US3731238A - Balun transformer with a single magnetic core and impedance transforming means - Google Patents

Balun transformer with a single magnetic core and impedance transforming means Download PDF

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US3731238A
US3731238A US00218139A US3731238DA US3731238A US 3731238 A US3731238 A US 3731238A US 00218139 A US00218139 A US 00218139A US 3731238D A US3731238D A US 3731238DA US 3731238 A US3731238 A US 3731238A
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winding
balun
turns
impedance
core
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E Jones
R Tanner
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Fluke Electronics Corp
Technology for Communications International
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Communications Technology Corp
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H7/00—Multiple-port networks comprising only passive electrical elements as network components
    • H03H7/42—Networks for transforming balanced signals into unbalanced signals and vice versa, e.g. baluns
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H7/00—Multiple-port networks comprising only passive electrical elements as network components
    • H03H7/38—Impedance-matching networks

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  • ABSTRACT Several embodiments of a high power wideband balun transformer are disclosed. In each embodiment a single high permeability magnetic core is incorporated. It is wound with a balun winding which converts an unbalanced impedance Z at unbalanced terminals to an equal balanced impedance across a gap region.
  • Balanced terminals are connected to the gap region of the balun winding by impedance transforming means, so as to provide a balanced impedance Z which is not equal to the unbalanced impedance Z
  • the impedance transforming means are the outer conductors of the balun winding a selected number of turns from the gap region, Z is less than Z
  • the impedance transforming means are additional windings wound about the core Z is greater than 2, and the ratio 2 /2 is a function of the number of turns N, between the balanced terminals and the number of turns N of the balun winding.
  • Apertures insulating boards are employed to control the spacings between the windings and the core, and the spacings between adjacent turns of the balun winding and the impedance transforming winding.
  • the present invention generally relates to balun transformers and, more particularly, to an improved balun transformer of the type in which windings are wound on a single magnetic core and which is capable of high power and/or wideband operation.
  • balun transformer or simply a balun, is a device which transforms an unbalanced impedance with respect to ground to abalanced impedance.
  • Some baluns are of the non-impedance transforming type in that they transform an unbalanced impedance only to an equal balanced impedance, while other type baluns transform the unbalanced impedance to a different balanced impedance.
  • the latter type may be referred to as an impedance transforming balun.
  • baluns In the prior art the bandwidth of operation of baluns has been extended by winding the balun. winding on a high permeability magnetic core.
  • One example of such a prior art balun is shown in Antenna Engineering by Walter 1... Weeks, page 173, published by McGraw-Hill Book Company, Library of Congress Catalog Card Number 68-13106.
  • the single core balun described therein is of the nonimpedance transforming type.
  • An impedance transforming balun is also described in the same book. However the latter requires two cores and is capable of only a 4:1 impedance transformation ratio.
  • Another object of the invention is to provide a new single core balun with a wide range of unbalance to balance impedance ratio.
  • a further object of the invention is to provide a novel high power broadband impedance transforming balun.
  • FIG. 1 is a top view of one embodiment of the invention
  • FIG. 2 is a cross-sectional view of a basic embodiment of the invention with insulating boards
  • FIG. 3 is a top view of another embodiment of the invention.
  • FIG. 4 is a simple diagram useful in explaining one of the embodiments of the invention.
  • FIG. 5 is a view of another embodiment
  • FIGS. 6 and 7 are top and side views respectively of yet another embodiment of the invention.
  • FIG. l' is a dia gram of one embodiment of a balun in accordance with the present invention in which an unbalanced irnpedance Z is transformed into a balanced impedance 2,, which is smaller than Z
  • the balun winding is formed of a coaxial transmission line 11 of a characteristic impedance 2,, which makes a plurality of turns, defined N,,, through the center of a magnetic core 12.
  • Both ends of the outer conductor or shield 14 of the coaxial line 11 are conductively joined together and connected to ground, which represents one unbalanced terminal.
  • the other unbalanced terminal, designated by numeral 15, is connected to the inner conductor 16 of the coaxial line '11 at one end thereof.
  • the unbalanced impedance 2, is between terminal '15 and ground.
  • the coaxial line 11 is completely severed at its midpoint to form a gap 18 and the center conductor 16 is either removed from one half, such as the left half of the line, or is shorted to the outer con ductor of that half at gap 18.
  • the center conductor 16 in theright half of the line is joined across the gap to the shield 14 of the left hand half of the line 11.
  • the shield 14 of line 1 l is in effect being used as a balanced autotransformer.
  • the balanced impedance, defined as Z,,, across terminals 25 and 26 is related to the balanced impedance at the gap defined as Z which equals Z by the relationship B G( s p U( r/ p)
  • N /2 is the number of turns between each balanced terminal and ground
  • N IZ is the number of turns between the gap and ground.
  • the unbalanced current which flows in the center conductor 16 and the inside of the outer conductor or shield 14 and which is designated 1, divides into a balanced current I, and a remainder current I which flows on the outside of the shield R4 of the right and left halves of the balun winding 11.
  • the symmetry of the balun winding, formed by the outside of the shield 14, insures that the balanced terminals of the balun are electrically balanced with respect to ground at all frequencies.
  • the range of frequencies over which Z is essentially equal to 4/9 2 is determined by the range of frequencies over which the current 1,, is small with respect to 1,.
  • the impedance of the winding is inductive and the magnitude of the impedance is directly proportional to the product of frequency f, core cross sectional area A, core relative permeability ,u and the square of the total number of turns, i.e., (N,,)
  • the impedance Z of the balun winding is resistive with a magnitude determined by the transformer losses.
  • the impedance Z of the balun winding is capacitive and its magnitude decreases to a small value at the frequency f,, when the electrical length of the winding is approximately a full wavelength.
  • the magnitude of the capacitive reactance at a frequency f is proportional to (f f) and inversely proportional to the diameter of the balun winding. Therefore, the maximum bandwidth of operation for a low power balun is achieved by using a high permeability core (high p.,.) with a small cross sectional area A, wound with a large number of turns N, of a small diameter coaxial line.
  • balun V In the design of a high power balun V is a known function of operating power level, and the lowest operating frequency is specified. These parameters set the requirement on the product N -A which must be made large enough to keep B below its upper limit at the lowest operating frequency. It is often found necessary to use large diameter, low resistance conductors, particularly for the balun winding, and to immerse the balun in high dielectric strength oil to provide the required insulation resistance between the windings and to transfer heat away from the windings and core.
  • the permitivity of the oil reduces the frequency f, at which the balun winding is a full wavelength long and the large diameter of the winding reduces Z at frequencies slightly below f
  • the oil and winding conductor diameter have no appreciable effect on Z at low frequencies.
  • a high power balun has a core of large cross sectional area A, few turns N, and great winding length, so that in comparison to a low power balun the low frequency limit of operation is raised and the high frequency limit of operation lowered, resulting in reduced overall bandwidth.
  • FIG. 3 is a diagram of a novel balun in accordance with the present invention, capable of providing a balanced impedance Z,,, which is greater'than the unbalance impedance Z
  • This balun in addition to the balun winding 11, includes a balanced impedance transforming winding 30, consisting of wire segments 31 and 32, which connects the balanced terminals 25 and 26 to the outer conductor 14 across the gap 18.
  • the balanced impedance transforming winding 30 comprises a wire 31 which is wound about the left side of the core, interconnecting terminal 26 to the outer conductor 14 which is on the right side of the gap.
  • Winding 30 also includes a wire 32 which is wound about the right side of the core, interconnecting terminal 25 to the outer conductor 14 which is on the left side of the
  • the balanced impedance Z,, across terminals 25 and 26 is related to the balanced impedance Z across the gap, and therefore to the unbalanced impedance Z which equals Z by the relationship B U( p) (4)
  • N 6 since the number of turns from the gap to ground is 3.
  • N, l8, since the total number of turns from each balanced terminal to ground consists of six turns of the balanced impedance transforming winding 30 and three turns of the balun winding l it for a total of nine.
  • balun winding 11 and the impedance transforming winding 30 are wound symmetrically with respect to ground, the balanced terminals are electrically balanced with respect to ground at all frequencies.
  • the low frequency limit of operation of this type balun is determined both by the impedance considerations and the power considerations herebefore discussed for the balun shown in FIG. I.
  • the impedance match of this balun at high frequencies is also influenced by the decrease in the impedance Z of the balun winding at the high frequencies as is the case with the aforedescribed balun.
  • an additional consideration applies to the particular balun of FIG. 3 due to the incorporation of the balanced impedance transforming winding 30.
  • This winding and the outer shield 14 of the balun winding 11 form, in effect, a two conductor transmission line.
  • this line is an appreciable fraction of a wavelength, in order to achieve reflectionless transmission along it, it is necessary to adjust the characteristic impedance Z of this line so that it is related to the impedance level along the balanced impedance transforming winding which varies from Z Z U at the gap to Z which in the example equals 9Z at the balanced terminals 25 and 26.
  • the characteristic impedance of a two-conductor transmission line is a function of the logarithm of the ratio of the distance between the two conductors and their diameters. In the present invention their diameters are not changed. However, the distance between them is varied by increasing the spacings between the turns of the balun winding and the turns of the balanced impedance transforming winding 30 from the gap 18toward the balanced terminals 25 and 26.
  • the average characteristic impedance of the equivalent transmission line For operation at frequencies where the electrical length of the balanced impedance transforming winding is less than approximately one-third wavelength it is sufficient to make the average characteristic im pedance of the equivalent transmission line equal to (Z XZ /2. For operation at higher frequencies it is necessary to continuously vary the characteristic impedance Z from a valve of Z /2 at the gap region along the length of the winding to a value 2 /2 at the balanced terminals. This is achieved by controlling the spacings between the turns of the balun winding 1 l and the adjacent turns of the balanced impedance transforming winding 30. This aspect of the invention may be further explained in conjunction with FIG. 4.
  • the two-conductor transmission line formed by the outer shield 14 of the balun winding 11 and the impedance transforming winding 30 is represented by conductors 11a and 30a.
  • the insulating boards 20 and 21, shown in FIG. 2 are particularly useful for controlling the spacings between the turns of the windings. This is achieved by providing properly spaced holes 22 in each board and winding the two windings therethrough.
  • the length of the balanced impedance transforming winding 30 increases in terms ofa wavelength at higher frequencies. Also, its length becomes significant when a large magnetic core is employed for high power I operation. However, by controlling the spacings between the turns of the two windings, the effect of the length of the balanced impedance transforming winding is greatly reduced, thereby enabling the novel balun to operate at high frequency and at high power levels.
  • each of the wires of the balanced impedance transforming winding 30 is shown wound on a different half of the core 12. If desired, each wire can have some turns on each core half. This may be advantageous to minimize the voltage difference between adjacent turns of the balun and the balanced impedance transforming windings, thereby minimizing the dielectric stress on the winding insulation. Such a winding arrangement may also be advantageous in achieving the required variation in characteristic impedance Z of balanced impedance transforming windings.
  • FIG. 5 is a diagram of a high power balun with a balanced impedance 2,, which is equal to 49/4 2
  • both the gap 18 and the balanced impedance transforming winding .30 occur on opposite sides of the high permeability magnetic core 12. Voltage differences between adjacent turns of the two windings throughout the balun are minimized. Consequently, dielectric stress on the winding insulation and on the core is minimized.
  • the embodiment of FIG. 5 is preferred where the high power limit of the balun is controlled by the allowable dielectric stresses.
  • the balun winding may include one half winding consisting of an unbalanced three-wire transmission line, in which the center wire forms the center conductor of the transmission line and the two outer wires, which operate in parallel the outer conductor of the line.
  • the other half of the balun winding consists of a single conductor.
  • FIG.- 6 Such an arrangement is shown in FIG.- 6.
  • the balun winding is designated generally by numeral 40, and the three-wire transmission line is designated by numeral 41.
  • Numeral 42 designates the center wire, and the two outer wires are designated by numerals 43 and 4d.
  • the transmission line 41 is wound about the core 12 from the unbalanced end, at which the center wire 42 is connected to the unbalanced terminal 15 and the wires 43 and 44 are grounded. At the gap 45 the ends of the two outer wires are tied together by a wire section 45a and to the balanced terminal 25, as shown in FIG. 7. The latter is a side view of the balun of FIG. 6 and is useful in highlighting the connections at the balanced end 45.
  • the center wire 62 is connected to the single conductor 46 which forms the other half of the balun winding and to the other balanced terminal 26.
  • the single conductor 46 is wound about the other half of the core and its other end is grounded.
  • Conductor 46 has a selfinductance per unit length which is equal to the self-inductance per unit length of the two outer conductors 43 and 44.
  • the balun with the three-wire transmission line winding has the advantage over the balun with the coaxial line winding in that higher values of balun winding characteristic impedance 2 can be achieved, while maintaining a large diameter for the center conductor.
  • baluns capable of high power and/or wideband operation of the non-impedance and primarily of the impedance transforming type.
  • Common to all baluns is the use in each ofa single core on which the balun winding is wound.
  • a balanced impedance Z is present which equals the unbalanced impedance Z
  • the balanced terminals are connected to the balun winding at points other than at the gap region to provide a balanced impedance Z, which is smaller than the unbalanced impedance (see FIG. 1) or to the gap through an impedance transforming windings (see FIGS. 3 and 5).
  • the spacings between the balun and impedance transforming windings are controlled to minimize the effect of the length of the impedance transforming winding, in terms of wavelength at the higher frequencies, or due to the use of a large magnetic core.
  • two apertured insulating cards, placed on opposite sides of the core are used (see FIG. 2), with the various windings being wound through appropriately spaced holes in the cards.
  • the description includes embodiments in which a coaxial transmission line or a three-wire transmission line is used for the balun winding.
  • a balun transformer comprising:
  • balun winding means wound about said core said balun winding means comprising first winding means wound about one portion of said core from said unbalanced terminal means to a gap region and second winding means wound about another portion of said core from said gap region to said unbalanced terminal means, whereby a balanced impedance is present across said gap region which is substantially equal to an unbalanced impedance, definable as 2 across said unbalanced terminal means;
  • impedance transforming means for connecting said first and second winding means to said first and second terminals respectively, to provide a balanced impedance, definable as Z across said first and second terminals, Z being not equal to said unbalanced impedance, with the number of turns of said balun winding means between said gap region and said unbalanced terminal means being Al /2 and the number of turns about said cores between each of said first and second terminals and said unbalanced terminal means being N,/2 where N, N whereby 2.
  • a balun transformer as described in claim 1 further including first and second insulating means on opposite sides of said core, said insulating means defining winding-spacing apertures, with said balun winding means being wound about said core, through said apertures, so that the spacings of the windings from said core and the spacings between winding turns are controlled by the apertures through which the winding turns are wound.
  • balun transformer as described in claim 1 wherein the number of turns of said balun winding means between said gap region and said unbalanced terminal means, defined as N l2 is greater than the number of turns about said core between each of said first and second terminals and said unbalanced terminal means defined as N,/2, whereby Z Z 5.
  • said impedance transforming means comprise impedance transforming windings, extending from said first and second balanced terminals coupled to said first and second windings and wound about said core whereby 2,, Z
  • said impedance transforming means comprise a first impedance transforming winding connected between said first balanced terminal and said first winding means and wound about said core whereby the total number of turns between said first balanced terminal and said unbalanced terminal means is definable as N,/2, and a second impedance transforming winding connected between said second balanced terminal and said second winding means are wound about said cores so that the total number of turns between said second balanced terminal and said unbalanced terminal means is N,/2, whereby N N and Z, Z
  • a balun transformer as described in claim 6 further including first and second insulating means on opposite sides of said core, said insulating means defining winding spacing apertures, with said balun winding means and said impedance transforming windings being wound about said core through said apertures, which control the spacings between the windings and the core and preselected spacings between turns of said balun winding means and said impedance transforming windings.
  • impedance transforming means coupled to and including at least a portion of said balun winding means and further coupled to first and second balanced terminals for providing a balanced impedance across said first and second balanced terminals, definable as Z,;, which is not equal to Z 9.
  • the arrangement as recited in claim 8 wherein 2, is a function of the number of wound turns, definable as N,/2, of said impedance transforming means between each of said first and second balanced terminals and said unbalanced terminal means and the number of wound turns, definable as N,,/2, of said balun winding means between said gap region and said unbalanced terminal means, whereby 10.
  • N /2 is less than N,,/2 with each of said first and second balanced terminals being directly connected to said balun winding means at a point which is an equal number of turns from said unbalanced terminal means.
  • said impedance -transforming means include first and second conductors, each with first and second ends, means connecting said first and second balanced terminals to the first ends of said first and second conductors respectively, means connecting the second ends of said first and second conductors to said balun winding means at points which are an equal number of turns from said unbalanced terminal means, with each of said first and second conductors having at least a portion thereof wound about said core, so that the number of turns between each of said balanced terminals and said unbalanced terminal means is the same, being equal to N,/2, which is greater than N /2.
  • balun winding means comprises a coaxial cable and said winding control means comprises first and second insulating boards, spaced on opposite sides of said core, and defining spaced apertures through which said coaxial cable and said first and second conductors are wound, whereby the spacings between turns are controlled by the apertures spacings.

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US00218139A 1972-01-17 1972-01-17 Balun transformer with a single magnetic core and impedance transforming means Expired - Lifetime US3731238A (en)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4631504A (en) * 1984-05-28 1986-12-23 Showa Musen Kogyo Kabushiki Kaisha Impedance conversion transformer
WO2011008866A1 (en) * 2009-07-15 2011-01-20 Varian, Inc 1:9 broadband transmission line transformer

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
AT393571B (de) * 1988-06-09 1991-11-11 Siemens Ag Oesterreich Transformatorkombination zur bandbreitenvergroesserung fuer hochfrequenzleistungstransistorstufen

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3195076A (en) * 1961-07-07 1965-07-13 Westinghouse Electric Corp Impedance matching balun employing a ferrite core
US3305800A (en) * 1963-08-15 1967-02-21 Tektronix Inc Electrical transformer circuit
US3428886A (en) * 1965-04-15 1969-02-18 Fujitsu Ltd Broad frequency band transformer
US3518596A (en) * 1968-07-29 1970-06-30 Gen Electric Coil wire fastening device and method

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3195076A (en) * 1961-07-07 1965-07-13 Westinghouse Electric Corp Impedance matching balun employing a ferrite core
US3305800A (en) * 1963-08-15 1967-02-21 Tektronix Inc Electrical transformer circuit
US3428886A (en) * 1965-04-15 1969-02-18 Fujitsu Ltd Broad frequency band transformer
US3518596A (en) * 1968-07-29 1970-06-30 Gen Electric Coil wire fastening device and method

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4631504A (en) * 1984-05-28 1986-12-23 Showa Musen Kogyo Kabushiki Kaisha Impedance conversion transformer
WO2011008866A1 (en) * 2009-07-15 2011-01-20 Varian, Inc 1:9 broadband transmission line transformer

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DE2302171A1 (de) 1973-07-19
GB1417093A (en) 1975-12-10
DE2302171B2 (de) 1977-06-23

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