US2948896A - Weatherproof antenna and reflector and method of making the same - Google Patents

Description

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NEATHERPROOF ANTENNA AND REFLECTDR AND METHOD oF MAKING THE SAME Filed Sept. 8, 1952 2 Sheets-Sheet 2 Vul/un Pams :fyi

INVENTOR.

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ATTORNEYJ United States Patent O WEATHERPROOF ANTENNA AND REFLEC'IOR AND METHOD 'OF MAKING THE SAME Frederick R. Hart, Jr., West Concord, Mass., assignor to The Gabriel Company, Cleveland, Ohio, a corporation of Ohio Filed Sept. 8, '1952, Ser. No. 308,363

28 Claims. (Cl. 343-873) The present invention relates to antennas and methods of making the same, and more particularly to microwave reflectors and methods of fabricating such reflectors.

Radio-frequency antennas are presently manufactured in a number of different ways. Reflectors, such as those ofl paraboloidal contour, for example, are customarily manufactured with the aid of a forming tool that is pressed by an operator against a spinning surface of conducting sheet metal'in order to impart the desired paraboloidal or other contour to the sheet. technique is not only time-consuming, but it is subject to the skill or lack of skill of the operator, and it does not lend itself to eflicient mass production. Such a technique, moreover, is not adapted for the production of unsymmetrical reflectors. Reilectors have also been made of perforated or expanded metal or Wire mesh and the like. The perforated or expanded metal or the wire mesh is fused, or otherwise secured to a frame. This technique too, is subject to the disadvantage that it requires a considerable length of time for manufacture and its results depend to a large extent upon the skill of the assembler. Securing of the reflector sections to the frame, indeed, requires a weld that is extremely diflicult to accomplish Iwithout producing distortion of the frame. Very small reflectors have also been die-stamped, but such a process is prohibitive for large-size antennas and Ait involves the high cost of expensive dies and machinery for reflectors of different contours and sizes. Reflectors of this nature must frequently be mounted within Weatherproof housings and the like. l

An object of the present invention is to provide a new and improved weatherproof antenna and method of making the same that is particularly adapted for the microwave radio-frequency range and that is not subject to the above-mentioned disadvantages.

A yfurther object is to provide a new and improved weatherproof antenna and method of making the same, that 'lends -itself to efficient mass production irrespective of the contour or size of the antenna, that requires a minimum of specialized and expensive equipment for manufacture, and that is not subject to the skill or lack of skill of the operator or assembler.

Still a further object is to provide a new and improved light-fweight reflector and method of making the same.

An additional object is to provide a new and improved technique for the formation of reflectors of unsymmetrical contours such as sections of a paraboloid offset from the axis thereof.

Other and further objects will be explained hereinafter and will be more particularly pointed out in the appended claims.

Conducting and dielectric materials have heretofore been combined in antenna structures. Loop and screen antennas have, for example, been Woven fabrics and similar materials to improve their appearance and to protect them; aircraft antennas and the like have r'ice been encased in dielectric housings or coverings for such purposes as eliminating static electricity, providing streamlining and protecting against atmospheric conditions; layers of dielectric and conducting material have previously been used for Waveguiding and focusing structures; and dielectric materials have been combined with conducting elements to produce desired radiation characteristics. The present invention, on the other hand, utilizes conducting and dielectric materials in a new and different manner to form light-weight, weatherproof radio-wave propagating surfaces. ln summary, the present invention relates to a method of making a weatherproof antenna of predetermined shape that comprises assembling upon a form of the predetermined shape one or more fabric layers with a porous radiowave conducting sheet, such as a wire-mesh screen having apertures of dimensions small compared to the wavelength of the radio Waves in connection with which the antenna is to be utilized. Liquid Weather-proofing material is applied to the assembled layers and sheet which are pressed, preferably by suction, into conformity With the form and into intimate interpenetrating relation, with the liquid weatherprooiing material forced into the apertures of the sheet. The assembled layers and sheet are maintained pressed together until the weather-proofing material hardens in order to unite the sheet, the layers and the hardened weather-proofing material into a unitary weatherproof antenna. Preferred method steps are hereinafter described, as are preferred antennas and reflectors formed in accordance therewith.

The invention will now be described in connection with the accompanying drawings,

Fig. `l of which is a perspective view, partly cut away, illustrating some of the steps of the preferred fabricating method of the present invention;

Fig. 2 is a fragmentary section upon a larger scale taken upon the line 2 2 of Fig. l, looking in the direction of the arrows;

Fig. 3 is a perspective of a reector formed in accordance with the present invention, partly broken away to show the details of construction;

Fig. 4 is a section taken upon the line 4--4 of Fig. 1 looking in the direction of the arrows, illustrating a further step in the process of fabrication; and

Fig. 5 is a fragmentary section upon an enlarged scale taken upon the line 5 5 of Fig. 3, looking in the direction of the arrolws.

For purposes of illustration, the preferred method of the present invention will be described as applied to the formation of paraboloidal reilectors. It is to be understood, however, that the invention is not so limited, but is applicable to other reflector shapes and to antennas of various configurations, as later discussed.

Referring to Fig. l, a form 3, as of wood or any similar preferably inexpensive material, is utilized for forming the reflector into the desired paraboloidal shape. This shape 4is shown to be unsymmetrical since the particular illustrated reflector is to be of the oEset-type before mentioned.

Super-posed upon the form 3 is or are one or more fabric layers 5, as of fiberglass cloth and the like. Upon this fabric layer or these fabric layers 5, a porous radiowave conducting sheet 7 is assembled. The sheet 7 may, for example, take the form of a wire-mesh screen the apertures of which are small compared to the dimensions of the radio waves with which the system is to be utilized. For the microwave range, for example, screening having apertures of a fraction of an inch, such as a quarter or an eighth of an inch, is satisfactory. Assembled upon the Wire-mesh screen or other porous sheet 7, are further layers 9 and 11 of fabric material, such as the beforementioned fiberglass cloth. A tubular marginal reilector frame or bead 13, as of metal, may also be incorporated in the reflector structurally to support the assembled fabric layers and conducting sheet and to support Welded or otherwise secured outwardly projecting studs 15 for mounting the reflector in its ultimate mounting structure. The over-all dimensions of the fabric layers 9 and 11 and the radio-wave conducting sheet 7 are preferably slightly less than those of the bottom fabric layer or layers in `order to maintain an uncovered border 17 of the fabric layer or layers 5 that may be folded back over the tubular marginal frame 13. Holes may be punched in the folded-back border y17 to permit the studs to extend therethrough. The edge of the folded-back border 17 of the fabric layer or layers 5 is preferably overlapped by the edges of the fabric layers 9 and 11, as shown at 19, in order securely to lock the frame 13 to the assembly.

Liquid or molten weather-proofing material 21, such as preferred polyester resin and the like, is applied to the assembly in order to impregnate the fabric layers. In Fig, l this impregnation is shown performed by causing the plastic material 21 to flow from a duct 23 upon the upper fabric layer 11 of the assembly. If desired, on the other hand, the fabric layers may themselves be initially impregnated with such plastic resinous material. The form 3 with the assembled layers 5, 9, 1'1 and the sheet 7 may be carried upon dollies 25 so that it may be rolled within a rubber or similar bag 27, Fig. 4. The bag may then be sealed except for an outlet 28 which may be connected to a vacuum pump 29 in order to evacuate the bag. The suction created by such evacuation exerts pressure upon the assembled layers and sheet forcing them into conformity with the shape of the form 3. Continued pressure of this nature forces the assembled layers and sheet into intimate interpenetrating relation and drives the weather-proofing material 21 through the fabric layers and into the apertures of the sheet 7, Fig. 5. The suction is maintained until the material 21 hardens, thus uniting the layers 5, 1,1, 9, the sheet 7, the marginal frame 13 and the hardened weather-proofing material into a unitary, rigid, weatherproof reflector. While this evacuation system 29 has been found most desirable for the curing or hardening of the reflector assembly, pressure may be exerted in other well-known ways to make the layers conform to the desired shape and to drive the weather-proofing material through the fabric layers and within the pores or apertures of the screen or other conducting sheet, in order to lock the assembly together. If desired, moreover, this curing process may be effected at elevated temperatures, though reectors have been successfully formed in this manner, at room temperature.

It has been found that reflectors thus manufactured are extremely light weight. They are thus economical to ship and may require less powerful antenna rotating and other structures than equivalent reflectors formed from metals such as aluminum. The reflectors, moreover, have been found to withstand abuse from atmospheric conditions and to perform electrically and mechanically at least as well as present-day communication, radar and similar refiectors manufactured by the more costly previously described techniques. The structural rigidity compares favorably, indeed, with present-day braced reflectors of all-metal and other re-enforced structures even though no bracing or re-enforcing is utilized. This technique, as before stated, moreover, is adapted to the manufacture of reectors of any desired symmetrical or unsymmetrical configuration merely by substituting inexpensive forms 3 of the desired contour. This is to be contrasted with the present-day spinning techniques for forming reflectors, in accordance with which only symmetrical devices can be made, and these devices must be cut up and joined if it is desired to form unsymmetrical reflectors. Unlike such spinning techniques, furthermore, the present invention does not require skill on the part of the operator or assembler. Reflectors of the same contour but of different dimensions can, of course, easily be made with the same form 3 merely by covering different areas of the form with the layers. This technique is Ialso to be contrasted with the before-mentioned diestamping and similar refiector-forming techniques in accordance with which separate expensive equipment is necessary for each reflector of different dimensions and configuration. The fabrication method of the present invention, moreover, in addition to providing such low-cost operations, performable by unskilled labor, and such great flexibility, has been found to speed up greatly the production time, so that but a fraction of the time presently required to form reflectors is necessary.

It is to be understood, as before stated, that the same technique may be applied to forming other types of antennas such as driven elements. A dipole antenna, for example, may be constructed in this same manner by assembling the fabric layers and the wire screen or other porous sheet 7 about a mandrel form 3. As another example, reflectors of other configuration such as plane reflectors may be manufactured in accordance with this technique merely by utilizing a planar form 3. It is also to be understood that it is not necessary to utilize wire screening. If desired, for example, thin porous or expanded metal may also be utilized. Other fabric layers than the preferred fiberglass may also be utilized, though fiberglass, when combined with the before-mentioned polyester resins, has been found to result in the making of extremely strong and long-lasting devices. The number of layers illustrated and described are preferred for microwave reflectors, but more or less layers may, of course, be employed. Adequate structures for many purposes, indeed, have been made with the interpenetrated layers on one side only of the screen 7.

Further modifications will occur to those skilled in the art, and all such are considered to fall within the spirit and scope of the invention, as defined in the appended claims.

What is claimed is:

1. A method of making a weatherproof antenna that comprises assembling one or more fabric layers with a porous radio-wave conducting sheet, applying liquid weatherproofing material to the assembled layers and sheet, pressing the assembled layers and sheet together to bring them into intimate interpenetrating relation and to force the liquid weatherproofing material into the pores of the sheet, and maintaining the assembled layers and sheet intimately pressed together until the weatherproofing material hardens in order yto unite the sheet, the layers and the hardened weatherproofing material into a unitary weatherproof antenna.

2. A method of making a weatherproof antenna that comprises covering a radio-wave conducting wire antenna of predetermined configuration with fiberglass strands to which are applied resinous material, pressing the strands and wire antenna together to force the strands adjacent the antenna to conform to the said configuration, and maintaining the strands and antenna intimately pressed together until the resinous material hardens in order to unite the antenna and the strands into a unitary structurally strengthened weatherproof antenna.

3. A method of making a weatherproof antenna that comprises assembling one or more fabric layers with a porous radio-wave conducting sheet, applying liquid weatherproofing material to the assembled layers and sheet, disposing the assembled layers and sheet within an envelope, subjecting the envelope to suction in order to press the assembled layers and sheet together into intimate interpenetrating relation and toforce the liquid weatherproofing material into the pores of the sheet, and maintaining -the `assembled layers and sheet intimately pressed together until the weatherproofing material hardens in order to unite the sheet, the layers and the hardened weatherproofing material into a unitary Weatherproof antenna.

4. A method of making a weatherproof antenna of predetermined shape for use with radio Waves of predetermined Wavelength that comprises placing upon a form of the predetermined shape a radio-Wave conducting sheet having apertures of dimensions small compared to the predetermined wavelength, assembling one or more fabric layers with the sheet, applying liquid weatherproofing material to the assembled layers and sheet, pressing the assembled layers and sheet into conformity with the form, at the same time pressing them into intimate interpenetrating relation and forcing the liquid weatherproofing material into the apertures of the sheet, and maintaining the assembled layers and sheet pressed together until the weatherproofing material hardens in order to unite the sheet and the layers and the hardened weatherproofing material into a unitary Weatherproof antenna.

- 5. A method of making a Weatherproof reflector of predetermined shape for use with radio Waves of predetermined Wavelength that comprises placing upon a form of the predetermined shape a radio-wave conducting sheet having apertures of dimensions small compared to the predetermined Wavelength, assembling one or more fabric layers with the sheet, applying liquid weatherproofing material to the assembled layers and sheet, pressing the assembled layers and sheet into conformity with the form, at the same time pressing them into intimate interpenetrating relation and forcing the liquid weatherproofing material into the apertures of the sheet, and maintaining the assembled layers and sheet pressed together until the Weitherproofing material hardens in order to unite the s heet and the layers and the hardened weatherproofing material into a unitary Weatherproof reflector.

6. A method of making a weatherproof reflector of predetermined shape for use with radio Waves of predetermined wavelength that comprises placing upon a form of the predetermnied shape a first fabric layer or layers, assembling thereupon a radio-wave conducting sheet having apertures of dimensions small compared to the predetermined Wavelength and a further fabric layer or layers of overall dimensions slightly less than those of the first fabric layer or layers in order to maintain an uncovered border of the first fabric layer or layers, placing upon the said border a marginal frame, folding the border back over the marginal frame, overlapping the edge of the folded-back border with the edges of the further fab-ric layer or layers, applying liquid weatherproofing material to the assembled layers and sheet, pressing the assembled layers and sheet into conformity with the form, at the same time pressing them into intimate interpenetrating relation and forcing the weatherproofing material into the apertures of the sheet, and maintaining the assembled layers and sheet pressed together until the weatherproofing material hardens in order to unite the layers, the sheet, the marginal frame and the hardened Weatherproofing material into a unitary Weatherproof reflector.

7. A method of making a weatherproof reflector of predetermined shape for use With radio waves of predetermined Wavelength that comprises placing upon a form of the predetermined shape a first fabric layer or layers, assembling thereupon a radio-wave conducting sheet having apertures of dimensions small compared to the predetermined wavelength and a further fabric layer or layers of overall dimensions slightly less than those of the first fabric layer or layers in order to maintain an uncovered border of the first fabric layer or layers, placing upon the said border a marginal frame, folding the border back over the marginal frame, overlapping the edge of the folded-back border with the edges of the further fabric layer or layers, applying liquid weatherproofing material to the assembled layers and sheet, disposing the form and the assembled layers and sheet Within an envelope, subjecting the envelope to suction in order to press the assembled layers and sheet into conformity with the form and at the same time together into intimate interpenetrating relation forcing the weatherproofing material into the apertures of the sheet, and maintaining the assembled layers and sheet pressed together until the weatherproofing material hardens in order to unite the layers, the sheet, the marginal frame and the hardened weatherproofing material into a unitary Weatherproof reflector.

8. A method of making a Weatherproof reflector of predetermined paraboloidal shape for use with radio waves of predetermined Wavelength that comprises placing upon a form of the predetermined paraboloidal shape a radio-wave conducting screen the apertures of which are small compared to the predetermined Wavelength, assembling one or more fabric layers with the screen, applying liquid weatherproofing material to the assembled layers and screen, pressing the assembled layers and screen into conformity with the paraboloidal form, at the same time pressing them into intimate interpenetrating relation and forcing the liquid weatherproofing material into the apertures of the screen, and maintaining the assembled layers and screen pressed together until the weatherproofing material hardens in order to unite the screen and the layers and the hardened weatherproofing material into a unitary Weatherproof paraboloidal reflector.

9. A method of making a weatherproof reflector of predetermined unsymmetrical paraboloidal shape for use with radio Waves of predetermined Wavelength that comprises placing upon a form of the predetermined unsymmetrical paraboloidal shape a radio-Wave conducting screen the apertures of which are small compared to the predetermined Wavelength, assembling one or more fabric layers with the screen, applying liquid weatherproofing material to the assembled layers and screen, pressing the assembled layers and screen into conformity with the paraboloidal form, at the same time pressing them into intimate interpenetrating relation and forcing the liquid weatherproofing material into the apertures of the screen, and maintaining the assembled layers and screen pressed together until the weatherproofing material hardens in order to unite the screen and the layers and the hardened weatherproofing material into a unitary Weatherproof unsymmetrical paraboloidal reflector.

l0. A method of making a Weatherproof reflector of predetermined paraboloidal shape for use with radio Waves of predetermined Wavelength that comprises placing upon a form of the predetermined paraboloidal shape a first fabric layer or layers, assembling thereupon a radio-wave conducting screen the apertures of which are small compared to the predetermined Wavelength and a further fabric layer or layers of overall dimensions slightly less than those of the first fabric layer or layers in order to maintain an uncovered border of the first fabric layer or layers, placing upon the said border a tubular marginal frame, folding the border back over the marginal frame, overlapping the edge of the foldedback border With the edges of the further fabric layer or layers, applying liquid weatherproong material to the assembled layers and screen, pressing the assembled layers and screen into conformity with the form, at the same time pressing them into intimate interpenetrating relation and forcing the weatherproofing material into the apertures of the screen, and maintaining the assembled layers and screen pressed together until the weatherproofing material hardens in order to unite the layers, the screen, the marginal frame and the hardened weatherproong material into a unitary Weatherproof paraboloidal reflector.

ll. A method of making a Weatherproof reflector of predetermined paraboloidal shape for use with radio Waves of predetermined Wavelength that comprises placwhich are small compared to the predetermined 'Wavey lengthI and a further fiberglass-cloth layer or layers ofV overall dimensions slightly less than those of the first iberglassfcloth layer or .layers in order to maintain an uncovered border of the first fiberglass-cloth layer or layers, placing upon the said border a tubular marginal frame, folding the border back over the marginal frame, overlapping the edge of the folded-back border with the edges of the further fiberglass-cloth layer or layers, applying liquid polyester resin to the assembled layers and screen, pressing the assembled layers and screen into conformity with the form, at the sa-me time pressing them into intimate interpenetrating relation and forcing the polyester resin into the apertures f the screen, and maintaining the assembled layers and screen pressed together until the polyester resin hardens in order to unite the layers, the screen, the marginal frame and the hardened polyester resin into a unitary weatherproof paraboloidal reflector.

12. A method of making a weatherproof reflector of predetermined paraboloidal shape for use with radio waves of predetermined wavelength that comprises placing upon a form of the predetermined paraboloidal shape a first fabric layer or layers, assembling thereupon a radio-wave conducting screen the apertures of which are small compared to the predetermined wavelength and a further fabric layer or layers of overall dimensions slightly less than those of the first fabric layer or layers in order to maintain an uncovered border of the first fabric layer or layers, placing upon the said border a tubular marginal frame, `having outwardly projecting reflector mounting studs, folding the border back over the mar- |ginal frame with the studs protruding through the foldedback border, overlapping the edge of the folded-back border with the edges of the further fabric layer or layers, applying liquid weatherproofing material to the assembled layers and screen, pressing the assembled layers and screen into conformity with the form, at the same time pressing them into intimate interpenetrating relation and forcing the weatherproofing material into the apertures of the screen, and maintaining the assembled layers and screen pressed together until the weatherproong material hardens in order to unite the layers, the screen, the marginal frame and the hardened weatherproofing material into a unitary weatherproof paraboloidal reflector.

13. A weatherproof radio-wave antenna for operation with a predetermined radio wavelength comprising one or more fabric layers pressed together into intimate interpenetrating relation with a porous radio-Wave conducting sheet and united by weatherproofing material forced into and hardened within the pores of the sheet, the said pores being of dimensions small compared with the said wavelength.

14. A weather-proof radio-wave antenna for operation over a predetermined band of wavelengths constituted of a radio-wave conducting wire antenna embedded within a fiberglass plastic covering comprising strands of fiberglass plastic material united with and hardened as a unit to the wire of the antenna, the cross-sectional geometrical configuration of the plastic covering being substantially the same as the cross-sectional geometrical configuration of the wire antenna, though of larger dimensions, but the thickness of the plastic covering being a negligible portion of the said wavelengths.

l5. A weather-proof radio-wave antenna constituted of a radio-wave conducting wire antenna embedded within a plastic covering comprising strands of plastic material united with and hardened as a unit to the wire `of the antenna, the thickness between opposite sides of the plastic covering of the antenna slightly varying toward the free end thereof.

16. A weather-proof radio-wave antenna constituted of a fiberglass plastic covering forced into intimate relation with a radio-wave conducting wire antenna and united therewithV with the plastic hardened to the wire of the in and integrated with a hardened plastic covering com-y prising resincoated `fiberglass strands generally coextensive with the wire antenna and united therewith with the resin-coated strands hardened to the wire antenna in intimate structural strengthening contact.

19. A weather-proof radio-wave antenna constituted of a radio-wave conducting wire antenna having one or more wire strands of zig-zag configuration embedded within a hardened plastic covering comprising fiberglass strands pressure-applied to conform to the said configura'- tionand held in intimate structural strengthening contact with the wire antenna.

20. A weather-proof radio-wave antenna constitued of a radio-wave conducting wire antenna embedded within and integrated with a hardened plastic covering substantially thicker than said antenna and having fiberglass strands therein held in intimate structural strengthening connected contact with the antenna.

21Y A weather-proof radio-wave` antenna constituted of a radio-wave conducting wire antenna surrounded by and united with a pressureapplicd hardened plastic protective covering of substantial thickness on all sides of the antenna and containing fiberglass strands intimately contacting the antenna and structurally connectedr with and strengthening the same.

22. A weather-proof radio-wave antenna constituted ofk a radio-wave conducting wire antenna embedded within aA plastic covering containing fiberglass strands, said cofvering being pressureapplied and hardened to the antenna fto integrate the covering to the antenna with fiberglass strands in intimate strengthening structural connection and contact with the antenna.

23. vA weatherproof radio-wave paraboloidal reflector for operation with a predetermined radio wavelength comprising one or more fabric layers pressed together into intimate interpenetrating relation with a porous radiowave conducting sheet and Iunited by weatherprocflng material forced into and hardened within the pores of the sheet, the said pores being of dimensions small compared with the said wavelength.

24. A weather-proof radio-wave reflector for operation with `a predetermined radio wavelength comprising a radio-wave conducting screen embedded between and in intimate interpenetrating` relation with layers of fiberglass-cloth impregnated with polyester resin hardened within the apertures of the screen, the said apertures being of dimensions small compared with the said wavelength.

25. A weatherproof paraboloidal radio-wave reflector,

comprising a radio-wave conducting screen of paraboloidal shape embedded between and in inimate interpenetrating relation with fabric layers impregnated with weatherproorfing material hardened within the apertures of the screen and'having a tubular marginal frame locked` in place by the border of one or more of the fabric layers folded over the frame.

26. A weatherproof paraboloidal radio-wave reflector comprising a radio-wave conducting screen of parabshape embedded between and in intimate interpenetrating relation with one or more fabric layers on each side thereof impregnated with weatherprooling materiali hardened within. the apertures of the screen and having a' tubular marginal frame locked in place by the border off the layer or layers on one side of the screen folded over the frame and overlapped by the border of one or more of the layers on the other side.

27. A weatherproof paraboloidal radio-wave reector comprising a radio-Wave conducting screen of paraboloidal shape embedded between and in initimate interpenetrating relation with fabric layers impregnated with weatherproong material hardened within the apertures of the screen and having a tubular marginal frame provided with outwardly projecting mounting studs and locked in place by the border of one or more of the fabric layers folded over the frame with the studs protruding through the folded border.

28. A weatherproof paraboloidal radio-wave reector comprising a radio-wave conducting screen of paraboloidal shape embedded between and in intimate intel-penetrating relation with one or more berglass-cloth layers on each side thereof impregnated with polyester resin hardened within the apertures of the screen and having a tubular marginal frame locked in place by the border of the layer or layers on one side of the screen folded over the frame and overlapped by the border of one or more of the layers on the other side.

References Cited in the file of this patent UNITED STATES PATENTS 1,475,623 Egerton Nov. 27, 1923 10 1,639,656 Kersten Aug. 23, 1927 1,708,811 Von Auw Apr. 9, 1929 2,304,263 Luty Dec. 8, 1942 2,312,227 Yant Feb. 23, 1943 2,417,586 Crosley Mar. 18, 1947 2,439,137 Kelle-r Apr. 6, 1949 2,478,165 Collins Aug. 2, 1949 2,525,070 Greenwald et al Oct. 10, 1950 2,560,599 Ryan July 17, 1951 2,594,740 De Forest et al. Apr. 29, 1952 2,596,184 Sutton May 13, 1952 2,601,266 Ducatman et al. June 24, 1952 2,602,037 Nelb July 1, 1952 2,605,203 Silver July 29, 1952 2,639,248 Overholt May 19, 1953 2,681,412 Webster June l5, 1954 2,690,411 Seymour Sept. 28, 1954 2,747,180 Brucker May 22, 1956 2,750,321 Koppelman June 21, 1956 FOREIGN PATENTS 582,168 Great Britain Nov. 7, 1946 OTHER REFERENCES Modern Plastics, December 1950, p. 92.

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