Abstract:
An apparatus for launching a surfacewave onto a single conductor transmission line provides a launch including a flared, continuously curving cone portion; a coaxial adapter portion; a wire adapter portion for contacting the wire conductor which allows for a multiplicity of wire dimensions for either insulated or uninsulated wire, or a tri-axial wire adapter device enabling non-contacting coupling to a wire; and a longitudinal slot added to the flared cone, wire adapter, and coaxial adapter portions of the launch to allow direct placement of the launch onto existing lines, without requiring cutting or threading of those lines for installation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application claims the benefit of the filing date of U.S. Provisional Patent Application, Ser. No. 60/432,099, filed 09 Dec. 2002. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   REFERENCE TO A MICROFICHE APPENDIX 
   Not applicable. 
   TECHNICAL FIELD 
   The present invention relates generally to methods and apparatus for surfacewave transmission, and more particularly to an improved method and apparatus for launching a surfacewave onto a single wire transmission line. 
   The present invention improves the performance and utility of previous launch devices for converting coaxial mode transmission to and from surfacewave mode transmission on a single conductor transmission line. This technology is related to transmission lines made from single conductor line having a thick outer dielectric sheath, an insulated wire, and known as a “Gobau line”, “G-line”, SWTL (surface wave transmission line), or “singlewire”, and is also related to lines made using thinly insulated conductors, as well as completely uninsulated conductors having no outer dielectric sheath at all. It includes conductors fabricated from multiple parallel strands, twisted or untwisted, and either insulated from each other or contacting each other, as well as single solid conductors of elliptical or rectangular cross-section. 
   BACKGROUND INFORMATION AND DISCUSSION OF RELATED ART 
   Previous launch devices for single conductor transmission lines have used a simple conical shape structure to excite the surfacewave mode onto an insulated single conductor transmission line. In addition to having excess transmission attenuation due to losses in the dielectric insulation, these designs suffer from significant impedance mismatch, unwanted conversion to radiating modes and resultant transmission attenuation of the surfacewave mode when a broad range of frequencies is supported, and require both a longer cone and wider cone mouth in order to excite a surfacewave mode onto the single conductor. Special effort is also required to mount these previous designs onto the conductor, requiring that the transmission line be broken so that the launch may be threaded onto the line in order to be attached. This presents both mechanical and electrical challenges and limitations to the designer and installer of such a device, particularly if the launch apparatus is to be installed onto a pre-existing single conductor line. 
   The foregoing reflects the current state of the art of which the present inventor is aware. Reference to, and discussion of, this art is intended to aid in discharging Applicant&#39;s acknowledged duty of candor in disclosing information that may be relevant to the examination of claims to the present invention. However, it is respectfully submitted that none of the prior art discloses, teaches, suggests, shows, or otherwise renders obvious, either singly or when considered in combination, the invention described and claimed herein. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method and apparatus for launching a surfacewave onto a single conductor transmission line. The inventive apparatus provides launch of a surfacewave onto either insulated or uninsulated single conductors making application to a variety of existing lines not only practical, but simpler and more economical. This furthers the use of existing lines, such as high tension power main wires, for surfacewave mode transmission of UHF and microwave energy. Previous known references and teachings on the subject have restricted usage to insulated wires. 
   The invention includes a flared cone portion constructed of either a continuously curving “horn” or a combination of two or more straight conical sections of different flare angle approximating a curved structure, providing improved impedance match, improved broadband and multiband transmission performance and improved conversion to surfacewave mode with a physically smaller horn section when compared to prior, single conical section designs. 
   The invention further includes a coaxial adapter portion which adapts conventional coaxial transmission cable to the coaxial transmission line mode which is present at the narrow end of the cone or horn section of the launch, and which can function simultaneously on two different frequency ranges separated by more than an octave. 
   A first embodiment of the invention provides a wire adapter device for contacting the wire conductor which allows for a multiplicity of wire dimensions for either insulated or uninsulated wire. A second embodiment provides a tri-axial wire adapter device enabling non-contacting coupling to a wire. 
   The invention also provides a longitudinal slot incorporated into to the flared cone, wire adapter, and coaxial adapter sections of the launch which allows simple and easy placement of the launch onto existing lines, and requires no cutting or threading of those lines for installation. Use of such a slot is not inconsistent with good electrical characteristics of the launch. 
   It is therefore an object of the present invention to provide a new and improved surfacewave launch apparatus. 
   It is another object of the present invention to provide a new and improved surfacewave launch adapted for use on insulated or uninsulated single conductors. 
   A further object or feature of the present invention is a new and improved flared cone apparatus with improved broadband and multiband transmission performance. 
   An even further object of the present invention is to provide a novel coaxial adapter for conventional coaxial transmission cable. 
   A still further object of the present invention is to provide an improved wire adapter for contacting a wire conductor. 
   An additional object of the present invention is to provide an improved method for placement of a surfacewave launch on an existing line. 
   Other novel features which are characteristic of the invention, as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings, in which preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration and description only and is not intended as a definition of the limits of the invention. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. The invention resides not in any one of these features taken alone, but rather in the particular combination of all of its structures for the functions specified. 
   There has thus been broadly outlined the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form additional subject matter of the claims appended hereto. Those skilled in the art will appreciate that the conception upon which this disclosure is based readily may be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
   Further, the purpose of the Abstract is to enable the U.S. Patent and Trademark Office and the public generally, and especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The Abstract is neither intended to define the invention of this application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way. 
   Certain terminology and derivations thereof may be used in the following description for convenience in reference only, and will not be limiting. For example, words such as “upward,” “downward,” “left,” and “right” would refer to directions in the drawings to which reference is made unless otherwise stated. Similarly, words such as “inward” and “outward” would refer to directions toward and away from, respectively, the geometric center of a device or area and designated parts thereof. References in the singular tense include the plural, and vice versa, unless otherwise noted. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawing wherein: 
       FIG. 1  is a schematic view of a prior art surfacewave launch device; 
       FIG. 2  is a side elevation cross-sectional view of a first embodiment of an improved surfacewave launch apparatus of this invention; 
       FIG. 3A  is an end elevation view of the surfacewave launch of  FIG. 2 , while  FIG. 3B  is an enlarged end elevation view of the coaxial adapter and wire adapter portions of the surfacewave launch of  FIG. 3A ; 
       FIG. 4A  is an end elevation view of a wire adapter of this invention as installed on a single wire conductor, while  FIG. 4B  is a top view of a wire adapter as installed on a single wire conductor, showing the taper to the single wire conductor; 
       FIG. 5  is a side elevation cross-sectional view of an alternate dualband embodiment for a coaxial adapter of this invention; and 
       FIG. 6A  is a perspective view of the flared horn portion of an alternate embodiment of the surfacewave launch of this invention; while  FIG. 6B  is a cross-sectional view of the flared horn of  FIG. 6A , and  FIG. 6C  is a detail view of an optional rolled edge for the outer mouth of the flared horn. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a schematic view of a prior art surfacewave launch device  10 . Prio art surfacewave launch  10  includes conical launch  12  and coaxial connector  14 , both connected to wire conductor  16  covered by an outer dielectric sheath  18 . Mounting of the prior art launch device  10  requires that wire  16  be broken, so that the launch may be threaded on. This presents both mechanical and electrical problems to the installer of such a device, particularly if the launch is to be installed onto a pre-existing single conductor line. 
   Referring to  FIGS. 2 ,  3 A,  3 B,  4 A,  4 B,  5 ,  6 A,  6 B,  6 C, wherein like reference numerals refer to like components in the various views, there is illustrated therein a new and improved surfacewave launch apparatus of this invention. 
     FIG. 2  is a side elevation cross-sectional view of a first embodiment of the improved apparatus  20  for launching a surfacewave onto a single conductor transmission line  21  (either with or without an outer dielectric covering). Surfacewave launch  20  includes a flared horn or cone section  22 , a coaxial adapter section  24 , and a wire adapter portion  26 . 
   The entire launch device may be cast or formed from a single piece of metal if desired. However, for the purpose of this description, construction of the inventive apparatus will be divided into sections for clarity as follows: (a) the construction of the coaxial adapter; (b) the construction of the flared cone or “horn”; and (c) the construction of the wire adapter. Construction here is described for use with coaxial connections with 50 ohm characteristic impedance, but other designs are possible by modifying the dimensions. Similarly, this description shows a launch apparatus designed to operate simultaneously on two US ISM bands centered around 2.44 GHz. and 5.3 GHz. Other choices are possible by changing the dimensions of the multiband choke sections and the size of the open end of the flared cone, as is well known in the art. 
   In the first embodiment of the improved surfacewave launch apparatus of this invention, as depicted in  FIGS. 2 ,  3 A,  3 B,  4 A,  4 B, construction of the coaxial adapter portion  24  consists of the multiband choke sections  30 , comprised of a 19 ohm section  32 , a 50 ohm section  34 , and another 50 ohm extension section  36 , along with a shorting end 38 (opposite non-shorted end  39 ) and a mounted coaxial connector  40 . The coaxial connector  40  is mounted with its outer (ground) connection attached to the outer sleeve  42  which is provided with a hole for the connecter pin  44  ( FIG. 3B ) or inner conductor to pass through. The pin extends into the inside of the sleeve  42  and makes low impedance electrical contact with the wire adapter  26 . The flared horn  22 , wire adapter  26 , and coaxial adapter  24  each include a longitudinal slot  23  ( FIG. 3A ),  46 , and  47  ( FIG. 3B ), respectively, provided for mounting onto the single conductor line without breaking the line. 
   The wire adapter  26  shown here makes electrical contact with the single wire conductor  21  at downstream terminus  48  beyond the mouth  50 , the widest dimension of the flare on the flared cone  22 , effectively providing an electro-mechanical attachment to the wire at that point. The other wire adapter terminus  49  is preferably only a mechanical attachment. 
     FIGS. 4A and 4B  illustrate the wire adapter  26  as installed on a single wire conductor  21 . This connection is required to provide a low impedance between the wire adapter and the single conductor wire across both operating bands of the launch. It is desirable for the wire adapter  26  to be as thin as is mechanically practical since its dimensions decrease the impedance at the mouth of the flared cone. Additionally, there is an impedance discontinuity at the terminus  48  ( FIG. 4B ) of the wire adapter  26  which is minimized when the step in diameter between the wire adapter and the wire is small. Tapering the end of the wire adapter, as along tapered section  52  ( FIG. 4B ), helps minimize this discontinuity. The length of the taper is preferably at least a quarter wave at the lowest operating frequency. 
   Direct electrical contact between the terminus  48  of the wire adapter and the wire  21  is provided by metal contacts such as “tacks”  54  which are driven through the wire adapter  26 , through any dielectic which is present, and into the wire conductor. These tacks may be further secured with a dielectric compression band of the “TyWrap” (cable tie) variety. 
   Good electrical contact is also required among the end short  38 , the end  56  ( FIG. 2 ) of the outer sleeve  42  and the wire adapter  26 . 
     FIG. 5  is a side elevation cross-sectional view of an alternate dualband embodiment for a coaxial adapter  60  of this invention. This embodiment utilizes a triaxial structure providing a bushing/triax outer conductor  62 , a triax intermediate conductor  64 , and using the single conductor/triax center line  66  as the third and central conductor of the assembly. This method allows coupling to and launching of the surfacewave mode onto the central line but requires only a single electrical and mechanical contact onto that line, i.e., shorting point  68  located at the end, shorting block  70 . Two separate coaxial cavities are formed, one between the intermediate line  64  and the central line  66 , and a second between the outer conductor  62  and the intermediate line  64 . The intermediate line  64  is tapered at the open end  72  where the coaxial cable contact is made at connection  74 . Coupling  76  couples the adapter section to the narrow end  78  of the flared horn. The two coaxial cavities, along with the tapered line provide good coupling to the central line across two separate bands without requiring any physical contact which could be problematic for outdoor use due to environmental concerns. 
   For the construction of the flared cone section in both embodiments, the cone or “horn” section may be cast or formed from either metal or from a non-conducting material and metalized after fabrication. As for the coaxial adapter section, for lowest losses copper or silver should be plated onto the current carrying surfaces or used directly for the entire interior of the flared cone section. 
   While the flared cone may be fabricated from multiple flat sheet metal subsections to approximate the desired exponential tapered shape, the preferred method of construction is to create a three-dimensional curved surface which exactly represents the desired exponential taper. This taper is such that the resulting impedance of the coaxial line formed by the flared cone outer conductor and the wire adapter inner conductor ranges from the coaxial adapter extension section impedance (50 ohms) to a higher impedance which is that of a coaxial line having an outer conductor inner dimension the same as the mouth of the flared cone, and an inner conductor of the same dimensions as the wireless adapter. 
   In the first embodiment of  FIGS. 2 ,  3 A,  3 B, the flared cone diameter is exponentially tapered between these two end limits, as shown. The flare angle  72  of the flared cone  22  ( FIG. 2 ), measured from the non-shorted end  39  ( FIG. 2 ) of the coaxial adapter portion  24  to the center of the opening, is preferably between 40 and 60 degrees. 
   In the second embodiment illustrated in  FIGS. 6A ,  6 B,  6 C, the diameter throughout the midsection of the flared cone  100  is substantially exponentially tapered while the change of taper at the ends (narrow end  102  and mouth  104  ) falls to zero. This arrangement can improve the broadband characteristics of the launch. To arrive at particular dimensions, it is useful to consider the entire cone from the perspective of a broadband coaxial line matching transformer. As with the first embodiment of  FIGS. 2 ,  3 A,  3 B, the higher impedance at the mouth  104  of the flared cone  100  should be as high as possible, and preferably over 200 ohms.  FIG. 6B  shows the dimensions of this second embodiment. The values on the left indicate the horn&#39;s inside radius, and the values on the right are the location of that radius, relative to the reference point which is at the intersection of the vertical line of symmetry and the narrow end  102  of the horn. Thus, these numbers are actually coordinate pairs showing the radius of the horn as you go alone it&#39;s length, increasing toward the wide, mouth end  104  of the horn. Also analogous to the flared cone of the first embodiment, the cone 100 includes a longitudinal slot  106  ( FIG. 6A ) enabling direct mounting onto a wire. 
   As detailed in  FIG. 6C , the edge  108  of the outer mouth  104  of the flared horn may be rolled smoothly rather than simply terminating. This can be advantageous for reducing surfacewave to radiated mode conversion and improving transmission characteristics of the surfacewave mode. 
   Construction of the wire adapter and end short is also different for the two embodiments. In the first embodiment of  FIGS. 2 ,  3 A,  3 B,  4 A,  4 B, the wire adapter  26  serves to allow a variety of wire shapes and sizes to be used with the launch. 
   For the case where circular wire is being adapted, the wire adapter  26  may have a circular internal shape, exclusive of the longitudinal slot  46 , in which the circular single wire lays (see  FIGS. 3B and 4A ). For this case, the wire adapter  26  may be constructed by cutting copper tubing lengthwise with a band saw. The end short  38  may be constructed from a copper disk, slightly larger than the outer diameter of the outer sleeve  42 , and providing a hole of the same diameter as the wire adapter outer diameter (see  FIGS. 2 and 3B ). 
   In the second embodiment ( FIG. 5 ), no physical contact is required, so it is possible to accommodate multiple wire diameters by simply providing different hole diameters in the end, shorting block  68 . The electrical design of the dual coaxial cavity structure is tolerant of considerable variation in line size without a great deal of sacrifice in performance. For radically different central wire diameters, it may be necessary to modify the dimensions of the coaxial cavities as well as the flared horn. 
   Assembly of the launch also differs with the two embodiments. For the first embodiment ( FIGS. 2 ,  3 A,  3 B,  4 A,  4 B), a hole the diameter of the coaxial connecter pin  44  is provided in the wire adapter  26 . 
   Depending upon requirements, mechanical strength and hermeticity may be improved by filling the flared cone  22  and coaxial adapter  24  with a low loss, low dielectric constant material. However, if this is done, dimensions may have to be modified to achieve the desired impedances. 
   For the first embodiment of the launch, as shown in  FIGS. 2 ,  3 A,  3 B,  4 A,  4 B, dimensions and materials may be as follows: 
                                               Flared Horn Length   3.5 inch           Flared Horn Mouth Diameter   3.5 inch           Outer Sleeve Inner Diameter   .60 inch           Wire Adapter Outer Diameter   .26 inch           Wire Adapter Length   8.0 inch           Single Wire Diameter   .23 inch           19 ohm section Diameter   .44 inch           19 ohm section length   .91 inch           50 ohm section Diameter   .26 inch           50 ohm section length   .61 inch           50 ohm extension length   1.4 inch           Coaxial Connector   type SMA or N                        
All material except for the connector is copper.
 
   For the second embodiment of the launch, as shown in  FIGS. 5 ,  6 A,  6 B,  6 C, dimensions and materials may be as follows: 
   
     
       
             
             
           
         
             
                 
             
           
           
             
               Flared Horn Length 
               3.5 inch 
             
             
               Flared Horn Mouth Diameter 
                 7 inch 
             
             
               Coupling 
               1″ US Schedule L copper 
             
             
                 
               coupling 
             
             
               Bushing 
               1″ to ½″ US Schedule L 
             
             
                 
               copper bushing 
             
             
               Triax Intermediate Conductor 
               ½″ US Schedule L copper 
             
             
                 
               pipe 
             
             
               End, Shorting Block 
               Aluminum 
             
             
               Central Wire Diameter 
               .25 to .32 inch 
             
             
               Coaxial Connector 
               type SMA or N 
             
             
               Dimension A (length of tapered portion of 
               1.78 inch 
             
             
               intermediate conductor) 
             
             
               Dimension B (length by which central 
               1.40 inch 
             
             
               coaxial cavity is greater than outer coaxial 
             
             
               cavity) 
             
             
               Dimension C (length of constant impedance 
                .85 inch 
             
             
               portion of central coaxial cavity that is 
             
             
               common with outer coaxial cavity) 
             
             
               Dimension D (length of central coaxial 
                .75 inch 
             
             
               cavity clamped by shorting block) 
             
             
                 
             
           
        
       
     
   
   The completed surfacewave launch may be mounted to an existing single wire conductor as follows. For the first embodiment, illustrated in  FIGS. 2 ,  3 A,  3 B,  4 A,  4 B, the wire adapter attachment devices (metal contacting tacks  54 ) are first installed to establish good electrical contact and mechanical robustness. 
   After mounting the device the singlewire transmission line may be used over the entire frequency and band ranges supported just as other types of transmission lines fitted with coaxial connectors would be. Transmitters, receivers, filters and frequency selective devices may be added external to the device and connected to the coaxial connector to suit the desired application. Although shown in the figures as a coaxial cable connector, this connector may also be for direct connection to electronic circuitry located immediately adjacent to the coaxial section of the inventive launch, thus allowing the launch to be part of an integrated communications assembly. 
   Accordingly, the present invention may be characterized as a launch apparatus for launching a surfacewave onto a single conductor transmission line, the launch apparatus comprising a flared cone portion; a coaxial adapter portion connected to the flared cone portion; a wire adapter portion for coupling the coaxial adapter portion to the line; and a longitudinal slot in the flared cone portion, coaxial adapter portion, and wire adapter portion to enable direct placement of the launch apparatus onto the line for installation. 
   Alternatively, the invention may be characterized as a method for launching a surfacewave onto a single conductor transmission line, the method comprising the steps of providing a launch apparatus having a flared cone portion, a coaxial adapter portion connected to the flared cone portion, and a wire adapter portion for coupling the coaxial adapter portion to the line; providing a longitudinal slot in the flared cone portion, coaxial adapter portion, and wire adapter portion; and placing the launch apparatus over the line for installation. 
   The above disclosure is sufficient to enable one of ordinary skill in the art to practice the invention, and provides the best mode of practicing the invention presently contemplated by the inventor. While there is provided herein a full and complete disclosure of the preferred embodiments of this invention, it is not desired to limit the invention to the exact construction, dimensional relationships, and operation shown and described. Various modifications, alternative constructions, changes and equivalents will readily occur to those skilled in the art and may be employed, as suitable, without departing from the true spirit and scope of the invention. Such changes might involve alternative materials, components, structural arrangements, sizes, shapes, forms, functions, operational features or the like. 
   Therefore, the above description and illustrations should not be construed as limiting the scope of the invention, which is defined by the appended claims.