US2644928A - Directional transmission line transducer - Google Patents

Description

July 7, 1953 NORTON 2,644,928

nmsc'rxomu. TRANSMISSION LINE mmsoucsa Filed June 9, 1948' 2 Sheet-Sheet 2 I III/ 1111 v i i 'IIIIIIIIIIIl/IIII/lIlIIlIIIIllII/Il ATTORNEY Patented July 7, 1953 DIRECTIONAL n sMl sl n LINE 'rn NsnUcun Lowell E. Norton, Princeton Junction, 'N.J.',- as,-. si ner to R d o (inven io of Amer a. a an rot on 9. De awar Application June 9, 1948, Serial No. 31,850

, 8 Claims. (01.333-33) This invention relates to directional trans- I ducers, and more particularly to directional transducers for use with transmission lines.

When speaking of transmission lines herein,

the term is used generically to denote an elec- I trical line for the conductienof electromagnetic energy, so that the term includes hollow pipe waveguides, as well as two-conductor transmission lines such as coaxial transmission lines or coaxial waveguides, and open wire transmission lines. In using transmission lines for the conduction of energy from a source or generator to a load, it is ordinarily desirable that the load presented to the generator shall be substantially free 01 changes due to changes in the load at the far end of the line. This is particularly true with magnetron generators for example. It is well known that magnetron generators are particularly sensitive to changes in load which cause the generator to change the frequency of the generated waves, or to increase or decrease the amplitude of the generated waves. Thus, the phenomenon of puIling in connection with magnetrons is well known to the art. This problem becomes particularly acute when the final or useful load,'such as an antenna, to which the power from the 'generator'is fed, is physically located some distance from the generator. along a transmission line whichis many wavelengths long when high frequencies are employed. As a result slight changes in the antcnnaioading are magnified due to the long electrical length of the line. The magnified change in load presented to the generator may be minimized by placin i t ine a directienel t a uce A- The power is usually fed tothe antenna" It is another object of the invention to provide a directional transducer relatively insensitive to load changes.

These and other objects, advantages, and novel ieatures of the invention will be more clearly understood from the following description taken in connection with the accompanying drawing in which:

Figure 1 is a schematic diagram of an open wire transmission line between a high frequency generator and a load and employing the directional transducer of the invention;

Figure 2 is a face view of a directional transducer of the invention as used in a rectangular waveguide transmissionline;

Figure 3 is a face view of a'directional transducer of the invention using circular waveguide;

directional transducer is a devicewhich is-preff erential to the passage of electromagnetic energy in one direction over passa e inthe reverse di rection. When such a directional transducer is inserted in the transmission line between the generator and the load, ,and preferably placed near the load, the efiect oi. slight changesQ-in'the load is considerably minimised because theimpedance thereafter presented to the transmission line at its junction with the directional coupler new changes only slightly with load changes.

It is an objectof the'present invention to-pro: vide a novel directional transducer.

It is a further object "of the inventiontoprovide such a transducer which "is of particularly simple structure.

It is a further object of the invention to provide a transmission line directional t-ransducereiima proved directional characteristics.

Figure 4 is a sectionalview of a directional transducer of the invention in a rectangular waveguide in fwhich'the coupling" between the shunt section and the, waveguide proper is less than unity;

Figure 5 is afacevieizv of adirectional trans.- ducer employing a plurality of directional trans-.- ducer sections of rectangular waveguideaccording to theinvention'; I a

Figure 6 is a sectional view of a directional transducer of the invention using a highly resistive impedance termination.

In accordance 'with the invention, my directional transducer comprises a section of transmission line; a reentrant transmission line portiion, and an impedan e mination. preferably.

energy absorbent, inserted in the reentrant portion." With :the termination impedance of proper value and properly placed, the desired directional transducer and other desirable characteristics are secured, as will appear more fully hereinafter.

Referring now more particularly to Figure 1, there is shown a generator I0, which may be a magnetron or any other type of ultra-high frequency generator. The generator may be matched to the line H through the transducer by any of various well known means, not shown. The generator [0 is coupled to a load [2 through an-euectwww rc ransmissio li '4 and a re ion l t ansduc r efh invention c isin he eciien lint tr s n wh h may he .cons de da or ien of th in l4. and 81sec? tion Qf'transrnission line I8 reentrant with line ll andj s nt ith ero ien The l [2: may: also be matched to the line by any ct various well knownmeane. not shown. The imnedanoe Z1 terminates the section It. at a point 4 intermediate to two junctions with line H. The section, looking into the section in the direction section [6 has a length a. The distance from in which the passage of incident energy is the junction 20 between the transmission line favored, appears to be a substantially stable load. section [8 and the section H at the point of This may be stated still another way qualitatively incident energy and the point at which imped- 5 by saying that whatever impedance may appear ance Z1 terminates the line section l8 hasalength on the remote side of the line, that side of the c. The distance between the point at which imline nearer the generator sees a load which tends pedance Z1 is shunted across the line section l8 to receive energy but from which energy is reand the junction 22 of line section l8 on the load fiected, if at all, in relatively constant amounts. side of line H is b. This results because the transducer tends to pre- The directional transducer of Figure 1 may be vent the return of energy reflected from whatanalyzed by considering the impedance Zr lookever load may be placed on the remote side of ing into the transducer arrangement at the juncthe directional transducer. tion 20, that is looking in the direction from the The action of the transducer shown in Figure generator the loadh o wing Set Of l is not truly susceptible of qualitative analysis.

equations may be derived by inspection of the The only proper approach to ascertain the action figure: thereof is byv inspection and study of Equation 9. The new transducer presents to the line a EQUATIONS 1 to 8 substantially constant impedance within moder- (1) ate limits of variation of the various parameters.

This is equivalent to saying that the relative variations in input impedance, Z'r, are less than the I 221 .b COS d-H S111 5 relative changes in terminal impedance, Zn, and

' therefore the mis-matches and reflections due (3) to the undesired changes in Zn have less efiect upon the generator than they would have pro- I I I (4) duced without the network. Nevertheless, before I +1 17:1 showing that such a result occurs, at least for m certain selected values of the parameters in- E m= T= r 00$ B 5111 5 volved in the equation, we may qualitatively re- E ,1,=E,.=E, cos {ib+iI, Z sin 5b (7) view the structure of Figure 1 with a view to a better understanding of its operation.

cos fic+dnzk Sm (8) The generator looks into a'line at the end of In the foregoing equations, the T5 are current which the line looks into an impedance. This e pressions, the Es are voltage eXD eS and impedance is the impedance Zr of the equations 6 s t propagation constant Thus above. At-the junction 20 at which this imped- 27F ance is calculated, it will be observed that the incident energy splits in two directions, neglecting for the moment any portion which may be where x is the line wavelength. All of the lines 40 reflected. One portion travels the line section have a characteristic impedance Zk. The sub- Hi to the junction 22 and the other portion scripts indicate the points at which the current travels the line section 18, and only a part of and voltage values are taken, and are markedon this second portion reaches the junction 22 be- I ,,=I, cos {Ba-H sin Ba Figure 1 in such a fashion as to be self-explanacause of the interposition of the line terminatory to those skilled in the art. From this set of tion Z1. A moments consideration will convince equations it is possible to derive the following eX- one that it is possible to choose the line lengths pression for Zr. and impedance Z1 in such a way that reflections fi s 1 s [fi( )]i Z T: l

' 1 2 sin [B(b+c)] iZ b b S n[B(a+c)] Z tZ 00S ;3(bc)cos [B(b+c)] 2 SlIl flai 1 +c)] cos +C)]}+ iZ sin [fi(r+b)] Z sin fib sin 6b I sin Ba Z sin [B(b+c)] Z1 K eoS [fl( -i- 1 1'Z sin 8b sin Ba i sin [M i-11)] sin 8 Z i'Z sin Ba sin db 1 Z sin flb+ sin 6b iZ sin [[3(b+c)] Z l 1 sin [B(a+b)] sin dc Z 'iZ sin Ba sin Bb The various characteristics of the directional from the useful load are substantially or even transducer section may be analyzed by means entirely cancelled at the junction 20 with reflecof Equation 9. Such analysis shows that this tions from impedance Z1, whereas energy from transducer section in general has directional junction 20 is transmitted at junction 22 toward transmissive characteristics. In other words, it the load. One ofthe important characteristics preferentially transmits energy in one direction of the new transducer arrangement, however, rather than in the reverse direction. The result cannot be explained in such a simple manner. of such directional characteristics may also be This characteristic is the insensitivity of Zr to expressed by saying that the directional coupler 7 changes in the impedance ZR.

perature variations. The other end of this quarter wavelength line section is connected in Figure 1 to form the termination impedance Z1, so that I Z 22,, If ZR may change to (1+A)ZR Where A is con- Z 'iZK cos or T sin 2iZKZ1 cos a-Zfi sin (1 cos a Z Z (l-2 sin? a) 1,Z s1n a, Z cos 04+ ZR cos ZR 2'LZ Z sin a I 2z'Z Z Z t-an a I v Z (23$in a) iZK -j-mZ Z s1n u+- R sin a It is clear, because of the periodicity of the funcsiderably less than one, say less than rt, it may tions involved that [5a, oh, and 80 mayeach be be shown that p changed by amounts of 21r without altering these I Z} results. Even with the assumption 1 already T v made, there are still many possible values to choose which make ZT relatively constant-with changes in ZR. If ZT remains constant in this sense, at a value of ZR, then the standing wave ratio between a generator and Zr is substantially independent of changes in ZR. Choose, for example, a as (ZN-1) 11', where N is an integer. Then Equation 10 then reduces to:

Again, it will be understood that any of the quantities ca, 181), so may be different by 21r from the above assumed values, without altering the results.

The termination ZR may be considered as th impedance of the useful load. It will be understood that Za is subject to various changes, for example, changes due to ambient temperature variations. Now, it is desired that at least for small changes in ZR. the impedance Zr shall remain small. Thepresent structure has a further advantage. Not only is the per unit change in ZT with respect to per unit changes in ZR, small, or even zero at selected values, but the second rate of change, of ZT with respect to Zn is also small. This characteristic is even more important than it is that the first rate of change be small. The importance of this advantage is that over a substantial range of values of ZR the rate of per unit change of Zn: with respect to per unit changes in Za is small. In other words, the slope of the curve of ZT plotted with ZR asan independent variable is small and rem-ains'small over a wide range of values. This range of values may readily be found to the desired degree of accuracy and for the desired limiting values of the slope, by making the plot of Z: against ZR to scale. To calculate it is very difficult. When we say the slope is small, We mean that it is fractional, that is, less than one. 1

Let

be the characteristic impedance of a section ofa quarter wavelength transmission line atone end of which there is a termination havingan impedance substantially equal to the lo-ad impedance ZR and undergoing substantially the same changes, for example, those due to ambient temapproximately, which indicates that the change in Zr'is only about 45% as great as that in .Za.

As a second example if Z1 is madeof a material like barium titanate which has a decreasing re-' sistivity with field strength, then by suitable choice of ZK, Z121, and Z1 it is again possible to reduce the variation of Z'r with changing ZR.

For Z1:Za,. and assuming that Zn changes to ZR(1+A) where A is considerably less than one, then it may be shown'that which indicates a change in Zr only about 37% as great as the. change in ZR. Thus it will be observed that with such disparate values as Zl=ZK /2ZR and Z1:Za, for the two conditions, the difference in the rate of change of Zr with ZR variesonly by about 8%.

Referring now to Figure 2, which is a sectional view of a directional transducer of the invention, a quarter wavelength section 30 of hollow pipe rectangular cross-section transmission line is shuntedby a section 32 of similar hollow pipe line. The section 32 of transmission line is reentrant on the section 30 having junctions therewith at 34 and 36. A line termination 38 is inserted in the section 32 at a distance an odd number of half wavelengths from the junction 36 and the same odd number of half wavelengths plus a quarter wavelength distant from the junction 34.

Thus the length 'of the entire section 32 is twice an odd number of half wavelengths plus a quarter wavelength, as may be the case for the specific. values chosen for Figure 1 to derive Equation 10. Since ca, 81 and Bo occur in periodic functions with a periodicity of Z any of or that side of the line transmission which is to be favored. A similar directional transducer is shown in Figure 3 in face view, in which the waveguide transmission line is cylindrical in cross-section and the construction and nature 7 thereof will be obvious to those skilled in the art from what has been said heretofore.

It will be observed that in Figures 2 and 3 the coupling of the reentrant section is effected y utilizing a complete opening formed at the intersection of the internal surfaces of the line section and the reentrant section at the junctions. A somewhat different arrangement is shown in Figure 4 illustrating a directional transducer of the invention for use with rectangular Waveguides. In Figure 4 the junctions are at 50 and 52. The section of the main line is 54 and the reentrant section is 56. The termination in line 55 is shown at 58 as a waveguide stub, similar to the stubs utilized for such terminations in Figures 2 and 3. However, at all of the junctions shown in Figure 4, the coupling between, for example, the reentrant section 56 and the line section 54 at 59 and 52, are formed by windows in section 54 allowing only limited communication at the junction between section 54 and the reentrant section 56. I often prefer the use of such window couplings, because they reduce the coefficient of coupling between the line proper and the reentrant section and thus tend to avoid reflections back in the direction from which the incident energy arrives. The directional transducers of Figures 2, 3 and 4 may be analyzed by the same methods employed in the analysis of the coupler of Figure 1, and with the similar results. The appropriate application of these formula to the waveguide structures will be understood by those skilled in the art from what has been said heretofore.

A preferred embodiment of the invention is that illustrated in Figure 5 which includes a plurality (three of which are shown) of such directional transducers as those shown in Figure 4 placed in tandem along the line. Directional effects are enhanced by using a plurality of the directional transducers of Figure 4 arranged as shown in Figure 5, each of which contributes to the directional transducer effect and yet, because of the small coefficient of coupling with the reentrant portions, reflections from "the transducer of the incident energy back in the direction from which it arrives are reduced. With a plurality of such sections, any change in impedance looking into the first section due to the change in a load on the end of the other section will be reduced as the product of the fractional change, at least for very small changes to a first approximation, which would be resultant from one alone; thus supposing that for each of two such sections, the change in Zr is about /2 for a given change in ZR in the notation of Figure 1. When combined the change in Zr of the two sections will be about A; for the same change in the load. Lossy material may be included in the terminations 59, as shown.

Figure 6 shows a directional transducer of the invention which is inserted in a coaxial line. The termination impedance Z'r in this instance comprises a resonantcavity 60 coupled to the reentrant branch 62 by a line stub 64. The cavity is coupled to the line stub 64 by an exciting probe 66.v The cavity is filled with an energy absorbent material 68 to increase the resistive component to a desired value. A high resistive component is generally desirable in the termination impedance Zr. Coupling may be further reduced by removing a part of the contacting portions 62a and 62b to provide capacitive coupling of the shunt section.

What is claimed is:

1. A linear passive network directional transducer comprising a transmission line section substantially a quarter wavelength long at the operating frequency, a reentrant transmission line section having junctions with said quarter Wavelength section at the ends thereof, an impedance termination in said reentrant section at a point an odd number of half wavelengths from one said junction and an odd number of half wavelengths plus a quarter wavelength distant from the other said junction, said impedance termination terminating the portions of said reentrant line section on each side thereof in a, highly resistive power absorbing impedance having a value different substantially from that of the characteristic impedance thereof.

2. A linear passive network directional transducer comprising a first transmission line section having a length \/4+(n11) at the operating frequency, a reentrant transmission line section having junctions with said first section at the ends thereof, and a highly resistive power absorbing impedance termination in said reentrant section at a point (2nz1) \/2 from one said junction and (2n31) \/2+ \/4 from the other said junction, n1, m and n3 each being a positive whole number, A being the line wavelength at the frequency of operation the impedance of said termination being substantially different from the characteristic impedance of said reentrant section.

3. A linear passive network directional transducer comprising a first transmission line section having a length \/4+(n1-1) at the operating frequency, a reentrant transmission line section having junctions with said first section at the ends thereof, and an impedance termination in said reentrant section at a point (2n-1) \/2 from one said junction and from the other said junction, m, m and 11,3 each being a positive whole number, i being the line wavelength at the frequency of operation, said termination impedance including a highly resistive power absorbing component and differing substantially in value from the line characteristic impedance.

4. The directional transducer claimed in claim 2, wherein said transmission line sections are hollow pipe wave guides.

5. The directional transducer claimed in claim 2 wherein said transmission line sections are two-conductor transmission line sections.

6. The directional transducer claimed in claim 1, the coupling between the sections at said junctions being very much less than unity coupling.

7. The combination comprising a plurality of transducers as claimed in claim 1, connected in tandem.

8. The combination comprising a plurality of transducers as claimed in claim 2, connected in tandem.

LOWELL E. NORTON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,147,809 Alford Feb. 21, 1939 2,190,131 Alford Feb. 13, 1940 2,226,686 Alford Dec. 31, 1940 2,251,997 Goldmann Aug. 12, 1941 2,519,734 Bethe Aug. 22, 1950

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