WO1990004210A1 - Electromagnetically non-reflective materials - Google Patents
Electromagnetically non-reflective materials Download PDFInfo
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- WO1990004210A1 WO1990004210A1 PCT/US1989/004469 US8904469W WO9004210A1 WO 1990004210 A1 WO1990004210 A1 WO 1990004210A1 US 8904469 W US8904469 W US 8904469W WO 9004210 A1 WO9004210 A1 WO 9004210A1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01Q—ANTENNAS, i.e. RADIO AERIALS
- H01Q17/00—Devices for absorbing waves radiated from an antenna; Combinations of such devices with active antenna elements or systems
- H01Q17/008—Devices for absorbing waves radiated from an antenna; Combinations of such devices with active antenna elements or systems with a particular shape
Definitions
- This invention relates to materials which interact with incident electromagnetic energy and, more specifically, to materials which exhibit diminished reflectance of said electromagnetic energy.
- One of the many uses for electromagnetic energy is in the detection and location of objects from which such energy is reflected.
- One more commonly known example of this is radar.
- the radar-discernable shape in the backscatter direction exhibited by an object by virtue of its reflectance of electromagnetic energy is known as its radar cross section. Since the practical development of radar in the 1940's, researchers in the field of electromagnetic identification, location and classification have been concerned with radar cross section evaluation, reduction, and management. Even for objects which possess great physical dimensions, it is possible to tailor shape and constituent materials so as to reduce the object's radar cross section.
- a vast amount of research from both the theoretical and experimental points of view has been directed toward the development of particular target shapes and the synthesis of new materials which may advantageously be employed in radar cross section management and reduction.
- the materials useful in this regard are those which obscure objects by reduction of reflection of incident electromagnetic radiation.
- the potential uses for materials which effectively absorb electromagnetic energy or substantially preclude its reflectance include the construction of photonic and microwave/millimeter wave components, radar absorbers for low observables or other applications, radomes, antennae, and microwave and millimeter wave anechoic chambers.
- Figure I depicts a chiral structure, a helix. It and its enantiomorphs are useful in the practice of the invention. Summary of the Invention
- Chirality relates to the lack of bilateral symmetry in an object. Any object or structure that is not congruent with its mirror image is said to be chiral. An object so defined has the property of handedness and may be either right-handed or left-handed. If an object is right-handed, its mirror image or enantiomorph is left-handed and vice versa.
- Figure 1 presents a chiral object (single-turn helix). For example, a spring, screw, and mobius are chiral. The concept of chirality applies to objects of all sizes. The employment of such chiral structures makes it possible to overcome the previously noted limitations.
- Electromegnetic chirality embraces both optical activity and circular dichroism.
- Optical activity refers to the rotation of the plane of polarization of optical waves by a chiral medium while circular dichroism indicates a change in the polarization ellipticity of optical waves by such a medium.
- Electromagnetic chiral materials can be described by the following chiral constitutive relations:
- ⁇ o 8.854 x 10 -12 F/m
- ⁇ o (1.257 x 10 -6 H/m) are the permittivity and permeability of free space.
- the constants ⁇ , ⁇ , ⁇ c are, in general, complex and have values that are determined by material properties, such as the shape, dimension, concentration and number of the chiral objects of which chiral material is composed.
- the electric field and displacement vectors E and D are parallel as are the magnetic field intensity and magnetic flux density vectors H and B.
- the pair E and D is perpendicular to the pair H and B.
- the ratio of E to H is given by the chiral intrinsic impedance n c .
- D and B yields:
- the location, shape and identification of targets can be achieved from a knowledge of waves reflected or scattered from the target boundaries. These boundaries can be looked upon as variations or discontinuities of electrical parameters. If the reflected or scattered waves can be reduced significantly, the location and shape of targets will not be determined. In other words, the targets will become electromagnetically "invisible.”
- chiral structures employable in the practice of this invention can be naturally- occurring or man-made.
- a preferred chiral structure is the single-turn wire helix given in Figure 1, having total stem length 21, loop radius a, and thickness as noted therein.
- Preferred materials for constructing helices include copper, gold, silver, iron, and aluminum.
- chiral structures can be produced, for example, by molding, extruding or otherwise shaping a suitable metal, alloy, polymer or other conducting structure.
- chiral structures are embedded in a suitable host material which is generally constituted so as to contain the chiral moieties and to cause them to adhere to or form articles or coatings upon articles.
- a suitable host material which is generally constituted so as to contain the chiral moieties and to cause them to adhere to or form articles or coatings upon articles.
- Polymerizable materials such as acrylics, epoxies and the like are exemplary host materials.
- Other solidifiable materials may be used as well.
- Chiral structures and their possibly lossy host material, taken together form mixtures which satisfy equation VIII can be referred to as a Chirosorb.
- Chirosorb can be applied directly to objects which are to be obscured.
- Suitable host media comprise liquids, polymeric, polymerizable or otherwise solidifiable materials, and certain solids with varying degrees of loss.
- the chiral material employed to impart electromagnetically non-reflective properties to a given object may be homogeneous or may comprise chiral structures of varying size, shape, and constitution to provide broadband obscuration. The invention is further described in connection with the following prophetic examples.
- Elemental copper having conductivity of about 5.0 x 10 7 mhos is drawn into a cylindrically shaped wire having diameter (t) of about 0.1 millimeters.
- the wire is then shaped into single-turn helices having stem half-length (1) and loop radius (a) of about 3.0 millimeters, as shown in
- n/n o and relative expected chiral impedance (n o /n o ) respectively, are then determined using a microwave receiver.
- Materials in accordance with the invention may form or be applied to articles to diminish their reflectance of microwave energy. Thus, they may be applied to substructures to affect such diminuition.
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Abstract
Materials which reflect diminishingly small electromagnetic energy are disclosed. In accordance with preferred embodiments, composite materials are made by embedding chiral structures in a host medium. The composites so obtained have low reflectance for electromagnetic radiation. These materials laid upon a radar target reduce the target radar cross section.
Description
ELECTROMAGNETICALLY NON-REFLECTIVE MATERIALS
Background of the Invention
This invention relates to materials which interact with incident electromagnetic energy and, more specifically, to materials which exhibit diminished reflectance of said electromagnetic energy.
One of the many uses for electromagnetic energy is in the detection and location of objects from which such energy is reflected. One more commonly known example of this is radar. The radar-discernable shape in the backscatter direction exhibited by an object by virtue of its reflectance of electromagnetic energy is known as its radar cross section. Since the practical development of radar in the 1940's, researchers in the field of electromagnetic identification, location and classification have been concerned with radar cross section evaluation, reduction, and management. Even for objects which possess great physical dimensions, it is possible to tailor shape and constituent materials so as to reduce the object's radar cross section. A vast amount of research from both the theoretical and experimental points of view has been directed toward the development of particular target shapes and the synthesis of new materials which may
advantageously be employed in radar cross section management and reduction. Among the materials useful in this regard are those which obscure objects by reduction of reflection of incident electromagnetic radiation.
In addition to radar cross section management and reduction, the potential uses for materials which effectively absorb electromagnetic energy or substantially preclude its reflectance include the construction of photonic and microwave/millimeter wave components, radar absorbers for low observables or other applications, radomes, antennae, and microwave and millimeter wave anechoic chambers.
For a material to effectively absorb electromagnetic energy - - and thus exhibit diminished or zero reflectivity - - certain conditions must be satisfied which are difficult to meet in practice. In addition, many materials previously employed in the art of radar cross section reduction and other purposes are anisotropic. These anisotropic materials pose problems in the design, manufacture, and application of electromagnetically non-reflective objects.
It is therefore an object of this invention to provide isotropic electromagnetically absorptive materials which can be made with relative ease. It is another object of this invention to provide materials which might advantageously be employed in the construction of objects having low radar cross section. It is still another object of this invention to provide materials which exhibit near- zero reflectance of incident electromagnetic wave energy.
Brief Description of the Drawing
Figure I depicts a chiral structure, a helix. It and its enantiomorphs are useful in the practice of the invention. Summary of the Invention
It has been found in accordance with this invention that the incorporation of chiral structures embedded in a host layer upon the surface of an object can reduce that object's reflectance of electromagnetic energy. A medium composed of a host material and embedded chiral structures is denoted "chiral material."
Chirality relates to the lack of bilateral symmetry in an object. Any object or structure that is not congruent with its mirror image is said to be chiral. An object so defined has the property of handedness and may be either right-handed or left-handed. If an object is right-handed, its mirror image or enantiomorph is left-handed and vice versa. Figure 1 presents a chiral object (single-turn helix). For example, a spring, screw, and mobius are chiral. The concept of chirality applies to objects of all sizes. The employment of such chiral structures makes it possible to overcome the previously noted limitations.
Electromegnetic chirality embraces both optical activity and circular dichroism. Optical activity refers to the rotation of the plane of polarization of optical waves by a chiral medium while circular dichroism indicates a change in the polarization ellipticity of optical waves by such a
medium. These phenomena, known since the mid-nineteenth century, are due to the presence of the two unequal characteristic wavenumbers corresponding to two circularly polarized eigenmodes with opposite handedness. Books on the fundamentals of electromagnetic chirality which discuss the chiral constitutive relations include Post, E. J., Structure of Electromagnetics. North-Holland, Amsterdam (1962) and Kong, J. A., Theory of Electromagnetic Waves. Wiley Interscience, New York (1975). Incorporated herein by reference is the more recent work of Jaggard, D. L., Applied Physics. 18, pp. 211 (1979), which treats the interaction of electromagnetic waves with chiral structures, and Bassiri, et al., "Electromagnetic Wave Propagation Through a Dielectric Chiral Interface and Through a Chiral Slab", Journal of the Optical Society of America A, Vol. A5 No. 9, pp. 1450-1459 (1988), which discusses the reflection of electromagnetic waves from chiralachiral interfaces.
Electromagnetic chiral materials can be described by the following chiral constitutive relations:
D = εE + iξcB
(I)
H = i£cE + (1/μ)B.
(II) where E, B , D and H are the electromagnetic field vectors , and where = " = iμ") = μº(ξc' iξ are complex
permittivity, permeability and chirality admittance of the medium, respectively. Here εº (= 8.854 x 10-12 F/m) and μº =
(1.257 x 10-6 H/m) are the permittivity and permeability of free space. The constants ε, μ, ξc are, in general, complex and have values that are determined by material properties, such as the shape, dimension, concentration and number of the chiral objects of which chiral material is composed.
Using the above constitutive relations and the source-free Maxwell equations, one obtains the following chiral Helmholtz equation: = (III)
where C is any one of E, H, B, and D, with k = ω(με) 1/2 where ω is the radian frequency of the time harmonic fields. There exist two eigenmodes of propagation; a right-handed and a left-handed circularly polarized plane wave with wavenumbers k± = + (IV)
When ε, μ, ξc are complex quantities, the two wavenumbers k+ and k_ are also complex with unequal real and unequal imaginary parts. The media described by Equations I and II are isotropic and reciprocal as opposed to magnetically biased ferrites which exhibit Faraday rotation. For each eigenmode, the electric field and displacement vectors E and D are parallel as are the magnetic field intensity and magnetic flux density vectors H and B. The pair E and D is perpendicular to the pair H and B. The ratio of E to H is given by the chiral intrinsic impedance nc . Specifically,
(k±/k±) x E = n cH (V) where k± is the wave vector associated with the wavenumber k± and nc = n/(1 + n2ξc 2)1/2 with n(= μ/ε) as the background or host intrinsic wave impedance. Similarly, the relation between D and B yields:
(k±/k±) x D = B/nc. (VI)
The absolute value of the dimensionless quantity ηξc bounded by zero and unity. It is this quantity that is a measure of the degree of chirality of the medium. Table I summarizes and compares the characteristics of a chiral medium and a simple isotropic medium.
When a monochromatic electromagnetic plane wave is normally incident upon the interface between a chiral medium and free space, it splits into two transmitted waves proceeding into the chiral medium, and a reflected wave propagating back into free space. The reflected energy can be obtained from a knowledge of the amplitude reflection coefficient. This reflection coefficient is written as
(VII)
In radar, the location, shape and identification of targets can be achieved from a knowledge of waves reflected or scattered from the target boundaries. These boundaries can be looked upon as variations or discontinuities of electrical parameters. If the reflected or scattered waves can be reduced significantly, the location and shape of targets will not be determined. In other words, the targets will become electromagnetically "invisible."
To achieve target invisibility, the reflection coefficient of Equation VII must be reduced to zero which occurs when no=nc. More explicitly.
where = Ψ = ξc'2 - ξc -2 and Φ = 2ξc'ξc'. The additional degrees of
freedom introduced by the choice of ξc' and ξc" to achieve the zero reflectivity condition nº = nc , plus the isotropic characteristics of chiral media, are important advantages of chiral materials over conventional radar absorbing materials.
An exceedingly wide variety of chiral structures is amenable to the practice of this invention, so long as such structures exhibit an effective capacity to conduct electric current and have the same handedness. Chiral structures employable in the practice of this invention can be naturally- occurring or man-made. A preferred chiral structure is the single-turn wire helix given in Figure 1, having total stem length 21, loop radius a, and thickness as noted therein. Preferred materials for constructing helices include copper, gold, silver, iron, and aluminum. As will be appreciated by those skilled in the art, chiral structures can be produced, for example, by molding, extruding or otherwise shaping a suitable metal, alloy, polymer or other conducting structure. These chiral structures are embedded in a suitable host material which is generally constituted so as to contain the chiral moieties and to cause them to adhere to or form articles or coatings upon articles. Polymerizable materials such as acrylics, epoxies and the like are exemplary host materials. Other solidifiable materials may be used as well. Chiral structures and their possibly lossy host material, taken together form mixtures which satisfy equation VIII can be referred to as a Chirosorb.
Chirosorb can be applied directly to objects which are to be obscured. Suitable host media comprise liquids,
polymeric, polymerizable or otherwise solidifiable materials, and certain solids with varying degrees of loss. The chiral material employed to impart electromagnetically non-reflective properties to a given object may be homogeneous or may comprise chiral structures of varying size, shape, and constitution to provide broadband obscuration. The invention is further described in connection with the following prophetic examples.
Materials which can be either naturally occurring or man-made may be employed. Chiral molecular species are also suitable in accordance with certain embodiments of the invention. Thus, natural or synthetic molecules or molecules having chirality introduced by electromagnetic forces may be used. EXAMPLES
Example 1 - Construction of Helices
Elemental copper having conductivity of about 5.0 x 107 mhos is drawn into a cylindrically shaped wire having diameter (t) of about 0.1 millimeters. The wire is then shaped into single-turn helices having stem half-length (1) and loop radius (a) of about 3.0 millimeters, as shown in
Figure 1.
Example 2 - Incorporation of Helices into Lossy Host Medium
The components of an Eccosorb (Emerson and Cumings) lossy material preparation are mixed in an open-top cardboard box having known internal volume. Before the preparation
solidifies, an appropriate number of the copper helices constructed in Example 1 is uniformly added to reach the desired concentration (N) helices per square centimeter. The matrix is stirred well and allowed to fully solidify. Example 3 - Comparative Testing
The cardboard-boxed specimens prepared in Example
2 are independently positioned and the open side thereof exposed to incident electromagnetic energy having known frequency (f) produced by a microwave horn antenna. The magnitude of the specimens' approximate relative impedance
(n/nº) and relative expected chiral impedance (nº/nº) respectively, are then determined using a microwave receiver.
The experimental method is repeated, employing copper helix concentrations, Eccosorb preparations, and electromagnetic energy frequencies as indicated in the following table.
The expected data will demonstrate that the preparations incorporating the helics exhibit significantly diminished microwave reflectances.
Those skilled in the art will appreciate that numerous changes and modifications to preferred embodiments of the invention and that such changes and modifications may be made without departing from the spirit of the invention. It is therefore intended that the appended claims cover all such equivalent variations as fall within the true spirit and scope of the invention.
Materials in accordance with the invention may form or be applied to articles to diminish their reflectance of microwave energy. Thus, they may be applied to substructures to affect such diminuition.
Claims
1. A composition comprising chiral structures of size and conformation effective to substantially diminish reflectance of electromagnetic radiation incident upon objects comprising said composition.
2. The composition of claim 1 further comprising a carrier medium, said medium facilitating the application of said composition to a substructure.
3. The composition of claim I further comprising a lossy host medium.
4. The composition of claim 1 wherein said composition is polymerizable.
5. The composition of claim 2 wherein said composition solidifies subsequent to application.
6. A composition comprising chiral structures of size and conformation effective to substantially absorb electromagnetic radiation incident upon objects comprising said composition.
7. The composition of claim 6 further comprising a carrier medium, said medium facilitating the application of said composition.
8. The composition of claim 6 further comprising a lossy host medium. 3.
9. The composition of claim 6 wherein said composition is polymerizable.
10. The composition of claim 6 wherein said composition solidifies subsequent to application.
11. A method of making a composition comprising chiral structures of size and conformation effective to substantially diminish reflectance of electromagnetic radiation incident upon objects comprising said composition, said method comprising the steps of providing chiral structures and admixing said chiral structures with a carrier medium.
12. A method of making a composition comprising chiral structures of size and conformation effective to substantially diminish reflectance of electromagnetic radiation incident upon objects comprising said composition, said method comprising the steps of providing chiral structures and admixing said chiral structures with a lossy host medium.
13. A method of making a composition comprising chiral structures of size and conformation effective to absorb electromagnetic radiation incident upon objects comprising said composition, said method comprising the steps of providing chiral structures and admixing said chiral structures with a carrier medium.
14. A method of making a composition comprising chiral structures of size and conformation effective to absorb electromagnetic radiation incident upon objects comprising said composition, said method comprising the steps of providing chiral structures and admixing said chiral structures with a lossy host medium.
15. A body having diminished reflectance of incident electromagnetic radiation, said body comprising a composition comprising chiral structures of size and conformation effective to secure said diminution.
16. The body of claim 15 wherein said composition is coated upon said body.
17. A body which absorbs incident electromagnetic radiation, said body comprising a composition comprising chiral structures of size and conformation effective to secure said absorption
18. The body of claim 17 wherein said composition is coated upon said body.
19. A method of making a body having diminished reflectance of incident electromagnetic radiation comprising the steps of providing a structural underlayment and coatingly adhering to said underlayment a composition comprising chiral structures of size and conformation effective to substantially diminish the reflectance of electromagnetic radiation incident upon said body.
20. The method of claim 19 further comprising the step of molding, extruding, or otherwise shaping the underlayment upon which the composition comprising chiral structures has been coatingly adhered.
21. A method of making a body which absorbs incident electromagnetic radiation comprising the steps of providing a structural underlayment and coatingly adhering to said underlayment a composition comprising chiral structures of size and conformation effective to absorb electromagnetic radiation incident upon said body.
22. The method of claim 21 further comprising the step of molding, extruding, or otherwise shaping the underlayment upon which the composition comprising chiral structures has been coatingly adhered.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US25510788A | 1988-10-07 | 1988-10-07 | |
| US255,107 | 1988-10-07 |
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| Publication Number | Publication Date |
|---|---|
| WO1990004210A1 true WO1990004210A1 (en) | 1990-04-19 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| PCT/US1989/004469 WO1990004210A1 (en) | 1988-10-07 | 1989-10-06 | Electromagnetically non-reflective materials |
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5202535A (en) * | 1991-06-28 | 1993-04-13 | Grace N.V. | Chiral absorber |
| US5229773A (en) * | 1991-06-28 | 1993-07-20 | Grace N.V. | Chiral absorbing structure |
Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2293839A (en) * | 1940-06-25 | 1942-08-25 | Rca Corp | Centimeter wave absorber |
| US3039172A (en) * | 1958-01-07 | 1962-06-19 | Walter G Egan | Protective clothing for radar workers |
| US4006479A (en) * | 1969-02-04 | 1977-02-01 | The United States Of America As Represented By The Secretary Of The Air Force | Method for dispersing metallic particles in a dielectric binder |
| US4538151A (en) * | 1982-03-31 | 1985-08-27 | Nippon Electric Co., Ltd. | Electro-magnetic wave absorbing material |
-
1989
- 1989-10-06 WO PCT/US1989/004469 patent/WO1990004210A1/en unknown
Patent Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2293839A (en) * | 1940-06-25 | 1942-08-25 | Rca Corp | Centimeter wave absorber |
| US3039172A (en) * | 1958-01-07 | 1962-06-19 | Walter G Egan | Protective clothing for radar workers |
| US4006479A (en) * | 1969-02-04 | 1977-02-01 | The United States Of America As Represented By The Secretary Of The Air Force | Method for dispersing metallic particles in a dielectric binder |
| US4538151A (en) * | 1982-03-31 | 1985-08-27 | Nippon Electric Co., Ltd. | Electro-magnetic wave absorbing material |
Non-Patent Citations (4)
| Title |
|---|
| BIGG, D.M., ADV. POLYM. TECH. 4(3-4) 251, 1984. See the Abstract. * |
| DAVIS, M. et al., FARADAY TRANS 1, 68, 1041, 1972. See Figure 1. * |
| GEORGER, J.H. et al., & J. AM. CHEM. SOC. 109, 6169, 1987. See column 2, page 6169 and the Experimental Section. * |
| SEZGINER, A. et al., RADIO SCIENCE, 18, 639, 1963. See the Abstract and the results. * |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5202535A (en) * | 1991-06-28 | 1993-04-13 | Grace N.V. | Chiral absorber |
| US5229773A (en) * | 1991-06-28 | 1993-07-20 | Grace N.V. | Chiral absorbing structure |
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