US 2812502 A
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Nov. 5, 1957 w. H. DOHERTY TRANSPOSED COAXIAL CONDUCTOR SYSTEM Filed July 7, 1953 lV//M/- /v//J/f/ W///V/////A A TTOR/VE y United States Patent O TRANSPOSED COAXIAL CONDUCTOR SYSTEM William H. Doherty, Summit, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 7, 1953, serial No. 366,510
3 Claims. (ci. ssa-96) This invention relates to improvements in electromagnetic transmission structures or tuned cavities.
The object of the present invention is to reduce the losses in transmission lines or resonant cavities.
It has previously been proposed to use Litzendraht condoctors, consisting of a large number of strands of line wire that are insulated from each other except at the ends where the various wires are connected together, to reduce losses at lower frequencies. At frequencies above about one megacycle, however, capacitive effects and stranding irregularities make Litzendraht conductors impractical.
In accordance with the present invention, losses in conductive surfaces at high frequencies are reduced by the use of -two or more superposed conductive layers which are insulated from one another `and which are periodically transposed, forcing current to flow in all the layers, instead of just in the skin of a single surface. In one specific embodiment of the invention, the center conductor of a coaxial line is made up of a series of thin concentric `cylindrical conducting shells which are periodically transposed. `In contrast to Litz wire, the present structures are eminently suitable for high frequencies, the laminated conducting elements do not require any direct electrical connection, and the currents are induced in the conducting laminations by an electromagnetic wave passing down a wave guiding passageway.
Other objects and various features and advantages of the invention will be developed in the course of the detailed description of the drawings. In the drawings:
Fig. l is a schema-tic illustration of a coaxial line having a transposed multi-layer center conductor;
Fig. 2 is a cross sectional view of a transposed conductor structure which could be used for the center conductor of the coaxial line of Fig. l; and
Fig. 3 shows an alternative transposed conductor structure.
Referring more particularly to the drawings, Fig. l shows by way of example and for purposes of illustration a coaxial line energized by a high frequency signal generator 11. The coaxial line includes an outer conductor 12, and an inner conductor having two concentric layers 13 and 14. These layers are periodically transposed as indicated at transposition points 15, 16, 17, and 13, and can be spaced apart from one another by the insulation layer 19. Similarly, the wave guiding passage` way between the outer conducting layer 13 of the central conductor structure and the outer coaxial conductor 12 can be completely filled with the solid dielectric layer 21. Spaced ldielectric beads may, however, be substituted for the layer 21 in a manner which is well known in the art. The thin conducting shells which make up the active elements of the composite center conductor are provided with an insulating core comprising the insulation layer 22 and a central wire 23. This central wire 23 is insulated from the conducting shells 13 and 14 throughout its length, and can be constructed of copper or steel to conduct power -to repeater points or to add strength to the cable structure, respectively.
Although the individual conducting shells or layers may be as thick as is considered desirable from a construction viewpoint, optimum electrical properties are obtained with thicknesses of not more than one or two skin depths. Another construction matter which should be noted is the alternative of constructing each conducting layer from untransposed laminations of conducting and insulating material. This alternative structural configuration improves the current distribution in the individual layers and avoids proximity eiiects due to currents in other layers, which may otherwise tend to limit the improvement in transmission obtained by applicants structure.
The purpose of this transposed multi-layer conducting structure is to increase the etective penetration or skin depth of the current, and thus reduce ltransmission losses. As applied to the device of Fig. l, the signal generator 11 excites an electromagnetic iield in the wave guiding passageway between conductors 12 and the composite center conductor structure, and current flows in the outer surface of the conducting shell 131 of the irst transposition section of the center conductor. As conductor 141 is shielded from the iield by conductor 131, little or no current flows in this section. At transposition point 15, however, the outer conducting shell 131 is connected to the inner conducting shell 142 of the second transposition section. Current is thus carried by both the inner and outer conducting shells of the composite center conductor in the second and all successive transposition sections of the transmission line, giving substantially higher conductivity and lower losses than if the center conductor were solid and the current carried by the outer surface or skin of the single conductor. It is obvious that, as alternatives to the foregoing, the current can be introduced into the structure on the inner conductor, or on both or all conductors simultaneously.
In order to assure a uniform current distribution between the two inner concentric conductive surfaces 13 and 14 and to minimize distortion of the field between the outer conductor 12 and the composite inner conductor, the transposition sections should be relatively small as compared to one-quarter of the propagation wavelength kp of the signal which is transmitted down the coaxial line. Thus, although -any transposition interval less than one-quarter wavelength gives substantially improved results as compared with the usual type of coaxial line, transposition lengths of one-sixteenth wavelength or less are preferred. In addition, it might be noted that under appropriate conditions transposition intervals of several hundred feet or more might still fullill the above-noted limitations. This would correspond f to greatly increasing the length of the straight sections between the transition points in the structures shown in the drawings.
Figs. 2 and 3 illustrate specific structures involving interleaved tabs which may be used for the center conductor structure of the coaxial line of Fig. l.
In Fig. 2, a detailed view of one form of construction is shown, with all elements except the inner conducting layers 13 and 14 and the intermediate insulation layer 19 deleted for purposes of clarity. At the transposition point 15 between the first and second transposition sections, the conducting tab 27 interconnects the outer conducting shell 131 of the first transposition section with the inner conducting shell 142 of the second section, and the tab 28 interconnects inner and outer conducting elements 141 and 132, respectively. In order to allow adequate clearance for the tabs and still provide maximum conductivity, each tab has an angular extent of approximately 120 degrees. With the two tabs located at diametrically opposed points of the conducting cylinders, this arrangement allows 6() degrees clearance as the tabs pass each other. One of the two diametrically opposed tab clearance spaces is shown at 29 in Fig. 2 between tabs 27 and'28. The structure described for transition point is duplicated at points 16, 17, 13 and at successive transition points.
In Fig. 3 a composite center conductor structure hay: ing three layers 31, 32, 33 is shown, with the increased number of conductors serving to further reduce the resistive losses in this center conductor structure. With a` view toward maintaining good conductivity between conducting shells, only two transitions are effected at each transition point. In order to carry out this'design,V the outer conducting cylindersor shells 311, 312, 313 are offset with respect to the inner conducting shells 331, 332, 333, and the intermediate conducting shells 321 through 325 are of approximately one-half the length of the inner and outer conducting cylinders. In addition, these intermediate cylinders 321 through 325 are located longitudinally so that the spaces between adjacent cylinders successively coincide with the spaces between the inner cylinders 331 through 333 and the spaces between the outer conducting cylinders 311 through 313. At these common spaces between cylinders or transition points tabs similar to those shown in Fig. 2 cross-connectY successive cylinders of diterent radii so that the current induced in the outer conducting sections 31 is progressively directed down-V wardly through the intermediate conducting sections 32 to the inner conducting shells 33, and then is directed back to the outer surface of the central conductor structure again. Referring to the outer conducting shell 311 atthe lefthand end of the conductor structure shownin Fig. 3, current is led away fromthis outer shell 311 and inwardly to the conducting cylinder 322 by means of the conducting tab 37 which overlies and interconnects these two cylinders. Incidentally, this tab 37 has a substantial peripheral extent and has'veryY nearly the same diametrically opposed relationship with tab 3S as tabsv27 and 28 of Fig. 2 have for oneanother. After traversing the length of conducting cylinder 322, the current is led inwardly through tab 41 tothe inner conducting shell 322 andV then is led back outwardlyto section 325 via tab 42. By examining the other interconnections shown in; this structure, it can. readily be seen that there. are.
the invention is by no means limited to this arrangement. For example, the outer conductor Vof a coaxial linecould have several transposed layers. included in its structure. It is generally desirable for shielding purposes, however, vto have the outermostconducting layer continuous. Furthermore, the principles of the invention are also applicable to resonant chambers and wave guides.
In the case of wave guides,. one or more ofV the walls are of multi-layer construction and are periodically transposed in much the same manner as suggested above for the center conductor structure of the coaxial line.
As mentioned hereinbefore the individual conducting layers need not be connected together at the ends of the cable for high frequency transmission. However, for the transmission of lower frequencies, various or" the conducting layers may be connectedin parallel' to provide one or more pairs of conducting paths.
It is to be understood that the above-described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.
What is claimed is:
l. In an electromagnetic wave transmission system for use over a predetermined range of wavelengths, an extended coaxial transmission line comprising a continuv` ous outer conducting member, a composite internal coaxial conductor comprising a plurality of conducting and insulating laminations including an inner conducting layer, an outer conducting layer, and one or more intermediate conducting layers, and means for transposing said conducting laminations at intervals along the transmission line length, the means for transposing the said inner layer with one of said intermediate layers being spaced axially from the means for transposing one of said intermediate layers with the said outer layer.
2. In an electromagnetic wave transmission. system for use over a predetermined range of wavelengths, an extended coaxialv transmission line as claimed in claim l wherein the'means for transposing said conducting laminations comprises a plurality of substantially flat conducting tabs having a greater width than thickness.
3; In an electromagnetic wave transmission system for usel over a predetermined range of wavelengths, au extended coaxial transmission line comprising a continuous outer conducting member, a composite internal coaxial conductor comprising a plurality of conducting and: insulating laminations including an inner conducting layer, an outer conducting layer, and one or more intermediate conducting layers, means for transposing said conducting laminations at intervals along the transmission line length, the means for transposing the said inner layer with one of saidintermediate. layers being spaced axially from the means. for transposing one of said intermediate layers with thel said outer layer, and means including a signal source for launching wave energy onto said conducting laminations in parallel.
References Cited` in the fileV of this patent UNITEDf STATES PATENTS 2,115,761 Blumlein May 3, 1938 2,416,790 Barrow Mar. 4, 1947 2,684,993 Bowers July 27, 1954 FOREIGN PATENTS 272,407 Great Britain June 16, 1927V