US 2750321 A
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June 12, 1956 E. KOPPELMAN 2,750,321
ANTENNAS AND MATERIAL FOR FABRICATION THEREOF Filed Sept. 4. 1951 EDWARD KOPPELMAN lNl/ENTOR 7'TOPNEV United States ANTENNAS AND MATERIAL FOR FABRICATION THEREOF Application September 4, 1951, Serial No. 244,876
14 Claims. (Cl. 154-127) This invention relates to a structure consisting essentially of one or more layers of an initially nonconductive woven fabric rendered electrically conductive by the deposition of metal thereon and sealed within an envelope of a solid synthetic resin.
A purpose of the invention is to provide a material of the above description which is particularly adapted to the fabrication of antennas for sending and receiving radio and radar waves.
A purpose of the invention is to provide a material for the fabrication of electrical shielding elements.
A'purpose of the invention is to provide an antenna of very light weight as related to its sending or receiving capacity.
A purpose of the invention is to provide an antenna which is completely waterproof and incapable of being shorted out by rain, fog or dew.
A purpose of the invention is to provide an antenna which may be formed as part of the skin of an airplane without weakening of the structure.
A purpose of the invention is to provide a material for the construction of directional antennas in which the sending or receiving surface may readily be arranged in a true plane.
A purpose of the invention is to provide an antenna of rod-like form which is both light and stilf and which may be supported from a single point.
A purpose of the invention is to provide an antenna having any desired configuration of effective surface, how'-' ever elaborate, and in which such configuration will be maintained during the life of the structure.
A purpose of the invention is to provide a sheet mate rial having a high relation of strength to weight. A
A purpose of the invention is to provide a material in which fibre-glass cloth is rendered electrically conductive or interceptive by the deposition of thin films of metal on one or both of its surfaces.
A purpose of the invention is to provide a laminated sheet material of light weight in which both tensional and fiexural strength are materially increased by the'deposition of films of metal on one or more of the component woven fabric laminae.
The structure of a simple antenna of the type above described is illustrated in the attached drawings, in which Fig. 1 is a plan view of an antenna of plane form, and
Fig. 2 is a section through the same on any medial line.
In these figures, and 11- are layers of a solid synthetic resin; 12 is a sheet of metallized woven fabric so proportioned as to leave a marginal zone 13 in which the resin layers coalesce to produce a tight seal, and 14 is any electrical conductor contacting the metallized surface and projecting beyond the boundary of the marginal seal.
It will be noted in connection with Fig. 2 that the cross hatching of the resin is carried through fabric layer 12 to indicate that the two layers are joined by the penetration of the resin through the interstices in the fabric; that for certain purposes the fabric layer 12 may be pluralized; that for certain purposes the conductor 14 may be omitted mm o 2. and, finally, that the relative thickness of the assembly is grossly exaggerated in this figure.
The basic elements of the invention are the provision of a woven, nonmetallic fabric having films or continuous minute particles of metal deposited on its component fibers in such manner as to avoid choking of the interstices between the fibers, and the embedding of the metallized fabric in a sealed envelope of a waterproof resin, the resin being caused to pass through the interstices in the fabric to form a unitary resin mass.
The fabric onwhich the metal is deposited may be of organic fibre such as cotton, linen, ramie, hemp or jute, or may be inorganic, as for example fibre-glass. The last named is the preferred material for use in the fabrication of antennae. Metallic fabrics such as woven wire cloth are unsuitable for the purpose of the invention and should not be used.
The metal used for metallizing the fabric may be any metal which is reasonably stable to the atmosphere, which has good electrical conductivity, which is not brittle and which has a reasonably low melting point. Cadmium, lead, tin, zinc, copper and aluminum are all suitable and any other metal meeting the above requirements may be substituted. Zinc is the preferred metal by reason of its combination of low cost, moderate melting point and fairly high electricalconductivity.
The mesh of the fabric is variable over a wide range. One limit is that the space between fibres or threads must not exceed the length of the wave to be sent or received. At the other end of the scale, the fabric must not be so tightly woven that the interstices will be filled by the deposited metal, or so nearly choked that the particular resin used for the envelope will not penetrate it under the'conditions of fabrication. Within the wide range thus permitted it will usually be desirable to select a fairly close weave as affording a greater fibre area for the re ception of metal and thus a more efiective antenna than a relatively open weave.
The deposition of the metal on the fabric is preferably but not necessarily performed by spraying a molten metal against the fabric in the manner customary in metallizing for other-purposes. This is a well known art and no specific directions are required. In following the usual practice, in which a rod or wire of the metal is melted by a high temperature flame, it is well to hold the pressure of atomization at a level at which the velocity of the metal particles does not cause them to injure the fabric. This is particularly desirable in metallizing fibreglass cloth.
A modified practice, which is economical in the spraying of large areas of cloth, is to maintain a supply of metal in molten condition and feed the liquid metal in a controlled stream into an atomizing jet of air or gas. This modification effects economy of fuel, permits'closer regulation of temperature of application, and also permits some reduction in both temperature and velocity.
' A sprayed metal may be applied to either or to both sides of the fabric. For many purposes an application to one side is sufficient and gives an antenna having the same capacity as one of equal area in which both sides are coated. For other purposes, as for example in making shields for delicate electrical apparatus, it will be found that a single-side coating is only partially reflective while the double coating permits total reflection.
An alternative method of applying the metal is to draw a web of the fabric through a bath of the molten metal and apply high velocity jets of air or gas at right angles to the emerging surface before the metal on the surface has had time to solidify. This method is of limited applicability as while the air blast will free the interstices between the fibres, the coating itself is dense rather than porous as in spraying. Further, the method is restricted Patented June 12, 1956.
to the use of metal alloys of melting point sufficiently low to avoid damage to the fabric, and is difficult to manipulate to produce a fabric of which less than the entire area is metal coated. Notwithstanding these drawbacks, the immersion method may be used and will often be found economical in the metallizing of large areas of fabric.
Contoured antennae, i. e., antennae provided with tongues, strips or islands of conductive material separated by nonconductive material, are often required. The desired forms may be provided either by cutting them from fabric of which the entire surface has been metal coated, or by masking portions of the surface of the fabric during the spraying operation and thus forming the desired pattern of coated surface on a continuous sheet of fabric.
The first named method is effective but requires more care in placing the metallized fabric in the mold, to avoid shifting or distortion of form, and the masking method is preferable if the form of the metallized portion is elabo rate or the spacing narrow.
The quantity of metal applied to the fabric will ordinarily be limited to the minimum quantity which will produce the required conductivity or reflectivity as the case may be. This is particularly desirable in cases where weight is an important consideration, as while the weight of metal is a very small fraction of the total weight of the assembly, it is often desirable to hold the weight at the absolute minimum of functionality.
Where strength is the main consideration, as in the construction of the laminates below described, it may be desirable to metallize more heavily.
The resin used for the enclosing envelope may be selected from almost the entire range of known synthetic resins, both thermosetting and thermoplastic, and even some naturally occurring resins may be used. The fixed essentials are that the resin shall be substantially nonconductive electrically, and that at some stage in the fabrication it shall reach such state of fluidity as to permeate the cloth and bind the exterior layers of resin solidly together. Given these essentials any preferred resin may be used, the choice being governed by consideration such as cost of material and of fabrication, and the requirements for strength and stiffness in the situation in which the antenna is to be used.
The fabrication of the antenna will follow methods which vary with the known manipulation of the particular resin used. In very general terms, the metallized fabric, in a flat sheet or rolled into rod-like or conical form, is placed with the resin in a suitable mold, great care being taken that no portion of the metallized surface contacts the wall of the mold. The contents of the mold are then subjected to such conditions of heat or pressure or both as to cause the resin to flow and to permeate the fabric, and the finished antenna is withdrawn from the mold after solidification.
Laminated sheets having a high relation of strength and stiffness to weight may be formed by pluralizing the layers of fabric, and various combinations of this type may be produced to meet different requirements.
For example, in the tail elements of a plane, the use of a light weight antenna which can function as part of the skin of the plane often results in a local saving of weight which effects a highly important reduction in construction cost For this purpose the plate may consist of a plurality of layers of uncoated fabric and a single layer (preferably that facing outwardly) of metallized fabric.
A particularly strong and rigid laminated plate may be formedby the use of a plurality of layers of fibre-glass cloth embedded in resin as above described. In such practice it is preferable to metallize all of the fabric layers, as both tensional and flexural strength are markedly increased by the metal coatings.
The reason for this improvement is not entirely certain, though it appears to have its origin in the tendency of the fibres which, in the uncoated cloth, are in contact at the intersections, to abrade and break when they are shifted over each other by tension or flexure. Apparently the metallic coatings lock the intersecting fibres in fixed relative position and thus inhibit this source of loss of strength.
Whatever the reason, I have found in actual testing by A. S. T. M. methods that a laminate in which the fibreglass fabric is metallized will often or usually show an increase of more or less seventy percent in tensile strength over a laminate having the same number of uncoated fibreglass fabric layers, as well as materially enhanced rigidity.
The combination of metallized fabric, and in particular of metallized fibre-glass fabric, with an enclosing envelope of resin, offers marked advantages over other known materials for the fabrication of antennas. Neither the prime materials nor the fabrication are unduly expensive. The metallized cloth or metallized portion may be brought to any desired configuration and, if formed by masking, will positively retain the given form through the molding operation.
Rod antennas, even when of considerable length, are stiff enough to withstand high wind pressure when supported from one end only. Flat plates may be brought to any desired curvature by pressure at the softening temperature, when formed with the use of a thermoplastic resin. No insulation of the antenna from the supporting means is required, only the projecting conductor which connects with the electrical apparatus requiring insulation. When used for shielding, a plate having one layer of coated fibre-glass cloth, when coated on both sides, has the full reflectivity of a polished aluminum plate. And finally, as the entire exterior surface of the antenna consists of nonconductive resin and the coated fabric is sealed except for the projecting connector, leakage and grounding are entirely and permanently prevented if the connector be properly insulated.
I claim as my invention:
1. A structure adapted to send and receive electrical waves: a sheet of nonmetallic woven fabric having metal deposited on its surface in quantity suflicient to render said sheet electrically conductive but insufficient to render said sheet impermeable to fluids, the metallized fabric encased between layers of a waterproof, solid, electrically nonconductive resin, said layers being bonded together through pores in said fabric and around the periphery of said sheet, and an electrical terminal electrically connected to said metallized fabric.
2. Structure as described in claim 1, wherein said electrical terminal is electrically bonded at one end to said metallized fabric and projected through said encasing resin.
3. Structure as described in claim 1, in which said deposited metal is in the form of minute discrete particles in electrically conductive contact.
4. Structure as described in claim 1, in which said metal is deposited on both sides of said sheet.
5. Structure as described in claim 1, in which said metal is deposited in a desired configuration on only a portion of the area of said sheet and said resin bonds extend beyond the margins of said metallized areas.
6. Structure as described in claim 1, in which said woven fabric is fibre-glass cloth.
7. The method of fabricating an electrical antenna comprising: depositing molten metal on a portion of a nonmetallic woven fabric sheet in quantity suflicient to render said portion of said fabric sheet electrically conductive, the quantity of said metal being so restricted as to leave said portion of said fabric sheet porous; placing said sheet of said metallized fabric between layers of a synthetic resin, said layers extending beyond the bounds of said metallized portion of said sheet and beyond the bounds of said sheet, subjecting the assembly of fabric and resin to conditions by which said resin layers are bonded together through pores in said fabric including the metallized portion thereof and the edges of said metallized sheet are sealed by the bonding together of the resin layers extending beyond the bounds of said sheet, and electrically connecting an electrical conductor to said metallized portion of said sheet.
8. A rigid load carriable antenna in the form of a laminated sheet consisting of a plurality of layers of woven fiberglass cloth encased in a solid waterproof resin, said layers being bonded together by a permeation of said resin through the pores of said cloth layers, a deposit of metal on both sides of at least one of said layers so as to increase the structural strength of said antenna, and an electrical terminal electrically connected to the deposited metal.
9. In combination with an antenna, a structure adapted to send and receive electrical waves: a sheet of nonmetallic woven fabric having metal deposited on its surface in quantity sutficient to render said sheet electrically conductive but insufficient to render said sheet impermeable to fluids, the metallized fabric encased between layers of a waterproof solid electrically nonconductive resin, said layers being bonded together through pores in said fabric and around the periphery of said sheet.
10. Structure as described in claim 9 in which said deposited metal is in the form of minute discrete particles in electrically conductive contact.
11. Structure as described in claim 9 in which said metal is deposited on both sides of said sheet.
12. Structure as described in claim 9 in which said metal is deposited in a desired configuration on only a portion of the area of said sheet and said resin bonds extend beyond the margins of said metallized areas.
13. Structure as described in claim 9 in which said woven fabric is fiberglass cloth.
14. In combination with an antenna, a rigid load carriable structure member in the form of a laminated sheet, consisting of a plurality of layers of woven fiberglass cloth encased in a solid Waterproof resin, said layers bonded together by permeation of said resin through the pores of said cloth layers, and a deposit of metal on both sides of at least one of said layers so as to increase the structural strength of such member.
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