US 2848696 A
Description (OCR text may contain errors)
S. E. MILLER ELECTROMAGNETIC WAVE TRANSMISSION Aug. 19, 195s 2 Sheets-Sheet 1 Filed March 15, 1954 var aff A /NI/ENOR 5. E. M/LLER By @f4 ATTORNEY Aug, 19, 1958 5E. MILLER 2,848,696
ELECTROMAGNETIC WAVE TRANSMISSION Filed March 15, 1954 CONDUCTOR W/TH OX/DED SURFACE LOSS L O W D/ELE C TR/C WEA THE R PROOF SHIELD H/GH LOSS SPR/NG MA TER/AL MATER/AL STEEL HEA 7'/NG ELEMENT /A/l/EA/ TOR 5. E. M/LLER ATTORA/EV 2. Sheets-SneekI 2 nLncrnoMaoNnric ivi/Avn rnANsr/nssioN Stewart E. Miller, Middletown, N. 5., assigner to Beil Telephone Laboratories, incorporated, New York, N. Y., a corporation of New York Appiication March 15, 1954, Serial No. 416,316
1 Qlaim. (Ci. 333-95) This invention relates to electromagnetic wave transmission systems, and more particularly, to the transmission of the circular electric or TEM mode of wave propagation over long distances in a guided wave transmission system which either through choice or inherency does not follow a perfectly straight path.
The propagation of microwave energy in the form of T501 waves in circular wave guides is ideally suited for the long distance transmission of wide band signals since the attenuation characteristic of this transmission mode,
unlike that of all other modes, decreases with increasing frequency. However, one difficulty with this method of transmission is that the TEM mode is not the dominant mode supported in a circular wave guide, and consequently energy may be lost to other modes also capable of transmission therein. In an ideal wave guide which is perfectly straight, uniform and conducting, the propagation of TEM waves therethrough is undisturbed, but slight imperfections in the guide and especially curvature of the wave-guide axis may excite waves of other :modes and produce serious losses. These losses are attributed mainly to the fact that the bending of the guide produces a coupling between the desired TBM and other transmission modes, mainly the TMll mode.
Recognizing that the coupling between these modes may be likened to the coupling between traveling waves on coupled transmission lines in that an exchange of energy will take place between the waves when they travel together at the same phase velocity of propagation, the prior art has provided a large number of devices for negotiating bends or turns in the guides. Thus, the phase velocity of the TMll mode (which is normally equal to that of the TEM mode) is changed relative to that of the TEM mode, to increase the relative differences in their propagation constants and to reduce the effective coupling therebetween.
Of the several prior art devices operating according to this principle, one of the most satisfactory is described as a spaced ring line. It comprises a plurality of conductive ring-shaped sections, coaxially arranged, and successively spaced from each other at a uniform distance by insulating material. As will be considered in more detail hereinafter, the desired circular electric TEM mode produces circumferential currents which are not appreciahly interfered with by the ring structure. The undesired modes produce predominantly longitudinal currents which are seriously affected by the division of the guide into the short cylindrical sections and therefore these modes are given velocities of propagation different from the TEM mode. While this structure is very satis factory for a short section of line and may be used at a specifically contemplated bend, it is too expensive and too ditdcult to fabricate as a long section to be used as the transmission line itself and thus to include within its length all bends that cannot be anticipated.
It is therefore an object of the present invention to Lil 2,848,696' Patented Aug. 19, 1958 transmit circular electric mode wave energy over long distances without its degenerating into other modes.
It is a further object of the invention to provide a circular electric Wave mode transmission medium which may be economically fabricated in long lengths.
In the copending application of I. R. Pierce, Serial No. 416,315, filed March 15, 1954, it is disclosed that a helical conductor of diameter greater than 1.2 free space wavelengths will propagate a properly excited circular electric TEM mode with a different phase constant than the TMll mode. This provides a substantial decoupling between these modes, and is in addition, a structure that is easily and economically wound in sections 0f arbitrarily long lengths. In accordance with the present invention, the respective TEM and TMm modes propagated along the resulting helical transmission path are exposed in a special way to electrically dissipative or lossy material so that they have substantially different attenuation constants. It will be shown that if the difference between the attenuation constants is large, the coupling between the TEM and TMm modes may be made arbitrarily small, resulting in a minimum of degeneration of the TEM mode, without however, any substantial amount of energy being actually lost in the dissipative material.
In the specic illustrative embodiments of the invention to be described in detail hereinafter the dissipative material comprises a cylinder-like casing having an inside surface contiguous with the outermost part of the helical conductor. Thus the longitudinal currents of the TM mode must pass through the material, giving to the wave a large attenuation constant while the component of the circumferential currents of the TEM mode which follows the helix conductor is unaffected.
Features of the invention reside in the particular physical arrangements of these components and in the manner of assembly thereof.
These and other objects, the nature of the present invention, and its various features and advantages, will appear more fully upon consideration of the Various specitic illustrative embodiments shown in the accompanying drawings and analyzed in the following detailed description of these drawings.
In the drawings:
Fig. l -diagrammatically illustrates a guided microwave communication system employing the circular electric wave and having a long distance transmission line of the type provided by the present invention;
Figs. 2 and 3 illustrate the transverse electric and magnetic field patterns of the TEM and TMm waves, respectively;
Fig. 4, partially in cross section, illustrates the construction of a small section of transmission line in accordance with the present invention;
Figs. 5 and 6 are cross-sectional views showing first and second alternative constructions of sections of transmission line in accordance with the invention; and
Fig. 7 shows the structure and also one method of assembling the structure of another alternative embodiment of the invention.
Referring more specifically to Fig. 1, a long distance guided microwave communication system is schematically shown. The system is characterized as long to distinguish it from the short distances found in terminal equipment and to define a system in which the factor of transmission attenuation becomes relatively important. The length of such a system would be measured in terms of thousands of feet and perhaps miles as opposed to several inches or a few feet in the terminal equipment. This System comprises a terminal station 11 which may be a transmitter, or if this is an intermediate station, a repeater 11 which is to be connected to a receiver or subsequent repeater comprising station 12. The circular electric TEol mode is the mode in which energy is transmitted between stations and since this mode is not usually produced or utilized directly in the components of a station, transducers 13 and 14 are interposed between stations V11 and 12 and the long distance transmission line 15. Transducers 13 and 14 may be of any suitable well known `types for'converting TEM wave energy to and from a dominant wave mode conguration. For example, they may be structures of the types disclosed in United States Patent 2,656,513 granted to A. P. King, October 20, 1953, or of the copending applications of S. E. Millen'Serial No. 245,210, filed September 5, 1951, which on May 29, 19.56, issued as United VStates Patent 2,748,35Q, and S. E. Miller, Serial No. 357,665, led May 27, 19,53.
lLine, 15 is not completely straight along its entire length since in practical installation it is substantially impossible to maintain the line along a precisely straight path over a long distance. Intentional bends may also he included in order that the linev may follow right of ways .or turn corners. It is these bends that produce the characteristic moding or degeneration of the TEM into TMH wave power. As noted above, this moding is ascribed to the fact that these waves have substantially the same phase constants, i. e., phase velocity and wavelength and, therefore, interact Strongly in a manner analogous to coupled transmission lines.
Fig. 2 illustrates the distribution of the electric and magnetic fields in a transverse section of a circular conductive -wave guide supporting the TEol transmission mode. This wave i s designated the circular electric type inasmuch as the electric eld, shown by the solid lines 16, consists of circular lines coaxial with the guide and lying transversely thereto Without any longitudinal ccmponents. The transverse component of the magnetic field, indicated by the dotted lines 17, forms at variouspoints along the guide in a radial pattern. The electric field intensity attains a maximum approximately half Way between the axis and surface of the guide and drops to zero at the surface. The current ow associated with the TEO! wave is predominantly circular around the per riphery of the guide as illustrated in Fig. 2.V
The configuration of the transverse TMH mode is shown in Fis. .3 and is Similar t that 0f a Shielded ccnductor pair. The magnetic field pattern is entirely transverse without any longitudinal components and is indicated by the dotted lines 13 encircling the respective poles P and P. Since the magnetic lines must form closed paths, they tend to spread out near the center of the guide and to crown close together at the inner surface mostly near the axis passing through poles P and P', thus inducing a considerable longitudinal conduction of current in the wall of the guide. The direction of this current flow is shown conventionally by the symbols 19 and 20 on Fig. 3.
Fig. 4 shows in detail a short section of the transmission line 15 of Fig. 1. Thus line 15 comprises a conductor 41 wound in a helix having an internal diameter d. Conductor 41 may be solid or stranded and may comprise a base metal such as iron or steel plated by a highly conductive material such as copper or silver. Adjacent turns such as 42 or 43 of the helix are electrically insulated from each other, and this may be provided by a small air gap such as 44. The pitch distance ot"V the helix, i'.V e., the distance between Vthe center of turns 4,2 and 43, and therefore the pitch angle of the helix, should be as small as consistent with the aboveL mentioned insulating' requirement. This distance in all events must be less than one quarter wavelength and is kpreferably such that theY gap 44 between adjacent turns is less.- than.
the diameter ofl conductor 41.
The space between adjacent turns of helix 41V, i. e.,-
gap 44, is exposed to electrically dissipative or lossy. material.
This may bey done by'enclosing helix 41 in,y a 75 substantially equal to the outside diameter of helixV 41. Casing 45 also serves as a protective and supporting structure for helix 41 and may therefore be either semi-rigid, forming a more or less permanent structure, or pliable, forming a flexible one.
Casing 45 may then be covered with'a nonfcorrosive conductive shield 46 which serves to protect the line from outside mechanical influences such as weather, moisture and insects and from electrical influences such as stray radiation from adjacent transmission lines. lt is desirable that the cross Section of helix 41 be maintained as nearly circular as possible. This condition may be maintained by the resilience of casings 45 and 46. It may, however, be maintained by employing for the con ductor of helix 41 a spiral of spring steel plated by a highly conductive material. Thus the effect of the spring itself will maintain the desired circularity and shield 45 may be wound of overlapping, thin strips or may be made of a woven braid.
The inside diameter ci of helix 41 is related to the i diameter of the circular pipe guide which would transmit waves of the same frequency. Thus the diameter d must be greater than the critical or cut-off diameter for the TEM mode in a circular guide. is equal to 1.22%, where )to is the wavelength in free space of the longest wave in the transmission band. In
Vpractice d might be in the range 1.25 to 1() times the smallcomponent of the wave is presented with a small reactance caused by the discontinuity:between adjacent turns. This reactance will have the eifect of changing the phase velocity of the total wave very slightly. Very little of the total TEM current willpass through the resistive material of casing 45 and therefore the attenuation constant of the TEM Wave is changed very little.
The TMm mode, however, has a predominantly longitudinal current tlow along the wave-guiding path and will be seriously affected by the discontinuity between adjacent turns of helix 41. Not only is the phase constant of this mode increased by the `reactance of each discontinuity, but the longitudinal currents are forced to ow through the dissipative material of casing 45. lt may be shown that the amount of degeneration of the TEM mode into the TMll mode along a bent section of waveguide is proportional to the coeicient of coupling between these two modes along such bend and may be expressed C: RA.,
in which r is the radius of the circular wave-guiding path, )to is the free space wavelength of the wave energy,
and R is the radius of the bend. While this expression not depart substantially from results predicted bythisk expression.
This cut-off diameter Y The loss to the TEM wave in a bend is governed by the ratio is much greater than unity, that the attenuation coecient for the TED; Wave in a bend is equal to 2 ani-i' C rThus it is seen that the quantity al1-aol is made large by increasing the factor an much more than the factor am, the amount of degeneration and loss for T E01 may be made arbitrarily small. Therefore the loss of casing 45 should be intentionally made as large as possible.
As indicated hereinbefore it is desirable that adjacent turns of the helix be as close together as consistent with the requirement that they be insulated from each other. A novel embodiment of the invention is shown in Fig. by which both of these requirements are met. Thus in Fig. 5 helix 51 is illustrated as being Wound of a conducting material of the type that forms a high resistance oxide coating upon the surfaces thereof exposed to air, such as aluminum. Thus an oxide coating 52 forms upon helix 51 which provides a uniform insulating separation between adjacent turns. A principal advantage of this structure is that if the transmission line is damaged in any Way, i. e., by too sharp a bend or by prolonged friction between the helix turns, the oxide insulating layer will be self healing. Furthermore, the necessary spacing between turns may be made very small. Lossy casing 53 of the type already described is laid over helix 51. Since materials such as aluminum have little resilience, a helix 54 of spring steel may be wound over casing 53 to maintain the circularity of helix 51. An outer weather proof casing 55 may then form the outside shield.
In Fig. 6 another embodiment of the invention is shown that is suitable for application where unusual mechanical strength is required of the transmission line, such as in an underground or underwater application. 'Ihus the line comprises a solid core 61 of low loss dielectric material. Core 61 may be extruded from a semiflexible material such as polyethylene. Helix 62 is wound directly upon core 61. A high loss casing 63, in accordance with the invention is applied over helix 62.
In Fig. 7 a novel method and apparatus for assembling a transmission line is shown in diagrammatic form. 'Thus the conductive helix 71 is wound upon a suitably held mandrel 72 of diameter equal to the required internal diameter of the helix. Over this is spirally wound a Cil 6 tape or ribbon 75 of the lossy, resistive material suspended in a binder of thermal setting plastic such as Minnesota Mining and Manufacturing Company type EC-880. Over this is Wound the supporting spring 73 or protective casing or both as a particular application may require. As this assembled combination is slipped or drawn from the free end 76 of mandrel 72, it is passed through an oven or near a heating element 74, which causes successive turns of ribbon 75 to melt, fuse or blend together to form a substantially unitary cylindrical casing. Alternatively, a ribbon of lossy, resistive material may be coated on one or both sides by a thermal bonding adhesive before it is wound. Upon heating, the adhesive binds all of the assembled components together.
It should be noted that While the conductive and supporting helical members in the above described embodiments have been illustrated as having substantially circular cross-sections certain advantages have been found to making these members of elongated materials of rectangular cross section. In particular the helix should be wound with the Wide dimension of the rectangular cross section extending parallel to the axis of the helix. This provides a maximum of longitudinal coverage along the helix with a minimum of the material being used.
In all cases it is understood that the above described arrangements are illustrative of a small number of the many possible specic embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can readily be devised in accordance with these principles by those skilled in the art Without departing from the spirit and scope of the invention.
What is claimed is:
In an electromagnetic wave transmission system, means for producing the circular electric mode of said wave energy, means for utilizing said circular electric Wave energy, transmission means connecting said utilizing means to said producing means comprising an elongated member of conductive material wound in a substantially helical form and a jacket of electrically dissipative material surrounding said conductive material, said helix having a diameter greater than one Wavelength of said Wave energy, said helix having adjacent turns thereof electrically insulated from each other to expose all longitudinal current components to said electrically dissipative material at least once every quarter wavelength of high frequency wave energy propagated along said transmission means and spaced substantially less than one quarter Wavelength to expose circumferential current components of said circular electric mode to said dissipative material to a substantially smaller degree than the exposure of said longitudinal current components.
References Cited in the tile of this patent UNITED STATES PATENTS 2,416,177 Hollingsworth Feb. 18, 1947 2,626,371 Barnett et al. lan. 20, 1953 2,779,006 Albersheim Jan. 22, 1957 FOREIGN PATENTS 427,599 France June 1, 1911 869,734 France Nov. ,17, 1941