US 3078461 A
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Description (OCR text may contain errors)
Feb. 19, 1963 w. J. DWYER $073,451
DISHED, ANNULAR, RADIO FREQUENCY ABSORBER AND METHOD OF MANUFACTURE Filed April 7, 1958 ates ate 3,78,4l Patented Feb. 19, lfifiii This invention relates to an improved radio frequency absorber for absorbing stray, reflected, incoming signals in antenna and to an improved method for making the ante.
Resistor mats and resistor honeycombs now used in antenna systems are sometimes formed of absorbing resistive materials that are difficult to mechanically machine into the desired configuration.
it is the object of the invention to provide a resistor element for absorbing stray reflected signals from the concave reflector of an antenna which can be first assembled into a laminated block and then can be mechanically machined into dished, annular shape without shredding, tearing, chipping or breaking during the process.
Another object of the invention is to provide a dished, annular, radio frequency absorber formed of flexible tapes having an electrically resistive coating and bonded to strips of self supporting, cured plastic. The electrically resistive tape has a given resistive value and a given carhon-binder formulation to eliminate the possibility of reflection back to the reflector.
A further object of the invention is to provide a radio frequency absorber with little or no absorption effect on incoming signals passing therethrough but with substantially total absorption effect on stray signals angularly reflected back toward the element from a concave metal reflector.
Other objects and advantages of the invention will be apparent from the claims, the description of the drawing and from the drawing in which:
EEG. i is a perspective view in section of a dished, annular, radio frequency absorber constructed in accordance with the invention.
FIG. 2 is a front view thereof.
FIG. 3 is a diagrammatic exploded view of the device of this invention in use in a typical antenna system and H68. 4 and 5 are diagrammatic views of the steps in the method or" making the device.
As shown in 3 an incoming radio frequency signal is polarized as it passes through the polarizing member 22 in the direction of the open headed arrows, which denote the direction or" travel of the radio frequency energy. The polarized signals then strike the concave metal mom er 3., vhich is a reflector and reflects the signals to the focus All of the incoming signals are not reflected by the member 21 to the focus 45, some of the signals, shown by closed arrow heads, being reflected outwardly, with a loss of polarization, at various angles of incidence. These stray reflected signals interfere with the incoming polarized signals and the absorber 23 of this invention is designed to absorb all such stray signals. The crystal l8 accepts the signals for the control system of the missile in a well known manner.
The absorber 23 is of dished, annular configuration with an outer rim portion 24 of substantial depth for spacing purposes. The central portion 25 is uniform in thickess and includes the central opening 26. The front peripheral face 27 and the rear face 28 are parallel and smooth and the front recess 29 is also smooth faced.
Absorber 23 makes use of an electrically resistive tape material similar to the resistor material described in my United States Patent No. 2,781,277 issued February 12,
2 1957. As described in that patent, an asbestos tape is coated with an electrically resistive material such as carbon ground into a very fine powder, which is mixed with a binding agent such as silicone to form a liquid suspension. Solvents are driven off by an air-drying process and the tape is then cured by heating in an oven at a temperature of about 300 C. The resulting asbestos resistive tapes are designated 31 herein. The electrically resistive film applied to the asbestos backing tape in this invention is preferably a mixture of epoxy resin, phenolic resin, carbon, graphite and butyl Cellosolve (Z-butoxy ethanol) as a solvent. It may be applied by spraying, in a series of layers, as in my above patent or otherwise, to accurately control and produce in the tape 31 the resistive characteristics desired in an RF absorber for use at a specified frequency.
A plurality of identical, thin, flexible, asbestos, electrically resistive tapes 31 are laminated alternately between a plurality of identical, self supporting strips 32 of cured plastic or plastic foam with the edges of the tapes and strips in front to back special arrangement. The tapes and strips are bonded to each other by means of layers of binder material 33 such as epoxy resin cured in an oven at C. for 20 minutes to form a unitary, laminated, block 34. The strips 32 are preferably of rubber-polymer foam, commercially available as Cooper formula Hycar (butadiene co-polymer with acrylonitrile).
The resulting laminated block 34 is shown in FIG. 5 and, because of its asbestos resistive tape and cured plastic laminations, can be mechanically machined into the desired form as shown diagrammatically in FIG. 6 such machining may be by lathing or drilling with a suitable machine tool 35, the drill 36 forming the central opening 26 and the cutter 37 forming the tapered dished front recess 29 with its smooth face 33 and the faces 27 and 28. The materials do not chip, fracture or tear during the cutting or smoothing operations. Angularly spaced holes 41 may be drilled in the block 34, around the inner periphcry of the annular absorber.
in the preferred embodiment illustrated, the annular absorber 23 is approximately six inches in outside diameter and one and one half inches inside diameter. The rim face 27 is spaced from the rear face 28 a distance of about .815 inch in depth and the rim face 27 is about .210 inch wide. The central portion 25 of member 23 is about .310 inch in depth, or thickness, the plastic strips 32 are about .365 inch in width and the asbestos resistive tapes 333. are about .010 inch in width.
As shown in FIG. 3 an absorber, or grating trap 23 is positioned in the antenna assembly between the polarizing member 22 and the concave metal reflector member 21, and parallel to the spaced parallel polarizing elements 39 of polarizing member 22, whereby if the polarizing lines are vertical the absorber lines are also vertical as shown.
The incoming radio frequency signals pass through polarizing member 22 and pass through absorber 23 without substantial deterrence because the resistive tapes .31 are thin and edgewise to the signals, and the cured plastic strips 3-2 while relatively wide are dielectric and do not retard, or interfere with, the polarized signals. The concave metal member 21 reflects the incoming signals to the focus 4d of the antenna and any unpolarized stray signals reflected outwardly enter the absorber 23 to be totally absorbed without interfering with the incoming signals.
The asbestos resistive tape must be of a predetermined resistive value for example, 300 ohms per square and of a predetermined carbon binder formulation, for example, 'epoxy resin; phenolic resin; carbon; graphite; and butyl Cellosolve as a solvent to properly absorb the reflected radio frequency signals Without re-reflection. The carbon binder material is applied in a relatively uniform thickness of fil-m to the tape and the ohmic value of the resulting product is then measured. If the measurement indicates that the ohmic value per square is not correct for the particular frequency desired, the proportions of the carbon binder formulation are changed to give either a lower or higher per square ohmic value until the desired value for example, 300 ohms per square is obtained. Thereafter the tapes 31 may be accurately produced using the formulation determined to be correct by the above trial and error method.
The depth of the portions 24 and 25 of absorber 23 and the spacing of the resistive tapes 3 1 depend on the particular selected frequency of the incoming signal and can be calculated by one skilled in the art. H
Reference is made to pages 247248 of Radar Engineering by Donald Fink, published in 1947 by McGra-w- Hill Book Co. Inc. The position of the absorber 2 3 in relation to the paraboloidal reflector 211 is critical in that it be placed near the focus indicated at 45 but external to the face plane indicated at 44. The formula where p is the distance measured along, the axis from a point on the axis known as the focus to the curve shows that an incoming signal passing through the face plane 44, contacting the paraboloid 21 and reflected thereby will reflect toward the focus 45, since the incoming signal is parallel to the x axis by way of the polarizing segment 22. The terms x and y in the above formula are the axes of the parabola with the x axis at right angles to the face plane of the parabola and the y axis at right angles to the x axis and parallel to the face plane all as shown in FIG. 168, page 248 of the above mentioned article.
Such R-F energy as is not absorbed at the focus 45 will be reflected in an outward direction from the paraboloid 211. This R-F energy is scattered energy and passes through the face plane 44 of the paraboloid at all angles of scatter. This scattered energy enters the absorber 23 at all angles and due to'the end spacial arrangement of the resistance tapes 31 plus their depth, the scattered energy contacts the strips broadside at various angles and is absorbed. V
T he resistance value of the absorbing resistance material 31 approximates that of the characteristic impedance of empty space which is 377 ohms per unit square. This remains more or less constant for any unit regardless of frequency and wave length.
The distance between resistance tapes 3 1 is determined by the fact that all tapes must be placed directly behind the metal lines of the polarizing segment 22. Therefore, since metal lines of the polarizing segment are spaced according to the wave length at any given frequency, the resistance tapes are also spaced according to the wavelength at any given frequency. For example, the particular element described herein is designed to operate on a one quarter wavelength. Using the wavelength formula where f-is' the frequency in cycles per second, 7\ is Wavelength in feet if v is velocity in feet per second, the spacing of metal polarizing lines for the unit operating at a specific frequency can be determined. The spacing of the metal grid lines in this unit at its specific frequency of operation turns out to be 0.125 inch on centers. The spacing of the tape resistors 31, and the thickness of the plastic strips 32, is .365 inch as stated above because it is not necessary that there be a tape resistor in alignment with every metal line of the grid 22.
l. A dished, annular, radio frequency absorber adapted to be interposed between the polarizing grid and reflector of a microwave antenna, said absorber comprising a laminated, unitary, self supporting body of dished, annular shape formed of spaced, parallel, electrically resistive, thin, flexible, asbestos tapes of substantial depth, edgewise facing, normal to, and external of the face plane of said reflector, said tapes being each bonded to and supported by a pair of relatively thick, parallel strips of dielectric plastic material, whereby polarized incoming signals pass through said body with minor absorption but stray signals reflected from said reflector member are absorbed by the resistive tapes of said absorber. 2. Acombination as specified in claim 1 wherein said dished, annular body includes a hat central portion of uniform thickness and a rim portion having a rim face at a predetermined distance from said central portion for spacing saidcentral portion from the face plane of said reflector member. H
3-. A combination as specified in claim 1 wherein said parallel strips of dielectric plastic material are of cured rubber polymer plastic foam and the faces thereof, opposite the face plane of said reflector, are smooth.
4. A combination as specified in claim 1 wherein said electrically resistive tapes are of a predetermined carbonbinder formulation having a predetermined ohmic value per square bonded to a flexible asbestos backing and said dielectric strips are of cured plastic foam.
'5. The method of making a. dished, annular, radio frequency absorber which comprises bonding a plurality of identical, relatively thick, self supporting strips of cured plastic alternately, in parallelism, with a plurality of identical, relatively thin, flexible tapes of asbestos having a coating of carbon particles bonded thereto, to form a laminated block and then mechanically machining said block into dished configuration and drilling said block into annular configuration.
6. A dished, radio frequency absorber comprising a unitary, self-supporting body of dished, annular shape, said body being formed of a plurality of alternate, thin, parallel, flexible, asbestos tape resistors of substantial depth and uniform width and relatively thick, parallel strips of self-supporting, dielectric, plastic material of substantial depth, and uniform width, said tape resistors and said plastic strips being adhesively united to each other to edgewise face the front and back faces of said body and the thickness of said strips being predetermined to space said tapes apart according to the wavelength of a predetermined frequency. a
7. The method of making a dished annular radio frequency absorber which comprises the steps of forming a plurality of identical strips of cured plastic foam, each having a width corresponding to the wavelength of a selected radio frequency, then bonding said identical plastic strips alternately, in parallelism with a plurality of identical, flexible, asbestos tape resistors, each having a selected resistance value, to form a laminated block and then machining said block into dished, annular configuration by lathe cutting a rim therein and drilling an axial bore therein.
References Cited in the file of this patent UNITED STATES PATENTS 2,511,610 Wheeler June 13, 1950 2,610,250 Wheeler Sept. 9, 1952 2,724,112 Hepperle Nov. 15, 1955 2,736,895 Cochrane Feb. 28, 1956 2,822,539 McMillan Feb. 4, 1958 FOREIGN PATENTS 890,069 Germany Sept. 17, 1953 OTHER REFERENCES NR'L Report 4137, D'arkfiex-A Fibrous Microwave Absorber, by H. A. Tanner et 211., April 20, 1953, Naval Research Laboratory, Washington, DC.