact with the signal, which is induced into slotline  216 . The balanced stubs  212 ,  214 ,  220 ,  224  create a very broadband unbalanced-to-balanced current transformer (balun) having minimal insertion loss, wherein the output signal current becomes well behaved a very short physical distance away from junction  210   a ,  210   b . Micro stripline  208  may be meandered to include multiple quarter-wavelength transformers to provide additional degrees of freedom for impedance matching. The combination of micro stripline sections  206  and  208  provides the first tuning capability. The six-port network sections  212 ,  214 ,  220 ,  224 , etc. converts the unbalanced current to a balanced current with a minimal insertion loss. The balanced signal (not shown) then travels along slotline  218  to an antenna (not shown) coupled at terminus  218 . 
     Slotline  218  is shown in the embodiment chosen for purposes of disclosure, as a constant width section. In alternate embodiments, however, slotline  218  could be implemented as a variable width section. The width could vary as an exponential curve, for example, or the like. In summary, the inventive balun consists of three interacting sections: the uniform/non-uniform slotline; the six-port network; and the meandering micro stripline. 
     Because the inventive balun  200  may be readily manufactured using well known printed circuit technology, it is inexpensive. This means that changes to customize the balun operating frequency range may be made readily by changing printed circuit exposure masks. All other manufacturing processes remain unchanged. Also, because it is typically implemented on thin, flexible material (e.g., 10 mil FR4), the balun  200  may be added as a layer in a composite patch antenna lay-up. The inventive balun  200  exhibits excellent broadband performance and reliability even when thermally stressed. 
     Referring now to FIGS. 5 and 6, there are shown front and back plan views, respectively, generally at reference number  500 , of an alternate embodiment of the balun shown in FIGS. 3 and 4. A thin substrate  502 , typically 10 mil FR4 material, supports metallized patterns  504   a ,  504   b  disposed on both the front and back sides of substrate  502 , respectively. On the back side of substrate  502  (FIG.  6 ), an input pad  506  allows for the connection of an external, unbalanced transmission line (not shown) to a micro stripline  508  which terminates at junction  510   a . Unlike micro stripline  208  (FIG.  3 ), micro stripline  508  is a meander line, which allows an even smaller balun to be constructed compared with the embodiment shown in FIGS. 2 and 3. 
     An open circuit stub leg  512  and a short-circuited stub leg  514  are electrically connected to and radiate from junction  510   a . Open circuit stub  512  may exhibit a flair at its outboard terminus which may be used to help control the tuning and/or “Q” of the balun  500 . 
     On the front side of substrate  502 , a relatively large amount of metallized pattern  504   a , typically copper, remains. A slotline  518  etched in metallized pattern  504   a  extends from junction  510   b  to a terminus  518 . Slotline  518  may be flared in the vicinity of terminus  518  either to act independently as an antenna or to facilitate coupling to an attached radiating element (not shown) to which the balun  500  may be coupled. Typically, terminus  518  may be coupled to any type of balanced radiating elements such as dipoles, slots, spirals, log-periodics, etc. 
     A short-circuited slotline branch  520  and an open-circuited slotline branch  524  are electrically connected to and radiate from junction  510   b . Open circuit slotline branch  524  is a meander line which defines a relatively large, irregular space  526 . Junction points  510   a  and  510   b , located on opposite surfaces of substrate  502 , are aligned directly over one another but are not directly electrically connected. 
     The combination of strip line  508  and slotline  516 , in cooperation with stubs  512 ,  514 ,  520 ,  524 , form the six-port network described in detail hereinabove. 
     The inventive balun  200  has exhibited virtually identical VSWR performance to its expensive, aluminum plate, prior art versions. 
     Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention. 
     Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.