US 2517969 A
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Aug. 8, 1950 BROWN 2,517,969
REACTANCE COMPENSATION SYSTEM Filed March 50, 1945 IN VEN TOR.
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197' TOR/V5 Y Patented Aug.- 8, 1950 1 REACTANCE COMPENSATION SYSTEM George H. Br'owm' lrinceton," N J., assignor to Radio Corporation of America, a corporation of I .f "Delaware mania March 30,1945, Serial No. 585,611;
This invention relates to tion, and more particularly to improvements in the art of. neutralizing-the effects of reactances which are unavoidably present in radio frequency power transmission circuits As is .well known to those. skilled in the art, thefrequency band width through which a transmission circuit operates efiiciently to transfer power is a function of the shunt reactances in the circuit. q l The principal. object of, the present invention is 'to provide improved'methodsof and means for compensating the effects of reactance in radio frequencytransmission circuits. v.
Another object is toprovide improved methods of and means for obtaining broad-band operation of radio frequency transmission circuits." A further object is to provide improved methods of and means for compensating the reactances of shmv't-circuited quarter wave line sections'such as are'used in line balance'convertor systems and the like.
The invention will be described-with reference to the accompanying drawing, of which:
Figure 1 is a schematic circuit diagram of a radio frequency transmission circuit embodying the instant invention,
Figure 2 is a schematic diagram of a' compensated line balance convertor in accordance with prior art practice,
Figure 3 is a schematic diagram of a compensated line balance convertor in accordance with the present invention, and l Figure l is a modified form of the device shown in Figure3.'-
Referring to Figure 1, it is assumed that aload I, of impedance R, is'to be energized from a source 3, having an internal impedance R, through a transmission line 5 having a characteristic impedance Zn. The "value of Z is made equalto the impedance R of the load I and'the source 3, ,so that in the absence of any further circuit elements, the load I and line will bematched to the source 3 at all frequencies, providing efilcien t transfer of power independently of frequency.
Suppose that it is necessary to connect'across the load I a transmission line section 7, shortcircuited at its end away from the load It is well known that short-circuited transmission lines exhibit characteristics similar to those of parallel resonant circuits. Thus at the frequency "at which the line i is one quarter Wavelength long, a very high impedance, resistive in character, is presented across the load I. At lower frequencies,
theline I acts like an inductance connected to the load Lwhile at higher frequencies the line 1 presents a capacitive reactance across the load I.
As long as the reactance of the line 1 isvery much greater than the impedance R of the load I, there is substantially no-impeclance mismatch caused by, the presence of theline l. l-Iowever, at
reactancecompensa all frequencies outside of a relatively narrow band, the reactance of the line I will be of the same order as the impedance R; or lower. At any of these frequencies, a major portion of the current from the source 3 will flow through the line I, causing serious impedance mismatch and preventing efficient transfer :of energy from the source 3 to the load I.
In accordance with the present invention, a short-circuited half wavelength line section 9 is connected in series with the load I and the quarter wave line 1. By proportioning the conductors of the line $170 provide a surge impedance Z2 which is a predetermined function of the load impedance R and the'impedance Z1 of the line 1,
the line '9 may becaused to introduce reactance variations which areequal and opposite to those of the line '1, through'a substantial range of frequencles.
Thefreactance.presented by the quarter wave line 1 across the load I, at a frequency f, is:
I; jR tan i We (-3 i2) jZ tan rationalizing,
seam @591 5 The reactive component Xe of 23. is:
. -41 Xa- RZi tan 0 I The negative sign indicates that Xa is negative (i. e. capacitive) at frequencies higher than fo,1.
and positive (inductive) at frequencies lower than The reactance of the half wave line 9 is:
and the rate of change of reactance with change 7 in frequency is:
dX z2 7 df f0 When f=fo;
m 1 (8) f )f=f Z2 0 Since the derivative 2 2 Eh) f )f=fc f f= Substituting Equations 5 and 8,
W I lfah ii) and 1 2 From Equation 11, it is evident that the characteristic impedance Z2 of the half wave line 9 may be determined from the load impedance R and the characteristic impedance Z1 of the shunt quarter wave line i! to provide substantial cancellation of the reactance in the neighborhood of the resonant frequency in.
The described method of reactance compensation may be appliedto the design of compensated line balance ccnvertors, such as are used for transferring radio frequency power between balanced and unbalanced circuits or lines.
Referring to Figure 2, one well-known type of line balance convertor includes a cylindrical conductive sleeve I I surrounding a quarter wave length section I3 at the end of a coaxial transmission line I5. The line I5 comprises the unsymmetrical or unbalanced circuit of the convertor'. The sleeve II is connected tothe outer conductor of the line section I3 by a conductive disc [7. A pair of transmision lines I9 and 2| are connected to the inner and outer conductors respectively of the line section I3. The lines I9 and 2i comprise the balanced circuits of the convertor. The sleeve II is slightly more than one half wavelength long, and is closed at its upper end by a disc 23. rod 25 extends coaxially within the sleeve II A quarter wavelength 75 tions to the unbalanced line I5.
' across'the end of the balanced line 2|.
4 from the disc 23 to the point of connection of the inner conductors of the lines I5 and I9.
A pair of quarter wave stub lines 21 and 29 are connected to the lines I9 and H respectively at points one quarter wavelength from the connec- The stubs 2'! and 29 are similar to the upper portion of the convertor structure formed by the sleeve II and the rod 25.
In the structure of Figure 2, the outer conductor of the line section I3 cooperates with the lower portion of the sleeve II to act as a quarter wavelength short-circuited line, connected The rod 25 and the upper portion of the sleeve I I act as a similar quarter wave line across the balanced line I9, so that the reactances across the lines I9 and 2I vary in the same Way with variation in frequency, and the lines remain balanced.
As mentioned above, the reactances of the quarter wave lines will decrease to such an extent as to introduce serious impedance mismatch when the frequency of operation is materially different from the'resonant frequency of the lines. This is counteracted to some extent by the quarter wave stubs ZI and 29, which vary in reactance in the same manner as the quarter wave lines II, 25, and II, I3. The reactance of the stub 21 is inverted by the final quarter wave sectionof the balanced line I9, providing an equal and opposite reactance at the point of connection to the line I3. This reactance cancels that of the rod 25 and sleeve II. The stub 29 acts in the same manner to cancel the reactance presented by the outer conductor of the line section I3 and the sleeve II. A compensated converter of the type shown in Figure 2 will operate efficiently over a fairly wide frequency band, but has the disadvantage of structural complexity.
Referring to Figure 3, a line balance converter embodying the reactance compensation of the present invention includes a sleeve 3|, one half wavelength long at the mean operating frequency, surrounding the final half wavelength section I4 of the unbalanced line I5, and connected to the outer conductor thereof by a disc I8. A larger sleeve III, similar to the sleeve II of Figure 2, surrounds the final quarter wavelength portion 32 of the sleeve 3I, and a conductor 33, one quarter wavelength long and of the same diameter as the sleeve 3|. The conductor 33 is connected to the sleeve III at its end by a disc I23.
The inner conductor ofthe unbalanced'line I5 is connected to the conductor 33. The balanced lines .I9 and 2| are connected to the adgacent ends of the conductor 33 andthe sleeve The quarter wave lines formed by the sleeve III with the conductor 33 and thesleeve 3| are shunted across the balanced lines I9 and H respectively, and are in series with each other between the adjacent ends of the conductors 3| and 33. The unbalanced line is connected to these points through the half wave short-circuited line section I4, 3|. It is evident that the arrangement is equivalent to that shownin Figure 1, except that-twoloads (the balanced lines I9 and 2I) are connected in series with each other, and each is shunted by a quarter wave line. The reactances are'compensated as described in connection with Figure 1, and the proper, value for the characteristic impedance Z2 of the half wave line section is still indicated by Equation 11 above, since both numerator and r ar rled-n li. 5110 be noted, however, that, the characteristic impedance of the l'ine-"l5 mustbe-twice'that 'of the lines I9 and 2| to provide proper matching This is'also true of the systemLL-of Figure 2...}:
Under. certain conditions, it .will be 'found that the-impedance Z2 must be so lhighthat-.itQis mechanically awkward to construct} the device .of Figure 3 with the proper ratio of diameter's'o'f'the elements III and-3| toeach' other and still have the diameter of the sleeve 3 I-,--in the proper-ratio to;.that of the outer conductor t the li ection 1. 1 In? hisvht he truc ure 1 1 is preferable. The final half wave portion I4 is surrounded, as in Figure 3, by a half wave sleeve I 3i, and the final quarter wave portion of the sleeve I3I extends into an outer sleeve 2I I, similar to the sleeve II I of Figure 3. The conductor 33 of Figure 3 is replaced by a half wave sleeve I33 with a coaxial inner conductor I34 connected thereto at its outer end. The inner conductor of the unbalanced line I5 is connected to the inner conductor I34, which is of the same diameter as the outer conductor of the line I5.
The system of Figure 4 is equivalent to that of Figure 5, except that the half wave line is in two sections, connected in series with each other. With this arrangement the characteristic impedance Z2 of the lines I4, I3l and I34, I33 is one half the impedance Z2 required in the system of Figure 3. The reactances are compensated in the same manner as in the systems of Figures 1 and 3.
Although the invention has been described with reference to short-circuited line sections of lengths one quarter and one half wavelength respectively, the principle applies to lines which are certain multiples of the described lengths. Any of the sections described as one quarter wavelength long may be replaced by sections m. quarter wavelengths long, where m is any odd integer, and the one half wavelength sections may be replaced by sections 12 half wavelengths long, where 11. is any integer. The impedance Z2" of the compensating (half wavelength) sections is then determined by the relation:
The invention has been described as an im-.
proved method of shunt reactance compensation, in which a half wavelength short-circuited line is connected in series with the element whose reactance is to be neutralized. By proper adjustment of the impedance of the half wavelength line, compensation is effected throughout a band of frequencies. Typical applications of the invention to line balance convertor systems have been described. It is to be understood, however, that the invention may be used in other similar situations where undesirable reactance is present.
I claim as my invention:
1. A line balance convertor including an unbalanced-to-ground circuit comprising a coaxial transmission line, a tubular conductor substantially one half wavelength long surrounding the final half wavelength portion of said line and other end of said second tubular conductor and is connected atone of its ends to the, end thereof and which has a diameter equal to theoutside diameter of the first-mentioned tubular conduce tor, said coaxial line, said cylindrical conductor, and saidfirst and second tubular conductors be ing concentric about'a common axis, the other end of said-cylindrical conductor being adjacent to and facing the end of said final portion of said first-mentioned tubular conductor, and a balanced-to-ground circuit comprising one conductor connected tothe end of the inner conductor of said coaxial line of said adjacent end of the cylindrical condhctor and another conductor connected-to"the' adjacent end of said first-mem. tioned tubular conductor. v 3 I f "2. A radio frequency network for transferring energy between twoterminationseach of impedance Rf'over an in-series circuit of an unbalanced-to-ground transmission line and a balanced-to-ground transmission line comprising an element in shunt to said circuit at the point of connection of the lines, the element being parallel resonant at a wavelength x and exhibiting parallel resonant characteristics similar to those of a short circuited line of length substantially where m is any odd integer and the line has a surge impedance Z1, a reactance compensating device for said element comprising a transmission line, of length substantially where n is any integer and of surge impedance Z2, short circuited at one of its ends and connected at the other in series with one of the conductors of the balanced-to-ground line near its point of connection to the unbalanced-to-ground line, the surge impedance Z2 of said line being such as substantially to satisfy the relation and a second reactance compensating device to compensate any residual reactance effects of the first compensating device, the second reactance compensating device comprising a transmission line of equal length and surge impedance to the first compensating device and being short circuited at one of its ends and connected at its other end in series with the other conductor of the balanced-to-ground line near its connection to the unbalanced-to-ground line.
3. A line balance convertor including an unbalanced-to-ground circuit comprising a coaxial transmission line, a tubular conductor substantially one half wavelength long surrounding the final half wavelength portion of said line and connected to its outer conductor at a point one half wavelength from the end thereof, a second tubular conductor substantially one half wavelength long one end of which surrounds the final quarter wavelength portions of said first-mentioned tubular conductor and the coaxial line within it and is connected to the former at a point substantially midway along its length, a second coaxial line which is one half wavelength long and short circuited at one end, has an inner conductor whose outside diameter is equal to that of the outer conductor to said first-mentioned line, and has an outer conductor whose inside and outside diameters are equal to those of "7 said first-mentioned tubular conductor; a quarter wavelength portion of the open end of said second: coaxial line being surrounded by the other end of said second tubular conductor and having the midpoint of its outer conductor connected thereto, all of said first and second coaxial line and said first and second tubular conductors being concentric about a common axis,. the open end of said second coaxial line being adjacent to and facing the end of said final portion of said first. tubular conductor, and a balanced-toground circuit comprising one conductor connected to one end of the inner conductor of said first-mentioned coaxial line over a connection including in series the conductors of the. second coaxial line and another conductor connected to the adjacent end of said first-mentioned and tubular conductor. I v
GEORGE H. BROWN.
8 REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS Number Name Date 2 ,174,963 Braaten Oct; 3, 1939 2,314,764 Brown Mar. 23; 1943 FOREIGN PATENTS Number Country Date 542,780 Great Britain Jam27, 1942 860.22g
France S pt. 24, 1940