US2637781A - Series reactance transformer - Google Patents

Description

May 5, 1953 J. I. BOHNERT SERIES REACTANCE TRANSFORMER Filed Sept. 14, 1945 FIG.

TO GENERATOR FIG. 4

FIG.5

LOA D TO GENERATOR INVENTOR. BOHNERT JOHN I.

ATTQIQVEY Patented May 5, 1953 UNITED STATES, OFFICE.

' m les answ a John I. Bohnert, Cambridge, Mass, assignor, by inesne assignments, to the Uni; 8' States of America as r presen eg; y the S.....9 a f $1 ew mis n avisaia ist a a Naa so;

i7 fllaims, 1 .8 A) i 2 My present invention relates in general to ap- 2 iliustrates in electrical scheme the .oql isYr paratus for eliminating high standing wave raalent circuit of the transformer of Fig. 1; tics in transmission line systems, and in p'artic- Fig.3 is a graph showing the react-ance curs/e uler to series reactance transf'orineiiffor' use in I I e a transmission line system having a freq'u y 9 transformer of Fig. 1;

sensitive termination to provide a relatii/ 'elycon- Fig. 4 illustrates in longitudinal section a stant input impedance to such a line overjarelatransformer of my invention installed in a holof the inductive and capacitive members of the tive'ly wide band of frequencies. lbw ave guide; and

When it is sought to transmit electromagnetic Fig'. '5 illustrates in electrical scheme the equivenergy of high frequencyirom ges-learners l alen t circuit 01f the apparatus of Fig. .i. frequency sensitive load throngh laYftrairisniissioh Referring now to Fig. 1, a coaxial line they: line system, it is necessary thatthe freguencyof ing anlouter conductor 2, and an inner conductor operation be kept relatively'constant ijf' the m:- 3 has afiix'ed to the innerk'conducter a projection pedance of the load is to remain relativel'y con- 4 extending longitudinally or axially with respect s'tant during transmission. Variations iethe 11 to theiw-a'veuide I. l'l'he projection t is under? eg fi y may cause the load edance t o 'belc'nt' at each end '5 and 3 in a direction parallel come inductive or capacitive depen ngupon the tolthe axis of the lin i to form first and second direction of variation. A variation ;1-n the load shout iciivcuitedltransmission line sections or stubs impedance will be reflectegl as a variationinF-the I and B'respectivelir between the projection 4 and input impedanceto the transmissionf'liri'e at the 29 the innerconductor 30f the coaxial line i. These generator. This in'turn is due to Ethef-fact'that sections are separated by a support 9 forthe the variation in the load impedance fwith'freprojection .4. The section of line contained ice.-

quency changes causes high standing wavratibs tween the projection A and the outer Wall .12 has in the transmission line. finch -h'ighl-stariding a-:low.er-l'nn pedance than the main line I, having wave ratios resultin'loss'ofpowerandfrequently a lesser spacing between conductors than the alsoin arc-overgin-the transmissionlinefbotflof said mainiline =1. .-flhe length of this section of Which are undesirable. lower impedance line is determined by the axial "It is an object of my invention to provide ,a length lof the lprojection 4, which length is prefseries reactance transformfil fi use 'a'nsenablysubstantially equal to one-half or an inmission line which will preserve "substantially no te'gralnumber'ef half wave lengths of energy at unaltered over a wide iband of fre'ciucncies the the resonance or kmid-loandoperative frequency. input impedance-to a transmission line which has Ifhe projection .14 projects toward the outer conafrequency sensitivetermination." ductor'ii." a distance: determined by the spacing It is a further object of my invention to pro required between these two members to prevent vide such an impedance matching-transformer voltage breakdown, and further by desired; im1- that will have no effect upon aptunedgtransrnisnedancejvalues inlthe .short circuited sections .11 sion'linesystem'when the system is operating at andira. Whe1i Ithe coaxial line i that is :bein the resonant frequency for-a --frequency sensitive used istnot extremelysmall; the projection iimay load, have an outer diameter as great as one-eighth of It is a stillfu1ther-object-of -my -i-nvention.=to 40 arwaveslengthofttheienergysloeing carried in-,-the previdesuch a series -reactance transformerlthat lineal. r will be simple to construct in the ordinarytrans- .iwi-dth and depth :of each short .circuitcd mission line of thecoaxialor,h'ollowmave guide stubls'Lorsflrare deterrninedhy the desirecliinpem type, ancey-alues; ilfihe first stub 11 hasa ciepth alwasss Other and furthercobjeets andfeaturesof greatermen one quarter but ,Iessthan' one-half present invention willrbecomeqapparent;upon.q;a the length f a ;;Wave of energy therein at any careful consideration of ,uthe dollowing detailed f ieqnencyj I net-operativefrequency "hand, while description When tak nytoeether-n t mherac mhew-se ond. tubefihes eaien a ways e s -t an panying drawing, the fignres of which illustrate one qu ll ter oftth e said length typical embodiments of the inye tion ,an fitllbsfl and fi mayiee mann r h;th ac bqdi ncntsmaah lement rm a eri s sioered -tooperate. -band operative frequ'e yrilr -Ffig. 1 illustrates.in longitudinal.sectiona series error; as will he hereinnelow s e a c t n fe me -Y W m'Q ra e ma n mel-tyn o essel. sion line;

g noyrto 2, there, is illustrated .a ion line system having a frequency senlong, this stub I will have the electrical effect of a capacitor, and is illustrated as acapacitor I in Fig. 2. It is to be noted that the inductor. I4 and the capacitor I5 are in series with each other; The half wave section of line contained between the projection 4 and the outer wall 2 in Fig. 1 is in Fig. 2 a section of line I3 having a lower im- 4 X0, as illustrated at the corresponding lower frequency operating points 24 and 25 respectively in Fig. 3.

The effect of the hereinabove discussed changes in the natures of the circuit elements 'I' and 8 of the transformer of my invention during frequency variations is to counterbalance simultaneous changes in the nature of the impedance of the load so as to preserve substantially unaltered the input impedance of the line I over a band of operating frequencies. The half wave line section I3 will become effectively shorter or longer than a half wave in the presence of frepedance 21 than the main line II, located between the two reactive elements I4 and I5. A generator (not shown) will see'an impedance Z0 upon looking into the line II.

In operation, my transformer is effective to cause the range through which the input impedance of the line It (or the line i) 'would vary during a particular rangeof operative frequency variations to become smaller than the normal range of input impedance variation for the same amount of frequency variation. This punching together of the input impedance limits within a frequency band may be considered to be accomplished in steps by placing first one andthen the other of the stubs l and 8 in the line I. By properly locating the transformer in the line I with respect to the termination or load, a point maybe found such that the addition. of a first reactance with one sense will cause a certain amount of launching, and the additionof a sec 0nd reactance with the opposite sense will cause still more bunching and at the same time match the line I to its load. The stubs l and 8 provide such first and second reactances. The desired bun'cliing of input impedance variation limits may be had without effectingan impedance match between the line I and its load, it being an object of my invention only to maintain the said input impedance substantially constant.

In" the case where the line I is matched to its.

load (or the line II to the load IZin Fig. 2) the stubs I and 8 should preferably be series'resonant at the resonance frequency for the line and load. The points and ii in Fig. 3 illustrate this condition, showing the reactances Xi. of the shorter (inductive) stub 8 and X0 of the longer (capacitive') stub I as equal in magnitude and opposite in sense. These reactances will mutually cancel each other, so that the stubs I and 8 will be effectively not present in the line I at the resonance for a shorter operating wave length. The

shorter (inductive) stub 8 will ha v'e a relatively larger inductive reactance XL, and the longer (capacitive) stub l a relatively smaller capacitive reactance X0. With a drop in frequency the stubs fl and 8 become effectively shorter. shorter stub 8 will then have a relatively smaller inductive reactance XL, while the longer stub 1 willhave a relatively larger capacitive reactance quency variations in the same manner as the stubs I and 8. When this section it has a length other than exactly one half a wave length, its effect upon the transmission line I will be to multiply the impedance of the line I at the point of entering the half wave section, which imdepending upon the direction of frequency shift. This effect is small compared to the effect of the stubs 1 and 8, and may be neglected.

Thus it may be seen that, when properly located in a line having a frequency sensitive termination, my transformer will maintain the input impedance to the line, as seen for example from a generator, substantially constant over a relatively wide range of .frequencies. In the event that a load having the opposite frequency characteristics from one with which my transformer may be in use is used, the transformer may be reversed in the line I or moved along the line a distance of the order of one half a wave length and satisfactory operation will be therewith obtained.

The limits of the band of frequencies over which the transformer of my invention will operate-satisfactorily are reached when either. of the twoshort circuited stubs I or 8 becomes exactly one quarter of a wave length long. In these limits one or the other of these stubs I or 8 will have substantially infinite impedance, as may be readily appreciated by reference to the graph of Fig. 3. An infinite impedance in series with the transmission line has the effect of openingthe transmission line circuit. Thus at a cer tain frequency above the mid-band frequency, the shorter stub 8 will be exactly one quarter of a wave length long, and become effectively an infinite impedance in series with the line I, while at another frequency below the mid-band fre quency the longer stub 1 will be exactly one quarter of a wave length long, and become effectively an infinite impedance in series with the line I.

' Fig; 4 illustrates a transformer of my invention incorporated in a hollow wave guide trans-- mission line5 l, having opposite walls 5| and 52 and an internal spacing D between said walls. The transformer of my invention is installed in the guide 50 on the opposite inner surfaces of these'walls 5| and '52, one or more projections 53 and 54 being affixed' on said opposite inner surfaces and projecting toward each other into the wave guide 50. The projections 53 and 54 extend axially or longitudinally with respect to the wave guide 50 and are substantially parallel toeach other. Similarly to the projection 4 in the apparatus of Fig. 1 the projections 53 and 54 form a second wave guide section having an in ternal spacing d, which section corresponds to the h lf Wave transmission line section W of F s- 2- The pr je t ons 53 an 5 a e llbbbil ti ly one-half the le gt o wave he Wa e sn d e t on bo nded y th m of ene gy at th mid-band operative frequency al o. in th ame manner as in the case of the projection l; of the apparatus of Fig. 1, the ends of the projections 53 a d 54 are u d cu t f rm sho and one. short-circuited transmission line stubs 55 and 55, r p tively. between e ch pro ec ion 53 o 54 and the neighboring wall 5| or 52 respectively. A pair of StllbSlEfi and 56 are separated from each other by a support 5,! for each projection 53 and 54. The shorter stubs 55 are, as in the case of the apparatus of Fig. 1 always less than onequarter of the length is of a wave therein at any frequency in the operative frequency band. while the longer stubs 56 are always greater than One-quarter but less than one-half of the length As of such a wave. The outermost distance a projection 53 or 54 extends from the inner surface of the wall 5! or 52 respectively to which the projection is aflixed is again preferably determined by the impedance considerations in the stubs 55 and 56, and the spacing at which voltage breakdown will take place between the two projections 53 and 56. It is satisfactory to space each of the projections 53 and 54 inwardly a distance substantially equal to one-sixteenth of the length of a wave of energy in the wave guide 50. Thus the cross-sectional spacing d of; the half-wave length section of the line bounded by the two projections 53 and 54 may for example be substantially K; as great as the cross-sectional spacing D between the walls 5| and 52 or the wave guide 5d.

Because of the above mentioned ratio between the spacings D and, d it can be reasonably predicted that the half wave length section of guide bounded by the projections 53 and 54. will not become a wave guide beyond cut-oil. For example in the case where the wave guide is a cylindrical wave guide having a diameter D large enough to permit the passage of the TMm mode but small enough to inhibit the passage of the TEzi mode, the diameter D will desirably be slightly less than .97A, where A is the wave length of energy being carried in the wave guide 50. A small diameter d which is of 97% would be approximately .85i. However, the small diameter it would have to be reduced to .76)\ before the TMM mode were totally inhibited. Thus it can be seen that the transformer of my invention can be placed in a cylindrical wave guide without interfering with a normal mode of operation of that wave guide. Those skilled in the art will realize that the transformer of my invention may be placed inside other forms of waveguides without substantially interfering with the normal mode of operation of such wave guides.

Referring now to Fig. 5, it is apparent that the apparatus of Fig. 4 is similar in nature to that of Fig. 1. The load l2 having an impedance Z may be placed at one end of the line 9! and the other end of the line 90 connected to a generator (not shown). The shorter and longer shortcircuited transmission line stubs and 56 on the upper wall 5| are represented by inductive and capacitive circuit elements 54 and I5 respectively, while those on the lower wall 52 are represented by similar circuit elements l4 and i5 respectively. The one-half wave length section of line contained between the two projections 53 and 54 may be represented by a half-wave line section 93. Inasmuch as the elements I4 and I5 are me ely licates of'the corresponding elements 4 and Hi. these elements may be lum e togethe with their respective opposites to yield in eff ct. t ci uit of Fig. 2.. A further nalys s of t appa a us of Fig. 4 i accor ance with the ir- Qu t Of his. 5 c n thus be seen to be substan ial y identical to the analysis of theapparatus of Fig. 1. The apparatus or Fig. 4 functions in substantially the sam manne as he ap aratus of Fig. 1 when installed in a wave guide transmission line system, with one end of the guide 56} connected to a genorator (not shown) and the other end connected to a load having an impedance characteristic that is frequency sensitive. I

A th u h I a hown t e t ans orm r of my invention having short-circuited transmission line stub ha a e r spectively less han and greater than one-quarter ofja wave length long and projections comprising those stubs which are ubsta ially one-half wave len th l n it is o be understood that the aforesaid projections, such as for ex mple the Projection. 4 in the apparatus of Fi 1 may be n int ral number f half-wave lengths long. Likewise the Short circuited stubs 1 and 8 of Fig. l or corresponding stubs 55 and 56 of Fig. 4 may be made in the neighborhood of three-quarters of a wave length long, or any odd number of quartenwave lengths of energy in those stubs. Therefore it is to be understood that my invention is not to be limited except as may be required by the prior art and spirit of the appended claims.

I claim:

1.. A broad-banding matching transformer for use in coaxial transmissioniines comprising, a first coaxial transmission line having an inner and an outer conductor, and an axially extend ing projection coaxially affixed to, said inner CQIl-r ductor and projecting toward said outer conductor a portion of; the distance between said conductors to form a section of a second coaxial transmission line between said projection and said outer conductor, the axial length of said projection being substantially equal .to an integral num ber of half wave lengths in said second coaxial line of energy at the mid frequency of the operative frequency band, said rojection being undercut at each end in an axial direction to form first and second similar short circuited transmission line stubs between said projection and said inner conductor, said first stub being of a length always less than one quarter of a wave length therein of energy at any frequency in said operative frequency band, and said second stub being of a length always greater than one quarter but less than one half of said last mentioned wave length.

2. A broad-banding matching transformer for use in hollow wave guides comprising, a first ho1 low wave guide, and one or more axially extending projections affixed on opposite inner walls of said wave guide and projecting toward each other a portion of the distance between said walls to form a section of a second wave guide between said projections, the axial length of said projections being substantially equal to an integral number of half wave lengths in said second wave guide of energy at the mid frequency of the operative frequency band, said projections being each undercut at each end in an axial direction to form first and second similar short circuited transmission line stubs between each of said walls and the thereunto aflixed projection, said first stub being of a length always less than one quarter of a wave length therein of energy at any frequency in said operative frequency band, and said second stub being of a length always greater than one quarter but less than one half of said last mentioned wave length.

3. A broad banding transformer for use in transmission line systems comprising an open ended transmission line section of over-all length substantially equal to an integral number of halfwavelengths of energy therein at a predetermined frequency positioned within said transmission line, said section being formed to be capacitively reactive at one end thereof and inductively reactive at the other end thereof.

4. A broad banding transformer for use ina transmission line comprising, a section of overall length substantially equal to an integral number of-half-wave lengths of energy therein at a predetermined frequency positioned within said transmission line, said section having annular openings at one end thereof coaxial with said section and of a depth shorter than a quarter-wave length of said energy therein at said predetermined frequency, and having annular openings at the other end thereof of a depth greater than a quarter-wave length of said energy therein at said predetermined frequency.

5. Apparatus for maintaining the input impedance of a transmission line having a frequency sensitive termination substantially constant throughout a band of frequencies deviating from a mid-frequency comprising, a conducting structure positioned within said transmission line and cooperating therewith to form an open-ended transmission line section having a length substantially equal to a half-wave length at said mid-frequency, said structure being undercut at each end to a depth in a direction longitudinally of said transmission line to provide a capacitive reactance in series with said transmission line at one end of said section and an inductive reactance in series with said transmission line at the other end of said section, said reactances being series resonant throughout said band of frequencies'.

6. A broad-band impedance matching transformer for use in a transmission line comprising, a conducting structure positioned within said transmission line and arranged to form an open- 8. ended transmission line section of overall length substantially equal to an integral number of halfwave lengths of energy at the mid-frequency of said band having an impedance smaller than the impedance of said transmission line, said structure bein undercut at each end in a direction longitudinally of said transmission line to form first and second similar short circuited transmission line stubs between said transmission line and said structure, said first stub always being shorter than one-quarter wave length of energy therein at any frequency in said band of frequencies, and said second stub being of a length always greater than one-quarter but less than one-half of said last-mentioned wave length.

7. A broad-band impedance matching transformer for use in a hollow wave guide system comprising, a section of hollow wave guide, and a pair of projections secured to opposite inner walls of said Wave guide and extending axially thereof, said projections extending toward each other and forming an open-ended transmission line section having a length substantially equal to an integral number of half-wave lengths at the midfrequency of said bands, said projections being each undercut at each end in an axial direction to form first and second similar short-circuited transmission line stubs between each of said walls and the thereuntosecured projection, said first stub always being shorter than one-quarter of a wavelength of energy therein at any frequency in said band, and said second stub being of a length always greater than one-quarter but less than one-half of said last-mentioned wave length.

JOHN I. BOHNERT.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Re. 26,859 Potter Sept. 13, 1938 1,898,180 Hansell Feb. 21, 1933 2,233,166 Hahn Feb. 25, 1941 2,241,582 Buschbeck May 13, 1941 2,438,912 Hansen Apr. 6, 1948 2,530,418 Alvarez Nov. 21, 1950

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