US2576255A - Reflecting fabric - Google Patents

Description

Nov. 27, 1951 E. HUDSPETH ET Al.

REFLECTING FABRIC Filed June 15, 1948 SH RT 0% .ms ED U VH N. IL T T E M M E JOHN P. NASH BY ATTORNEY Patented Nov. 27, 1951 UNITED srrEs Partir otros Emmett L. Hudspeth, Springfield, Pa., and John P. NashNeenah, Wis., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the'vNavy Application,June 15, 1948, Serial No. 33,204

' 9 claims. (c1. 34a-1s) This invention relates to a collapsible reiiect- 'ing surface and more particularly to a metallic fabric surface for -reecting electromagnetic waves.

This application is a 'continuation in part of our copending application, Serial No. 520,986, filed October 8, 1945, now Patent No. 2,534,716, for Collapsible Corner Reflectors.

It is frequently desirable to provide collapsible antenna units and reflectors for microwave electromagnetic systems. Preferably such units are constructed of a fabric reecting surface supported on a rigid framework. The fabric should be flexible so that the unit when collapsed will occupy a minimum amount of space. When the antenna or reector is erected, the fabric should be free from wrinklessags or other surface irregularities that might distort the field pattern of the antenna or the reflector unit. Also, for optimum operation the threads :of the fabric should make good electrical contact at the points of crossing.

Heretofore, electromagnetic reflectors have been constructed of sheet metal having al plurality of small openings formed therein. The sheet metal thus formed resembles. a fabric woven of ilat strips of metal. However, sheet metal formed in this manner is not flexible enough to be used in collapsible reector units. Woven fabric has also been employed for reflecting units. However,

there are several disadvantages in using a woven fabric for a collapsible reflecting unit. Woven fabric of electrically conductive material will not hold a uniform mesh spacing and will notlie in a plane unless it is carefully stretched over a rigid framework. If steps are taken to fasten the conductive threads at the crossing points'toV maintain the proper mesh spacing and to improve the electrical contact at these points, the fabric usually becomes too rigid to be used in collapsible units.

It has been discovered that knitted metallic fabrics, when stretched at only a` few points, will tend to lie in a plane and under tension` throughout. It will be readily understood that a knitted fabric, being under tension throughout the knit, will provide adequate surface contact between adjoining loops vof the knit to cause the entire fabric to become electrically conductive.

Therefore, it is an obje-ct of the present invention to provide a exible reflecting surface suitable for use in collapsible electromagnetic reflecting units.

It is a further object 'of the present invention to provide a flexible reecting surface thatww-ill inet stitch.

lie in a flat plane when supported at only a. few

points.

Another object of the present invention 1s to provide a reflecting surface for electromagnetic waves that can be folded into a small volume and which exhibits suflicientA resiliency to liel in a flat plane when supported at only a few points,V

about its periphery.

Other objects, features and `advantages of the invention will become apparent from' the following description taken in connection with the accompanying drawings in which g Fig. 1 isv an enlarged view of a portion of aflexible. reflecting surface formed in accordance with the present invention;

l'ig.,2` is an -enlarged view of a second type of reflecting surface formed in accordance with the present invention;

Fig. 3 is a reflecting wall constructed in accordance with the present invention;

Fig. 4 is a second reflecting wall constructed in accordance with the present invention; and

Fig. 5 illustrates a corner reflector employing reflecting walls constructed in accordance with the present invention. .Y

Reference is now made more particularly to Fig. 1 of the drawing wherein there isshown a fabric of metallic strands 8, l0, l2 and ,I4 interlaced in a series of loops to form a knitted fabric. In the fabric shown in Fig. 1, the loops formed at the junction of strands 8 and l!! and strands l2 and I4 lie on the face of the fabric as shown in Fig. l, while the loops formed at. the junction of strands I il and l2 lie on the reverse side of vthe fabric as shown in Fig. 1. The stitch shown in Fig. 1 is known in the knitting art as the "garter stitch.

Fig. 2 illustrates asecond type of fabric composed of interlaced metallic strands (I6, 1.8 20 and 22. In the fabric shown in Fig. 2, the loops formed at the junction vof all of the strands lie on the reverse side of the fabric. This type of stitch is known in the knitting art as the stock- The type of fabric shown in Fig. 2 has one surfacethat is smoother than either surface of the fabric illustrated in Fig. 1f. Hown ever, the fabric of Fig. 1 is slightly more elastic than the fabric illustrated in Fig. 2. Generally, either of the two types of metallic fabric illustra-ted herein, or other fabrics having similar characteristics, may be employed in reflecting surfaces constructed in accordance with the present invention.

Various electrically conductive materials may beI used to form the reflecting Vsurface' such as copper, brass, and stainless steel, although these materials are not as satisfactory as materials such as silver-plated copper wire and Monel metal. A cotton thread wrapped with a silverplated copper spiral has also proven satisfactory. Of yall these materials Monel metal is preferred due to its' characteristic resistance to corrosion and consequent ability to retain a high electrical surface conductivity under adverse conditions. Materials such as brass and copper wire are generally not as satisfactory as Monel metal or silver-plated copper wire due to the rapid oxidation of the copper and brass which results in a relatively lowV surface conductivity of the metal. However, these materials may be satisfactorily used when coated with a protective coating such as oil to prevent such oxidation. The contact between the knitted-loops is sufficient to displace the oil or other substance at such points, exposing clear surfaces of metal to one another, yet the remaining surface is protected against corrosion and oxidation by the protective coating. A high electrical surface conductivity of the wire in the mesh is desired in order that the reflectivity of the reflector be independent of the polarization of the incident beam of electromagnetic radiation.

. In order to provide a minimum of wind drag, it is desirable that the spacing between Athe loops of the knitted fabric `be as large as possible. However, the knitted fabric, being used to re- Vilect electromagnetic radiation wtihin a band of frequencies, must contain a sufficient number of loops per inch to reflect properly the highest frequency electromagnetic radiation that will be used in conjunction with it. It has been discovered that the spacing between the knitted loops cannot be much greater than one-sixth of a wave length of the shortest wave radiation which will be used in conjunction with the knitted fabric. Therefore, the spacing between .the knitted loops cannot be much greater than substantially one-sixth of the Wave length of the highest frequency radiation included in the band of frequencies to be used in conjunction with the reflector.

Fig. 3 illustrates a triangular reflecting wall constructed in accordance with the present invention.

Preferably, the sides of fabric 24 of Fig. 3 are arcuate rather than straight for reasons that will appear presently. The edges of fabric 24 are preferably finished with an elastic or inelastic binding tape 26. This tape may be sewed, stitched, or otherwise fastened to the edges of fabric 24. Hooks 28 or other suitable attaching means are provided at the corners of the reflecting surface for securing the reflecting surface to a rigid supporting structure.

. A reflecting surface of the type shown in Fig. 3 may be formed by cutting fabric 24 from a larger sheet of knitted material or the fabric 24 may be knitted in the desired shape by well known means. If fabric 24 is knitted in the desired shape the fabric is preferably made of a single strand of conductive thread. Reflecting surfaces constructed in this manner are slightly more elastic than reflecting surfaces cut to shape from larger sheets of knitted material. However, reflecting surfaces cut from larger sheets of knitted material are superior to reflecting surfaces constructed of woven fabrics.

The size of the reflecting surface in relation to the supporting structure is such that the fabric is slightly stretched when fastened in posi- 4 tion. The arcuate shape of the sides of fabric 24 will permit the fabric to be stretched even though an inelastic binder 26 is employed. Hooks may be provided at intervals along the edges of fabric 24 if desired. However, unless the reflecting surface is very large, tensions applied at the three apices will cause the reflecting surfaceA to lie in a plane and under tension throughout the surface.

Fig. 4 illustrates a rectangular section of reflecting wall formed in accordance with the .present invention. Again the sides of the fabric 30 are made arcuate and are finished with a binding tape 32. In the rectangular section, it is .preferable to provide attaching hooks 34 at the four corners of the panel.

' Other reflecting surfaces of regular or irregular shapes may be constructed in the manner outlined above for use with any suitable type of supporting framework.v p

Fig. 5 illustrates a` corner reflector formedno twelve reflecting surfaces 36 of the type illustrated in Fig. 3. Reflecting surfaces 36 are supported by six rods 38 mounted on acentralcube 40. Rods 38 are detachably secured to cubeV 4D. A reflector constructed in this manner possesses the desirable characteristics 0f being simple to erect, having low wind drag, and'having ajrelatively small volume when collapsed.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obviousto those skilled in the art that various changes and modifications may be made therein Ywithout departing from the scope of the appended claims.

What is claimed is:

l. A collapsible reflecting surface for reflecting electro-magnetic energy, said surface being formed of a series of knitted spaced loops of electrically conductive. thread, said surface having a binder secured to the edgesthereof, each of said edges .having a concave arcuate shape whereby said surface will .remain under tension throughout when supported at the corners thereof.

2. A collapsible reflecting surface for reflecting electromagnetic radiation within a band -of frequencies, said surface being formed of a series of knitted spaced loops of electrically conductive thread having a high surface conductivity, said surface having an inelastic binder secured to the edges thereof, each of said edges having aconcave arcuate shape whereby said surface will remain iiatand under tension throughout when supported at the corners thereof.

3. A collapsible triangular reflecting surface for reflecting electromagnetic energy, said surface being formed of a series of knitted, spaced loops of electrically conductive thread, said surface having a binder secured to the edges thereof, each of said edges having a concave arcuate shape, whereby said surface will remain flat and under tension throughout when supported at the corners thereof. A

4. A collapsible reflecting surface for reflecting electromagnetic energy, said surface being formed of a series of knitted spaced loops of electrically conductive thread, said surface having at least three edges, at least one of said edges being in the form of an arc having a concave shape as viewed from without said surface and a binder secured to said edges whereby said surface will be remain flat and under tension throughout when supported from the corners thereof. v

5. A collapsible reflecting surface for reflecting electromagnetic energy, said surface being formed from a single electrically conductive thread disposed in a series of knitted spaced loops, said surface having at least three edges, each of said edges being in the form of an arc having a concave shape as viewed from without said surface, and an inelastic binder secured to said arcuate edges whereby said surface will remain flat and under tension when supported at the corners thereof.

6. A collapsible reecting surface for reecting electromagnetic energy, said surface being formed of a fabric of knitted spaced loops of electrically conductive thread, said fabric having at least three edges, and a binder extending along each of the edges of said fabric and secured to said fabric, at least one of said edges of the unstressed fabric having a concave arcuate shape whereby said surface will remain flat and under tension throughout `when supported at the corners thereof.

7. A collapsible reflecting surface for reecting electromagnetic energy, said surface being formed of a fabric of knitted spaced loops of electrically conductive thread, said fabric having at least three edges, and a binder extending along each of said edges of said fabric and secured to saidfabric, each of said edges of the unstressed fabric having a concave arcuate shape whereby said surface will remain flat and under tension throughout when supported at the corners thereof.

8. A collapsible triangular reflecting surface for reecting electromagnetic energy, said surface being formed of a fabric of knitted spaced loops of electrically conductive thread and an inelastic binder extending along the entire length of each of the edges of said fabric, each of said edges of the unstressed fabric having a concave arcuate shape whereby said surface will remain flat and under tension throughout when supported at the corners thereof.

9. A collapsible triangular reflecting surface fcr reflecting electromagnetic energy, said surface being formed of a fabric of knitted spaced loops of electrically conductive thread and an inelastic binder extending along the entire length of each of the edges of said fabric, said binder being secured to said fabric throughout the length of said binder, each of said edges of the uns tressed fabric having a concave arcuate shape whereby said surface will remain flat and under tension throughout when supported at the corners thereof.

EMMETT L. HUDSPETH. JOHN P. NASH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 213,363 Wakeman Mar. 18, 1879 281,804 Simonson July 24, 1883 750,903 Shrum et al Feb. 2, 1904 1,779,771 Hartwell Oct. 28, 1930 1,829,401 Kamrath Oct.- 27, 1931 1,965,542 Colvin July 3, 1934 2,419,549 Griesinger et al. Apr. 29, 1947 2,423,648 Hansell July 8, 1947 2,450,417 Bossi Oct. 5, 1948 2,455,469 Caspar Dec. 7, 1948 2,463,517 Chromak Mar. 8, 1949 2,471,828 Mautner May 31, 1949 OTHER REFERENCES Monel, Bulletin H-3, The International Nickel Co., Inc., 67 Wall Street, New York, N. Y., pages 6 and 7, September, 1940.

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