US3016503A

US3016503A - Helix wave guide

Info

Publication number: US3016503A
Application number: US862665A
Authority: US
Inventors: John R Pierce
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1959-12-29
Filing date: 1959-12-29
Publication date: 1962-01-09
Anticipated expiration: 1979-01-09
Also published as: BE598551A; DE1149764B; NL258016A; GB903617A; NL130387C

Description

Jan. 9, p RcE HELIX WAVE GUIDE Filed Dec. 29, 1959 RECEIVER on REPEATAR TRANSMITTER T5 0!? MP TRANSDUCER FIG. 2

INVENTOR J. R. PIERCE A TTOPNEJ 3,di6,5a3 Patented Jan. 9,1952

3,016,503 HELIX WAVE GUIDE John R. Pierce, Berkeley Heights, N.J., assignor to Bell Telephone Laboratories, incorporated, New York, N.Y., a corporation of New York Filed Dec. 29, 1959, Ser. N 862,665 11 (Jlaims. (Cl. 333-95) This invention relates to electromagnetic wave transmission systems and more particularly to an improved form of transmission line for the TE circular electric mode of wave propagation.

United States Patent 2,848,696, issued August 19, 1958 to S. E. Miller, discloses that a closely wound helical conductor of diameter greater than 1.2 free space wavelengths of the transmitted energy is a transmission medium suitable for propagating a properly excited TE mode. Upon this wave guiding structure the TE mode produces wall currents which are circumferentially directed and which are satisfactorily supported by the small pitch helix even though no continuously conductive c'ircumferential path is provided. At the same time, the unwanted modes into which the TE mode has a tendency to degenerate, produce Wall currents which are directed in a longitudinal direction parallel to the guide axis. The dielectric gaps between adjacent helix turns, across which these currents must pass, affect the production and propagation of the unwanted modes. Generally, on the helix guide the TE mode has a phase constant substantially different from that of the TM and other unwanted spurious modes. By virtue of this difference in phase constant, decouping between the modes is provided. Additionally, the helix may be surrounded with a jacket of electrically dissipative material to increase the attenuation constant difference between the TE mode and the unwanted modes and thereby further to reduce the tendency of the TE mode to convert to spurious wave forms.

Sue: a transmission medium is ideally suited for long distance transmission of wide band signals since attenuation of power in the TE mode decreases with increasing frequency. Used in long lengths, the helix type wave guide serves to negotiate both accidentally and intentionally introduced bends and turnswith relatively low conversion loss. Used in shorter lengths, the helix guide acts as a filter to purify the TE energy by attenuating spurious mode components, particularly of the TM and TE modes.

Since an increase in the magnitude of the difference in phase constants and attenuation constants presented by t e guiding structure to the desired mode on the one hand and to the undesired modes on the other hand increases the efficiency of the helix guide by reducing mode conversion, it is desirable to increase this differenceas much as possible.

It is therefore an object of the present invention to reduce by new and improved means the tendency of the circular electric mode in a helix type guide to degenerate into unwanted modes. I

It has become apparent however, that for some applications of the helix guide, the obtainable differences in the propagation constant components and therefore the electricalperformance of the helix guide is below the desired level. Several reasons may be advanced for this limitation. First, the surface impedance presented to the unwanted mode currents by the dielectric jacket surrounding the helix is oftentimes of an undesirable value. Second, the finite size and spacing of the adjacent helix wires create a capacitive grid between the propagating spurious mode wave energy within the guide and the external dielectric jacket, thereby partially shielding the jacket from the wave energy and permitting only partial penetration of the undesired currents into the jacket.

Third, the dielectric constants of available lossy jacket materials are generally too high for effective guide performance.

In the copending application of H. G. Unger, Serial No. 679,929, filed August 23, 1957, it is disclosed that a dielectric separation between helix wires and lossy jacket in a radical direction introduces an inductive effect which may be used to compensate for the capacitance of the wire separation and to transform the impedance presented to unwanted mode currents by the jacket to a more desirable value. In the copending application, the compensating medium takes the form of an isotropic insulating layer of dielectric material of low'to moderate dielectric constant. Such a medium favorably affects the propagation characteristics of the helix guide by changing the impedance presented by the lossy jacket to the longitudinal gap currents associated with the unwanted wave modes. Since the theory of mode discrimination in the helix guide rests upon the provision of a substantially continuous conductive path for TE mode wall currents and a substantially discontinuous conductive path for unwanted mode wall currents, the use of an insulating medium as a dielectric impedance transformer does not offend widely held c011 cepts.

A further object of the invention is to improve the impedance transforming means interposed between the helix and the surrounding lossy jacket of a helix guide.

' In accordance with the present invention, it has been discovered that the performance of a helix wave guide may be improved by surrounding the helix with an anisotropically conductive jacket. As used in this specification the phrase anisotropically conductive will be understood to refer to a heterogeneous anisotropy which produces an inductive effect. That is, the anisotropy arises from discrete conductive and dielectric regions disposed in alternate relation circumferentially about the helix, each such alternate region extending longitudinally parallel to the central axis of thehelix over the extent of the modified guide section. Such an anisotropicallyconductive jacket, by providing an inductive effect as a result of its substantial conductivity in the longitudinal direction and negligible conductivity in the circumferential direction considerably improves the guide performance. This is so even though the significant characteristic difference between the wanted and unwanted Wave modes on a helix guide is the existence of longitudinal currents of the latter modes at the helix wall.

It is therefore a more specific object of the invention to improve helix guide performance by introducing discrete regions of longitudinal conductivity into the media surrounding the helix winding. 1

In accordance with the 'invention, a conductor which is closely wound in helical form with a diameter at least equal to 1.2 free space wavelengths of the energy to be transmitted is provided with a composite external jacket comprising both dielectric media and conductive media. The conductive media are anisotropic in nature; that is, the conductive path provided in a direction parallel to the axis of the helix is continuous for longitudinal currents while the conductive, path provided in a circumferential direction around such axis is discontinuous for circumferential currents.

According to a preferred embodiment of the present invention, the desired longitudinal conductivity is provided by longitudinally extending wires which overlay the exterior surface of the helix winding. These longitudinal wires are in turn surrounded by further media which may be exclusively dielectric, or both dielectric and conductive. The anisotropic conducting jacket serves to transform the impedance of its surrounding media to a value conducive to maximum effect upon the propagating unwanted wave modes and at the same time to com- 3 pensate the inherent capacitive shielding effect of the helix wire grid.

These and other objects, the nature of the present invention, its various features and advantages, will appear more fully upon consideration of the specific illustrative embodiments shown in the accompanying drawing and described in detail below.

In the drawing:

FIG. 1 diagrammatically illustrates a guided microwave communication system employing TE waves and including a long distance helix wave guide section;

FIG. 2 is a partially broken away perspective view of a section of helix guide including an anisotropically conductive jacket in accordance with the invention; and

FIGS. 3 and 4 are perspective views of additional helix guide structures having anisotropically conductive jackets.

Referring more particularly to FIG. 1, a long distance guided microwave communication system is shown in diagrammatic form. The system is characterized as long to distinguish it from the short distances found in terminal equipment and to indicate an application of allhelix guide in a long distance communication system. 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. The system comprises a terminal station 11 which may be a transmitter, or, if this is an intermediate station, a repeater which is to be connected to a receiver or subsequent repeater comprising terminal station 12. The energy to be transmitteed between these terminal stations is in the TE wave mode. It may be the case that this mode is neither produced nor utilized directly in the components of a given station and therefore the

transducers

13, 14 are interposed between the stations 11, 12 and the extremities of long distance helix guide 15. The

transducers

13, 14 may be of any suitable type for converting TE wave energy to and fro-m a dominant mode configuration. For example, they may be structures of the types disclosed in United States Patents 2,748,350 granted May 29, 1956 or 2,848,690 granted August 19, 1958 to S. E. Miller, or in the copending application of E. A. J. Marcatili, 11"., Serial No. 706,459, filed December 31,

. 1957 now US. Patent 2,963,663issued December 6, 1960.

It may also be the case that the TE wave mode is utilized directly in the components of the terminal stations in which case the

transducers

13, 14 would be unnecessary.

FIG. 2 is a partially broken away detailed view of a helix guide 20 in accordance with the invention which may be used as long distance guide section 15 in FIG. 1. Guide comprises an elongated conductive member 21 of relatively fine wire closely wound in a circular helix, surrounded by jackets '23 and 25. Conductor 21 may be a solid or stranded copper wire or it may comprise a metal such as iron -or steel plated with a highly conductive metal such as copper or silver. Adjacent turns of the helix are electrically insulated from each other, and this insulation may be provided by small air gaps 22, as shown, or the adjacent turns may touch, insulation being provided by an enamel or plastic coating on the conductor itself. The pitch distance and pitch angle of the helix, i.e., the distance between the centers of adjacent turns, is preferably as small as is consistent with the insulating requirement. This distance must in all events be less than one-quarter wave-length and is preferably such that the gaps 22 have a width which is less than the diameter of conductor 21.

Surrounding helically-wound conductor 21 and overlaid on the outer surface thereof is anisotropic conducting jacket 23 which, as illustrated, comprises a plurality of conductors or wires 24 which extend longitudinally parallel .to the axis of helix guide 20. Jacket 23 may comprise conductors similar to conductor 21, which are illustrated in FIG. 2 as having a circular transverse cross section, but this is by way of illustration only and is not intended to be limiting. Each of conductors 24 is insulated from the other and from the helical conductor 21. As before, this insulation is most easily accomplished by providing an insulating coating on the surface of each conductor. When the conductors 24 are coated with insulation, they may touch, the insulation providing the spacing between adjacent conductors to permit energy to penetrate beyond the jacket 23. If only conductor 21 is coated, a small air gap must be provided between each of conductors 24. As shown in FIG. 2, jacket 23 comprises a single layer of conductors. It might well be desirable to overlay the helix with a plurality of layers of conductors but, in any event, for best operation of the guide over a broad frequency band, the radial thickness of jacket 23 should not exceed one-quarter wavelength of the highest operating frequency to be transmitted.

Surrounding anisotropic jacket 23 is dielectric jacket 25, which may serve a plurality of different functions. Mechanically, jacket 25 lends support to the helix-longitudinal wire combination and holds the respective component parts thereof in proper relative relationship. If mechanical support isthe sole purpose of jacket 25, it may comprise a cylinder of low loss dielectric material such as polyethylene. However, it is most common in helix guide applications that there be at least one dielectric jacket which is electrically lossy surrounding the helix wires. The term electrically lossy is understood to refer to a material which is capable of converting wave energy incident thereon into heat energy. When jacket 25 is lossy, the attenuation constants of the unwanted modes are considerably increased over those associated with a lossless helix or ordinary copper guide. Typical examples of lossy materials suitable for jacket 25 are carbon loaded plastics, tin-oxide coated glass fibers, and carbon coated paper or string.

In the typical operation of a helix guide as shown in FIG. 2, and incorporated into FIG. 1, wave energy principally in the TE mode but with a finite unwanted mode level enters at one end of guide section 15 and is propagated therealong. The circumferential wall currents set up by the TE mode are carried by the helical conductor 21 whereas the longitudinal wall currents set up by unwanted modes are exposed through gaps 22 to the surrounding

jackets

23, 25 in which. these currents set up radially propagating waves. The discrete regions of longitudinal conductivity presented by jacket 23 causes it to appear as an inductive reactance to the radially propagating modes, thereby compensating for the capacitance of the helical conductor and causing the jacket impedance to assume a value conducive to nearly complete coupling of the energy represented by these modes into the jacket 25. The wave energy within jacket 25 is then absorbed by the loss mechanism within the jacket. When the desired impedance match is attained, wave energy in the TE mode in guide 15 does not easily convert to TM and other unwanted modes since both'the attenuation and phase constant differences between the desired and undesired modes are nonzero. This result follows from the axiom that the tendency to mode conversion is minimized by maximizing the propagation constant diiferences between the wanted and unwanted modes.

FIG. 3 is a perspective view of a helix guide structure 30 having advantages over and structural differences from the guide shown in FIG. 2. Specifically, guide 30 comprises elongated conductive member 31 of relatively fine wire wound in a circular helix, surrounded by an anisotropic jacket 32. Conductor 31 may be a solid or stranded copper wire or. it may comprise a metal such as iron or steel plated with a highly conductive metal such as copper or silver. Adjacent turns of the helix are electrically insulated from each other, and the insulation may be provided by small air gaps 33, as shown, or the adjacent turns may touch, insulation being provided by an enamel. or plastic coating on the conductor itself.

The pitch distance and pitch angle of the helix, i.e., the distance between the centers of adjacent'turns, is preferably assmall as is consistent with the insulating requirement. This distance must in all events be less than one-quarter wavelength and is preferably such that gaps 33 have a width which is less than the diameter of conductor 31.

Surrounding helically-wound conductor 31 is jacket 32 comprising a plurality of conductors 34 each covered with a coating'35 of electrically lossy material. Conductors 34 are preferably metallic and maycorn'prise material similar to that of helical conductor 31. Lossy coatings 35 provide the desired attenuating mechanism within jacket 32. As a specific example, jacket 32 may comprise copper wires which are coated with carbon impregnated paper pulp. Alternatively the metallic wire may be coated with carbon loaded plastics, or any other similar lossy material. Since the conductors 34 should be insulated from one another, coatings 35 should possess dielectric insulating properties in addition to their loss properties.

As shown in FIG. 3 the jacket 32 comprises a plurality of coated conductors wound about the helical conductor 31 with a wide circular pitch; that is, the direction of the central axis of a given one of conductors 34, indicated by line 36, is related by a small angle a to the direction of the central axis of the guide itself, indicated by line 37. In theory, on should be of the order of one or two degrees. However, the inductive effect produced by the conductors varies as cosine a and therefore, in practice or could be increased to twenty-six degrees with a consequent reduction of only ten percent in inductive etfect. Jacket 32 thus exhibits a substantial conductivity in a direction parallel to the axis of guide 30 but very low conductivity in a circumferentialdirection. One particular advantage of this coated conductor type anisotropic jacket lies in its ease of fabrication. Manufacture of guide 30 is facilitated since loss and anisotropic conductivity are imparted to the guide at the same time as jacket 32 is formed by simultaneous winding of the coated conductors about the helix. As illustrated in FIG. 3 jacket 32 comprises only one layer of coated conductors. The attenuating properties of the jacket may be increased without detracting from over-all guide performance by winding a plurality of layers of such conductors over the inner helix.

FIG. 4 is a perspective view of an alternate embodiment of the helix guide shown in FIG. 2 or PEG, 3. As disclosed in the copending application of G. T. Kohman et al., Serial No. 679,835, filed August 23, 1957, now U.S. Patent 2,966,643, issued December 27, 1960, the jacket which surrounds the central helix of a helix type wave guide may comprise successive laminated wrappings of resin impregnated Fiberglas. In FIG. 4 conductors 40 are randomly distributed within the Fiberglas laminations 41 which surround the helically wound conductor 42. Helix 42 and the Fiberglas portion of jacket 41 are proportioned as disclosed in the above-mentioned Kohman et al. application. Conductors 40 are randomly distributed throughout the laminations as the Wrapping process associated with the winding of jacket 41 proceeds. Conductors 40 may extend longitudinally parallel to the axis of the helix guide 43 as in P G. 2. or they may be wound with wide circular pitch, as in FIG. 3. In either case the presence of conductors 40 within jacket 41 imparts longitudinal conductivity to the jacket which substantially improves the electrical transmission characteristics of the ordinary Fiberglas jacketed helix guide,

Certain aspects of the structures shown in FIGS. 3 and 4 are being claimed in the copending application of H. T. Friis-H. G. Unger Serial No. 862,664, filed December 29, 1959.

In all cases it is understood that the above-described arrangements are merely illustrative of the many specific 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:

1. A transmission medium for electromagnetic wave energy in the circular electric wave mode comprising an elongated member of conductive material wound in the form of a helix having a longitudinal axis with adjacent turns of said helix electrically insulated from each other, means exhibiting anisotropic conductivity overlaying the outer surface of said helix, and an electrically lossy dielectric jacket surrounding said means.

2. The medium according to claim 1 in which said means exhibit maximum conductivity in a direction parallel to said axis.

3. The medium according to claim 2 in which said means comprise a layer of metallic conductors extending parallel to said axis.

4. In an electromagnetic wave energy transmission system, the combination of means for launching the circular electric mode of said wave energy, means for receiving said Wave energy, and means interconnecting said launching means and said receiving means, said interconnecting means comprising a conductor wound into a helix having a longitudinal axis with adjacent turns of said helix electrically insulated from each other, and anisotropically conductive means for improving the transmission characteristics of said helix surrounding and contiguous with said helix.

5. The combination according to claim 4 in which said means exhibiting anisotropic conductivity comprise a plurality of conductors extending in a direction parallel to said longitudinal axis.

6. In combination, an elongated member of conductive material wound in a substantially helical form with a helix diameter greater than 1.2 free space wavelengths of said energy and with adjacent turns insulated from one another and spaced apart a distance less than onequarter wavelength of said energy, means for exciting the circular region encompassed by said helix in a desired hollow pipe wave mode whereby unwanted mode currents extending between said adjacent helix turns in the direction of energy propagation through said helix are generated, a dielectric jacket surrounding said helix, and a plurality of elongated conductors extending in the direction of said unwanted mode currents interposed between and in contiguous relationship with said helix and said dielectric jacket.

7. In combination, a high frequency electromagnetic wave energy transmission line comprising conductive means defining a low-loss transmission path having a circular cross section in planes transverse to the direction of transmission of said energy therealong, a medium having anisotropic conductivity surrounding said conductive means along the entire extent thereof, and means for exciting said circular cross section in the circular electric wave mode, said conductive means and said medium being electrically coupled by a plurality of regularly Spaced gaps in said conductive means, the outermost surface of said medium exhibiting dielectric properties.

8. In combination, a helix type wave guide for circular electric mode wave energ an anisotropically conductive layer having maximum conductivity in a direction parallel to the axis of said helix overlaying said helix and a dielectric jacket surrounding said anisotropic layer.

9. The wave guide according to claim 8 in which said dielectric jacket is electrically lossy.

10. A transmission medium for electromagnetic wave energy in the circular electric wave mode comprising a conductive helix with adjacent turns spaced apart and insulated from each other, an electrically lossy medium surrounding said helix and exposed through said spaced turns to mode currents set up at said helix in a direction parallel to the direction of propagation of said wave energy through said transmission medium, and conductive means for providing substantial longitudinal conductivity and substantially zero circumferential conductivity disposed in the region between the outer surface of said helix and the outer surface of said lossy medium; and means for exciting said medium in said wave mode.

11. A transmission medium for wave energy in the circular electric Wave mode comprising an" elongated 10 member of conductive material wound in the form of a helix with adjacent turns electrically insulated from each other, a conductive medium presenting substantially zero conductivity in the circumferential direction surrounding said helix, and an electrically lossy dielectric medium for presenting dissipative resistance to wave energy having current components parallel to the axis of said helix surrounding said conductive medium.

References fitted in the file of this patent UNITED STATES PATENTS 2,197,122 Bowen Apr. 16, 1940 2,610,250 Wheeler Sept. 9, 1952 2,848,696 Miller Aug. 19, 1958 FOREIGN PATENTS 751,153 Great Britain June 27, 1956 OTHER REFERENCES Hosono et al.: IRE Transactions on Microwave The- 15 cry and Techniques, vol. MIT N0. 3, July 1959, pages