Stock material or miscellaneous articles – Structurally defined web or sheet – Longitudinal or transverse tubular cavity or cell
Patent
1991-04-08
1992-11-17
Ryan, Patrick J.
Stock material or miscellaneous articles
Structurally defined web or sheet
Longitudinal or transverse tubular cavity or cell
428198, 428323, 428328, 156 60, B23B 900
Patent
active
051642425
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD OF THE INVENTION
This invention relates in general to a structure that prevents ice accumulation and which attenuates electromagnetic waves. In particular, this invention relates to a structure placed on an electrically conductive surface, for example the surface of an airplane or ship, which prevents ice accumulation through the use of a pneumatic deicer and which also attenuates electromagnetic waves through a dual band absorber, and which is weather and wear resistant.
BACKGROUND OF THE INVENTION
Electromagnetic wave attenuating material has commonly been placed on ships, planes, and other vehicles, particularly those used in the military, for several reasons. The radar system used on a ship generates radar signals. These radar signals may reflect off the ship's own structure, creating false echoes or ghost images. These false echoes may interfere with the proper navigation of the ship, for example, because of increased clutter on radar. These false echoes may also be represented as false targets on the radar. Electromagnetic wave attenuating material (also known as radar absorbing material) can be bonded to selective areas of the ship where reflection of the ship's radar signals commonly occurs. The electromagnetic wave attenuating material attenuates the radar signals, preventing back reflection.
Another application of electromagnetic wave attenuating material involves antenna radiation pattern shaping. The presence of conductive objects near an antenna can alter the established free space propagation characteristics of the antenna. However, placing electromagnetic wave attenuating material in those conductive areas near the antenna eliminates the problem. The use of electromagnetic wave attenuating material on ships, planes, and other vehicles also reduces the vehicle's cross-sectional area as seen by the radar. Reducing the radar cross section reduces the vehicle's signature, that is, its ability to be detected by radar.
Attenuation of electromagnetic waves is represented by the following equation: amplitude after passing through the electromagnetic wave attenuating material. Attenuation generally occurs as a result of two mechanisms. One mechanism involves destructive interference between a first wave and a reflected second wave which is 180.degree. out of phase with the first wave. Another attenuation mechanism occurs by the absorption of the electromagnetic wave energy.
With regard to the absorption of the electromagnetic wave energy, the energy of an electromagnetic wave is a function of the distance it travels through a medium, as represented by the following equation: the distance traveled in the medium and .delta. is the "skin depth." For a good conductor, .delta. is proportional to: ##EQU1## where .mu. is the magnetic permeability and .sigma. is the conductivity. Magnetic materials such as ferrites, iron and cobalt-nickel alloys are used to alter the permeabilities of materials. For example, the magnetic materials can be embedded in a rubber or elastomer. Increasing a material's magnetic permeability value increases the material's absorption of electromagnetic wave energy.
To achieve destructive interference between electromagnetic waves, typically a thin material is provided that is effectively one-fourth wavelength of the electromagnetic wave energy wavelength incident upon the material. The electromagnetic waves incident upon this material will be reflected or transmitted depending upon the properties of the material, as represented by the following equation: ##EQU2## where: Z.sub.o =.mu..sub.o /e.sub.o permittivity. Increasing the permeability of a material, such as by embedding magnetic particles in an elastomer, will cause electromagnetic waves to be partially reflected off of this material and partially transmitted through this material if the electromagnetic wave was traveling through a medium having a lower Z.sub.l (for example, if the permeability of the medium is lower than the permeability of the material). To achieve destructive cancellation of electromagneti
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Webster Steven D.
Williams Randy B.
Bahta Abraham
Ryan Patrick J.
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