US 2970284 A
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J11m- 1961 G. J. E. GOUBAU 2,970,284
WAVEGUIDE-TO-COAXIAL LINE TRANSDUCER Filed July 3, 1958 3 Sheets-Sheet 1 ELEC , MAGNETIC y ENERGY ENTERING PORT ENERGY ENTERING PORT INVENTOR, GEORG J. E. GOUBAU.
. ir 2 d g? ATTORALEX Jan. 31, 1961 G. J. E. GOUBAU WAVEGUIDE-TO-COAXIAL LINE TRANSDUCER 3 Sheets-Sheet 2 Filed July 5, 1958 ill l- INVENTOR GEORG J. 5 sous/w.
9141M? 2% @an agmrg ATTORNEY.
Jan. 31, 1961 G. J. E. GOUBAU 2,970,284 WAVEGUIDE-TO- Filed July 3, 1958 COAXIAL LINE TRANSDUCER 3 Sheets-Sheet 3 JIIIWIIIIIIIITW,
INVENTOR, GEO/5'6 J. 5. souanu "for this purpose.
A generalobjectis to improve transducers" fonelec'tri- WAVEGUIDE-TO-COAXIAL LINE TRANSDUCER -Georg J. E. Goubau, Eatontowm NJ assignor to the UnitedStates-of America a's'representedby the Secretat-y of. the Army 7 Filed July 3,1958, set; No. 746,571
1 Claim; 01; ass-#34 (Granted under Title? 35', US: Code (1952 sec. 266) The invention described hereinrnay be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.
The-invention relates to guided electromagnetic wave transmission systems and particularly to transducers for use-in suchsystems to couple electrically transmission devices or lines having impedances of different types and values, such as a waveguide line and a'coaxial line.
netic waveenergy in the highfrequency range above about 50 megacyclcs per second hecauseof its extremely low attenuation over a wide band of frequencies in this range and extremely high power carrying capacity (H. H.
B'arlow The Relative Power-Carrying Capacity of High Frequency Waveguides, Proc; IEE (London), Part E, vol. 90, pp. 21-27, January 1952). As describedsin the aforementioned patent, the SFTL comprises an elongated con- "ductor such as a wire,. adapted to propagate electromagnetic wave energy alongits outer. surface because of "special conditioningofthat:surface, for example; by the provision of a dielectric coating thereon, so as to reducethe phase velocity and thereby concentrate the field ofthe transmittedtw'ave" adjacent the conductor.
A SWTL is in effect a transmission device with coaxial terminations, and if his used" in" combination with a Waveguide transmission system requires a special adaptor -for properly electricallycoupling it towav'egu'ide terminals in that system. 'Ihe-standard'adaptors which'have been provided for this purpose, which are usually designed for connecting. SO-ohm cables, are not particularly suitable because they tend to reduce the usable frequency bandwidth oftheSWTL, Since the impedance of the surface wave is in the order of 200 to 300 ohms, the launchers provided for exciting the surface wave inthe SWTL must take care of an impedance transformation having a ratio of il or more. This not only increases the costof the launchers but also reduces their cfiiciency because of inherent transformation losses. Even more important is the fact that thecoa'xial termination of the standard adaptors limits the power-handling capacity of an associated surface Wave transmission line particularly inthe centimeter-frequency range where the physical dimensionsof the coaxial terminations are quite small. r
One object of the present invention is to provide an adaptor for coupling a surface wave transmisison line;
Withfcoaxial terminations to the waveguide terminals in a waveguide transmission system, which willnot have the above-mentioned disadvantages of the prior art adaptors Patented Jan. 31, 1961 cally coupling transmission devices or lines having terminating impedances of different types and values in a high frequency signal wave transmission system, from the standpoint of providing a more etficicnt transfer of the high frequency signal wave power between them while minimizing reflections at the junction point tending to increase power losses.
A related object is to provide at relatively low cost etficient impedance matching at frequencies in the ultrahigh or super-high frequency ranges, for example, at frequencies in the order of 500 megacycles per second, between a coupled waveguide line and another line having a coaxial termination.
A more specific object is to provide efli'cient electrical coupling between a Waveguide. line and a surface wave or other. transmisison line having a coaxial termination ina high frequency signal wave transmision system without appraciably reducing the power-handling capacity of the coupled lines in the system.
The transducers in accordance with the invention for attaining these objects comprise two tandem-connected portions of different constructions. Both portions are three-port line junctionsbut one of them, to be referred to hereinafter as thefinput unction,,has three waveguidecports and theother, to be referred to hereinafter as the output junction, has two waveguide ports and one coaxial port, The input junction operates to divide the electromagnetic signal waves entering an input waveguide port' into two equal wave energy portions and to propagate those portions separately through-different ones of its two output ports consisting of two substantially straight sections of hollow-pipe waveguide: of identical cross-sections and'having one side Wall in common, withthe electric fields of the propagated wave energy portions oppositelydirected with respect to the common wall.
The two input ports of the output junction are formed by two parallel sections of hollow-pipe waveguide with identical cross-sections and having one side wall'in' common. These two waveguide sections are also identical in cross-section width, and are respectively connected directlyto the two output waveguide ports of the input junction so as toreceive the propagated wave energy portions from the inputjunction. The output junction operates to combine these wave energyportions and to formcacoaxial'wave mode. The formation of this coaxial wave mode is achieved by' cutting away aportion of thecorninon wall of the adjacent waveguide section constituting'the two input ports of the output junction and effectively extending the center'of this wall to the output end-of the transducer, by, the'attachment thereto of a longitudinally extending cylindrical conductor or wire to provide the inner conductor of the coaxial output port of which the extended outer. walls of the two waveguide sections form the outerconductorj This coaxial output port provides a high output. impedance: for the transducer and is adapted for direct connection to the coaxial termination of the surface wave or other transmission line or device to'be electrically coupled through the transducer to a waveguide line.
One feature of the invention is' the gradual cutting away of the common Wall of the'two adjacent waveguide sections of the output junction, forexample, in tapered fashion, to provide a gradual transformation of the electric field of the propagatedwave energy, in order to 'increase the effective frequency bandwidth of the coaxial .guide junction in its input;
Figs. and 6 show perspective views, partially broken away to show structural details, of different embodiments of the output junction of a transducer in accordance with the invention;
Fig. 7 shows a perspective view of a modification in accordance with the invention of a portion of the output junction of Fig. 6;
Fig. 8 shows a perspective view of an embodiment of a complete transducer in accordance with the invention, in which the coaxial output port of the output junction is derived in part from the rectangular waveguide sections forming the input ports of that junction; and
Figs. 8A, 8B and 8C are cross-sectional views along the lines A-A, BB and C-C, respectively, of Fig. 8, showing the wall construction and the field transformation at ditierent points along the transducer.
Fig. 1 shows a three-port input junction of a transducer in which the input port 1 comprises one section of hollow-pipe waveguide of rectangular cross-section and the two output ports 2 and 3 electrically coupled thereto comprise two substantially straight sections of hollow-pipe waveguide of identical rectangular cross-sections, having a common side Wall 4. In Fig. 2, the input port 1A of the three-port input junction shown therein comprises a section of hollow-pipe waveguide of circular cross-section and the two output ports 2A and 3A coupled thereto comprise two substantially straight sections of hollowpipe waveguide of identical semi-circular cross-section, having a common side wall 4A. Fig. 3 shows how the circular and semi-circular cross-section of hollow-pipe waveguide used for the input and output ports, respectively, in the input junction of Fig. 2 can be obtained by gradual deformation of waveguides of rectangular crosssection. As indicated on each of the figures, the input junction of Figs. 1, 2 or 3 operates to divide the electromagnetic wave energy entering the input port 1, 1A or 1B into two equal energy portions which because of the.
particular orientation of the cross-sections of the connecting waveguides are propagated separately through a different one of the two output ports 2 and 3, 2A and 3A, or 2B and 313, with the electrical fields E of the propagated wave energy portions oppositely directed with respect to the common wall 4, 4A or 4B.
The particular embodiment of the transducer of the invention shown in Fig. 4 employs the well known H-plane, T-type waveguide junction in its input. As shown, this junction comprises an input arm 5 comprising a straight section of hollow-pipe waveguide of rectangular crosssection forming the stern of the T and two branch arms 6 and 7 consisting of two hollow-pipe waveguides of 1 identical rectangular cross-sections connected at a com mon point to the output of the input arm 5, and forming the cross-arm of the T. This junction with the identical cross-sections of the arms 5, 6, and 7 oriented as shown operates to divide the electromagnetic wave energy entering the junction through its input arm 5 into two equal energy portions which are separately propagated into difierent ones of the output arms 6 and 7 with their electric fields E oppositely directed. The extensions of the two branch arms 6 and 7 are bent inwardly as shown until near their output ends the adjacent walls combine so as to provide two straight sections 8 and 9 of waveguide having a common wall 10, constituting the two output ports of the input junction, similar to the two output 4 ports 2 and 3 in the embodiment of Fig. 1 and operating in similar manner.
Fig. 5 shows one embodiment of the output junction of a transducer in accordance with the invention, in which the two input ports 11 and 12 are formed by two parallel straight sections of hollow-pipe waveguide of identical cross-sections of semi-circular configuration, having one sidewall 13 in common. .In Fig. 5, a portion of the outer wall of the waveguide sections 1 1 and 12 is broken away to show more clearly how the coaxial output port of the output junction is constructed. As indicated, part of the common wall 13 is removed and replaced by a cylindrical conductor or wire l4 attached at one end to the center of the common wall and extending longitudinally to the output end of the transducer. The conductor 14 forms the inner conductor of the output coaxial line section or port 15 for propagation of the coaxial wave mode, the outer conductor of which is provided by the extension of the outer walls of the two waveguide sections forming the input ports 11 and 12 of the output junction, to the ouptut end of the transducer. The dielectric disc 16 attached at a selected point along the cylindrical conductor 14 in the interior of the coaxial line output port 15 is used to compensate for the impedance discontinuities which occur at the junction of the conductor 14 and the common wall 13 by causing a reflection of equal amount and degrees out-of-phase therewith, and thereby to effect complete transformation energywise of the waveguide modes entering through the input ports 11 and 12 into the coaxial Wave mode leaving the output coaxial line section or port 15.
Although the dimension of the output coaxial line section or port 15 usually would be sufiiciently large so that this section can transmit the unwanted waveguide mode of the type TE the formation of which would limit the size and thus the power-carrying capacity of the coaxial portion in standard waveguide-to-coaxial transducers, this mode would not be excited in the output junction of Fig. 5 because of the opposite orientation of the electric fields of the propagated wave energy with respect to the common wall 13.
Fig. 8 shows one embodiment of a complete transducer in accordance with the invention adapted for producing an increase in the effective transmission frequency band- With of the output junction. Referring to that figure, the input junction of the transducer shown therein has an input port 17 formed by a single section of hollow-pipe waveguide of rectangular cross-section, open at one end and closed at the other by an axially movable plunger 18, and two output ports 19 and 20 respectively formed by two substantially straight sections of hollow-pipe waveguide of identical rectangular cross-section and having one side wall 21 in common. As shown, the inputs of the two Waveguide ports 19 and 20 are electrically coupled to the input waveguide port 17 through a suitable opening in one sidewall of the latter at a point intermediate its ends, and the cross-sections of the three waveguide ports 17, 19 and 20 are suitably oriented with respect to each other so that the electromagnetic wave energy propagated over the input guide or port 17 and fed into the branch waveguide sections or output ports 19 and 20 will be split into two equal energy portions which will be respectively separately propagated over a different one of these sections with the electric fields E of the propagated wave energy portions oppositely directed with respect to the common wall 21 of these sections, as indicated inthe figure by the direction of the arrows so designated. The two input ports of the output junction of the transducer shown in Fig. 8 comprise the extended portions of the two waveguide sections 19 and 20 of identical rectangular cross-sections. The coaxial output section or port of the output junction is formed in the following manner.
The full common wall 21 of the waveguide sections 19 and 20 extends only to a point 22 a distance in the order of a wavelength from the junction of that wall with the :input guide 17. Beyond this ipoint the portion of the common wall 21 above and below a common center line are cut away gradually in tapered fashion as shown. The taper preferably begins from the middle of the wall and extends to the output-end of the transducer, :or, :as shown, to the point 23 along the length of the guides 19 and 20, a short distance from the output end of the transducer where all portions of the common wall 21 are eliminated and only the extensions of the outer walls of the guides 19 and 20 remain. A cylindrical rod or wire 24 having one end connected to the center of the common wall 21 at the point 22 and extending longitudinally to the output end of the transducer, forms the inner conductor, and the extended outer walls of the two waveguide sections 19 and 20 the outer conductor of an output coaxial line portion or port 25 of the transducer. The gradual (tapered) cutting away of portions of the common wall 21 in the manner described results in an increase in the bandwidth of the output junction. In this manner, a more gradual transformation of the electric field of the propagated waves can take place as illustrated by the field patterns shown in Figs. 8A, 8B and 8C.
The field transformation which takes place in the transducer when the electromagnetic wave energy propagates from a cross-section A-A where the two wave energy portions are separated by the common wall 21, through the region where the wall 21 is cut away in tapered fashion so that a varied amount of coupling occurs between the two energy portions, to the coaxial output of the transducer, is illustrated in the cross-sectional views of the transducer at dilferent point shown in Figs. 8A, 8B and 8C. The impedance matching produced by the transducer of Fig. 8 between a waveguide line connected to the open end of the input guide 17 and any line with coaxial termination connected to its coaxial output end 25 can be improved by proper positioning of the plunger 18 in the other end of the input guide 17.
Further improvement in the electrical coupling provided by a transducer in accordance with the invention can be obtained by reducing the phase velocity of the waves in the waveguide sections to a velocity which is only a little higher than the velocity in the coaxial line section of the output junction. One method of accomplishing this is illustrated in Fig. 6 applied to an output junction employing waveguides 26, 27 of identical rectangular cross-section having a common wall 28, for the two input ports, which common wall is cut away gradually in tapered fashion along its length beyond the point 29. A cylindrical conductor 30 extending longitudinally from the center of wall 21 at point 29 to the end of the transducer forms the inner conductor of the output coaxial line section or port 31 of which the extended outer walls of the waveguide sections 26 and 27 form the outer conductor. The required relation between the phase velocities of the propagated waves in the waveguide and coaxial line sections of the output junction is obtained by the use of similar metal ridge members 32, 33 in the interiors of the rectangular waveguide sections 26 and 27, which extend from the waveguide sections into the coaxial line section and taper ofi gradually in both directions. The ridge members 32, 33 may be located opposite to the common wall 28 and attached to the outer walls of the guides 26 and 27, respectively, as shown in Fig. 6, or may be attached to opposite sides of the common wall 28, as illustrated in the detail view of Fig. 6 showing only the ridges 32, 33, the common wall 28 and the cylindrical conductor 30 forming the inner conductor of the output coaxial line section or port. Instead of the metal ridge members, spaced dielectric slabs (not shown) in the interiors of the two waveguide sections 26, 27 may be used for accomplishing the same purpose.
The parallel waveguide sections used for forming the input and output ports it of the input punction and the input ports of the output junction and for deriving the coaxial output port of the output junction of the transducer, may
have configurations other than those illustrated in the embodiments of the transducer of the invention-shown in Figs. l'to 8. Also, although'only the use of the transducers ofthe invention for transferring electromagnetic wave power in the direction from a waveguide line-to a line having a coaxial termination has been described above, it is to be understood that they are also applicable for transferring electromagnetic wave power in the opposite direction, that is from a line with coaxial termination to a waveguide line.
When a surface wave transmission line is one of the lines coupled by a transducer in accordance with the invention, the cylindrical conductor forming the inner conductor of the coaxial output port of the transducer would be connected to the elongated conductor of the SWTL, and the output end of the outer walls of the extended waveguide sections forming the outer conductor of that output port would be connected to the wave guide horn or other surface wave launching means such as described in the above-mentioned patent, or the surface wave conductor may serve as conductor 30 of the coaxial section.
Various other modifications of the transducer arrangements illustrated and described which are within the spirit and scope of the invention will occur to persons skilled in the art.
What is claimed is:
A transducer for electrically coupling a waveguide line to another line having a coaxial termination comprising two tandem-connected line junctions one having three waveguide ports and the other two waveguide ports and one coaxial line port, the waveguide ports of said other junction being respectively connected in wave transmission relation with a diiferent one of two of the waveguide ports of said one junction through two sections of hollow-pipe line of identical cross-section having a common sidewall between them, the connection between the three waveguide ports of said one junction being such that the electromagnetic energy entering one port of that junction from said waveguide line is split into two equal portions which are respectively propagated through different ones of said two sections to the other two waveguide ports thereof with their electric fields oppositely directed with respect to said common wall, said common wall being partly eliminated beyond a point a given distance from the waveguide end of the transducer and a cylindrical conductor is connected to the center of that wall at said point and extending longitudinally to the coaxial line end of the transducer to form the inner conductor of said coaxial line port, the extended outer walls of said line section forming the outer conductor of said coaxial line port, the partial elimination of said common wall beyond said point being provided by cutting away that wall in tapered fashion on opposite sides of a center line to provide a more gradual transformation of the oppositely phased electric fields. to increase the efiective frequency bandwidth, the tapered cut beginning at the center point to which said cylindrical conductor is attached and extending lengthwise to the opposite outer edges of the common wall near the coaxial line end of the transducer, impedance matching between the coupled lines being provided by the use of a similar metal ridge section in the interior of each of said hollow-pipe waveguide sections attached to corresponding wall thereof at a point opposite the common wall, which ridge sections taper olt gradually longitudinally in both directions, to reduce the phase velocity of the waves in the waveguide portions of the transducer to a velocity which is only a little higher than that in the coaxial line portion of the transducer.
(References on following page) 1 5 References Cited in the file of this patent FOREIGN PATENTS H UNITED STATES'PATENTS 623,770 Great Britain May 23, 1949 :Alford Nov. 10 1953 OTHER REFERENCES Smith May 14, 1957 5 Ragan: Microwave Transmission Circuits, vol. 9, Worthington Dec, 24, 1957 MIT Radiation Laboratory Series, copyright May 21, 1948 Elliott Mar. 17, 1959 (pages 360-361 relied upon).
Robertson Feb. 9, 1960