Method and apparatus for launching a surface wave onto a single conductor transmission line

BACKGROUND OF THE INVENTION

<u>Technical Field</u>

0001The present invention relates generally to methods and apparatus for surface wave transmission, and more particularly to an improved method and apparatus for launching a surface wave onto a single wire transmission line.

0002The present invention improves the performance and utility of previous launch devices for converting coaxial mode transmission to and from surface wave 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. An example of a Gobau line is disclosed in document <patcit id="pcit0001" dnum="US3509463A"><text>US 3509463</text></patcit>

<u>Background Art</u>

0003Previous launch devices for single conductor transmission lines have used a simple conical shape structure to excite the surface wave 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 surface wave mode when a broad range of frequencies is supported, and require both a longer cone and wider cone mouth in order to excite a surface wave 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.

0004The 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'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.

<u>Disclosure of Invention</u>

0005The present invention provides a method and apparatus for launching a surface wave onto a single conductor transmission line. The inventive apparatus provides launch of a surface wave 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 surface wave mode transmission of UHF and microwave energy. Previous known references and teachings on the subject have restricted usage to insulated wires.

0006The 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 surface wave mode with a physically smaller horn section when compared to prior, single conical section designs.

0007The 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.

0008A 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.

0009The 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.

0010It is therefore an object of the present invention to provide a new and improved surface wave launch apparatus.

0011It is another object of the present invention to provide a new and improved surface wave launch adapted for use on insulated or uninsulated single conductors.

0012A further object or feature of the present invention is a new and improved flared cone apparatus with improved broadband and multiband transmission performance.

0013An even further object of the present invention is to provide a novel coaxial adapter for conventional coaxial transmission cable.

0014A still further object of the present invention is to provide an improved wire adapter for contacting a wire conductor.

0015An additional object of the present invention is to provide an improved method for placement of a surface wave launch on an existing line.

0016Other 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.

0017There 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.

0018Further, the purpose of the Abstract is to enable the national Patent Offices 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.

0019Certain 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.

<u>Brief Description of the Drawings</u>

0020The 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:

0021<figref idref="f0001">FIG. 1</figref> is a schematic view of a prior art surface wave launch device;

0022<figref idref="f0002">FIG. 2</figref> is a side elevation cross-sectional view of a first embodiment of an improved surface wave launch apparatus of this invention;

0023<figref idref="f0003">FIG. 3A</figref> is an end elevation view of the surface wave launch of <figref idref="f0002">FIG. 2</figref>, while <figref idref="f0003">FIG. 3B</figref> is an enlarged end elevation view of the coaxial adapter and wire adapter portions of the surface wave launch of <figref idref="f0003">FIG. 3A</figref>;

0024<figref idref="f0004">FIG. 4A</figref> is an end elevation view of a wire adapter of this invention as installed on a single wire conductor, while <figref idref="f0004">FIG. 4B</figref> is a top view of a wire adapter as installed on a single wire conductor, showing the taper to the single wire conductor;

0025<figref idref="f0005">FIG. 5</figref> is a side elevation cross-sectional view of an alternate dualband embodiment for a coaxial adapter of this invention; and

0026<figref idref="f0006">FIG. 6A</figref> is a perspective view of the flared horn portion of an alternate embodiment of the surface wave launch of this invention; while <figref idref="f0007">FIG. 6B</figref> is a cross-sectional view of the flared horn of <figref idref="f0006">FIG. 6A</figref>, and <figref idref="f0007">FIG. 6C</figref> is a detail view of an optional rolled edge for the outer mouth of the flared horn.

<u>Best Mode for Carrying Out the Invention</u>

0027<figref idref="f0001">FIG. 1</figref> is a schematic view of a prior art surface wave launch device 10. Prior art surface wave 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.

0028Referring to <figref idref="f0002 f0003 f0004 f0005 f0006 f0007">FIGS. 2 through 6C</figref>, wherein like reference numerals refer to like components in the various views, there is illustrated therein a new and improved surface wave launch apparatus of this invention.

0029<figref idref="f0002">FIG. 2</figref> is a side elevation cross-sectional view of a first embodiment of the improved apparatus 20 for launching a surface wave onto a single conductor transmission line 21 (either with or without an outer dielectric covering). Surface wave launch 20 includes a flared horn or cone portion 22, a coaxial adapter portion 24, and a wire adapter portion 26.

0030The 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.

0031In the first embodiment of the improved surface wave launch apparatus of this invention, as depicted in <figref idref="f0002 f0003 f0004">FIGS. 2 through 4</figref>, 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 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, 46, and 47, respectively, provided for mounting onto the single conductor line without breaking the line. The lower impendance choke section 32, approximately 19 ohms, is created by increasing the outer diameter of the wire adapter over a portion of the distance between the connecter pin 44 and the end short 38. The inner diameter of the coaxial sleeve and the outer diameter of the wire adapter are chosen to provide the required impedances.

0032The 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.

0033<figref idref="f0004">FIGS. 4A and 4B</figref> 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 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, helps minimize this discontinuity. The length of the taper is preferably at least a quarter wave at the lowest operating frequency.

0034Direct 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" variety.

0035Good electrical contact is also required among the end short 38, the end 56 of the outer sleeve 42 and the wire adapter 26. Copper, brass and aluminum are all reasonable candidates for the construction of these coaxial adapter portion components. For lowest losses, copper or silver should be plated onto the current carrying surfaces or used for the entire coaxial adapter portion 24 and wire adapter portion 26.

0036<figref idref="f0005">FIG. 5</figref> 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 surface wave 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.

0037For 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.

0038While 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.

0039In the first embodiment of <figref idref="f0002">FIGS. 2</figref> and <figref idref="f0003">3</figref>, the flared cone diameter is exponentially tapered between these two end limits, as shown. The flare angle 72 of the flared cone 22 (<figref idref="f0002">FIG. 2</figref>), measured from the non-shorted end 39 of the coaxial adapter portion 24 to the center of the opening, is preferably between 40 and 60 degrees. The higher impedance at the mouth 50 of the flared cone 22, as described above, should be as high as possible, and preferably over 200 ohms. This prescribes a minimum diameter for the flared cone mouth 50.

0040In the second embodiment illustrated in <figref idref="f0006 f0007">FIG. 6</figref>, 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 <figref idref="f0002">FIGS. 2</figref> and <figref idref="f0003">3</figref>, the higher impedance at the mouth 104 of the flared cone 100 should be as high as possible, and preferably over 200 ohms. <figref idref="f0007">FIG. 6B</figref> shows the dimensions of this second embodiment. Also analogous to the flared cone of the first embodiment, the cone 100 includes a longitudinal slot 106 enabling direct mounting onto a wire.

0041As detailed in <figref idref="f0007">FIG. 6C</figref>, 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 surface wave to radiated mode conversion and improving transmission characteristics of the surface wave mode.

0042Construction of the wire adapter and end short is also different for the two embodiments. In the first embodiment of <figref idref="f0002">FIGS. 2</figref> and <figref idref="f0003">3</figref>, the wire adapter 26 serves to allow a variety of wire shapes and sizes to be used with the launch. The wire adapter 26 serves to "build up" the dimensions of whatever wire is being used in order to create a 50 ohm coaxial transmission line section no matter what type of wire is being used. It also serves as a means for making a low impedance connection to the wire and to provide mechanical mounting and robustness to the launch. Although not described here, it should be noted that it is also possible to create a low impedance UHF or microwave connection to the wire without actually contacting the wire through the use of open quarterwave choke sections which use either the wire dielectric or an additional dielectric between the wire adapter and the wire conductor.

0043For 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 <figref idref="f0003">FIGS. 3B</figref> and <figref idref="f0004">4A</figref>). 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 <figref idref="f0002">FIGS. 2</figref> and <figref idref="f0003">3B</figref>).

0044In the second embodiment (<figref idref="f0005">FIG. 5</figref>), 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.

0045Assembly of the launch also differs with the two embodiments. For the first embodiment (<figref idref="f0002 f0003 f0004">FIGS. 2 through 4</figref>), a hole the diameter of the coaxial connecter pin 44 is provided in the wire adapter 26. The coaxial connector 40 is first soldered to the outer sleeve 42 and then while the wire adapter 26 is held in position, the center pin may placed in the hole and soldered. This assembly is then soldered together with the 19 ohm section 32 and the end short 38, completing the coaxial adapter section 24. The flared cone 22 may then be soldered to the non-shorted end 39 of the coaxial section.

0046Depending 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.

0047For the first embodiment of the launch, as shown in <figref idref="f0002 f0003 f0004">FIGS. 2 through 4</figref>, dimensions and materials may be as follows: <tables id="tabl0001" num="0001"><table frame="none"><tgroup cols="2" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="45mm" /><colspec colnum="2" colname="col2" colwidth="25mm" /><tbody><row><entry>Flared Horn Length</entry><entry>3.5 inch</entry></row><row><entry>Flared Horn Mouth Diameter</entry><entry>3.5 inch</entry></row><row><entry>Outer Sleeve Inner Diameter</entry><entry>.60 inch</entry></row><row><entry>Wire Adapter Outer Diameter</entry><entry>.26 inch</entry></row><row><entry>Wire Adapter Length</entry><entry>8.0 inch</entry></row><row><entry>Single Wire Diameter</entry><entry>.23 inch</entry></row><row><entry>19 ohm section Diameter</entry><entry>.44 inch</entry></row><row><entry>19 ohm section length</entry><entry>.91 inch</entry></row><row><entry>50 ohm section Diameter</entry><entry>.26 inch</entry></row><row><entry>50 ohm section length</entry><entry>.61 inch</entry></row><row><entry>50 ohm extension length</entry><entry>1.4 inch</entry></row><row><entry>Coaxial Connector</entry><entry>type SMA or N</entry></row></tbody></tgroup></table></tables> All material except for the connector is copper.

0048For the second embodiment of the launch, as shown in <figref idref="f0005">FIGS. 5</figref> and <figref idref="f0006 f0007">6</figref>, dimensions and materials may be as follows: <tables id="tabl0002" num="0002"><table frame="none"><tgroup cols="2" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="46mm" /><colspec colnum="2" colname="col2" colwidth="60mm" /><tbody><row><entry>Flared Horn Length</entry><entry>3.5 inch</entry></row><row><entry>Flared Horn Mouth Diameter</entry><entry>7 inch</entry></row><row><entry>Coupling</entry><entry>1" US Schedule L copper coupling</entry></row><row><entry>Bushing</entry><entry>1" to 1/2" US Schedule L copper bushing</entry></row><row><entry>Triax Intermediate Conductor</entry><entry>1/2" US Schedule L copper pipe</entry></row><row><entry>End, Shorting Block</entry><entry>Aluminum</entry></row><row><entry namest="col1" nameend="col2" align="left">Central Wire Diameter.25 to .32 inch</entry></row><row><entry>Coaxial Connector</entry><entry>type SMA or N</entry></row><row><entry>Dimension A</entry><entry>1.78 inch</entry></row><row><entry>Dimension B</entry><entry>1.40 inch</entry></row><row><entry>Dimension C</entry><entry>.85 inch</entry></row><row><entry>Dimension D</entry><entry>.75 inch</entry></row></tbody></tgroup></table></tables>

0049The completed surface wave launch may be mounted to an existing single wire conductor as follows. For the first embodiment, illustrated in <figref idref="f0002 f0003 f0004">FIGS. 2 through 4</figref>, the wire adapter attachment devices (metal contacting tacks 54) are first installed to establish good electrical contact and mechanical robustness. A dielectric compression band (e.g. "TyWrap") may be used to further secure the wire adapter to the wire. Dielectric filler or covering may be inserted into the flared horn to meet environmental requirements. For the second embodiment, illustrated in <figref idref="f0005">FIG. 5</figref>, the end, shorting block 68 is drilled to form a bore to fit the wire, and is mounted in sandwich or clamshell fashion around the wire.

0050After 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.

0051Accordingly, the present invention may be characterized as a launch apparatus for launching a surface wave 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.

0052Alternatively, the invention may be characterized as a method for launching a surface wave 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.

0053The 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 scope of the invention. Such changes might involve alternative materials, components, structural arrangements, sizes, shapes, forms, functions, operational features or the like.

0054Therefore, the above description and illustrations should not be construed as limiting the scope of the invention, which is defined by the appended claims.