US2881425A

US2881425A - Method of producing radio wave reflector cords of varied length

Info

Publication number: US2881425A
Application number: US417524A
Authority: US
Inventors: Charles A Gregory
Original assignee: Individual
Current assignee: Individual
Priority date: 1954-03-19
Filing date: 1954-03-19
Publication date: 1959-04-07
Anticipated expiration: 1976-04-07

Description

C. A. UCI

April 7, 1959 GREGORY NG RADIO WAVE R 2,881,425 METHOD OF PROD EFLECTOR' CORDS OF VARIED LENGTH Filed March 19. 1954 FIG. I

v I \\v LlIIIIII IIIIIIIIIIIIIIIIIIIIIII I INVENTOR I GHARL ES 4.

GREGORY ATTORNEYS METHOD OF PRODUCING RADIO WAVE RE- FLECTOR CORDS OF VARIED LENGTH Charles A. Gregory, Arlington, Va., assignor to the United States of America as represented by the Secretary of the Navy of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a system and the production of means for a more effective airborne electronic countermeasure for military purposes by creating a false echo from enemy radar waves.

Window or chaff is the code name applied to airborne electronic reflector materials used for military purposes as a countermeasure against enemy radar. Generally, window, which comprises strips of aluminum of various lengths, is dropped from an aircraft to provide a reflector for enemy radar waves which will create a false echo, i.e., an echo which is not from the target.

Window, as presently produced, is cut from thin rolled sheet aluminum in thin strips of lengths to correspond to a deslre radio or radar wave length against which it is used; or is cut in long ribbons approximately onehalf inch wide and five hundred feet long, and dispensed from rolls for use against long wave length radio or radar waves. The disadvantages of the current methods of production of window, and of the materials used, are that even though made of a light weight electrical conducting material, the weight and space of the materials become limiting factors when large quantities, as required for military use, must be carried and dispensed from aircraft. Furthermore, the solid aluminum ribbon-like material does not permit its preparation in electrical lengths to correspond to the longer wave length radar frequencies without breaking the continuity of the roll. Another disadvantage of current and prior art window is that in the event of war, aluminum becomes a critical material at the time when millions of tons of window is required.

It is an object of this invention, therefore to provide resonant dipole reflectors in short lengths of much smaller cross section than presently employed, and to produce long ribbon material that can be electrically broken into resonant lengths, both types of which will create much greater echo power for the same weight per volume than the materials presently used, and to effect a saving in critical material (aluminum) in time of war.

Another object of this invention is the provision of radio or radar wave reflector having a base of light weight non-conducting material and having aluminum deposited thereon in desired electrical lengths.

Still another object of this invention is to provide a simple and inexpensive method of packaging window which will provide a saving of space when carried in military aircraft.

A still further object of this invention is the provision of an airborne countermeasure radio or radar wave reflecting material which may be packaged in a small volume.

Still other objects and advantages of this invention will become apparent to those skilled in the art after a consideration of the following detailed description and the Patented Apr. 7, 1959 appended claim taken in connection with the accompanying drawings in which:

Fig. l is a plan view of this invention;

Fig. 2 is a side elevation of the embodiment shown in Fig. 1;

Fig. 3 is a plan view of a second preferred embodiment of this invention;

Fig. 4 is an end in Fig. 3;

Fig. 5 is a View illustrating one method of the product of this invention; and

Figs. 6 and 7 are views illustrating a second method of packaging the product of this invention.

Referring to Figs. 1 and 2, it is seen that in a first embodiment of this invention, a very thin metallic coating 10 of an electrically conductive material is applied on a non-conducting ribbon base 11. Although aluminum is preferred because of its light weight and high reflecting qualities, metals such as copper and zinc may be readily employed. The base 11 is preferably a non-conducting substrate material such as tissue, cellulose, nylon, or other light weight non-conducting material. The metallic coating 10 may be applied to the base 11 by the vacuum evaporation process or by any suitable method. It is also seen that the metallic coating 10 is broken by the spaces 12 into predetermined electrical lengths. This may be accomplished by masking during the coating process, or by removing portions of the metallic coating 10, after the material has been applied to the base.

By breaking the metallic coating 10 into lengths corresponding to a dipole, or the half wave length of a given radio or radar wave, a resonant reflector which is ideally suited for lower frequency waves is produced. For example, a length of coating approximately 56 inches long will resonate at radio frequencies of 100 megacycles. dispensing of solid aluminum strips proved unsatisfactory since the individual strips intertwine and the resonating qualities are, therefore, lost. This invention makes it possible to dispense the individual strip in one long ribbon like streamer from a roll. Although the greatest advantages of this embodiment are demonstrated in the lower frequency ranges, it appears obvious that the metallic coating may be made of any length to resonate at any given radio or radar frequency.

The embodiment shown in Figs. 3 and 4, to which reference is now made, consists of a base 14 composed of materials similar to that employed in the base 11 of the embodiment of Figs. 1 and 2, but made in the shape of a fibrous cord. The metallic coating 15 is applied by the vacuum evaporation process, or any other suitable method as in Fig. l and materials such as aluminum, copper or zinc may similarly be used. Unlike the embodiment of Figs. 1 and 2, the metallic coating in this form of the invention is not broken. The coated fibers are cut into lengths corresponding to a dipole, or one-half wave length, :of the radio or radar wave. This type of reflector is particularly useful in frequency Inegacycles per second since a dipole at is only approximately of an inch long.

The reason it is possible to use a thin coating of conductive material on an insulated lightweight base, rather than using solid aluminum, as in the prior art, is because advantage is taken of the electro-magnetic phenomenon known as skin effect. It is known that at high freshowing one preferred embodiment view of the embodiment illustrated packaging the greater part of the electric current penetrates the conductor to only a skin thickness with the depth of penetration depending inversely on the frequency of the alternation of the electric waves. Theoretical calculations, as well as actual tests have demonstrated that for effective use of Window against frequencies near 10,000 rnegacycles per second the metallic coating required is of the order of only a few millionths of an inch. For frequency ranges of 100 to 500 megacycles per second, a depth of from 20 to 40 millionths of an inch is required.

In the case of the ribbon type base of Figs. 1 and 2, the thickness of the base material which has been used successfully has been approximately .00045 inch with a width of approximately .036 inch. in the case of a fibre type base, diameters on the order of .001 inch have been employed.

While the embodiment of Figs. 1 and 2 are most readily packaged in a continuous spiral roll, two methods of packaging the fibre coated embodiment of Figs. 3 and 4 have been illustrated in Figs. 5, 6 and 7.

Thus, in Fig. 5 is shown a method of packaging wherein the fibre coated reflector cords 16 are arranged in a cylindrical container 17 with the axis of the reflectors parallel to the axis of the container. With use of this method of packaging, it is seen that in a cylinder having a diameter of only 3 inches, an excess of 7 million coated fibershaving diameters of .001 inch can be contained therein. Compared to prior art methods of packaging solid window, there has been effected a vast saving of space.

A second method of packaging is illustrated in Figs. 6 and 7 wherein the continuous cord 16 is wound on a cylindrical core 18 to form a cylindrical package. By

' cutting the package in half along a line parallel to the axis of the core varying lengths of reflectors will be produced. Thus, assuming the core has a diameter of one inch, and assume the full package is three inches in diameter with a length of only two inches, approximately 400,000 dipoles, varying in length from 1.57 inches to 4.6inches is produced. Where the'frequency ofi operation of enemy radar is unknown, it is seen that the variation in length of the window produced is a desirable feature.

While certain specific embodiments of this invention have been described herein, it is obvious that many modifications and adaptations will become apparent to those skilled in the art. It is the intent, therefore, that this invention shall cover all such modifications and adaptations, and that it be limited only by the prior art and the scope of the appended claim.

'1 claim:

The method of producing radio Wave'reflector cords of varied length comprising the steps of coating a continuous fibrous base with metal to form a reflecting surfaced cord, winding said cord about a core to form a package, and dividing the formed package into parts by cutting therethrough along a line parallel to the core axis.