US2797393A - Composite wave guide - Google Patents

Description

June 25, 1957 FIG. [3

FIG. /A

ELE C TROMAGNE T/C WAVE CURRENT DENSITY B 2 m F C m m NNN me mTA UAMW mm fin DR PE K R T M a I H m F CURRENT DENS/TY INC/DENT FIG, 3 ELECTROMAGNETIC WAVE REFLECTED OFELECTR/C VECTOR INVENTOR- A. M. CLOGSTON A 7 TOR/VE V ate ireei htas This invention relates to electrical conductors and has as one of its principal objectives the improvement of electrical conductors with respect to skin effect. More specifically, this invention relates to composite conductors employing a multiplicity of spaced, thin, fiat conducting sheets.

This application is a division of application Serial No. 214,393, filed March 7, 1951, which issued as U. S. Patent 2,769,148 on October 30, 1956.

Due to the phenomenon known as skin effect, at high frequencies the current distribution through a conductor is not uniform. Consider, for example, the case of a solid conductor to which are applied waves of increasing frequency. At zero and sufliciently low frequencies, the current in the conductor is substantially uniformly distributed throughout and the resistance of the conductor and hence the conductor loss in the line is at a minimum. With increasing frequency, the current distribution changes so that the current density is a maximum at the outer surface of the conductor and decreases into the material at a rate depending on the frequency and the material. In the example given, the current density may be negligible at the middle of the conductor. Thus the current density in the conductor is associated with a power loss that is a function of the distribution of current density across the thickness of the conductor.

It is thus a more particular object of this invention to reduce the power loss associated with skin effect in electrical conductors and more specifically, in conductors including a multiplicity of spaced, thin, flat conducting sheets.

Further objects of the invention are to reduce the extent to which the power loss in such a conductor varies with frequency, and to make such power loss and consequently its contribution to the attenuation of a transmission line or other wave-guiding structure made up of such conductors substantially independent of frequency over a broad band of frequencies from the lowest frequency of interest to the highest. F or example, in practice such a band might be comparatively narrow or alternatively might be sufficiently wide as to accommodate a plurality of wide-band television channels.

This invention, in one of its more important aspects, resides in a composite electrical conductor that is sepa rated, transverse to the direction of desired wave energy propagation, into a multiplicity of insulated, fiat conducting elements or laminations of such number, dimensions and disposition relative to each other and the orientation of the electromagnetic Wave as to achieve a more favorable distribution of current and field within the conducting material. The smallest dimension of the laminations is in the direction perpendicular to both the direction of wave propagation and the magnetic vector. A convenient yardstick in referring to the thickness of the metal laminations and of the insulating layers is the distance 6 given by where 6 is expressed in meters, 1 is the frequency in cycles per second, ,1. is the permeability of the metal in henries per meter, and a is the conductivity of the metal in mhos per meter. The factor 6 measures the distance in which the current or field penetrating into a slab of the metal many times 6 in thickness will decrease by one neper; if e., their amplitude will become equal to 1/e=0.3679 times their amplitude at the surface of the slab.

This factor 6 will be called one skin thickness or one skin depth. In the case being considered, it is contemplated that the thickness of each lamination is many times (for example, 10, or even 1000 times) smaller than 6 (in general, the thinner the better) and that there will be many laminations (for example, 10, 50, 100 or more). The insulating layers are also made very thin. It has been found that when the conductor has such a laminated structure, a wave propagating along the conductor at a velocity in the neighborhood of a certain critical value will penetrate further into the conductor (or completely through it) than it would penetrate into a solid conductor of the same material. This results in a more uniform current distribution in the laminated conductor and consequently lower losses. Another way of looking at this result is to say that the effective skin depth is much larger in the laminated conductor than the skin depth 6 for a solid conductor of the same material as the laminations. The critical velocity mentioned above is determined by the thickness of the metal and insulating laminae, and the dielectric constant of the insulating laminae.

In certain circumstances, the metal laminations need not be continuous, as breaks therein will not cause the conductor to be inoperative at high frequencies.

The invention is applicable to wave guides, cable pairs, and single composite conductors for any of a great variety of uses-to mention just a few types of conductors wherein the present invention can be applied.

The invention will be more readily understood by referring to the following description taken in connection with the accompanying drawings forming a part thereof, in which:

Fig. 1A is a schematic representation of an electromagnetic wave propogating through space in the neighborhood of an electrical conductor;

Fig. 1B is a graph of current density vs. depth (distance away from the surface) in the conductor of Fig. 1A;

Fig. 2A is a schematic diagram showing respectively the directions of electric and magnetic field vectors and the direction of propagation of an electromagnetic wave near the surface of a composite conductor in accordance with the invention;

Fig. 2B is a graph having the same coordinates as in Fig. 1B, and showing the increased skin depth produced by the conductor of Fig. 2 as compared with that of Fig. 1A;

Fig. 3 is a graphical representation showing an electromagnetic wave being reflected from an extended'metal surface laminated in accordance with the invention;

Fig. 4 is a perspective View of a wave-guide structure in accordance with the invention.

Referring more particularly to the drawings, consider an electromagnetic wave propagating through space in the neighborhood of, and parallel to the surface of an electrical conductor such as copper, silver or aluminum, for example. This situation is shown diagrammatically in connection with theconductor 10 in Fig. 1A which can be representative of many phenomena. It can illustrate, for instance, the transmission of an electromagnetic wave through a coaxial line, or along an open or shielded two wire system, or a wave propagating through a metal Wave guide. It can also represent the situation in the vicinity of a transmitting or receiving antenna. Clearly a very broad class of electrical phenomena involving the transfer or periodic oscillation of electromagnetic energy in the vicinity of electrical conductors is represented in Fig. .LA.

The wave propagating in Fig. lA'is necessarily accompanied by electric currents flowing in the metal. Because of these currents, power is removed from the electromagnetic field and dissipated in the metal. This effect is nearly always undesirable. The distribution of this current in a direction away from the surface is shown in Fig. 1B where it has been assumed .that the conductor is thick compared to 6, if the frequency is sufficiently high. Because of the well-known skin effect, most of the current flows in a thin layer near the surface. The distance from the surface at which the current density has fallen to 1/e=0.3679 times its value at the surface is known (as mentioned above) as the skin depth and is denoted by 6. The distance is expressed in terms of the frequency (I) under consideration and the permeability (,LL) and conductivity of the metal in Equation 1 above. Within a given amplitude of the electromagnetic wave, the amount of power lost to the metal will be proportional to 1/617. Referring to Equation 1, it can be seen that the power loss is proportional to 1/ V0 so that normally the power loss is minimized by choosing a metal of high conductivity, such as copper or silver.

Suppose that it were possible to arbitrarily increase 6 without greatly changing a. It is clear that in such a situation the power loss from the electromagnetic wave would be greatly decreased. It has been discovered that it is possible to do just this thing, and the present invention is based on this discovery. A simple embodiment of the invention will be considered first and then more general cases will be discussed. Referring to Fig. 2A, there is again shown an electromagnetic wave propagating near the surface of an electrical conductor '20. The relationship of the electric and magnetic vectors and of the direction of propagation of the electromagnetic wave are shown. The conductor in Fig. 2A is no longer a solid piece of metal but is composed of many spacedlaminae 21 of metal of thickness W arranged parallel to the direction of propagation and parallel to the magnetic vector as shown. These laminae are veryth'in compared to 6 and are separated by empty space or anyappropriate dielectric 22 such as air, polyethylene, polystyrene, quartz, or polyfoam, for example, the thickness thereof being represented by t. Whatever the dielectric is, suppose that its dielectric constant is 6,, and suppose that the conductivity of the metal is u, as before. Fig. 2A is representative of many situations of which a few will be indicated later. The particular cases being considered in which the magnetic vector is parallel to the surface of the composite conductor are not representative of all cases, as will be indicated below.

Since the stack of metal laminae in Fig. 2A will not conduct direct current in a direction perpendicular to the plane of the laminae, it is possible by conventional means to measure an average dielectric constant associated with this direction. This average dielectric constant will be denoted by Zand is given by the expression ;=e1(1+W/t) farads per meter have in a medium of dielectric constant 2 and permeability 1. This condition can be arranged by properly disposing suitable dielectric material in all or part of the region traversed by the wave outside the stack.

Under the conditions mentioned, if W is small compared to 6, we can define an effective skin depth 6 by If the stack of laminations is several times 6 in thickness, the current density will decrease exponentially into the stack and be reduced by one neper at a distance below the surface equal to 6 This increased or effective skin depth is shown in Fig. 23. Furthermore, the effective conductivity Oe of the stack of laminations in the It is immediately observed that the power lost from the electromagnetic wave has been reduced by a factor For instance, if the laminae in a typical case are skin depth thick, the power taken from the wave will be only ,3 of the power that would be lost to a solid conductor.

The increased skin depth described above not only is effective in greatly reducing conductor losses, but has a further major concomitant advantage. Referring to Equation 1, it can'be seen that conductor losses generally increase as the square root of the frequency. This variation with frequency very often is equally as troublesome as the losses themselves. A simple but extremely wasteful way to reduce this effect is to make the metal conductor very thin. Suppose for instance that the skin depth is 6 at the highest frequency under consideration. If the conductor is not thicker than 5 the losses will clearlyremain uniform, but high, from very low frequencies up to this maximum. Similarly, with the arrangement of Fig. 2A the size of the stack can be limited to the thickness 6 determined by Equation 3 at the highest frequency, and thereby obtain uniform loss. But since 6 may be made as large as desired by making W small enough, this uniform loss can be achieved without accepting greatly increased losses at the lower frequencies. By way of example, it is desirable to make each insulatingand conducting lamina less than one thousandth of an inch thick. The general situation indicated in Fig. 2A can have many specific embodiments and variations.

Fig. 3 illustrates an electromagnetic wave being reflected from anextended metal surface 70 laminated as above to have a multiplicity of alternately positioned metal layers 71 and insulatinglayers 72 to reduce losses incident upon reflection; One component of this wave may be considered as traveling parallel to the surface and the-other component perpendicular to the surface. Clearly, the conductor losses associated with the component of the wave traveling parallel to the surface can be reduced below those encountered with a solid sheet of metal by use of laminations.

In Fig. 4, there is shown a section of awave guide 80, a pair of whosewalls 81 and 82 have been covered withthin metal laminations 83 separated byinsulation 84 as before. The twowalls 81 and 82 are connected bywalls 85 and 86 of solid metal. Here also, decreased attenuation can be realized if the electromagnetic wave propagates down the guide with a velocity in the neighborhood of that appropriate to the average dielectric constant of the stack. Obviously, other wave-guide arrangements utilizing this principle at e possible.

It is obvious that many changes can be made in the embodiments described above. The various embodiments and the modifications thereof described herein are meant to be exemplary only and they do not by any means comprise a complete list of conductors to which the present invention is applicable and it is obvious that many more will occur to those skilled in the art. It is intended to cover all such obvious modifications as clearly fall within the scope of the invention.

What is claimed is:

1. In an electromagnetic wave guiding system, a conductor medium comprising a multiplicity of flat elongated conducting portions spaced by means including insulating material, and means for launching high frequency electromagnetic Waves in said system, there being a suificient number of conducting portions to carry a substantial por-. tion of the current induced by said waves, the thickness of said conducting portions transverse to the direction of wave propagation down the length thereof being small compared with its appropriate skin depth at the highest frequency of operation with said high frequency waves and the spacing between at least some of said conducting portions being less than the skin depth, whereby the said conducting medium is substantially penetrated by the electric field of said waves.

2. The combination of elements as in claim 1 in which each of said conducting portions is a thin sheet.

3. The combination of elements as in claim 1 in which each of said conducting portions is a thin sheet and said sheets are arranged in two groups, the two groups being separated from one another by a larger distance than the spacing between any two of the conducting portions within a group.

4. The combination of elements as in claim 1 in which each of said conducting portions is a thin sheet and said sheets are arranged in two groups, the two groups being separated from one another by a larger distance than the spacing between any two of the conducting portions within a group, and there is dielectric material between the two groups.

5. The combination of elements as in claim 1 in which said conducting portions are arranged in flat stacks form'- ing opposite walls of a rectangular wave guide.

6. The combination of elements as in claim 1 wherein each of said conducting portions has a thickness of less than one-thousandth of an inch.

7. The combination of elements as in claim 1 wherein said flat conducting portions are arranged in a stack of more than ten portions and more than ten insulators, each of the conductors and insulators having a thickness which is less than one-thousandth of an inch.

References Cited in the file of this patent UNITED STATES PATENTS 2,008,286 Leib July 16, 1935 2,231,602 Southworth Feb. 11, 1941 2,433,181 White Dec. 23, 1947 2,676,309 Armstrong Apr. 20, 1954

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