Hybrid Mode Electric Transmission Line Using Accentuated Asymmetrical Dual Surface Waves

Abstract

A method and means for reducing net transmission losses and frequency dispersion within an electrical transmission line having dual conducting surfaces and carrying a hybrid mode electromagnetic wave comprising both a TEM component and a dual surface wave component. One of the two conducting surfaces is caused to have more surface resistance than the other by a predetermined amount thereby making the surface wave part of the field asymmetric and enhancing the energy particularly associated with the surface wave to the detriment of the other existing components such as the TEM field. The predetermined amount of surface reactance is controlled to reduce the product of transverse attenuation and phase-change coefficients for the hybrid wave thereby causing a reduction in its overall axial attenuation coefficient in the direction of propogation. Thus the wave is slowed down by the dielectric loading, its phase-change coefficient is increased and made more nearly proportional directly to frequency.

Parent Case Text

The subject matter of this specification constitutes a continuation-in-part of the subject matter of my previous application, Ser. No. 672,560, filed Oct. 3, 1967, now abandoned, which also relates to an electric transmission line.

Description

Minimizing net electrical transmission line losses and reducing frequency dispersion has long been recognized as an important and worthwhile goal in the electrical art. These objectives are often given special emphasis wherever long transmission distance and/or large amounts of power are involved to make such minimization economically imperative. However, before this invention, such efforts at minimization were largely restricted to increasing the conductivity of conductors, increasing the cross-sectional area of conductors, increasing the effective skin depth of electrical current in conductors, providing special line terminations and adding distributed or lumped inductive loading to the line. All of these previous efforts to minimize losses were made in the art while it was generally believed that the only significant mode of propagation in dual conducting surface line was a simple TEM mode.

It has now been discovered that the normal transmission mode of electromagnetic energy in a dual conducting surface transmission line, for example, a strip line or a coaxial cable, is, contrary to generally accepted belief, an overall hybrid mode and not a simple TEM mode. This hybrid mode comprises a combination of at least a dual surface wave field and a TEM field with the magnitude of the former depending to a large extent on the surface reactance of the two conducting surfaces. If, as is usually the case, the surface reactances of the two conducting surfaces are equal, then with a symmetrical disposition of conductors there will be a dual surface wave mode with fields of equal magnitude associated with each of the two conducting surfaces.

This invention is based upon the discovery that the net attenuation of electrical energy propagated along a two conductor line and the departure from the required linear relationship between phase-change coefficient and frequency may be reduced by purposefully increasing the surface reactance of one of the two conducting surfaces by a controlled amount. This controlled increase enhances the dual surface wave fields associated with the one surface having increased surface reactance to the detriment of other existing propagation modes thereby producing an asymmetrical hybrid wave which has been found to cause appreciably less net attenuation and less distortion than if no such asymmetry exists, provided that the amount of increased surface reactance is carefully controlled and kept within certain ranges or limits. In the preferred embodiment of this invention, the degree of asymmetry produced is such that the hybrid wave contour of minimum axial electric field occurs as close as possible. to the conducting surface opposite the one which is provided with increased surface reactance.

In other words, it is now established that a hybrid wave propagation of energy is not only unavoidable in any parallel conductor electrical transmission system but that the deliberate accentuation of the surface wave content thereof, to a limited and controlled extent, can be very beneficial. This technique, is, of course, applicable to the standard two-wire transmission line normally used for the lower poser frequencies as well as other lines such as coaxial and strip lines which are used for higher electrical frequencies.

The essence of this invention is based on the application of just sufficient reactive loading (achieved in the preferred embodiment with a thin dielectric coating) to one of the conductors thus enhancing and making asymmetrical the surface wave mode content up to the point where the overall net line attenuation and distortion is a minimum or thereabouts. In the preferred embodiment, this requires only a very thin layer of dielectric coating. If an excessive thickness of dielectric or other reactive loading is utilized, the net line attenuation will be increased from a minimum and, in fact, will eventually exceed the attenuation of the line without any coating or loading at all.

It has also been discovered that the production of an asymmetrical surface wave on a two conductor line by the previously described principles tends, at the same time, to reduce the undesirable frequency dispersion characteristics of the resulting line by making the axial propagation phase-change coefficient almost directly proportional to frequency.

The effect of adding a controlled amount of surface reactance to one of the two conducting surfaces (obtained in the preferred embodiment by a thin dielectric coating) causes very significant and complex changes in field distributions of electromagnetic fields within the space between the conducting surfaces. Over at least a small range of added reactance values, the result of such complex changes has been found to be a reduction in the product of transverse attenuation and transverse phase-change coefficients for the hybrid wave with enhanced asymmetrical surface wave content. At the same time, many other parameters of the hybrid wave are also changed. For example, the longitudinal or axial hybrid wave impedance and the longitudinal or axial phase-change coefficient are both increased and, as previously pointed out, the axial phase-change coefficient is also made more nearly proportional to frequency thereby producing a more desirable frequency dispersion characteristic.

A particularly important result of these changes is a reduction in the product of transverse attenuation and phase-change coefficients since the overall attenuation in the axial propagation direction of the hybrid wave with enhanced surface wave component is proportional to and thus directly dependent upon this product divided by the axial phase-change coefficient. Thus, over a predetermined range of added surface reactance, a controlled enhancement of the surface wave content increases the proportion of total energy propagating in that mode and may result in a reduced attenuation of the energy propagating in that mode. Thus, by carefully controlling the amount of added surface reactance, not only is the dispersion less but the reduction in overall net attenuation can exceed any additional losses that may be caused by the introduction of the added dielectric medium or other reactive loading. Actually, the transverse attenuation coefficient increases with added surface reactance while the transverse phase-change coefficient falls rapidly and the product is reduced over a limited range of surface reactance loading.

While in the prior art surface reactance has been added to a single conducting surface for the purpose of trapping a surface wave mode in the close proximity of the conducting surface and thus providing a single conductor surface wave transmission line, the amount of added surface reactance used for these single conductor surface wave transmission lines is greatly in excess of the limited range of surface reactance loading which may be profitably utilized with this invention. In fact, the excessive surface reactance loading required to achieve a single conductor surface wave transmission line would, in most cases, actually increase the net line attenuation in a dual surface transmission line supporting hybrid modes of propagation.

Accordingly, it is an object of this invention to provide means for producing an asymmetrical TEM dual surface wave on a two conductor line by adding a controlled amount of surface reactance to one conductor, the added reactance comprising a thin coating of a low-loss dielectric medium on the one conducting surface with the coating relative permittivity and thickness being carefully controlled to insure a greater decrease in attenuation (due to the resulting asymmetrical surface wave) than increase in attenutation (due to added losses in the coating medium) thereby insuring a reduced overall net line attenuation and at the same time reduced frequency dispersion.

A further object of this invention is to provide means for producing an asymmetrical hybrid TEM dual surface wave by adding a controlled amount of surface reactance to one conductor in the form of corrugations or grooves substantially at right angles to the direction of the hybrid wave propagation. These grooves preferably have a depth much less than a quarter wave length such that the surface reactance is of the same order as that employed with the dielectric coated conductor previously described.

It is also an object of this invention to apply the foregoing principles to standard a.c. power (50-60 Hz.). dual conductor transmission lines by using on one of the line conductors a controlled coating of a dielectric medium having a very high permittivity, as for example, a ferrite material.

Another object of this invention is application of the above principles to resonators where such resonators comprise a length of transmission line embodying the invention as previously discussed together with electrical reflectors or short-circuits at opposite ends thereof.

A more complete understanding of this invention may be obtained by carefully studying the following detailed explanation and the accompanying drawings of which:

FIG. 1 illustrates in cross-section a coaxial cable embodying this invention,

FIG. 2 is an axial cross-section of the cable illustrated in FIG. 1,

FIG. 3 is a graph generally showing the relationship between net line attenuation and thickness of the dielectric coating or other equivalent surface reactance,

FIG. 4 shows a spiral wrapping of a sandwiched dielectric material about a conductor according to this invention,

FIG. 5 is a detailed cross-sectional view of the sandwiched material taken along line 5-5 in FIG. 4,

FIG. 6 illustrates an axial cross-section of the center conductor of an alternate embodiment of this invention utilizing a corrugated or grooved conductor surface to achieve increases surface reactance,

FIG. 7 illustrates an end view of a low-frequency power cable embodying this invention,

FIG. 8 illustrates a plan view of the power cable shown in FIG. 7,

FIG. 9 illustrates a resonator constructed according to the principles of this invention, and pp FIG. 10 is a graph of experimentally determined approximately optimum dielectric thickness versus operating frequency.

The standard coaxial dual conductor line having an inner conductor 10 surrounded by an outer conductor 12 as shown in FIGS. 1 and 2 (for the moment disregarding dielectric coating 14) has long been known to support propagation of energy in a TEM mode characterized by radial electrical fields and circumferential magnetic fields. With this invention it has been discovered that the propagation of energy along such a dual conductor line as shown in FIGS. 1 and 2 actually includes significant surface wave modes as well as the long-accepted TEM mode which together constitute a resulting hybrid wave. It has been further discovered that, by a controlled enhancement of the magnitude of the surface wave fields associated with one of the two conducting surfaces, the overall net attenuation of hybrid mode energy propagated along the line may be reduced, while the phase velocity of propagation becomes substantially constant at all operating frequencies.

Referring now to FIG. 1, there is shown, in cross-section, a coaxial cable embodying this invention. Inner conductor 10 has a coating 14 with a thickness t of a dielectric material. If desired, the cable illustrated in FIG. 1 may be air-spaced in which case the space volume existing between dielectric coating 14 and outer conductor 12 would be filled with air or some inert gas and the conductor 10 would be held in proper spaced-apart relationship from outer conductor 12 by insulating spacers 16 as shown by dotted lines in FIG. 2.

It has been discovered that when a high frequency wave is launched into a cable as illustrated by FIGS. 1 and 2, the total energy propagation is in fact in the form of a hybrid mode comprising both a TEM mode and a dual surface wave mode having surface wave fields associated with each of the two conducting surfaces. The effect of dielectric coating 14, which increases the surface reactance of conductor 10, is to cause the surface wave field associated with conducting surface 10 to be enhanced, thus causing the total dual surface wave mode to become increasingly asymmetric. That is, more of the total propagated energy is caused to travel in the surface wave field associated with conductor 10 than before. As illustrated by the electric field pattern shown in FIG. 2, electric field 18 associated with conducting surface 10 is of a much greater magnitude than electric field 20 which is associated with conductor 12. At some point between these two electric fields there is a contour of minimum axial electric field which is preferably caused to be as close to conductor 12 as possible.

It has been also discovered that, as the thickness t of coating 14 is increased from zero value upwards, the net attenuation of axially propagating energy within the cable falls to a minimum and thereafter increases. A general indication of this relationship between an axial attenuation coefficient .alpha. and thickness t of coating 14 is shown in FIG. 3.

The thickness t of the coating 14 required to minimize net line attenuation of axial propagating energy is dependent upon many parameters including the relative permittivity of the dielectric coating and its value with respect to the permittivity of the main dielectric medium existing between dielectric coating 14 and outer conductor 12 and the frequency range of propagating waves. Although, as yet, no rigorously derived mathematical formulas have been shown to predict the optimum value of thickness t or other reactive loading, the following table of operating frequency versus approximate optimum thickness of dielectric coating has been discovered by an experiment in which the relative permittivity of the dielectric coating 14 is approximately two or three times greater than the relative permittivity of the surrounding dielectric medium of the transmission line.

Approximate Optimum Operating Frequency Thickness of Dielec- tric Coating __________________________________________________________________________ 10 Ghz. 0.0003-0.0015 inches 3 Ghz. 0.0005-0.0030 inches 100-300 Mhz. 0.0050-0.0080 inches 1-30 Mhz. 0.0100-0.0300 inches __________________________________________________________________________

The information given in the above table is presented in graphical form in FIG. 10 with operating frequency shown on a horizontal logarithmic scale and thickness shown by a linear vertical scale. Operation within the cross-hatched region will result in reduced net line attenuation as taught by this invention.

If the main dielectric is solid polythene, the inner conductor must be coated with a low-loss dielectric having a relative permittivity between 7 and 8, or thereabouts while if the main dielectric medium of the line is air, the coating may comprise polythene or polystrene. Instead of coating the inner conductor 10 with dielectric medium, the inner surface of outer conductor 12 may be coated, but it is preferable from the point of view of simplicity in manufacture to coat the inner conductor as shown in FIG. 2.

In the case of a cable filled with a solid dielectric medium where, as previously noted, it is necessary for the coating 14 to have a higher permittivity than the main dielectric medium, suitable materials for such a coating 14 are polystyrene loaded with titanium dioxide, calcium titanate, or strontium titanate. These materials may be applied as a coating to the inner conductor preferably by extrusion or as shown in FIGS. 4 and 5 by crushing the loading material into a powdered form and sandwiching the powder 22 between two layers of insulating flexible tape 24, for example, cellulose tape such as "Sellotape." The resulting sandwich 26 of flexible tape and powdered dielectric material is then wound spirally about inner conductor 10 to provide the necessary coating 14 as shown in FIG. 4.

In accordance with another embodiment of this invention shown in FIG. 6, corrugations or grooves 28 may be provided in the surface of one of the conducting surfaces (inner conductor 10 as shown in FIG. 6) instead of providing a coating such as coating 14 to increase the surface reactance. These corrugations or grooves 28 are preferably about the inner conductor if the transmission line is coaxial as shown in FIGS. 1 and 2. The circumferential grooves or channels 28 shown in FIG. 6 have effective depths much less than a quarter wave-length thereby producing a surface reactance comparable to that produced by the dielectric coating previously considered. The ratio of slot width b to slot pitch B is such that b /B is equal very nearly to the ratio of slot reactance to surface reactance. A cable constructed as described and illustrated in FIG. 6 behaves similarly to the cable illustrated in FIGS. 1 and 2 which utilizes the dielectric coating 14 in lieu of such corrugations or grooves to increase the surface reactance of conductor 10.

Obviously an advantage of transmission lines embodying this invention as described above is that since the overall net axial propagation attenuation of the line may be less than that of the usual type of line carrying a quasi-TEM wave, (i.e., one with most of the power in a TEM mode as would be the case without any surface wave accentuating means), the number of repeaters that are required when this form of line is used for a long distance signal transmission, as in a submarine cable, is reduced. Furthermore, these new lines result in noticeably less frequency dispersion than in a conventional cable so that the requirements for equalizing networks are reduced and the cables may be operated over wider frequency bands.

The previously discussed transmission lines are, of course, only representative of any dual surface electric transmission line having two conducting surfaces, such as, for example, coaxial lines, strip lines, twin lines, etc. While these transmission lines have their main application in the transmission of signals at hf and higher frequencies, the invention is also applicable to the transmission of power usually at frequencies of only 50 or 60 Hz. However, in order to increase sufficiently the surface reactance of a conductor at these low frequencies without utilizing excessively thick dielectric coatings, materials having very high permittivities are required. One suitable material is manganese zinc ferrite which has a relative permittivity of about 10.sup. 5, a relative permeability of 10.sup. 3 and a conductivity in the range of 3 .times. 10.sup.-.sup. 6 mho/m giving a loss tangent of 10.sup.-.sup. 2.

Because these ferrites are hard, brittle materials, the coating of a conductor with such materials may be in the form of a series of individual axially spaced apart rings of the material thus enabling the resulting reactively loaded cable to maintain the required degree of mechanical flexibility without damage to the dielectric material. Alternatively, the ferrite material may be sandwiched in powdered form between supporting flexible tapes, as previously discussed for higher frequency lines and depicted in FIGS. 4 and 5. The use of such sandwiched tapes wound spirally about the conductor will also result in less longitudinal or axial induced current within the ferrite material than with the ring configuration.

It is desirable to insure that as high a proportion of the total energy as possible is contained in the surface wave content of the field rather than in the TEM part and at 50 Hz. this feature is best achieved either by a pair of spaced apart conductors of circular cross-section or by a coaxial configuration of conductors. An example illustrating the first-mentioned arrangement for power frequency transmission lines is shown in FIGS. 7 and 8 in which two parallel spaced-apart conductors 30 and 32 of circular cross-section constitute a high power transmission line. Conductor 30 is coated with a series of axially aligned, individually spaced-apart rings 34 of manganese zinc ferrite.

Of course, in any of the previously described embodiments, it is permissible to apply surface reactance loading to both of the two conducting surfaces but in unequal amounts such that the requisite asymmetric TEM surface wave field is still produced by the supporting surfaces; however, it is preferable that only one of the conductors is coated or otherwise loaded with additional surface reactance.

One application for the low-loss transmission line of this invention is in a resonator as shown in FIG. 9. Here a fixed length l of electric transmission line constructed according to this invention is terminated at both ends by electrical reflectors or short-circuits 36 and 38. Electrical energy may be transferred to and from the resonator 40 by way of line 42 through reflector 38. As is usual with resonators of this type, resonance will be observed at a plurality of frequencies having wavelengths in the medium of resonator 40 corresponding to 2(1/n ) where n is an integer.

Although only a few embodiments of this invention have been specifically described above, it should be appreciated that this invention encompasses the whole of a dramatic new discovery of means for reducing the net line attenuation and dispersion of dual conductor electric transmission lines by providing surface-wave accentuating means for causing a significant portion of the total propagated energy to be in the form of an asymmetrical surface wave rather than in the usual TEM mode and for controlling this means both to cause a greater reduction in net line attenuation than any increase, thereof caused by the presence of said means and to cause a wider range of frequencies to travel along the line with the same phase velocity. Accordingly, many possible modifications of the disclosed embodiments are within the scope of this invention.

Claims

1. In a hybrid mode dual surface electrical transmission line, a new use for dielectric material on one conducting surface of said line which is of the type that inherently transmits a hybrid wave comprising at least a TEM mode and a dual surface wave mode and which has a pair of spaced-apart generally parallel conducting surfaces with a second dielectric material in between, said new use comprising the method steps of:

adding to said one conducting surface a predetermined thickness of the first-mentioned dielectric material having a permittivity greater than said second dielectric material to increase the surface reactance of said one surface and to product an accentuated asymmetrical dual surface wave mode thereby reducing the dispersion and attenuation of the resulting hybrid mode while simultaneously introducing additional line losses caused by the addition of said first dielectric material, and

controlling the predetermined thickness of said first dielectric material to produce a substantially constant phase velocity over a relatively wide frequency range and to produce an increase in overall line attenuation due to said additional losses that is significantly less than an accompanying reduction in overall line attenuation due to said accentuated asymmetrical dual surface wave whereby a net reduction in overall line attenuation is

2. A new use method as in claim 1, wherein said thickness controlling step

3. A method for decreasing frequency dispersion and net line attenuation in a two conductor transmission line system comprising a pair of spaced-apart generally parallel conducting surfaces having a dielectric medium disposed between said surfaces wherein a hybrid wave inherently exists having both TEM and dual surface wave components, said method comprising the step of

introducing a predetermined thickness of a further dielectric medium along the length of one of said conducting surface, said predetermined thickness and the relative permittivity of said further dielectric material being chosen to enhance the dual surface wave component relative to the TEM component and to make said dual surface wave component asymmetric whereby the frequency dispersion and net line attenuation of the overall hybrid mode are decreased in spite of added losses introduced by the addition of said further dielectric medium.