Transmission of guided waves

April 30, 1940.

S. A. SCHELKUNOFF

TRANSMISSION OF GUIDED WAVES

2,199,083

Filed Sept. 4, 1937

FIG. I

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FIG. 3 FIG. 4 FIG. 5 FIG. 6

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FIG. 7 FIG. 8 FIG. 9 FIG. IO

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FIG. I I FIG. !2 FIG. /3

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INVENTOR

S.A. SCHELKUNOFF

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ATTORNEY

Patented Apr. 30, 1940

2,199,083

UNITED STATES PATENT OFFICE

2,199,083

TRANSMISSION OF GUIDED WAVES

Sergei Alexander Schelkunoff, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York

Application September 4, 1937, Serial No. 162,422

Claims. (Cl. 178—44)

This invention relates to high frequency electromagnetic wave transmission systems and more particularly to the guided transmission of hyperfrequency electromagnetic waves.

In my copending application Serial No. 56,959, filed December 31, 1935, which issued on February 21, 1939, as U. S. Patent No. 2,147,717, it is shown that electromagnetic waves of certain kinds can be guided through a metallic pipe containing only a dielectric medium. Guided transmission of this kind is possible only at frequencies exceeding a critical or absolute cut-off frequency that is dependent on the transverse dimensions of the guiding structure, the guide thus presenting the characteristics of a high-pass filter. There are many types of such waves, each distinguished by its characteristic field pattern, and for each type there is a corresponding critical frequency, generally higher than the absolute cut-off frequency, which must be exceeded if a wave of that particular type is to be propagated within the guide. There is only one type of wave that can be propagated within the guide at frequencies so low as to approximate the absolute cut-off frequency, but at frequencies above the next higher critical frequency, not only a wave of this type but also one or more independent waves of different type, travelling at different velocities, will originate at the transmitter unless special precautions are taken to prevent it.

A principal object of the present invention is to provide a guided wave system of the kind described in which the absolute cut-off frequency, or minimum frequency of transmission, is substantially lowered without increasing the overall dimensions of the guide. Another object is to increase the frequency range over which the guide will transmit but a single type of guided wave.

The present invention in one of Its simplest embodiments comprises a hollow copper tube having a longitudinal copper fin or baffle extending radially inward from the periphery of the tube, together with means for launching electromagnetic waves of suitable character in the tube. The foregoing embodiment is typical and illustrative only, however, and the nature of the present invention and its various objects, features and advantages will appear more fully in the detailed description that is to follow. Reference will be made to the accompanying drawing, in which:

Figs. 1 and 2 illustrate typical transmission systems embodying the present invention;

Figs. 3 to 11 represent schematically various possible types of electromagnetic waves that may be propagated in one form of guide in accordance with the invention; and

Kgs. 12 and 13 illustrate two other embodiments of the invention.

The various types of electromagnetic waves that can be guided through a metallic pipe may be classified as explained in my copending application, supra, into two groups, transverse magnetic (TM) and transverse electric (TE), the former comprising those waves that are char- 5 acterized by the fact that the vector H representing the magnetic component of the field is wholly transverse to the direction of propagation and the latter comprising such waves as are characterized in that it is the electric vector 10 E that lies wholly transverse to the direction of propagation. Transverse magnetic waves can be described by means of characteristic distributions of electrical potential over any particular cross-section of the guide and transverse elec- 15 trie waves by means of similar distributions of magnetic potential over any particular crosssection. If x and y are the Cartesian coordinates of a typical point in a given cross-section, the functions T (i, y) governing the potential distri- 20 button satisfies the following equation:

(1) where χ is a positive real constant depending 25 upon the size and shape of the pipe and upon the particular characteristic field distribution of the wave to be transmitted. For transverse magnetic waves, T is also proportional to the longitudinal electric displacement current density, while for 30 transverse electric waves T is proportional to the longitudinal magnetic displacement current density.

The various possible characteristic field distributions are in accord with those solutions of 35 Equation 1 which satisfy appropriate boundary conditions on the surface of the pipe. Either T or its normal derivative must vanish on the boundary, depending upon whether the wave is transverse magnetic or transverse electric, re- 40 spectively.

Transmission of energy can take place only if the frequency is higher than a critical frequency depending upon _x. This cut-off frequency may be derived from the equation ω=2πί=_χο, where 45 c is the velocity characteristic of light in the dielectric medium within the pipe; the corresponding wave-length is

2tt

Y 50

If the space within the pipe is simply connected, _xis always greater than zero and there exists a lower frequency limit below which there is no transmission mode capable of transferring en- 55 ergy along the tube.

Equation 1 happens to be also the equation for the displacement of an oscillating elastic membrane, and the different transmission modes for electromagnetic waves can, with mathematical 60

2,189,083 exactitude, be associated with characteristic vibrations of the membrane representing the crosssection of the pipe. The boundary condition established by the presence of the metallic pipe in the case of transverse magnetic waves, viz., that longitudinal electric intensity must vanish at the periphery of the pipe corresponds mathematically with the boundary condition established by fixing the edge of the oscillating membrane. Similarly, in the case of transverse electric waves the boundary condition established by the presence of the metallic pipe, viz., that longitudinal magnetic intensity must vanish at the periphery of the pipe corresponds mathematically with the boundary condition of a free-edged oscillating membrane. In brief, the edge of the membrane is to be considered fixed or free, depending on whether the corresponding waves are transverse magnetic or transverse electric. The cut-off frequencies for the electromagnetic waves are proportional to the natural frequencies of the corresponding membranes. This fact is helpful in obtaining a qualitative idea of the relationship between various cut-off frequencies.

It has been observed by Lord Rayleigh with reference to oscillating membranes that each new constraint raises the gravest natural frequency of the membrane and that conversely a removal of a constraint results in lowering the gravest natural frequency. The same observation, applicant has found, may be made with reference to guided electromagnetic waves. Thus, if a metallic sheet is disposed longitudinally in a tubular metallic pipe carrying transverse magnetic waves so that it extends radially inward from the periphery of the pipe, it, as well as the pipe itself, imposes a constraint on the waves, just as clamping a circular membrane along a radius would impose a constraint on the mode of vibration of the membrane. Accordingly, the lowest cut-off frequency for transverse magnetic waves will be raised. On the other hand, if the pipe carries transverse electric waves the radial sheet removes a constraint, just as cutting the corresponding circular membrane from periphery to center would do, and both the pipe wall and the radial sheet establish free-edge boundary conditions. Accordingly, the lowest cut-off frequency for transverse electric waves will be lowered.

When the metal partition extends from the axis of the tube to its periphery, the proper values for x can easily be calculated. Thus in polar coordinates the potential distribution is given by (2) Τ&lt;ρ,φ) =Jp(xp) sin ρφ, or Τ(ρ,φ) =1ρ(χρ) COS Ρφ where φ is measured from the partition.

If the wave is transverse magnetic, then T(p,0) and Τ(/>,2π) must vanish. Consequently, we must choose the first form in (2) and let sin 2pjr=0, 2pir =nir, p=£’ n = l, 2, 3, . . . (3) A

Likewise, T must vanish on the boundary p=a of the tube; thus •J_n(x«)=0(4)

The smallest root of this equation corresponds to n=l; it is

-χ.α=τ (5)

The corresponding cut-off is given by

7TC j , n w_c=— and A_c=2a

If the wave is transverse electric, then the normal derivative of T must vanish on two sides of the partition and on the periphery. Hence, T must be given by the second form in (2) and χα must be a root of5 (β)

The smallest root occurs when n—1 and (6) becomes1®

Ji(xJ=0or^=2(7) ₅x»

Approximately we have . , „ 2πα. ,,

Χα=1.16,w

In the absence of a partition the lowest cut-off frequency for transverse electric waves in a tubu- jo lar metallic guide is that for the so-called ΤΕι,ι (or Hi.x) wave, and it corresponds to χ_ο=1.84, whereas with the partition the corresponding figure for χα is 1.16. Thus, the radial partition results in lowering the absolute cut-off frequency by almost 37 per cent. For transverse magnetic waves, on the other hand, the lowest cut-off frequency in the absence of a partition corresponds to χ_ο=2.40 whereas with the partition the corresponding figure is 3.14. Thus, for transverse 30 magnetic waves the lowest cut-off frequency is raised by nearly 31 per cent. An important consequence is that the frequency range between the absolute cut-off frequency and the next higher critical frequency is substantially widened. 35

Referring now to Fig. 1 there is shown an embodiment of the present invention comprising a metallic tube i, a radially disposed metallic baffle 2 extending longitudinally through the tube, and the wave source 3 connected between the axial 49 edge of the baffle 2 and the opposite point of the tube wall. The end of the tube I is closed by a metallic reflector 4 which may be in the form of a copper disc and which is disposed at such distance from the connecting leads associated with 45 the source as to obtain maximum transmission of energy along the guide. Hie optimum distance will be found to be approximately a quarter of a wave-length or an odd multiple thereof. The source 3 may be considered as simply a sine 50 wave source but it Is representative also of any source of intelligence-bearing high frequency waves, such as a carrier wave modulated with telephone, telegraph, or television signals, or the like. Similarly, at the receiving end of the sys- 55 tem, the detector 5 connected in the same manner as the source 3 is representative of any receiving device appropriate for signals of the character generated by the source 3. Preferably, the high frequency waves are transmitted in the 60 frequency range lying between the absolute cutoff frequency and the next higher cut-off frequency to obviate interference from spurious wave types.

Fig. 2 is quite similar to Fig. 1 except that the 65 metallic guide is shown as a metallic pipe 11 of rectangular cross-section closed at the end by a metallic cover and provided with a metallic baffle 22 extending inwardly along the mid-line of the upper surface and parallel to the lateral 70 surfaces. Where the physical dimensions of the source 3 make it inconvenient to dispose it in the position indicated in Fig. 1, it may be moved toward the end of the guide as shown in Fig. 2 and connected by parallel leads to the trans- 75

2,109,083 versely disposed conductors connecting the baffle and the lower surface of the pipe.

Fig. 3 is a cross-section of Fig. 1 and it shows the distribution of the transverse electric field 5 comprising the wave. The designation of this wave as a

TEi wave is consistent with the scheme of designation 1° employed in my copending application, supra.

Fig. 4 shows another type of transverse electric wave that can be sustained in the guide of Fig. 1. In its configuration it Is quite similar to the ΤΕι,ι wave, or as it is sometimes known, the Ηι,ι wave, 1® in an unbaffled tubular metallic guide. The source or receiver may be connected between the axial edge of the baffle and a point on the pipe such that the connecting leads are aligned with the electric field, or it may be disposed below the baffle, as illustrated, with connecting leads aligned with the electric field and terminating at the pipe. Aside from its other advantages, the baffle is useful in this case in suppressing any tendency for 2₈ the field to rotate out of alignment with the conductors of the receiving terminal.

Fig. 5 shows still another transverse electric wave, designated as the

2’¹ and a roughly Y-shaped terminal structure conforming with the electric field and adapted to generate or receive this type of wave.

„ Transverse magnetic waves of four different ³⁰ types are shown in Figs. 6 to 9 as they would appear in the guide of Fig. 1. The dotted lines represent the lines of magnetomotive force, which lie in transverse planes.

Figs. 10 and 11 show terminal structures adapted to generate or receive transverse magnetic waves of the types illustrated in Figs. 6 and 9, respectively. The structure shown in Fig. 10 comprises two coplanar metalic plates 6 and 7, configured and disposed over the end of the ^⁸ metallic tube so as to leave an elongated gap between them that follows or conforms with the lines of magnetomotive force in the wave. The translating device 3 is electrically connected to the plates 6 and 7 so as, in the case of a generator, to establish an alternating difference of potential across the gap.

In the terminal structure shown in Fig. 11 there are provided two approximately elliptical coplanar electrodes 8 configured and positioned to conform roughly with the magnetic lines depicted in Fig. 9, and the translating device 3 connected in operative relation therewith.

In connection with each of the terminal structures herein disclosed, any suitable means may ⁸⁰ be employed for purifying the wave generated, that is, for suppressing spurious wave types that may be produced incidentally to the production of the desired wave type. It is contemplated too U0 that the relatively inefficient terminal arrangements of Figs. 10 and 11 can be improved upon to effect a more efficient transfer of wave energy between the guide and the source or receiver.

Whereas the radial baffle has in each case been 70 described as extending from the periphery of the guide to its center it will be understood that this has been for specific illustrative purposes only and that the baffle may extend a greater or lesser distance as may be desired. The greater the K radial width of the baffle, the looser is the cou pling between the two portions of the guide. It will be obvious that the baffle, as the term is used here and in the appended claims, loses its character as such in the limit where it extends completely across the guide, or where otherwise it t serves only to subdivide the guide into a plurality of completely metallically enclosed, electrically independent passages. Where such independent passages obtain the interior of the guide is multiply connected, whereas the interior is simply 10 connected if the baffle is radial or if otherwise it is such that at substantially every cross-section along the guide there is a (lateral) dielectric connection between all parts of the interior.

From the foregoing description of the nature of Ίβ the invention and from the various examples illustrated it is evident that the baffle may be defined generally as comprising one or more essentially two-dimensional conducting strips disposed longitudinally within the guide. ⁸⁸

Although the embodiments of the invention hereinbefore described all utilize a single radial, or generaly transversely disposed planar baffle, it will be evident from the stated principles underlying these embodiments that the invention may ²⁸be embodied in a wide variety of other forms. Two illustrative modifications are shown in Figs. 12 and 13.

Fig. 12 shows a guide comprising an outer cylindrical metallic pipe enclosing three tubular ⁸⁸metallic partitions each having a longitudinal opening therein so that at all points along the guide the several semi-annular spaces are dielectrically connected. A radial metallic baffle „ 2 is provided extending from the periphery of the pipe to the innermost partition. The translating device 3 is connected by diametral leads from the innermost partition to a point on the outer pipe that is opposite the radial baffle. The several partitions and the radial baffle are effective in removing constraints, for transverse electric waves, and in permitting operation at lower frequencies. The guide shown in Fig. 13 is provided with a plurality of baffles lying in radial planes which also are effective in removing con- ⁸⁸straints.