US3003126A - Impedance transformer - Google Patents

Description

Oct. 3, 1961 H. JAslK IMPEDANCE TRANSFORMER Filed Dec. 8, 19.58

il M wm @Wham wNwwNN NNN @NM United States Patent 3,003,126 IMPEDANCE TRANSFORMER Henry Jask, New York, N.Y. (67--20A 193rd Lane, Fresh Meadows 65, N.Y.) Filed De'c. 8, 1958, Ser. No. 778,729 9 Claims. (Cl. S33-33) This invention relates to ya new and improved impedance transformer'and more particularly to a device for matching impedances of coaxial transmission lines.

By virtue of the theory and design of this device, it will be shown that with this invention, the value of any passive impedance may be transformed to any other value of impedance subject yonly to the minor limitation imposed by the small ohmic losses in the device.

In the radio frequency region above 50 mc., present coaxial line impedance matching devices include the double and triple stub tuner and the shunt stub tuner with a line stretcher. The double stub tuner has the limitation that it cannot match -all values of impedance while the other two devices are bulky and expensive to construct. This invention disclosed herein is compact and relatively inexpensive to construct and has no limitations on the range of impedances which it can transform.

It is therefore a principal object of this invention to provide a novel impedance matching device.

Another object of this invention is tov provide a novel impedance matching device having a wide range of impedance values.

Still another object of this invention is to provide a novel impedance matching device capable of transforming 4any value of impedance to any other value of impedance.

Still a further object of this invention is to provide a novel impedance matching device that is physically smaller than other devices.

An other important object of this invention is to provide a novel impedance matching Idevice that is physically compact with a relatively wide range of impedance matching capability.

The features of -my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself however, both as to its organiz-ation and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawing in which:

FIGURE l is a simplified schematic representation embodying the principles of my invention.

FIGURE 2 is a partial cross-sectional View of one embodiment of the novel device with the outer shell portion partially broken away to expose the internal construction.

FIGURE 3 is a front elevational View shown partially broken away, of the invention utilizing an alternate construction.

FIGURE 4 is an enlarged showing of la portion of the View of FIGURE 3.

The manner of operation of my novel tuner is readily understood from the Way in which it transforms the impedance of a matched load. With a fixed setting of the series arm, a total movement of the shunt arm of one half wavelength will cause the transformed impedance to move along a circular locus in the impedance plane. For another fixed setting of the series arm, the transformed impedance as a function of the shunt arm will move along another circular locus, which is rotated from the first locus by `an angle which depends on the change in the series arm. It can be readily shown that all of the circular loci will pass through one xed impedance point and will be tangent to the outer edge of the impedance plane. Thus, the transformed impedance covers the entire impedance plane.

It is of interest to note that if -a resistance termination of different value is used, the circular loci will now Vbe of different diameter, but will still have a common point of intersection. It can therefore be seen that the usefulness of my tuner is not restricted to a transmission line of a particular characteristic impedance. For instance, although the particular tuner shown in FIGURE 2 has been designed for use with a transmission line of approximately 50 ohms nominal impedance, it can still cover the entire impedance chart when used with transmission lines having an impedance as low vas 20 ohms or as high as ohms. The effectiveness of the tuner is limited only by the small irreducible ohmic losses of the transmission line elements and the moving contacts. For the embodiment shown, it has been possible to match impedances which if coupled in their unmatched condition would produce a voltage standing wave ratio in excess of 50 to l.

To provide low losses and smooth operation with long life, the transmission line surfaces on which the contacts ride may be fabricated of coin silver. The movable contacts are preferably multi-lingered beryllium copper rings which may be tipped with silver. The non-contacting parts may be silver plated. The apparatus is of simple mechanical construction and may readily be precision machined so `as to provide the smooth adjustment expected of a laboratory instrument.

Referring now to FIGURE l, there is shown diagraminatically coaxial lines 12 and 14 each of which terminates in a given characteristic impedance 15. Line 12 is shown as having the usual inner conductor 30 and outer conductor 32. Line 14 has similar inner and outer conductors 34 and 36 respectively. In series connection between conductors 30 and 34 are reactances 16, 18. While reactances 16 and 18 are herein shown as separate and distinct circuits it should be understood that when constructing my device reactances 16 and 18 are in reality biiilarly wound transmission lines.

Shunt reactance 24 is connected to the common point of the series Areactances 16, 18. Shorting bar 20 may be moved to any position along the length of reactances .16, 18 by the connecting arm 22 which is shown here as a dotted line. Shunt reactance 24 is shorted to the common outer conductor 27 by shorting bar 26 and the position of the shorting bar may be varied anywhere along the length of reactance 24 by connecting arm 28 also shown as a dotted line.

Although FIGURE l shows the reactances las helical windings, it should be noted that the dimensions of the windings are such that the windings behave as helical transmission lines. The shunt reactance 24 is obtained by taking the inner conductor of a coaxial transmission line and winding it into the form of a helix. In this way it is possible to greatly reduce the overall length of the transmission line. To enable the entire range of reactances to be obtained from reactance 24, its overall electrical length should be at least a half wave in length at the lowest operating frequency. To obtain this variation, the total unwound length of conductor which makes up the helical transmission line must be at least a half wave long.

Reactances 16 and 18 are a biiilarly wound transmission line consisting of two inner conductors in a conducting shield. As in the case of reactance 24, the overall electrical length of this transmission line must have a range offvariation of at least a half wave at the lowest operating frequency. Since the transmission line consisting of conductors 16`and 18 operates in the series mode, the dielectric material between the conductors will reduce the phase velocity so that the unwound length of the conductors may be less than a half wavelength.

Referring now to FIGURE 2 for the actual construction of my device, there is shown the input coaxial connectors 212 and 214 having respective outer conductor portion 236 and 232. These connectors are shown as conventional coaxial connectors. As may be more fully appreciated from the enlarged showing of FIGURE 4, the inner conductor 230, associated with connector 212 is connected to winding 216 while inner conductor 234, associated with connector 214 is connected to winding 218. Both windings 216, 218, are partially embedded, potted in, or plated onto a synthetic resin such as polystyrene or other suitable low loss dielectric material 217, to maintain structural rigidity. Shorting bar 220, in this instance, is in the form of a bell. The open end of the bell, having resilient spring like fingers contacting windings 216, 218, may thus be moved within the confines of the cylinder defined by the windings 216, 218, thereby performing the necessary shorting function on reactances 216, 218. Shorting bell 220 is moved by plunger iarm 222 which has a slip fit through end cap 235. The ends of windings 216, 218 furthest removed from the ends connected to the output connectors are connected in common to the shunt reactance 224 by metallic member 223. Member 223 also serves as a stop for shorting bell 220 and prevents the bell 220 from being withdrawn beyond the upper limits of windings 216, 218. Shunt winding 224 is also partially embedded in a plastic material 22S which may be similar to the potting material 217 used with windings 216, 218. As in the series windings, the shunt winding 224 has a shorting har 226 which in this instance is a toroid having a U shaped cross section. The legs of the U each have resilient spring like fingers for contacting both the windings 224 and the outer conductor 227. Plunger arms 228 are provided for moving the shunt shorting bar 226 and here too, forV ease of movement, a slip fit is provided in end cap 23S. To insure that both arms 228 move simultaneously and to prevent cocking the shorting bar 226, arms 228 are secured to end piece 233 by suitable locking means such as set screws 238.

Thus, to change the reactance of the series, bifilar windings 216, 218, plunger arm 222 is moved axially within the windings 216, 218. To vary the value of the shunt reactance 224, end piece 233 is also moved axially thereby varying the values of either the series or shunt reactances without changing the value of the other.

Referring now to FlGURE 3 there is depicted another embodiment of my invention. In this embodiment, windings 316, 318 correspond identically in structure and funetion with windings 216, 218 except that in this embodiment, the windings are constructed of a gauge of wire that is sufliciently heavy so as to be self supporting and obviate the need for the dielectric material of FIGURE 2.

In FIGURE 3, arm 322 moves shorting bell 320 within series windings 316, 31S to short the turns. Connecting piece 323 connects series windings 316, 318 to shunt winding 324 which in this case too corresponds identically in structure and function with shunt reactance 224 of FIGURE 2. In this embodiment winding 324 is also constructed of a wire gauge sufiiciently large to be self supporting and removing the need for any dielectric material. Here too, arms 328 serve to move shorting bell 326 axially to short shunt winding 324 to the outer conductor 327.

A device such as is shown in FIGURE 2 designed for use with a transmission line of approximately 50 ohms nominal impedance at frequencies below 150 mc. would employ a bifilar winding 216, 218 of a 1/16" wide conductor having a winding diameter of about 1/2, and a single winding 224, of the same conductor size, having a winding diameter of about Se. The outertube could be about 1% in diameter.

It is to be appreciated that the dimensions provided are not critical and are intended merely as a guide to those skilled in the art and is not intended to be limiting. The design of the helical sections for a particular frequency and impedance range is well within the capabilities of those engaged in the art.

It should also be appreciated that although the contacting arrangements shown have conductive contacts, essentially the same result can be achievedby using a non-contacting capacitive plunger which presents a low reactance at the required shorting point.

While I have described what is presently considered the preferred embodiment of my invention, it will be obvious to those skilled in the art that certain changes and modifications may be made therein without departing from the inventive concept and it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I olaim is:

l. A variable impedance matching transformer cornprising: first and second air-core coupled bifilar electrically conductive windings coaxially surrounding a central axis, a third electrically conductive winding axially displaced along said central axis and coaxial therewith, means connecting one end of said first and said second windings with one end of said third winding, first and second coaxial connectors having inner and outer conductive portions, each of said inner portions being connected to a respective one of the other end of each of said first and said second windings, a metal shield coaxially surrounding said first, second and third windings and connected to said outer conductor portions, first movable electrically conductive means for selectively interconnecting selected pairs of adjacent turns of said first and said second windings, means for moving said first movable means from a point external to said shield, and second movable electrically conductive means for connecting a selected portion of said third winding to said shield.

2. The transformer of claim l wherein said windings are partially embedded in low loss dielectric insulating material.

3. The transformer of claim l wherein said windings are self supporting.

4. A variable impedance matching device for coupling a pair of two-terminal terminated coaxial lines comprising a pair of series connected, air-core coupled, bifilar windings, each of said windings having an end adapted to be connected to one terminal of one of said coaxial lines; shorting means for interconnecting selected pairs of adjacent turns of said bifilar windings, and a two-terminal adjustable shunt inductive reactance having one terminal connected to the other ends of said bifilar windings to form a common junction and the other terminal nrranged to be connected, in common, to the other said terminals of the said pair of coaxial lines.

5. The combination of a variable impedance transformer and a first and a second two-terminal terminated coaxial lines coupled together by said transformer cornprising: a first winding; means coupling one terminal of said first coaxial line to one end of said first winding; a second winding wound bifilarly with said first winding and air-core coupled thereto; means coupling one terminal of said second coaxial line to one end of said second winding; movable shorting means arranged to electrically interconnect adjacent turns of said first and said second windings so that equal numbers of turns of said first and said second windings are at all times in series conne-ction with said first and said second loads; a two-terminal adjustable inductance having one terminal connected to the other ends of said first and second windings to form a common junction, and the other terminal connected, in common, to the other of said terminals of said first and said second coaxial lines.

6. The device of claim 5 including a metal shield surrounding said windings wherein said variable inductance is composed of a 4third winding partially imbedded in a low-loss dielectric insulating material `and includes means for shorting out to said shield a selected number of turns of said third winding.

7. The device of claim -5 including a metal shield surrounding said winding wherein said variable inductance is composed of a lthird Winding, self-supported in air, and includes means for shorting out to said shield a selected number of turns of said third winding.

8. The device of claim 5 wherein said first and said second windings are partially embedded in a high dielectric, 10W-loss insulating material.

9. The device of claim 5 wherein said first and said 6K second windings are of a heavy gauge wire and are self supporting.

References Cited in the file of this patent UNITED STATES PATENTS 1,874,242 Christopher Aug, 30, 1932 2,247,212 Trevor lune 24, 1941 2,709,788 Schmidt May 31, 1955 2,764,742 Cady et al. Sept. 25, 1956 2,844,801 Sabaroff July 22, 1958 2,855,599 Kandoian Oct. 7, 1958

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