Thermoplastic syntactic foam waffle absorber

#1. Field of the Invention

#The present invention relates generally to structural materials and, more particularly, to structural materials having radar absorption characteristics.

#2. Description of Related Art

#FIG. 1 is a cross-sectional view of the fiber loaded thermoplastic syntactic foam waffle absorber of the presently preferred embodiment showing features for various implementations; and

#FIG. 2 is an orthogonal perspective view of the thermoplastic syntactic foam waffle absorber according to the presently preferred embodiment.

#Turning to FIG. 2, an orthogonal view of the thermoplastic syntactic foam waffle absorber of the presently preferred embodiment is illustrated generally at 40. The thermoplastic syntactic foam waffle absorber comprises a base 22 and pyramids 43 of fiber loaded thermoplastic syntactic foam. The thermoplastic syntactic foam of the present invention has unique electromagnetic energy absorption and structural characteristics. The actual material comprising this foam is disclosed in a copending application entitled Thermoplastic Syntactic Foams and Their Preparation, which was filed by the Assignee of the present invention on Nov. 1, 1993, and which issued into U.S. Pat. No. 5,532,295. This fiber loaded thermoplastic syntactic foam operates on the electric field component of incident electromagnetic radiation, instead of operating on the magnetic field. Although magnetic absorbers operate on the magnetic field of incident electromagnetic radiation, often utilizing Lorence resonances in these magnetic fields, the present invention operates on the electric field of incident radiation and, more particularly, operates on the Debye resonances and the attending energy loss mechanism. Certain artificial dielectric fibers within the thermoplastic syntactic foam absorb the electromagnetic field energy, like small antennae. These resistive fibers absorb portions of the radiation and re-radiate the absorbed energy to other neighboring fibers, resulting in an energy loss. Additionally, the fibers of the thermoplastic syntactic foam of the present invention comprise lossy or resistant material, for converting this energy into heat through the resistance of the electrical currents in the fibers. The electric field is thus decreased by both the retransmission and the generation of heat, in order to provide an efficient absorber. Additionally, this thermoplastic syntactic foam provides very good structural characteristics, such as rigidity and low-weight, and can be used directly on aircraft, for example, to fabricate structural components.

#As illustrated in FIG. 1, the base 22 of the fiber loaded thermoplastic syntactic foam waffle absorber can be manufactured having a relatively small height 23, measured perpendicularly to a plane of the base of the thermoplastic syntactic foam waffle absorber. This small height 23, plus the pyramid height 35, is generally sufficient to provide a broadband frequency response to the thermoplastic syntactic foam waffle absorber, which in the preferred embodiment covers the S to Ku bands. In one presently preferred embodiment, the height 23 of the base 22 is approximately one quarter of an inch and the height of the fiber loaded thermoplastic syntactic foam pyramids is three quarters of an inch. In contrast, prior art absorbers were often manufactured in the order of feet, instead of inches, in order to provide comparable broadband frequency response. The fiber loaded thermoplastic syntactic foam waffle absorber 20 of the present invention further comprises a plurality of truncated fiber loaded thermoplastic syntactic foam pyramids 25. Each truncated pyramid comprises a bottom end 30 and a top end 32. The bottom ends 30 and the top ends 32 are preferably square in shape but may also be other polygonal shapes such as hexagonal. In the presently preferred embodiment, each bottom end comprises one square inch, and each top end comprises one sixteenth of a square inch. Thus, in the cross-sectional view of the thermoplastic syntactic foam waffle absorber 20 shown in FIG. 1, a width of each bottom end 30 is approximately one inch, and a width of each top end 32 is approximately one quarter inch. The height 35 of each pyramid, measured in the same direction as the height 35 of the base, is approximately three quarters of an inch.

#The truncated pyramids 25 provide a wide angle response, which covers incident angles up to approximately seventy-five to eighty degrees from normal, in the presently preferred embodiment. The base 23 of the fiber loaded thermoplastic syntactic foam does not have very good angle response without the truncated pyramids 25. The truncated pyramids 25 provide for the absorption and scattering of electromagnetic energy radiation at a variety of angles other than angles corresponding to incident radiation substantially perpendicular to the plane of the base 23.