US2207845A - Propagation of waves in a wave guide - Google Patents

Description

July 16, 1940. 1. WOLFF I 2,207,845

PROPAGATION 0F WAVES IN A WAVE GUIDE Filed May 28, 1938 z/NE 7 sum 1 L a m; A L a 5 A nE IE W 1 .Li: i v ,5 1.5 6' 1\\ lnventor Irvin WoLff Gttorneg Patented July 16, 1940 PROPAGATION OF WAVES IN A WAVE GUIDE Irving Wolff, Merchantville, N. J., assignor to Radio Corporation of America, acorporation of Delaware Application May 28, 1938, Serial No. 210,595

. 5 Claims.

My invention relates to the transmission of ultra high frequency electromagnetic waves, and more particularly to a method and apparatus for coupling a source of ultra high frequency waves to a wave guide.

It is a primary object of this invention to provide an improved apparatus and a corresponding method for propagating waves into a wave guide. Another object of my invention is to provide for the-proper termination of a transmission line in a wave guide. A further object-is to provide a system which is capable of propagating ultra high frequency waves of a wide range of frequencies through a wave guide. A still further object of my invention is to provide a resistive termination for a transmission line which is capable of propagating ultra high frequency waves through a wave guide without reflections.

This invention will be better understood from the following description when considered inconnection with the accompanying drawing, and its scope is indicated by the appended claims.

throughout the drawing.

Referring to the drawing, Figure 1 is an elevational view, partly in section, of one embodiment of my invention in which a Similar reference numerals refer to similar parts concentric transmission line is shown terminating in a wave guide, v I

Figure 2 is aside elevation, partly in section, of a preferred'embodiment of my invention,

' Figure 3 is a sectional view of an alternative type of 'dielectricelement having a nonuniform dielectric constant'along its length, and

Figure 4 is a side elevation, partly in section, of an embodiment of my invention in which a two-wire transmission line is terminated in a wave guide.

It has been known for some time that ultra high frequency waves may be propagated through I dielectric medium, such as a pipe or other container, or may merely be a discontinuity between the medium and air.

The problem of terminating a transmission line in a wave guide is analogous in many ways to the termination of a transmission line at an antenna. It is well known that an antenna must be matched to the characteristic impedance of the line in order to get an efficient transfer of energy, and. to avoid undesirable reflections.

tor.

Matching may be accomplished by resonating the antenna, but this provides efficient operation only at the resonant frequency. At other frequencies reflections are 'set up due to the reactive character of the antenna. To prevent these reflec- 5 tions from occurring over a wide band of frequencies, the antenna must be substantially resistive over the desired range. This, for example, is a feature of the so-called rhombic or wave antenna.

Similar problems arise in the propagation of ultra-high frequency waves through wave guides when the energy is delivered from an oscillator through 'a' transmission line. If the diameter of the outer tube of a concentric line bears the proper relation to the wave length of the wave, it might appear that propagation would be achieved by merely cutting off the inner conductor. However, the abrupt change from one condition to another is equivalent to a mismatched antenna in the analogy mentioned above, and undesired reflections are therefore set up."

It is therefore proposed that coupling may be effected by a system such as that shown in the drawing. v

Referring to Fig. l, a concentric transmission line I is shown coming in from the left. This line consists of an outer member 3 and an inner concentric conductor 5. Acoupling 1, such as a pipe joint, or any other suitable connector, is used to connect the transmission lineto awave guide 9, which continues to the right.

In the cut-away portion of these tubes, the inner conductor 5 can be seen. It is to be noted that this conductor is not abruptly ended, but graduallytapered off to a point. The length of this tapered section is preferably equal to a number of wave lengths.

The solid lines indicate the lines of electric 40 force that exist in the system. Within the concentric tube the lines of force are substantially radial from the conductor 5, as'shown at ll, due to the conduction currents through the conduc- Within the guide the lines of force are arranged as shown at I3. It will be noted that the conduction currents in the center conductor become displacement currents in the guide. This distribution of the lines of force has been fully described in various articles, and need not be explained in detail here. For example, see the article by G. C. Southworth in The Journal of Applied Physics for October, 1937, page 660, or an article by the same author in the July 1937 Proceedings of the Institute of Radio Engineers, page 807.

It will be noted that along the region adjacent to the tapered section the lines of force gradually change from one arrangement to the other. This gradual change is due to the increasingly high resistance of the inner conductor, which causes a gradual increase in the ratio of displacement currents to conduction currents. This arrangement, however, does not give the greatest efficiency because of the inherently high resistance losses.

Fig. 2 illustrates a preferred embodiment of my invention which makes possible a still further and more gradual change from the comparatively low resistance of the inner conductor to the medium of the wave guide. At the same time, the resistance losses due to the currents through the high resistance of the conductor are reduced.

The conductor 5 is tapered to a point, as before, but in addition a piece of low loss dielectric material i5 is positioned around the tapered section, forming an extension thereof. The dielectric material is also tapered to a point. The dielectric constant of this material should be higher than that of the Wave guide medium. Thus the lines of force transfer gradually to the dielectric inner conductor, and then, asthis tapers ofi, they assume their normal shape in the guide. The use of this dielectric material at the termination of the metallic inner conductor increases the ratio of displacement currents to conduction currents fora specific resistance of the metallic conductor. The dielectric thus offers an alternative low loss path in the region of the tapered metallic conductor, and thus reduces the resistance losses. For a still more gradual transfer, a series of dielectric elements of difierent characteristics can be used in a similar manner, or an arrangement illustrated in Fig. 3.

Fig. 3 is a cross-sectionalview of the end of one of the inner conductors which may be utilized in the devices shown in the preceding figures. The inner conductor 5 is terminated in a point as before. The dielectric element 2|, however, difiers from that shown in connection with the earlier figures in that it has a nonuniform electrical characteristic along its length. The increased density of the stipplingat the inner end of this element indicates a region having some degree of conductivity or,. alternatively, a relatively high dielectric constant. The conductivity o'r dielectric constant of the element tapers off until at the end the dielectric constant of the material approaches that of the guide medium. This may be accomplished by embedding particles in the dielectric material, or by other suitable means.

The embodiment shown in Fig. 4 illustrates the application of my invention to a two-wire transmission line. Two conductors l1 and 19 are the terminating ends of a transmission line which connects to a source of ultra highfrequency signals, which is not shown. The conductors are symmetrically placed within theguide 9. Each conductor is tapered in accordance with this invention, and functions in a similar manner.

I have thus described a system by which a concentric or a parallel wire transmission line may be .efficiently coupled into a dielectric guide by gradually changing from one condition to another.

Although the performance has been explained in terms of a transfer from line to guide, the arrangements shown are equally capable of transferring waves from the guide to a line.

I claim as my invention:

1. In combination, a transmission line, a'wave guide, and means for electrically'coupling said line and said guide, said means including a dielectric of gradually decreasing conductivity extending into said guide.

2. In a device of the character'described, a concentric line, a wave guide, and means coupling said guide to said line, said means com.- prising a tapered extension of said inner .conductor of said line extending Within said guide a distance equal to at least a wave length and terminated in a tapered insulator.

3. In a device of the character described, a I

parallel wire transmission line, a wave guide,- and means coupling said guide to said line, said means comprising a tapered extension of each of said parallel wires extending within said guide a distance equal to at least a wave length,.-and terminated in a tapered dielectric element.

4. In combination, a transmission line anda Wave guide, means for gradually transferring cone. duction currents in said line to displacement currents in'said guide, and means for reducing the resistance losses in said first-named means.

5. In a system for coupling a concentric transmission line to a wave guide, means for propagating waves into said guide, said means comprising a tapered extension of the inner con-r ductor of said line, and means for reducingthe resistance losses in said tapered extension.

IRVING WOLFE.

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