US2714706A - Power equalizer - Google Patents

Description

' Aug. 2, 1955 R. w. MASTERS 2,714,706

POWER EQUALIZER Filed July 25 1951 3 Sheets-Sheet l El L+xm R. W. MASTERS POWER EQUALIZER Aug. 2, 1955 R. W. MASTERS POWER EQUALIZER Aug. Z, 1955 Filed July 25 v1951 INVENTOR.. foef 2f/QW 9150.013214:

Unite POWER EQUALIZER Robert Wayne Masters, Columbus, Ohio, assignor to The Ohio State University Research Foundation, Columbus, Ohio, a corporation of Ohio Application July 23, 1951, Serial No. 238,063

17 Claims. (Cl. S33-9) My invention relates broadly to high frequency energy transmission systems and more particularly to a broadband power equalizer for distributing high frequency electrical energy in transmission systems.

One of the objects of my invention is to provide a construction and circuit arrangement for a broadband power equalizer for use in high frequency transmission systems in which echo signal energy is minimized and interference with primary signal energy substantially eliminated.

Another object of my invention is to provide a power equalizer for use in high frequency transmission systems associated with a selective network of energy absorbing resistors arranged to control the transmission of energy to separate loads from a central source.

Still another object of my invention is to provide a construction of power equalizer for high frequency signal transmission systems which provides substantial improvement in bandwidth and matching of transmission lines to loads while eliminating the transmission of echo signal effects.

Still another object of my invention is to provide an improved construction of broadband power equalizer which can be applied to two independent equal loads for the establishment of two-directional radiation patterns depending only upon the magnitudes of the currents in the radiators and not at all upon their relative phase.

Still another object of my invention is to provide a construction of high frequency broadband power equalizer in which an adapting network is built into the equalizer for controlling the transmission of high frequency energy to separate loads and determining the magnitude of the voltages and phase of the currents delivered to the loads while preventing the transmission of undesired signal components.

Other and furthers objects of my invention are to provide a construction of phase controlling and signal selective broadband power equalizer for high frequency transmission systems as set forth more fully in the specification hereinafter following by reference to the accompanying drawings in which:

Figure l is a theoretical diagram explaining the principles of my invention and showing the manner in which radio frequency power is distributed to separate loads over a wide frequency range while absorbing undesired energy components and preventing the transmission thereof; Fig. 2 is a longitudinal vertical sectional View taken through the power equalizer of my invention and showing the arrangement of absorbing resistors built into the structure of the power equalizer between the energy input portion and the power distribution portions to the separate loads; Fig. 3 is a longitudinal sectional view taken substantially on line 3--3 of Fig. 2; Fig. 4 is a vertical sectional view taken substantially on line 4 4 of Fig. 2; Fig. 5 is a vertical sectional view taken substantially on line 5-5 of Fig. 2; Fig. 6 is a vertical sectional view taken substantially on line 6-6 of Fig. 2; Fig. 7 is a vertical sectional View taken on line 7-7 of Fig. 8 and illustrating the spacer or support of insulation material employed between the inner 2,7l4,itl6 Patented Aug. 2, 1955 and outer tubes of the power equalizer; Fig. 8 is a vertical sectional view through the spacer or support taken on line 8 8 of Fig. 7; Fig. 9 is a longitudinal sectional view of the inner tube of the power equalizer showing the longitudinally extending slot therein and the arrangement of resistors showing the mounting of the resistors transversely of the bifurcated portion of the inner tube of the power equalizer; Fig. 10 is a fragmentary horizontal sectional view taken substantially on line lll-lll of Fig. 9; Fig. 11 is a perspective view of the outer tube of the power equalizer showing the input and output connections thereto; Fig. l2 is a perspective view of the input connection for the power equalizer; Fig. 13 is a perspective view of the inner tube of the power equalizer illustrating the mounting of the plurality of laterally disposed resistors of varying values disposed between the slotted portions of the inner tube and illustrating in juxta opposed positions the connecting means for establishing connection with the loads from the power equalizer; and Fig. 14 is a theoretical view showing the application of the power equalizer of my invention in matching the impedance of the input of a high frequency transmission system to two radiating, quarter-wavelength dipoles operating against a large ground plane.

My invention is directed to a power equalizer for connection between a coaxial cable system and high frequency radiating systems. The power equalizer includes a bridgetype network and by terminating two output arms in equal loads and making one arm one quarter wavelength longer than the other, it is possible to cause that portion of the energy which is reflected, when the loads mismatch their transmission lines, to be absorbed in a resistor system. The energy then delivered to and dissipated in or by the loads or antenna radiators is equally divided between them with relative phase equal to Heretofore power equalizers have associated a selective network with the absorbing resistor which placed undesirable frequency limitations on the operation of the device thereby hampering its usefulness in broadband applications. The present invention provides a method of circuitry whereby the absorbing load is divorced from all selective networks and can therefore be made to operate over as wide a band of frequencies as desired.

Referring to Fig. 1 of the drawings, radio frequency power of frequency f enters the device at the outer and inner tubular conductors 1 and 2, which together cornprise a coaxial transmission line. The generator or other transmission line is, in general, connected to outer and inner tubular conductors 1 and 2. An axial slot 3-4 is cut into the inner tubular conductor 2 of the coaxial line without materially disturbing the surge impedance of the coaxial line. The small alteration in surge impedance caused by large slots can be nullified by making the proper changes in the outer diameter of the portion of inner conductor containing the slot. The line is therefore bifurcated between points 3 and 4 for as large a distance as necessary or desirable, the slot being designated generally in Figs. 1-6 and in Figs. 9-10 and in Fig. 13 at 34. At the output end of the device each half of the bifurcated line section 29a and 29b connects to the inner conductor of a branch coaxial transmission line 5, 6, and 7, 8 so that the two branch lines receive incident waves of radio frequency energy at equal amplitude and phase. If the two loads represented by Z (a complex impedance) at points 9 and 10 are not perfect impedance matches for the transmission lines 5, 6, and 7, 8, a certain amount of the energy of the incident waves will be reflected back on these same lines. When the reflected waves have arrived back at 4, it is found that one of them is delayed more than the other by twice the equivalent electrical difference in length between the two branch lines. At fo this phase delay is designed to be exactly so that the retiected wave is exactly push-pull instead of pushpush which is the relationship of the incident waves at point 4. A resistor connected between the ends of the bifurcated line 3, 4, at point 4 will absorb all this reilected energy at fo frequency if the components including the fork length are all properly designed. None of the incident energy at this point is absorbed. The fork would be frequency selective, however, and cause the resistor not to absorb perfectly, at all frequencies, the reversed phase component of the reection. It will be apparent immediately that at frequencies different from fd, the reflected waves will not be exactly push-pull due to the change in electrical lengths of the transmissionV lines. The pair of reflected waves can be resolved into a push-pull pair of equal components and a push-push or unbalance component. None of the unbalance component can be absorbed because it is the same type (coaxial) wave as that incident upon the junction 4. It will later be shown that this component is, or can be made, very small compared to the push-pull pair. from causing a variation of the input impedance to the network, it has no effect upon power division or pattern. lt is the push-pull component of the reflected wave which destroys the equality of power division and causes pattern distortion, and its complete absorption at all frequencies in the operating band is therefore sought. It is the pushpull pair of components which are termed reversed phase components.

The novelty of the present invention therefore resides in the arrangement of a Wide-band absorbing means. Since the bifurcated section can be made as long as necessary it is proposed to make it oneaquarter or one-half wavelength long and till this section internally with a lossy material causing the bifurcated section to function as a shielded two-wire dissipative transmission line for the reversed-phase component of the reflected waves. These reversed-phase components will be absorbed completely (or to as high a degree as desired) throughout any given frequency band if the device is properly constructed.

The electrical difference in line length between the two branch lines can be maintained at one-quarter wavelength by means of a so-called line-stretcher which merely lengthens or shortens a line by means of sliding, contacting cylinders on the inner and outer conductors. By this means the unbalance component can be made zero at any given frequency. A variation of 20% from exactV quarter wavelength, however, still gives a very high degree of power equalization in the loads and permits absorption of almost all the reiiected energy.

The power equalizer as set forth herein will operate i satisfactorily over a much wider band of frequencies than previous types. The absorbing resistor may be made of a number of discrete resistors inserted at very short intervals in the length of split conductor between 3 and 4 rather than the continuous lossy material. The novelty of the device is further enhanced by the concept of the split fork with which the several resistors are associated.

Referring to Fig. 2 the outer conductor 1 comprises a metallic tube which is coupled through a metallic ring to the outer conductor 16 of somewhat smaller cylindrical section. The outer conductor 1 is provided with a metallic collar 17 at the input end thereof to which the circular plate 18 is attached through machine screws 19. The circular plate 18 provides a support for the screw threaded tting 20 connected therewith through screws 20a, which forms a coupling means to the input coaxial line from the high frequency source to the power equalizer. The tting 20 is provided with an annular recess 21 interiorly thereof which registers with a central aperture 22 in the plate 18 for centering and securing the spacer plate of insulation material shown at 23. The spacer plate 23 formed from insulation material is centrally apertured at 23a for the passage of the connecting pin 24 which is attached to the tip 25 extending from the cone-shaped mem- Aside ber 26 which is carried by the end plate 27 which extendsacross the end of the inner tube 2. The cone-shaped member 26 is centered by means of the plate 23 of insulation material so that inner tube 2 is positively spaced from the interior cylindrical wall of the outer tube 1. The inner tube 2 is increased in its thickness section at the position 28 displaced from the end plate 27 and is of uniform internal diameter for the remainder of the length thereof coextensive with the outer tube 1. The contiguous section 29 of inner tube 2V is reduced in section for the length thereof coextensive with the section 16 of the inner tube Vcommencing at the position 3i) and extending to the position 31 thereof. The end portion of the inner tube 29 commencing at the position 31 is reduced in section as represented at 32 and receives thereon the spider or support 33. The spider or support 33 is shown more clearly in Figs. 4, 5, 7 and 8 and consists of radially eX- tending arms of insulation material represented at 33a, 33h, 33e and 33d interconnected by the insulation material of the support and forming centering means between the inner tube 29 and the interior cylindrical surface of the outer tube 16.

The longitudinally extending inner tube constituted by sections 2 and 29 are slotted at the input end of the device at the junction of the section 28 of the inner tube with the section 2 thereof to provide the bifurcation 3-4 heretofore explained in connection with the theoretical diagram of Fig. l. This bifurcation divides the inner tube sections 2-29 into two longitudinally extending semicylindrical parts shown more particularly in Figs. 4, 5 and 9 at 29a and 29b and in Figs. 6 and 13 at 2a and 2b. The longitudinally extending slot thus formed extends from the solid cylindrical section of inner tube 2 at junction 28 throughout the balance of the length of the inner tube and for purposes of more clearly defining the diametrical linearly extending slot I have indicated the slot as a whole by reference character 34.

.The two semi-cylindrical portions of the inner tube 2-29 extend concentrically on opposite sides of the central longitudinal axis through the device and at the output end thereof are provided with pin terminals 35 and 36 carried by the sleeve section of reduced thickness indicated at 32 and extending in opposite directions on a diametrical line through the inner tube 29 and through the aperturedarms 33a and 33e in the spider or support 33.

The outer tube shown at 16 is provided with a pair of oppositely extending metallic iittings 37 and 38 having their inner ends shaped to conform with the circular cylindrical section of outer tube 16 and soldered thereto at their inner ends. The tittings 37 and 3S are tubular in structure and each include a tubular sleeve 39 and 40 of insulation material serving to center and space the doubleended connectors 41 and 42 from the inner walls of the fittings 37 and 3S. The connectors 41 and 42 thus insulated from the ttings 37 and 38 serve as connecting means to the inner conductor of transmission lines which connect to the load such as for example the load represented in Fig. 14 consisting of the two radiating quarterwavelength dipoles represented at 43 and 44 operating against a large ground plane designated at 45 and isolated by a quasi-infinite conducting screen 46, the connec* tions to the dipoles being completed through the outer conductors 47 and 48 which engage over the screw threaded ends 37a and 38a of the fittings 37 and 38. Suitable phase delay is obtained in one of the circuit connections as represented by the phase delay loop 49 represented in Fig. 14. A two-directional radiation pattern is thus obtained which depends only upon the magnitudes of the currents in the radiators and not at all upon their relative phase. The input impedance characteristic at the input position 50 of the system can be made to closely approach a constant over a wide frequency range without particular emphasis on the engineering design precision of the individual radiator impedance functions l 5 themselves. This is accomplished by the distribution of resistors linearly along the slotted portions of the inner tube 2-29. The structure constitutes a two-stage coaxial impedance transformer formed by the plurality of transversely arranged resistors shown generally at 51 and the stepped inner and outer conductors 1 and 2 going from a single 50-ohm coaxial input at 50 to two, parallel, 50-ohm, coaxial outputs S2 and 53 and containing the balanced absorbing resistor system 51. The principle of operation of this device, is of course, independent of the Values of input and output impedance as long as the two output impedances are like transmission lines of appropriate lengths or other suitable impedance functions, whose geometric mean is a constant, pure resistance. The construction of the present embodiment, however, allows standard 50-ohm cable to be used at all connections. The value of the absorbing resistor depends upon the output impedance values. The power equalizer for certain high frequency transmission applications may have an overall length of approximately 71/2 and an outside diameter of 11A and a spread of approximately 3" for connection with the load transmission lines. The absorbing resistor consists of the split live inch length section of inner coaxial conductor 2-29 which is loaded in ladder fashion with seventeen 1/a-watt carbon resistors S1 whose values are 4700, 4700, 2200, 2200, 1000, 1000, 470, 180, 100, 82, 56, 56, 39, 33, 18, l2, and l ohms. The lowest value resis* tor is located nearest the shorted end 3 of the slot 34. I have indicated the progressive values ofthe resistors 60 by showing in Figs. 2 and 9 the sequential increase in physical size of the resistors. The two 4700- ohm resistors 61 and 62 are placed abreast of each other at the open end 4 of the split section 29 in order to make use of their capacitive reactance for compensation purposes at that point.

Excellent results are obtained with the short length power equalizer of this invention. The impedance of the balanced resistor of the present embodiment, for instance, does not change more than 25 percent from a constant value over a range of frequencies extending from 605 to 1600 mc. in applying the power to two loads or antennas. The output end of the outer conductor 16 is closed by a plate 55 secured by means of screws 56 to the ring 7 connected with the end of the outer tubular conductor 16.

As long as the absorbing resistor system 51 remains constant at the proper value, the radiation pattern shape will be constant. lt will be understood that the loads referred to herein are symbolic only of various applications of my invention and may be a highly complex radiation or dissipating system operating upon the principles of my invention. Applications in which the radiation fields of the individual radiators overlap, and which are required to be phased 190 at the center frequency in order to obtain desired pattern eifects are also included in my invention.

It is not mandatory that the two loads be energized by means of two like transmission lines differing in length from a common driving point by one-quarter wavelength at a center frequency, fo, as shown. Other means whose transmission characteristics are more desirable in a given frequency range may be devised. Whatever the form of the adapting network, it will be assumed that it permits waves of equal magnitude to be incident on the loads at all frequencies in such a way that the phase difference between the waves reliected from the loads is qszvr/Z at fo. Equal power is delivered to the loads, and the currents are therefore equal in magnitude. lf the loads have a voltage reflection coeiicient, K, relative to the characteristic impedance, Zo, of the adaptor networks, then the magnitude of the apparent voltage reflection coeticient, K' at the input of the ideal power equalizer of my invention, is

lKl=lKl ICOS I (1) The two equal loads may be identical, linearly polarized Since eiiciency is reckoned as the percentage of the input power which is delivered to the loads, the efficiency of transmission through the ideal power equalizer is seen to be eos cos o l--IKI2 llicieney--l IK,I2 (3) These simplified equations are all based upon the assumption that the absorbing resistor completely absorbs the push-pull components of the waves reected from the loads at all operating frequencies. My invention makes it possible nearly to realize this desirable condition over a much wider band of frequencies than could be covered previously. Control of the phase angle, qt, which represents the time delay of a wave passing once through the adapter network, is another problem not governed by the improvement contributed by my invention. It will be noted that the Equations l and 2 obtain only when the absorbing resistor is perfect. They are good approximations for slightly imperfect absorbers. If a were at all frequencies, K would be zero and p would be unity. Perfect operation would be the result.

Television broadcasters are seriously concerned about echoes in the transmitting system which can produce secondary images annoyingly displaced from the primary image on a receiver screen. The relative radiated X percent field strength of the first echo arising from reflections' of reversed-phase components in a system using a power equalizer, whose absorber has a reflection coeicient of K" instead of zero is accomplished for conditions requiring maintenance and repair. For example, it is only necessary to remove the end plate 55 and the connectors 41 and 42 to completely free the inner conductors 2-29 and enable this entire assembly to be withdrawn longitudinally from within the interior of the outer conductor constituted by cylinders .1*16. The conical end 26 carrying pin 25 may be readily withdrawn from the aperture 23a in the apertured plate of insulation material 23 and the entire cornplement of resistors removed and made accessible for inspection and replacement. The reassembly is as simple as the disassembly process as the complete inner unit formed by tubes 2 and 29 supporting the multiplicity of laterally extending resistors is readily slipped through the open end of the cylindrical outer conductor 16 until the spider or support 33 is aligned with the sleeves 39 and 40 in the fittings 37 and 33 enabling the electrical connectors 41 and 42 to be transversely moved into position engaging the radially extending pins 35 and 36. The conical end 26 with pin 25 thereon engaging connector is readily centered in the aperture 23a of plate 23 of insulation material for establishing connection with the input end of the device.

- I have found the power equalizer as set forth herein highly efficient and practical in operation and while l have described my invention in certain of its preferred embodiments I realize that modifications may be made and I desire that it be understood that no limitations upon my invention are intended other than may be imposed by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as follows:

l. A power equalizer for high frequency transmission systems comprising inner and outer concentrically related hollow tubular conductors for transmitting high frequency waves, input connection means for high frequency energy adjacent the input ends of said conductors, output connection means for high frequency energy adjacent the output ends of said conductors, said inner conductor being bifurcated, a plurality of resistors electrically distributed in predetermined spaced positions along said inner conductor and connected between the bifurcated portions of said inner conductor in isolated relation to said outer conductor for providing a dissipative line for reversed-phase components of reflected waves and output connectors extending from said inner conductor radially through said outer conductor for forming said output connection means.

2. A power equalizer comprising a pair of concentrically disposed hollow tubular conductors arranged in electrically insulated relation one with respect to the other for transmitting high frequency waves, input connections for said conductors adjacent the input ends thereof, high frequency output connections for said conductors adjacent the output ends thereof, one of said tubular conductors being bifurcated linearly of the length thereof with the bifurcated portions of the said conductor extending in substantially parallel spaced relation with respect to each other, a multiplicity of electrical resistors disposed at spaced intervals in shunt electrical paths across the hollow interior of the bifurcated portions of said last mentioned conductor and providing a dissipative line for reversed-phase components of reflected waves and output connectors extending from the bifurcated portions of said inner conductor radially through the hollow tubular conductor that surrounds said bifurcated portions for forming said high frequency output connections.

3. A power equalizer for high frequency electrical systems comprising a pair of hollow tubular conductors supported in concentric spaced insulated relation one with respect to the other for transmitting high frequency waves, input terminal ends for said conductors, output terminal ends for said conductors, the inner tubular conductor having a longitudinally extending bifurcation therein dividing the said inner conductor into two opposite semi-cylindrical portions with the input end portions thereof integrally connected, a plurality of electrical resistors extending transversely between the semi-cylindrical portions of said inner hollow tubular conductor distributed in spaced positions along the length thereof providing a dissipative line for reversed-phase components of reflected waves and output connectors extending from said inner tubular conductor for forming said output terminal ends.

4; A power equalizer comprising concentrically disposed inner and outer electrically conductive hollow tubes, means for mounting said tubes in insulated relation, means for establishing high frequency electrical connections with the input ends of said tubes, means for establishing electrical connection with the output ends of said tubes, said inner hollow tube having a longitudinally extending diametrical slot therein dividing said inner hollow tube into semi-cylindrical portions, a multiplicity of transversely disposed electrical resistors having their opposite ends electrically connected with spaced distributed positions along the insides of the semi-cylindrical portions of said inner tube for forming progressively disposed shunt electrical paths between the semicyiindrical portions of said inner tube between the input cnd thereof and the output end thereof providing a dissipative line for reversed-phase components of reflected waves and output connectors extending radially from said semi-cylindrical portions of said inner tube through said outer conductive hollow tube and terminating in said means for establishing electrical connection with the output ends of said tubes.

5. A power equalizer as set forth in claim 4 in which the individual resistors progressively vary in value from a minimum adjacent the input ends of said tubes to a maximum adjacent the output ends of the tubes.

6. A power equalizer as set forth in claim f-lwherein said output connectors are disposed adjacent the output end of said inner tube and wherein said outer tube is provided with registering guides through which said connectors extend for completing electrical connections with output loads.

7. A power equalizer as set forth in claim 4 wherein the means for mounting said tubes in insulated relation comprises a disc of insulation material adjacent the input cnds of said tubes and a spider of insulation material adjacent the output ends of said tubes and wherein said inner tube extends axially through said disc and said spider in spacial relation to said outer tube and in which said spider has arms of insulation material registering with the means for establishing electrical connection with the output ends of said tubes.

8. A power equalizer as set forth in claim 4V in which a multiplicity of resistors are arranged abreast of the output ends of the semi-cylindrical portions of said hollow inner tube and form a multiplicity of shunt electrical paths across the slotted parts of said inner tube at the output end thereof.

9. A power equalizer as set forth in claim 4 in which the input end of said hollow inner tube terminates in a substantially conical tip having an axially extending conductive terminus thereon and wherein the means for mounting said inner and outer tubes in insulated relation comprises an apertured plate of insulation material carried by the input end of said Outer tube for receiving said axially extending conductive terminus and supporting said hollow inner tube in electrical-insulated relation to said outer tube and a sleeve of insulation material embracing the output end of said inner tube and having integrally connected arms of insulation material extend- '7 ing radially therefrom and establishing contacting relation with the inner surface of said outer tube for spacing said inner tube from said outer tube and wherein said output connectors pass radially through said apertured plate and through said arms of insulation material.

l0. A power equalizer for high frequency electrical systems comprising a coaxial impedance transformer including hollow inner and outer concentric conductive tubular members for transmitting high frequency waves, means for mounting said members in electrically insulated relation from the input to the output ends thereof, said inner tubular member being diametrically slotted from a position adjacent a solid input portion throughout the balance of the length of the hollow section thereof to the output end thereof to form two spaced sections,

, an absorbing resistor disposed within said hollow conductive tubular member and electrically connected between the spaced sections thereof, and terminal connections extending from the output ends of the spaced sectionsV of saidV hollow inner tubular member to electrical loads, providing a dissipative line for reversed-phase components of reflected waves.

l1. A power equalizer for high frequency electrical systems as set forth in claim 10 in which said absorbing resistor comprises a plurality of individual resistance elements disposed in spaced positions transversely of the spaced sections of said inner tubular member conjointly forming a dissipative transmission line for the reversedphase component of reflected waves at said output terminals.

12. A power equalizer for high frequency electrical systems as set forth in claim 10 in which the slotted inner tubular member forms a pair of semi-cylindrical conductors extending from a solid tubular section at the input end of said transformer to adjacent spaced concave sections adjacent the output end of said transformer, and wherein said absorbing resistor comprises an internally disposed lossy material electrically connected across the concave sections of said inner tubular member, the impedance of said absorbing resistor being substantially constant over the operating frequency of said system.

13. A power equalizer for high frequency electrical systems as set forth in claim 10 in which said absorbing resistor is constituted by two groups of resistance elements, one group of which is disposed linearly in positions distributed along the spaced sections of said hollow inner tubular member and another group of which is arranged abreast of the output end of the hollow inner tubular member.

14. A power equalizer for high frequency electrical systems as set forth in claim 10 in which said outer tubular member is formed by a pair of sections of diifering diameters.

15. A power equalizer for high frequency electrical systems as set forth in claim 10 in which said hollow inner tubular member has an internal diameter of uniform section throughout the length thereof.

16. A power equalizer for high frequency electrical systems as set forth in claim l0 in which said hollow inner tubular member has the input end thereof closed by a disc, and wherein a conical terminus is detachably mounted on said disc and terminates in a conductive member which is supported in electrical insulated relation to said outer tubular member.

17. A power equalizer for high frequency electrical systems as set forth in claim 10 in which the output end of said outer tubular member is enclosed by a detachable plate, and wherein said hollow inner tubular mem ber may be inserted into and removed from said outer tubular member when said plate is removed.

References Cited in the file of this patent UNITED STATES PATENTS 2,410,122 Mercer Oct. 29, 1946 2,459,857 Watts Jan. 25, 1949 2,471,515 Brown May 31, 1949 2,514,020 Wehner July 4, 1950 2,515,228 Hupcey July 18, 1950 2,550,409 Fernsler Apr. 24, 1951

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