US2563612A

US2563612A - Controlling transmission in

Info

Publication number: US2563612A
Authority: US
Publication date: 1951-08-07
Anticipated expiration: 1968-08-07

Description

Aug. 7, 1951 V c. N. NEBEL Filed Dec. 31, 1946 /2 27 2a 13 a i I K 14 14b U.(I.F N0./ A 510. we. 5'2??? 29:15: i 0 I GEN. ATT. L

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" oes-- .4 .3 .2 .l RELATIVE AMPLITUDE OF REFLECTED WAVES //v l/EN TOR C. N. NEBEL A NE) Patented Aug. 7, 1951 CONTROLLING TRANSMISSION IN MICROWAVE SYSTEMS Charles N. Nebel, Dcnville, N. J., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application December 31, 1946, Serial No. 719,356

4 Claims.

This invention relates to microwave signal transmission and more specifically to a method of and apparatus for compensating for wave reflections in such transmission systems.

Many microwave signal transmission systems comprise at least one section of rectangular wave guide. This wave guide may extend effectively in a rectilinear direction, and include one or more impedance irregularities due, for example, to coupling misterminations or variations in cross-section arising during manufacture. The misterminations may be frequently occasioned by the well-known choke or flange couplings. The variations in cross-section may also be due to the introduction of a bend or a twist into a rectilinear wave guide or to the fabricated corners in curvilinear wave guide which consists of several rectilinear sections of wave guide. Such impedance irregularies tend to cause wave reflections whereby standing waves tend to be produced in the transmission system to impair the efficiency thereof.

The present invention contemplates an arrangement for minimizing reflected waves in a microwave signal transmission system.

The main object of the present invention is to compensate for impedance irregularities inherent in a rectangular wave guide.

Another object is to control wave reflections.

A directional coupler is known in microwave signal transmission systems for separating refiected waves from incident waves for the purpose of their measurement. This coupler is essentially of the type disclosed in the copending application of W. W. Mumford, Serial No. 540,252, filed June 14, 1944. Any alteration of the microwave system that tends to reduce the amplitude of reflected Waves indicates a smaller standing wave or a decrease in the reflection coefficient of the impedance irregularities causing the reflected Waves in the system. It has been found that the amplitude change in the reflected waves becomes increasingly small as the standing wave tends to approach its ideal value 1.0. Thus, measurements of the amplitude of reflected waves provide highly sensitive indications of improvements in standing waves below a certain limit, say, for example, the value of 1.25.

In accordance with the present invention, the amplitude of the reflected waves that are inherent in a microwave signal transmission system for the above'noted reasons is utilized to determine the amount of correction requiredto be introduced into such system in order to reduce the standing wave therein to at least a tolerable limit, say, for example, to the value of 1:10 or therebelow so as to approach more closely the ideal value 1.0. This correction involves the introduction into the system of one or more additional reflected waves whose combined amplitude is substantially equal to that of the inherent reflected wave but degrees out of phase therewith whereby the inherent and additional reflected waves tend to cancel each other.

In a specific embodiment of the present invention, the foregoing compensation for reflected waves inherent in a known microwave signal transmission system is effected by producing of one or more preselected deformities in the inside surface of a rectangular wave guide being tested in the system by means of a suitable device employed in a predetermined manner. These deformities, as one illustration, constitute projections formed integral with the surface of the wave guide being tested at the opposite ends thereof and extending interiorly of the wave guide, each projection being spaced from the impedance irregularity to be compensated for and from each other in a sense related to the Wavelength of the signal waves being transmitted in the wave guide being tested. As a consequence of these projections additional reflected waves are introduced into the microwave system with such phase and combined amplitude that the additional waves tend to cancel the reflected waves and thereby tend to reduce the standing wave to at least the tolerable value.

The deforming device comprises a familiar type of vise-like apparatus having the usual two opposing jaws, one of which is movable and the other fixed. The movable jaw includes a smooth surface for engaging a small portion of the outer surface on one side of the wave guide, and the fixed jaw includes a pin-like projection for engaging a small portion of the outer surface of the opposite side of the same wave guide along the longitudinal axis thereof. As the movable jaw is gradually brought closer toward the fixed jaw to a predetermined extent, the pin-like projection on the latter jaw forms a dent in the outer surface of the wave guide. This results in a projection formed integrally with the inner surface of the wave guide being tested and extending interiorly thereof. One or more of these dents provided at properly spaced points on the wave guide being tested cause additional reflected waves which tend to cancel the initial reflected waves or to reduce the amplitudes of the latter waves and thereby reduce the standing Wave in the microwave signaling system as previously mentioned.

For determining the location and number of the dents, a measuring circuit adapted for use with signal waves whose frequencies lie in the ultra-high range is connected alternately to the reflected wave and incident wave outlet taps of the directional .coupler. Pursuant to certain readings in the measuring circuit when so connected, the first dent is located at one end of the wave guide being tested a distance of wavelengths from the impedance irregularity being compensated for, and each additional dent is located a distance of wavelengths from the next previous one, 11. being an odd integer and A the Wavelength of the signal waves in the interior of the wave guide being tested. This section of wave guide is then reversed end for end in the microwave system, and provided at its other end with one or more dents in a similar manner.

' A like procedure is followed for providing one or more dents in one or both ends of a bent and/or a twisted wave guide, of a wave guide having one or more fabricated corners, that is, curvilinear wave guide made up of -a plurality of rectilinear sections of wave guide, and in the auxiliary wave guides associated with the reflected and incident wave outlet taps of the directional coupler mentioned previously.

A feature of this invention is that wave guide previously rejected on the ground of causing standing waves lying above a tolerable limit, say for example, the value 1.10 mentioned hereinbefore, may now be rendered satisfactory in the manner aforesaid. This tends to result in an .economy of material, time and labor.

The invention will be readily understood from the following detailed description taken together with the accompanying drawing, in which:

Fig. 1 illustrates a known microwave signal transmission system modified in accordance with a specific embodiment of the invention;

Figs. 1A, 1B and 10 show portions of a microwave Wave system which may be individually substituted in Fig. 1 and modified in accordance with a. specific embodiment of the invention;

. Fig. 2 is a curve representing action obtainable in Figs. 1, 1A, 1B and 10;

Fig. 3 is a side elevational view of a device utilized for practicing the specific embodiment of the invention used in Figs. 1 and 1A, 1B and 10;

Fig. 4 is an enlarged fragmentary elevational view of Fig. 3; and

Fig. 5 is a cross-sectional view taken along the line {5-5 in Fig. 1.

Referring to Fig. 1, a known circuit for testing a section of tubular wave guide 14 for its standing wave ratio comprises in sequence a signal generator Ill, a suitable variable attenuator II, a suitable frequency meter l2, a directional coupler l3, the section 14 of tubular wave guide, and an adjustable resistive-card load l6. The wave guide 14, for the purpose of this illustration, maybe made of copper or aluminum, is

1%; inches by inch in cross-section, has a Wall thickness of approximately V inch, and will be assumed to include the impedance irregularities which are to be identified later on and to be compensated for in a manner to be explained subsequently. To the directional-coupler tap 11 for reflected waves are serially connected a section [8 of tubular wave guide and a crystal-com verter meter it! of known type including the usual pickup probe. To the directional coupler were for incident waves'are s'e'rially'connected a variable attenuator 2i and a fixed attenuator 22 of 20 decibels, both of which are of known design. The generator iil provides signal waves having a frequency Within the range at which hollow wave guides are useful in signal transmission systems. The electromagnetic waves being transmitted in the wave guide 44 are of the dominant mode having electric lines normal to the wide sides thereof.

The directional coupler 13 comprises two directional couplers, each of which is essentially of the type disclosed in Fig. 7 of the copending application of W. W. Mumford, supra; and as shown in Fig. 1, includes a main wave guide 25, an auxiliary wave guide 26 provided with a tap [7 at its left-hand end serving as an outlet for reflected power, and a conventional resistive card 2'! at its right-hand end;and an auxiliary wave guide 23 supplied with a tap 29 at its right-hand end serving as an outletfor incident power, and

the conventional resistive card 21 at its left-hand end. It will be understood that the taps I! and 23 comprise waveguides formed with suitable configurations for electrically connecting the auxiliary wave guides 26 and 28, respectively, to the apparatus associated therewith. Three openings 29 couple the main wave guide 25 to the auxiliary wave guide 25; and three openings 20a,

similar to the openings 29, couple the main wave guide 25 to the auxiliary Wave guide 28. These coupling openings provide substantially equal coupling losses for the respective auxiliary wave guides. t Y

In the operation of Fig. 1 to'obtain the SWR of the wave guide 14, the meter 19 is initially connected as shown in Fig. 1, andthe attenuator II is adjusted to' provide a preselected reading, say for example, ten per cent of the scale of meter 19. Next, the meter 19 is connected to the output terminal of the fixed attenuator 22, and then the attenuator 21 is adjusted to repeat the ten per centreading. on the meter IS. The sum of the reading of the variable attenuator 2| plus the ZO-decibel attenuation of the fixed attenuatorZZ constitute the front-to-back or F/B ratio of the wave guide It. This. sum is then referred to the curve in Fig. 2 to obtain the value of the SWR of thewave guide 14. Assuming, for example, an SWR of 1.10 as a tolerable maximum limit for the unknown wave guide [4, then the F/B ratio of this wave guide should be approximately 32.6 decibels; and the relative amplitude of the reflected wave should be approximately 0.3 shown in Fig 2 for the purpose of this illustration.

In accordance with a specific embodiment of the present invention shown in Figs. 1, 3, 4 and 5, the SWR of the waveguide I4 may be reduced in a manner that will now be explained from a value lying outside the tolerable limit of 1.10 to a value lying within this limit, or alternately to a value approaching more closely to the ideal value of 1.10 than the above-identified assumed tolerable limit.

Referring to Fig. 3, a vise-like clamp 30 comprises a U-shaped member 3| provided with a fixed pedestal 32 and a screw 33 adjustable by means of a handle 34 on its uppermost end. At the lowermost end of this screw is positioned a right-

angled members

31, 31.

a s-cadre member 35-1ncluding1a' fiatunderzsurface'fifi. i911 opposite vertical sides of member 35 is .a pairof The pedestal :32, as shown in Fig. 4, comprises a cylindrical *element 4|i mounted vertically onthe membe1n3| and whose upper-most end 41 is formed with La, substantially semispherical configuration whose diameter is approximately at inch. The handle 34 serves to raise and'lower the member 351With reference to the fixed pedestal :32 in the wellknown manner.

The operation of Fig. 3 .in-iconnectionwith.Figs.

'1 and 2is as follows: Letit be assumed that the 3|] is positioned on the left-hand end of the wave guide 4 in proximity of the coupling |4a :in the manner illustrated 'in Fig. .3. :In this connection it will be observed that the two rightangled members 31, 31 :serve to dispose the pedestal end 4| substantially on the center or longitudinal axis of the under-surface of the wave guide l4. Next, the handle 34 of the clamp 30 is so actuated as to increase gradually the pressure of the pedestal end 4| on the under-surface of the wave guide l4, \without permitting the pedestal end 4| to makea permanent dent in the under-surface of the latter wave guide. Simultaneously, therewith, the reading on the meter 9'is noted. A reading other than one decreasing below the 10 per cent pointed out previously would tend to indicate that the pedestal end 4| is improperly placed relative to the impedance irregularity in thecoupling |4a.

Accordingly, the clamp 30 is shifted :along the longitudinal axis of the wave guide 14 until there is found for the pedestal end 4| a position at which the pressure of the pedestal end '41 under control of the handle 34 :as previously :mentioned provides on the meter '19 a reading which tends to decrease below the initial 1011381 .cent. This position of the pedestal end 4| would tend to establish afurther impedance irregularity in the unknown wave guide l4 whereby .a further reflected wave is introduced into the system of Fig. 1. .Since the .further reflected wave tends :to reduce the reading on the meter l9, such further reflected wave tends to be out of phase with the reflected wave caused by the impedance irregularity in the coupling |4a. So long as the meter .reading'tends to decrease, this procedure of shifting and operating the clamp 30 is repeated until eventually the clamp 30 is located at a point on the wave guide l4 at which the maximum sensitivity of the decreasing meter reading is obtained. The further reflected wave produced at this point is substantially 180 degrees out of phase with the reflected wave caused by the coupling |4a.

In other words, the further impedance irregularity is located substantially wavelengths from the impedance irregularity in the coupling Ma, n being an odd integer and k the wavelength of the signal Waves in the wave guide 14. .It .is .known that the =0: two

impedance irregularities will -:substantial1y cancel each other when they Lare-spaced 4 wavelengths L-apart, provided their amplitudes are substantially equal. .At the point where the pedestal Lend 4| is located approximately wavelengths from the impedance irregularity in the coupling l4a, therhandle 34 of the clamp 30 .is actuated until the pedestal end is-driven to its maximum depth .into the under-surface .of the unknown waveguide .|4, whereby a circular dent 5.0 shownin Figs. .1,-=3 and .5.is permanently .produced in under-surface or one wide side of the unknown wave guide J4. This denthas-a depth of theorder .of one-@sixteenth inchianda diameter of approximately inch .for the size of the waveguide I4 previously mentioned, and results in the production of a tapering projection 50a, Fig. 5, formed integral with the inner surface of the lower wideside of the wave guide 14 and extending interiorly thereo'f. .In Fig. 5, the dent and projection are not shown to scale but .are somewhat enlarged .for the purpose of this :illustration; anditwill be obvious that the dimensions of the dent and projection may vary .ior different sizes of waveguide.

Shouldit happen thatthe effect 'of'th'e one dent 50 'fails to cancel the effect of the impedance irregularity in the coupling 14a, this means that "the amplitude 'of'the further-reflected wave'caused by the dent 50 is'less than the amplitude of the reflected wave caused by the impedance irregularity in the coupling Ma. Additional improvements in the effective cancelationof the reflected wave due to the impedance irregularity in the coupling |4a may be obtained by producing one or-more additional dents 50 in the wave guide I 4, each dent "being spaced wavelengths from adjacent dents. dent is located wavelengths from the impedance irregularity in the coupling |4a. .As a consequence, the famplitudes of the several further reflected waves due to the individual dents 50 effectively add *vectorially to cancel substantially the reflected wave caused by the impedance irregularity inherent in the coupling Ma.

The impedance irregularity in the flange 14b of the wave guide |4 may be compensated for by reversing the latter wave guide end for end in Fig. 1. so that the couplings Ma and 14b are interchanged in so far as their relative positions in Fig. l are concerned, and thereafter producing one or more dents 50 in the end of the wave guide .|4 adjacent the coupling I41) until the effect of the impedance irregularities due to the dents 50 substantially cancel the effect of the impedance irregularity inherent in the coupling |4b as here- .inbefore explained.

Over a range of samples of wave guide similar to the wave guide 14 in Fig. 1, it was found that a maximum of four dents 5|] introduced a suilicient number of impedance irregularities at each end of the sample wave guides to compensate for the impedance irregularity inherent in the coupling located at each end of the sample Wave guides; and further that such number of dents 50 had an adequate factor of safety to prevent a voltage breakdown between the projections 59a, Fig. 5, and the internal surface of the opposite side of the wave guide. In addition, the dent 50 and projection 50a have the advantages of (a) being pressuretight where the wave guide M is operated undergas pressure, and (b) not affecting the band-width characteristic of the wave uide 14. I

It has also been found that one or more dents 60, similar to dent 50, may be formed in the wave guide I4 at spaced points along the longitudinal axis thereof to correct for one or more impedance irregularities located elsewhere in the waveguide 14 but not in the couplings thereof and due, for example, to variations in the cross-section thereof. In this connection, it will be understood that one or more dents "60 may be located in the manner previously described relative to the production of dents 50 adjacent the ends of the wave guide [4.

An impedance irregularity in the coupling at each end of wave guidesections 52 and 53 shown in Figs. 1A and 13, respectively may be compensated for by substituting these individual sections to the right of the line aa for the section M in Fig. 1, and thereafter producing one or more permanent dents 50 and projections 50a in both ends of the respective sections, adjacent the couplings thereof in the mannerpreviously. explained in connection with wave guide 14 in Fig. 1. Additional dents may be produced in the bends of the

sections

52 and 53 in Figs. 1A and 13 to compensate for impedance irregularities inherent in these bends, and may be located in the manner previously described relative to the production of dents 50 in the wave guide section 14.

It will be understood that the aforenoted procedure may be employed to compensate for impedance irregularities in tubular wave guide sections whose initial SWR. is above the tolerable value 1.10 so as to reduce the SWR of these samples at least to this tolerable value, or alternately to a value therebelow, if possible. This would tend to reduce the quantity of wave guide rejected on the ground of having a SWIR. in excess of the tolerable limit of 1.10.

Figs. 1, 1C and 3 comprise an arrangement for compensating for impedance irregularities inherent in the directional coupler l3 and due, for example, to

imperfect coupling openings

29 and 29a, the resistive cards 27, and variations in cross-section. To accomplish this compensation, the apparatus included in the box 60a in Fig. 1C is substituted for the apparatus included in the box 6| in Fig. 1. Box 60a comprises the directional coupler I3 and the usual adjustable resistive-card load Hi. In the operation of Figs. 1, 1C and 3, the F/B ratio is measured by effecting the per cent readings on the meter [9 in the manner explained hereinbefore regarding the F/B ratio for the wave guide M in Fig. 1. This ratio for the directional coupler [3 in Fig. 1C should be approximately 35 decibels for the average sample thereof before the application of the compensating dents 50 and projections 50a thereto.

Then, with the meter [9 connected to the tap I! in Figs. 1 and 1C, the clam 30 is disposed on the auxiliary wave guide 26 in Figs. 1 and 10 in proximity of the three coupling openings 29, in a manner similar to that shown in Fig. 3, except one right-angled member 31 is removed from the clamp 30. As the pressure of the pedestal end 4| is gradually increasedon the under-surface of auxiliary wave guide 26 without producing a permanent mark therein as pointed out previously concerning wave guide M, the reading of meter i9 is noted. As this reading tends to decrease, then the clamp 30 is shifted a small distance to the right or left of the initial point and the clamp 30 operated again, So long as the meter reading tends to decrease, this procedure of the shifting and operating the clamp 30 is repeated until eventually the clamp 30 is located on a point of the auxiliary wave guide 26 at which the maximum sensitivity of the decreasing meter reading is obtained. The further reflected wave produced at this point is substantially degrees out-of-phase with the unwanted reflected Wave in the directional coupler at the reflected-wave tap. Accordingly, the clamp 30 is operated to produce a permanent dent 50 and projection 5041, Figs. 10 and 5, in the auxiliary wave guide 26 in the manner above described regarding unknown wave guide M. This dent, as above-mentioned, will be located approximately wavelengths from the impedance irregularity to be corrected for. Additional dents, each spaced a distance wavelength from the next previous one, are produced in the auxiliary wave guide 26 as required, as mentioned hereinbefore-concerning unknown wave guide [4. The F/B is then measured by connecting the meter I9 to the directional tap 20, and thereafter adjusting the attenuator 2i until the meter reading substantially equals the meter reading when the meter [9 was lastconnected to the tap H. The F/B ratio is the sum of .the reading of attenuator 2| plus the fixed ZO-decibel attenuation of fixed attenuator 22. The directional coupler I3 is then reversed end for end,

and the foregoing correction comprising one or more dents 50 and projections 50a are produced in the auxiliary wave guide 28. Both taps l1 and 20 are suitably corrected for the above-noted impedance irregularities, when the F/B ratio is greater than 50 decibels for each of these taps.

What is claimed is: 1. A tubular uniconductor wave guide for high frequency electromagnetic Waves having a plurality of permanent indentations in the wall thereof spaced along the guide in such correlation with the length of said waves in the guide as to have a cumulative reactive effect on said waves, said indentations projecting into the interior of said guide a distance small compared with the crosssectional dimensions of the interior of said guide.

2. A tubular uniconductor wave guide for high frequency electromagnetic waves including an inherent impedance irregularity in said guide, means for exciting high frequency electromagnetic waves therein, and at least one integral indentation in a wall of said guide in the form of a rounded point-like protuberance extending inwardly of said guide, said indentation being located substantially an integral multiple of onequarter wavelengths of the mean frequency of said waves in said guide away from said irregularity.

3. The structure according to claim 2 including a plurality of said indentations spaced apart longitudinally along a wall of said guide substantially an integral mutiple of one-half wavelengths of the mean frequency of said waves in said guide as to have a cumulative reactive effect on said waves.

4. A tubular wave guide for transmitting electromagnetic waves and including an inherent impedance irregularity causing wave reflection, an imperforate dent formed in a wall of said guide, said dent forming an integral point-like bulge extending inwardly of said guide of such dimensions and having such location with reference to said impedance irregularity in terms of the wavelength of the waves being transmitted in said guide as to introduce into said guide a further wave reflection in such sense as tends m to cancel the first-mentioned wave reflection. CHARLES N. NEBEL.

10 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,108,640 Bieling Feb. 15, 1938 2,374,498 Quayle Apr. :24, 1945 2,400,777 Okress May 21, 1946 2,403,289 Kormar July 2, 1946 2,407,068 Fiske et a1. Sept. 3, 1946 2,419,613 Webber Apr. 29, 1947 2,423,383 Hershberger July 1, 1947 2,432,093 Fox Dec. 9, 1947 2,433,368 Johnson et a1 Dec. 30, 1947 FOREIGN PATENTS Number Country Date 503,467 Great Britain Apr. 6, 1939