US2794144A

US2794144A - Traveling wave electron discharge devices

Info

Publication number: US2794144A
Application number: US312083A
Authority: US
Inventors: Roger E White
Original assignee: Deutsche ITT Industries GmbH
Current assignee: TDK Micronas GmbH; International Telephone and Telegraph Corp
Priority date: 1952-04-08
Filing date: 1952-09-29
Publication date: 1957-05-28
Anticipated expiration: 1974-05-28

Description

May 28, 1957 R. E. WHITE 2,794,144

` TRAVELING WAVE ELECTRON DISCHARGE DEVICES F'ad sept. 29, 1952 2 sheets-sheet 1 Q/lg O 'mal "31X ira vv l z5 l25 5 n UNSH/ELDED SHIELDED INVENTOR ROGER E'. WHITE STAND/NG WA VE RAT/0 F/Pfaz/ENCY /N /000 711% ATTORNEY j 'BY/ y l /Q 5.a 4.04.2 4.4 4.6 4.8 5.o 5.2 5.4 5.a 5.a 6.o 6.2 a4 6.a 6.a 7.o Ma?,

May 28, 1957 R. E. WHITE 2,794,144

TRAVELING WAVE ELECTRON DISCHARGE DEVICES Filed Sept. 29, 1952 2 Sheets-Sheet 2 1 INVENTOR ROGER E. WHITE ATTORNEY VRoger E. White, Nutley,

TRAVELIG WAVE ELEC'IRON DISCHARGE DEVICES v N. J., assignor to International Telephone and Telegraph Corporation, of Maryland Appiicfionseptember 29, 1952, seril'No. 312,083

2o claims. (ci. ais-ses) This invention relates to traveling wave electron discharge. devices and more particularly to transducer or transition arrangements for matching a low impedance .radio l.frequency line to theV 'relatively high impedance -an impedance match between the end of the helical line and the radio frequency terminal connections of the travcling wave-electron discharge device or tube. These-arrangements are particularly useful-in providing high gain within a tube vof given size. That lis to say, the longer the helical line can be made, the higher the gain. In the aforesaid copending application the transition arrangements therein disclosed for coupling between the R. F. terminal connections and thehelical line-are disposed transversely of the axis ofthe helix and made in such yform as to require but a small fraction of aninch of space-measured in an axial direction. The use of such type of transition thus reserves substantially all of the available-space between the source of electron supply and the electron-collector for the helical line.

It is one of the objects of this invention -to .provide still other uforms of transition arrangements which .also require very little axial space.

United States .Pacino a corporation 1 Another object is to provide transition sections .for v precise matching of the low impedance radio frequency coaxial terminal connections vto the high impedance helical line.

One of the features of this invention is the structural arrangement of the transition section, that is thespecial shaping of certain `of the conductors thereof so-as to provide gradual transition from the size of outer conductor of the coaxial terminal connection to the small conductor size of the heli-cal line, This is accomplished by tapering the transition section whether in the form of a specially designed post or istrip.

Another feature of the invention is the provision of a specially designed post or.stri'p conductor in collaboration with a planar conductor for transmission of radio 'frequency energy between the terminal connections and the helical line. By making the post or strip of a composite or laminated form with dielectric separation, the 'portion thereof connected to the helical line Vmay be isolated for the purpose of monitoring and/ or applying a D.-C.lbias or modulating voltage. Where the transition conductor is in the form of a fiat strip spaced 'from the planar conductor by means of a dielectric strip, the gradual transition to the size of the helical `conductor is obtained by tapering the strip between the `point of coupling with 'the terminal connection and the helical line. Regardless of the type of matching section, whether post 'or strip, the coupling to the radio frequency terminal 'connection `may be in either direction parallel to the helical linewithout the necessity of `constructing bends in "the terminal connection.

VvThe above=mentioned land other featuresV and "objects LCC of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a fragmentary View in longitudinal Section with'the envelope broken awayl of a traveling wave electron discharge device incorporating transition sections at the ends of the helical line in accordance with the principles of this invention;

Fig. 2 is a cross-sectional view of a modified form of transition section for the lforward part of the traveling Wave electron discharge device wherein the radio frequency coupling is extended-parallel to the helical line for terminal connection at the rearward end of the device;

Fig. 3 is a view in longitudinal cross-section of a form of transition similar to that illustrated in Fig. 1 but with a shielding arrangement to minimize radiation;

Fig. 4 is a curve showing the effect of the shielding of Fig. 3;

Fig. 5 is alongitudinal cross-sectional view showing still another-form of transition coupler;

Fig. 6 is a longitudinal cross-sectional view of still another embodiment of the transition arrangement;

Fig. 7 is a view in elevationy of the transition arrangement illustrated in Fig. 6 as viewed along line 7-7 thereof;

Fig. 8 is a fragmentary view in longitudinal section of .a traveling wave electron discharge device incorporating still another modification of the transition sections of this invention;

Fig. 9 is a view in elevation of the input transition section as viewed along line 9 9 of Fig. 8;

Fig. 10 is a view in elevation of the output transition section as viewed along line 10-10 of Fig. 8; and

Fig. ll is a cross-sectional view of still another embodiment of the transition section.

Referrin'gzto lFig. 1 of the drawings, a longitudinal sectionofthe'pertinent parts of the traveling wave electron discharge device is shown together with an illustration of one embodiment of the present invention. For disclosure of additional details of such devices to which this invention is applicable reference may be had to the copending. application of I H. Bryant and T. I.: Marchese, Serial No. 221,862, liled April 19, 1951. The tube generally comprises an electron gun unit 1 for producing a beam of electrons for liow axially of the device to a collector 2. The electron gun unit is either spaced from or madecontiguous to an alignment -or partition plate 3 which is positioned within a non-magnetic cylinder 4 against abutment 5 on the inner surface thereof. Carried on the outer surface of the cylinder 4 is either a permanentmagnet or magnetic coil 6 which serves to produce a magnetic field axially of the cylinder 4. At the rear of the cylinder is a sec-ond alignment or partition plate 7. Interposed between these two plates 3 .and 7 is a helical line 8, the line being supported by dielectric rods 9 or by a dielectric tube, whichever is desired. Any suitable dielectric, such as glass or quartz may be employed. The rods or tubing are supported on the

plates

3 and 7 in openings 10 and 11. yThe helical line 8 is connected between the input and output R.F. terminal connections 12 and 13 by means of

transition sections

14 and 15, respectively.

The transition section 14, for example, comprises a parallel line :arrangement which includes the plate 3 as one of the conductors and a post 16 as the other conductor. The post is connected at its base at 1'7 directly to the'plate 3 whereas the other end is connected or otherwise coupled to the helical line 8. By making the post equal approximately to a `quarter wavelength yor an odd multiple there-of 'of the operatingfrequency of the device, the section l14 becomes resonant and thereby establishes a standing wave with a voltage null at the short 17 andv a voltage max. at the end coupled to the helical line 8. The outer conductor 18 o'f the input R. F. terminal con nection 12 is connected to the plate 3 and the inner conductor 19 is connected to the post 14. This connection is located at a point between the voltage null and max. points so as to provide a proper impedance match for the low impedance coaxial connection 12. In order to provide a gradual transition and thereby avoid discontinuities to flow of R. F. energy between the size of the post at the connection to the coaxial line 12 and the size of the helical conductor, the post is tapered as indicated at 20.

The transition coupling 14 in Fig. 1 provides for propagation of radio frequency energy from the coaxial connection 12 to the helical line yS by means of a parallel line comprising the post 16 and the planar conductor sur- Eface of the plate 3. This propagation is believed to take place in the TEM mode. The transition section 15 is identical to the form shown at 14, the propagation however being in the reverse direction over the transition section.

An example of the proportions and spacing of the post conductor which may be employed for satisfactory transition coupling is as follows. The dimension d of the post 16 may be the same as the -diameter of the inner conductor 19. This relationship is not critical, however, and the post may be varied considerably with reference to the size of the inner conductor so long as the size of the post and the spacing thereof with respect to the planar surface of the plate 3 provides a proper transformation between the impedances of the coaxial line and the helix. For reasons of mechanical tolerances, however, d should not be much less than 0.1 inch. The dimension h may be taken from the equation 8 at 1.1f1

Where f1 is the lowest frequency to be handled. The dimension L is preferably selected approximately onequarter wavelength at 1.15f1. The dimension m is not critical and may be chosen within reason to meet the dimensions of the tube. The tapered region 20 makes a gradual transition from the large diameter of the post to the small diameter of the helical wire and again is not critical.

In the embodiment shown in Fig. 2, the post portion of the transition section is shown to be of a composite or laminated form. As illustrated, the post comprises a core conductor 21 which is surrounded with a sleeve of dielectric material 22 which in turn is surrounded by a conductive sleeve 23. The sleeve 23 is connected to the inner conductor 24 of coaxial terminal connection 25. The core 21 is connected to a lead 26 to which a D.C. bias or a modulating voltage may Ibe applied. The upper portions of the three

parts

21, 22, and 23 are tapered as indicated for smooth transition between the points of coupling. In Fig. 2 the transition section is shown to be located to the forward side of the partition plate 3a so that the coaxial coupling 25 may extend parallel to the helical line 8a to the rearward portion of the tube where both the input and output terminal connections for the radio frequency energy may be provided. The helical line 8a in this illustration may extend through the opening in the plate 3a and thereby extend the helical line to a point closely adjacent the source of electrons.

Tests have 4shown that there is very little radiation 75 4 from the transition coupling arrangements of Figs. l and 2 as borne out in the curve of Fig. 4. This small amount of radiation, however, may be eliminated by shielding the transition section substantially yas illustrated in `the embodiments of Figs. 3 `and 5. The transition posts shown in these two embodiments may be identical to either of the forms illustrated in Figs. 1, 2, 6 and 7. The shielding in Fig. 3 comprises a semicircular sheet 28 of -conductive material enclosing the post 16b in conjunction with the plate 3b. The forward portion of the helical line 8b is shielded by a cylindrical sheet 30 which is connected -as an extension of the -shielding portion 28.

Referring to the curves of Fig. 4, while the shielding of Fig. 3 is of some assistance, it will be noted that in the `absence of shielding there is negligible loss due to radiation. It will be noted, however, that the standing wave ratio curves of Fig. 4 indicate that the sections of the type shown in Fig. 3 provide standing wave ratios less than 2:1 over bandwidths as great as 60 percent of the frequency f1.

In Fig. 5, the shielding comprises a second plate 31 spaced on the opposite side of the post 16a` from the plate 3c which separates the electron gun 1c from the helical line 8c. As shown, these two plates may 'be interconnected at the ends of plate 31, the length being large enough to accommodate the part 16C and the helical line 8c. The post 16C may be mounted at one end of this double plate arrangement thus establishing the voltage null point `for the transition section. Each plate, of course, is provided with openings as indicated at 32 and 33, the opening 32 to accommodate the flow of the electron beam, the opening 33 being sufficiently large to receive the helical conductor and the supporting `structure, either rods or tubing as hereinbefore described in connection with Fig. 1. The width of plate 31 is preferably limited so as to form a rectangular waveguide in conjunction with plate 3c. The shielding arrangement of Fig. 5 not only shields against possible radiation eifects but 'also prov-ides structural features which rare advantageous in certain types of tube design. The post 16e, for example, may comprise a strip of conducting material sandwiched between two layers of dielectric material disposed between the

plates

3c and 31. The dimension h1 in this embodiment is preferably obtained from the expression In the embodiment illustrated in Figs. 6 and 7, the post is replaced by `a ilat strip 34 of approximately a quarter wavelength long or an odd multiple thereof of the operating frequency of the device. The strip is supported on a layer or strip of dielectric material 35. The dielectric may comprise any suitable dielectric, such as Fiberglas, Bakelite, or Teon. For further disclosure on this type of transmission line, reference may be made to the copending `applications fof D. D. Grieg-H. F. Engelmann, Serial No. 227,896, led May 23, 1951, and now abandoned, Serial No. 234,503, filed lune 30, 1951, now Patent No. 2,721,312 and M. Arditi-P. P-arzen, Serial No. 286,764, led May 8, 1952, now Patent No. 2,774,046. The strip 34 may be applied and shaped according to printed circuit technique including the process of etching or `slicing away unwanted portions of the conductive material. This type of strip conductor, spaced a small fraction of a quarter wavelength from the planar surface -of the lconductor plate 3d, provides for propagation of electromagnetic waves from the coaxial connection 12d in the TEM mode. The strip 34 is preferably of a Width suicient to provide satisfactory coupling to the coaxial transmission line, the portion disposed between this -coupling and the helical line being tapered as indicated at 36 to provide a gradual transition down to the size of the helical conductor. The dimension h for T this line over the planar conducting f surface Tof plate 3d may be obtained from the expression 377k 2h 1rd Z0 fv-V-( 1 grid-1 115]"l where h is the spacing `of the strip from the planar surface and d is the width of the strip 34 lat the point of coupling to the coaxial line 12d.

While the embodiments of the invention illustrated in Figs. 1 to 8 show the plate normally used as the planar conductor of the transition section to be an alignment plate of the tube, it should be understood that a plate or other planar conductor separate from any-alignment plate may be employed. In the embodiment shown in Figs. 8, 9 and 10, Vthe traveling wave tube therein shown is provided with two

alignment plates

37 and 38 at the forward and rear ends of the tube. An electron gun 39 is carried by the plate 37 while plate 38 supports atleast in part the input and output R. F.

terminal lines

40 and 41, respectively. The input terminal line 40 is shown to comprise a coaxial line having anouter conductor 42 vand an inner conductor 43 which extends to the forward end of the tube. While coaxial lines are herein shown for wide band purposes it should be understood that rectangular waveguides and other high frequency lines may be used. The input transition section 44-is shown to compr-i'sea-pla'nar conductor plate 45 and a strip conductor 46, the outer conductor42 being connected to the' plate 45 and the inner vconductor'43 being connected to the strip conductor 46. As 'best viewed inFigs. 9 and l0, the plate `conductor ofeach' of the transition sections is shown to be provided with a right angle flange 47 struck therefromtowhich one end of the strip conductor 46 iscoupled capacitively. This structural arrangement of the transition section provides for a tube of smaller diameter than where the transition section is made yof straight line conductorstransversely of the helix. As shown in Figs. 9 'and 10,Y the strip conductor comprises three leg portions, a tapered yportion 48 to which the helical line is coupled, the inner `conductor 43 being connected to the base of the tapered portion, a portion 49 disposed at right angles to the tapered portion 48 and a third portion 50 turned at right angles to the portion 49 and disposed in closely spaced parallel relation to the ange 47 to establish an R. F. capacitive short. The portion 50 is made -large and mounted to the tange 47 with a thin strip of dielectric material 51 interposed `therebetwe'en, the mounting being accomplished by suitable eyelets as indicated at 452.

Since'the parts of the transition sections at'the input and output ends of the helical line are identical reference characters like those used for the input section, Fig. 9, with the subscript a are employed to corresponding parts of the output section, Fig. l0. The only difference in the two sections shown is the manner of mounting. The plate 45 of the input transition 'section is carried by tie rods as indicated at 53, disposed between the

plates

37 and 38, the plate 45 being maintained in spaced relation to the plate 37 to provide space for a support 54 of dielectric material, Monel metal or other suitable material to assist in maintaining dielectric 'rods 55 in position for proper support of the helical line 56. The supporting structure 54 is preferably carried by the plate 45 although it may be carried by the plate '37, if desired.

The operation of the transition sections of this embodiment is substantially the same as hereinbefore described, the short coupling establishing the radio frequency voltage null being capacitive instead of a direct connection as shown in the embodiments of Figs. 1 to 8. The high voltage point of the transition, namely, the end of the tapered portions 48 and 48a are connected directly to the ends of the helical line 56. In transition section 44, the strip 46 is on the forward side of plate 45 and the helix extends through the opening in `plate 45 to ther strip vrIhe embodiment' shown 'in Fig. 1'1'is along the'iinesfof 'the composite ktransition section shown in Fig. V2.'v In' the coupling is obtained between the two strips. The center conductor 60 of the input coaxial line is coupled to'the strip 57 while the tapered end of strip 58 is coupled to the'helical line 61. The planar conductor of the section comprises a plate 62 to which the outer conductor' 63 of vthe input coaxial line is coupled. The end'of the strip 57 may be connected to an adjacent portion of the plate 6*'2'but is preferably coupled through ange 64 by way of strip 58 and a ilange 58a supported in close parallel relation to the flange 64 by a dielectric layer 65. By-making this coupling high in capacity it provides in leffect an R. F. short between the transition conductors 57, 58 'and the plate conductor 62, thus establishing a null point. By this construction, the helical line 61 and thevlamination '58, 58aare D. C. isolated with respect to the rest of the R. F. line. By means of lead 66 a D. C. bias or other voltage may be applied to the helical line for monitoring or modulation purposes.

While I have described above the principles of my invention in connection with specic apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of myinvention -as set forth in the objects thereof and in the accompanying claims:

I claim:

l. In a traveling wave device having a'helical wave energy propagating line of relatively high characteristic impedance and a radio frequency wave energy propagating line of relatively low characteristic impedance; a transition section coupling said low impedance line to said helical line comprising first and second conductors disposed in substantially parallel spaced relation, said conductors being coupled together at one point to establish a radio frequency voltage null, the length of 'said second conductor being such as to provide a resonant section at approximately the operatingfrequency of said device, said second conductor being so disposed as-to place a high voltage portion thereof 'in coupling relation to saidhelical line, means to couple said low impedance line to said conductors at a point between said'voltage null and said high voltage portion to match said low impedance line, and said second conductor having a tapered portion between the point of coupling to said'low impedance line and the point of coupling to said helical line.

2. In a traveling wave device according to claim l, wherein the cross-sectional size of said second conductor and the spacing between said rst and second conductors correspond to the characteristic impedance of said llow impedance line and the taper of said second conductor is such as to reduce gradually the cross-sectional size thereof to the cross-sectional conductor size of said helical line.

3. In a traveling wave device according toclaim 1, wherein the low impedance line is a coaxial line and the inner conductor thereof is connected to said second conductor while the outer conductor is connected to said first conductor.

4. In'a traveling waveY device according to claim 3, wherein said second conductor is laminated with dielectric material interposed between laminations and saidinner conductor is connected to one of said laminations and thehelical line conductor is connected to a second of said laminations.

5. In a traveling wave device according toclaim V4, wherein said `second lamination is provided with a :lead for application thereto of la voltage separateqfrom lthe: radio frequency energy propagated along saidse'clztitm.v 1

6. In a traveling wave device according to claim l,

lwherein said first and second conductors comprise subyfrornthe Voltage null point to and surrounding one end of said helical line, and said second conductorV is connected to said helical line.

7. In a traveling wave device according to claim 6, wherein said first conductor comprises wall portions disposed on opposite sides of said second conductor, said wall portions having openings therethrough in alignment with said helical line.

8. In a traveling wave device according to claim 1, wherein the first conductor of said transition section defines a planar surface and said second conductor comprises a strip of conducting material separated from said planar surface by a layer of dielectric material.

9. In a traveling wave device according to claim l, wherein the first conductor of said transition section comprises a planar conductor disposed transversely of said helical line with an opening therethrough in alignment with said helical line and said second conductor comprises a strip of conducting material disposed with one end terminating at said opening in connection with said helical line.

10. In a traveling wave device according to Iclaim 9, wherein said second conductor is on the side of said planar conductor away from said helical line and said helical line has one end thereof extended through the opening in said planar conductor in coupling relation with said first conductor.

1l. A transition coupler for use between a low impedance wave energy propagating line and a high impedance wave energy propagating line comprising first and second conductors disposed in substantially parallel spaced relation with the length of the parallel portions being such as to provide a resonant section at a given frequency, said conductors being coupled together at one point to establish a radio frequency voltage null, a voltage maximum point being thus provided a distance of approximately onequarter wavelength of said frequency from said null point, means coupling said low impedance line to said conductors at a point between said null and maximum points and means coupling said second conductor in the zone of said maximum point to said high impedance line, said second conductor being tapered between the point of coupling to said low impedance line and the point of coupling to said high impedance line.

l2. A transition `coupler according to claim ll, wherein said first conductor includes a pair of walls spaced on opposite sides of said second conductor, said walls being coupled together adjacent the point of coupling between said first and second conductors.

13. A transition coupler for use between a low impedance wave energy propagating line and a high impedance wave energy propagating line comprising first and second conductors disposed in substantially parallel spaced relation with the length of the parallel portions being such as to provide a resonant section at a given frequency, said conductors being coupled at one point to establish a radio frequency voltage null, a voltage maximum point being thus provided a distance of approximately one-quarter wavelength of said frequency from said null point, means coupling said low impedance line to sai-d conductors at a point between said null and maximum points and means coupling said second conductor in the zone of said maximum point to said high impedance line, said second conductor being laminated with dielectric material interposed between the laminations, the low impedance line being coupled to one of said laminations and said high impedance line being coupled to a second of said laminations, whereby said laminations provide for radio frequency coupling between said low impedance line and said high impedance line while isolating said lines with respect vto direct current.

14. In a transition coupler according to claim 13, wherein said one lamination comprises a sleeve of conductive material and said second lamination is a core of conductive material disposed axially of said sleeve.

15. In a traveling wave electron` discharge device having means at the forward end thereof for producing a beam of electrons for-fiow along a given path'and a helical line disposed along said path for propagation of radio frequency energy for interaction with the electrons of said beam.; the combination therewith of radio frequency input and output terminals at the rearward end of said device, a first conductor disposed transversely of said helical line adjacent the forward end thereof, a second conductor also disposed transversely of said helical line adjacent the rear end thereof, each of said conductors being in a form to present a planar surface and having an opening therethrough in alignment with said path, a line conductor disposed in close spaced substantially parallel relation to the planar surface of each of said first and second conductors to form therewith transition sections between said terminals and the ends of said helical line, means coupling the line conductor to the associated planar conductor to establish a radio frequency voltage null, the line conductors being coupled at a Zone of high voltage to the ends of said helical line, the line conductor of the forward transition section being disposed on the forward side of said planar conductor and the helical line having the forward end portion thereof extending through the opening in the planar conductor and coupled to the high voltage portion of the line conductor of said forward transition section, said input terminal being extended to a point adjacent the forward transition section and said output terminal being extended to the rear transition section, and each said terminal being coupled to the corresponding transition section at a point between said null and high voltage portions for impedance matching.

16. In a traveling wave electron discharge device according to claim 15, wherein the terminals comprise coaxial lines, the outer conductor of each coaxial line being connected to a corresponding one of the planar conductors and the inner conductor of the coaxial lines being connected to the corresponding line conductor of said sections.

17. In a traveling wave electron discharge device acr cording to claim 16, wherein the planar conductors each comprises a plate, and the line conductors each comprises a pair of legs disposed at an angle to each other, the coupling to the inner conductor of the coaxial line being at the junction of said legs, the end of one of said legs being coupled to the helical line and the end of the other of said legs is provided with a radio frequency short with respect to said plate.

18. In a traveling wave device having a helical wave energy propagating line of relatively high characteristic impedance and a radio frequency wave energy propagating line of relatively low characteristic impedance; a transition section coupling said low impedance line to said helical line comprising first and second conductors disposed in substantially parallel spaced relation, said conductors being coupled together at one point to establish a radio frequency voltage null, the length of said second conductor being such as to provide an approximately resonant section at the operating frequency of said device, said second conductor being so disposed as to place a high voltage portion thereof in coupling relation to said helical line, means to couple said low impedance line to said conductors at a point between said voltage null and said high voltage portion to match said low impedance line, the first conductor of said transition section comprising a planar conductor disposed transversely of said helical line with an opening therethrough in alignment with said helical line and said second conductor comprises a line conductor disposed with one end terminating at said opening in connection with said helical line, said line conductor having two legs disposed at an angle to each other and the low impedance line being coupled to the junction of said legs.

19. In a traveling wave device according to claim 18, wherein said planar conductor is a plate provided with a flange struck therefrom and one leg of said line conductor is provided with an extension disposed in capacitive relation with respect to said flange.

20. A transition coupler for use between a low impedance wave energy propagating line and a high impedance Wave energy propagating line comprising first and second conductors disposed in substantially parallel spaced relation with the length of the parallel portions being such as to provide a resonant section at a given frequency, said rst conductor having a planar surface, said second conductor having one end enlarged in a plane parallel to the planar surface of said rst conductor to provide a relatively high capacitive coupling therewith to Cil thereby establish a radio frequency voltage null, the other end of said second conductor being coupled in a zone of high voltage to said high impedance line, `and the low impedance line being coupled to said conductors at a point between said null and said high voltage zone, said second conductor having a tapered portion between the point of coupling to said low impedance line and said high voltage zone.

References Cited in the tile of this patent UNITED STATES PATENTS 2,138,953 Bohmefet al. Dec. 6, 1938 2,403,290 Korman July 2, 1946 2,432,057 Wheeler Dec. 2, 1947 2,516,944- Barnett Aug. 1, 1950 2,578,434 Lindenblad Dec. 11, 1951 2,637,775 Lund May 5, 1953