Absorptive Coating for the Reduction of the Reflective Cross Section of Metallic Surfaces and Control Capabilities Therefor

This invention relates to coatings for the reduction of reflective cross sections of metallic surfaces, and in particular, to a compact, wide band radar absorptive coating preferably having tuning and control capabilities.

Various absorptive materials have been used in the past to reduce radar cross section of selective objects. However, the materials used in the past, and their application, have been dependent upon the wave length of the incident wave. That is to say, the absorptive qualities of previously known materials have required a considerable thickness of the material in order to perform its function. In general, known prior art material provides a maximum reduction of radar cross section of about 20 db per wave length thickness of the material utilized for absorptive purposes. Known materials also are functional only in a very narrow band of radar frequencies. Consequently, the usefulness of the absorptive materials is exceedingly limited. For example, they are not suitable for use on high performance aircraft because the thickness of material required to make the aircraft non-reflective is too great. That is to say, the thickness required to offer a significant reduction in the reflective cross section of the aircraft is so great as to render its application to the aircraft impractical, particularly in view of the operation speeds of such aircraft.

The invention disclosed hereinafter overcomes these prior deficiencies by providing a material structure which is capable both of passive and active application as a absorptive coating. The coating of this invention exhibits wide band performance and is theoretically capable of reducing radar cross section by approximately 40 db. At high frequencies, the material may be used in its passive mode, a relatively thin thickness of material alone being sufficent to provide the absorptive qualities. At low frequencies, means are provided for injecting a control voltage within the coating, thereby enabling the material to provide absorptive properties, even where the thickness of the applied coating is less than the wave length of the incident wave.

One of the objects of this invention is to provide an absorptive coating for electromagnetic wave energy having improved performance characteristics.

Another object of this invention is to provide an absorptive coating for electromagnetic wave energy capable of use in either an active or a passive mode.

Another object of this invention is to provide an absorptive coating for electromagnetic wave energy suitable for use over a wide range of incident wave frequencies.

Another object of this invention is to provide a coating for a metallic surface having a PN junction therein.

Still another object of this invention is to provide a coating in which a control voltage may be injected across a PN junction of a coating for a metallic surface.

Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.

In accordance with this invention, generally stated, a coating for an incident wave reflecting surface is provided which includes an "N" doped layer material interposed between the reflecting surface and a "P" doped layer material so that a PN junction is provided in the coating. In its passive mode, the coating thickness is chosen to approximate the incident wave length and the layers are selected and arranged so that both conductivity and dielectric constant of the layers increases in a direction through the coating thickness toward the reflective surface to which the coating is applied. By attaching leads to the P doped and N doped layer portions of the coating, control voltages can be impressed to vary the electrical characteristics of the coating. In the embodiment illustrated, a third undoped layer coating is provided outboard of the first and second layers.

In the drawings, FIG. 1 is a view in side elevation of an aircraft employing coating of this invention;

FIG. 2 is a sectional view, partly broken away, taken along the line 2--2 of FIG. 1;

FIG. 3 is a enlarged, diagrammatic view of the coating shown in FIG. 2; and

FIG. 4 is a antenna employing the coating of this invention.

Referring now to FIG. 1, reference numeral 1 indicates one illustrative embodiment of aircraft having an outer skin 2 or metallic surface of generally reflective material. As is well known in the art, a variety of materials are utilized for the skin 2 and neither the metallic surface 2 nor the operational aspects of the aircraft 1 form any part of the invention described hereinafter. It is here sufficient to note that the aircraft 1 generally may be a high performance aircraft in which it is desirable to either suppress reflective incident energy or change its characteristics upon reflection. In order to accomplish this end, we provide the aircraft with an absorptive coating 3. The coating 3 includes a first layer 4, a second layer 5, and a third layer 6. The layers 4, 5 and 6 are diagrammatically illustrated in FIG. 3 which further illustrates the properties of the particular coating which are required to accomplish the desired end of the invention.

As shown in FIG. 3, a first Y axis 7 shows the dielectric constant of the material used in the coating 3 as a ratio .epsilon. divided by .epsilon..sub.o where .epsilon. is the dielectric constant of the particular material in question and .epsilon..sub.o is the dielectric constant of air. A second Y axis 8 depicts the conductivity of the material used in the coating 3, shown as the log to the base 10 of the conductivity in reciprocal ohms centimeter. The X axis depicts the material thickness in inches. As shown in FIG. 3, the dielectric material of the coating 3 is chosen so that the layer 6 has the lowest dielectric constant, the dielectric constant increasing through the layers 5 and 4. The conductivity of the layer 6 is chosen as 0 and the conductivity increases through the layer 5, exhibiting a characteristic dip at a PN junction 9. However, it increases substantially in the layer 4 to the boundary surface of the layer 5 and the metal 2. As indicated above, the layer 6 primarily serves as a protective layer. It may be used, however, to condition incoming electromagnetic wave energy to some extent so that the layers 4 and 5 better accomplish their function.

When coating 3 has a thickness of approximately one inch, the coating 3 will exhibit, in the passive mode of the coating operation, a broad frequency range absorptive characteristic. The one inch thickness of the coating 3 is useful against incident wave energy having a wave length not greater than the thickness of the coating. Thus, the coating 3 has an effective absorptive characteristic in its passive mode against a wide range of high frequency electromagnetic wave energy sources. When absorption of the incident wave requires a coating 3 thickness greater than approximately one inch, for example, when the electromagnetic wave originates at a low frequency source, the coating 3 may be used in an active mode to reduce reflected wave energy. Active mode operation is accomplished through the use of electrical leads 10 and 11, attached to the coating 3. The leads 10 and 11 are Ohmic contacts disposed periodically on the surface of the coating 3. As shown, the leads 10 and 11 are attached on opposite sides of the PN junction 9 at a pair of connection points 12 and 13, respectively. The leads 10 and 11 in turn are connected to a suitable source of electrical energy 14.

In the active mode of operation, the source 14 imposes an electrical signal across the PN junction 9, which alters the electrical characteristics of the coating 3, enabling the coating to act as an energy absorber even where low frequency wave energy impinges the coating. This is achieved by shaping the conductivity profile with the applied voltage such that the reflected wave is equal in amplitude and 180.degree. out of phase with the incident wave, thus resulting in extinction of reflection at the outside surface of the coating.

As will be apparent to those skilled in the art, use of the coating 3 in an "active" mode has wide ranging implications for coating 3 use. For example, a relatively small thickness of material can be used over a broad range of incident wave frequencies by altering the electrical characteristics through injection of some predetermined voltage across the PN junction of the coating 3. Thus, a relatively thin coating of material on the surface 2 can achieve a significant reduction of reflective wave energy in active mode operation. Conversely, the reflected wave energy can be increased by selective application of the insertion voltage across the PN junction. That is to say, the reflected energy can be enhanced, if that is a desirable goal.

The coating 3 may find application in a variety of applications. For example, in addition to being applied to the surface 2, the coating may be applied to a radome 15 of the aircraft 1, so that the radar tracking capabilities of the aircraft 1 itself are enhanced. That is to say, spurious energy reflection from the tracking radar of the aircraft 1 may be minimized by coating the radome 15 of the aircraft 1 in a predetermined manner, to enhance tracking capabilities along a particular desired direction. Likewise, FIG. 4 illustrates an antenna application in which a single antenna 16 has the coating 3 applied to it in a predetermined manner. With the coating 3 being used in its active mode, the single antenna 16 can be utilized in either search or track modes. The coating 3 also can be used in a phased array antenna to provide search and track and to provide data for a simplified altitude determination. The coating 3 also may find application in conjunction with a variety of microwave components, the coating 3 again being used to enhance component operation.

Although the coating 3 may comprise a variety of materials, we have found that coatings employing a layer 6 of low density polyacetylene or other low density inert plastic, a layer 5 of polyactylene doped with arsenic pentafloride and a layer 4 of polyacetylene doped with sodium achieves significant incident wave reduction in accordance with the principles of the invention discussed above. As will be appreciated by those skilled in the art, polyacetylene can be substituted for example by polyparaphenylene, polyphenylene sulfide, or amorphous classical semiconductor deposits to reduce diffusion of dopants.

Numerous variations, within the scope of the appended claims, will be apparent to those skilled in the art in light of the foregoing description and accompanying drawings. Thus, while particular thicknesses are indicated as preferred, the thickness of the coating 3 may be varied in other applications of this invention. Likewise, a number of materials, provided that they meet the dielectric constant and conductivity parameters set forth above, may be utilized for the layers 4, 5 and 6. Although certain applications were illustratively described in the invention described above, the invention is not limited to those particular applications. As previously indicated, the layer 6, while described in conjunction with the preferred embodiment, need not be used with other applications of the coating 3. These variations are merely illustrative.