Patent Publication Number: US-8114489-B2

Title: Composite material having low electromagnetic reflection and refraction

Description:
CROSS REFERENCE 
     The present application claims priority under 35 U.S.C. §119 on U.S. Provisional Patent Application No. 60/293,070 filed May 23, 2001. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with government support under Award No. DAAD19-00-1-0525 awarded by the Army Research Office. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related to materials having low electromagnetic reflection and refraction. The invention generally concerns materials provided to control electromagnetic reflection and refraction. 
     BACKGROUND OF THE INVENTION 
     The behavior of electromagnetic radiation is altered when it interacts with charged particles. Whether these charged particles are free, as in plasmas, nearly free, as in conducting media, or restricted, as in insulating or semi conducting media—the interaction between an electromagnetic field and charged particles will result in a change in one or more of the properties of the electromagnetic radiation. Because of this interaction, media and devices can be produced that generate, detect, amplify, transmit, reflect, steer, or otherwise control electromagnetic radiation for specific purposes. 
     The behavior of electromagnetic radiation interacting with a material can be predicted by knowledge of the material&#39;s electromagnetic materials parameters ∈ and μ where ∈ is the electric permittivity of the medium, and μ is the magnetic permeability of the medium. These parameters represent a macroscopic response averaged over the medium, the actual local response being more complicated to describe and generally not necessary to describe the electromagnetic behavior. 
     Reflection and transmission at the interface between two media are governed by the index of refraction η and impedance z of each medium. The index η and the impedance z are directly related to the reflection and transmission properties of a slab of material, and hence are the observable quantities that correspond directly to the electromagnetic performance of materials. The index of refraction η and the impedance z can be expressed in relative terms in relation to corresponding properties for free space as:
 
η=[(∈μ)/(∈ 0 μ 0 )] 1/2  
 
 z =(μ/∈) 1/2 /(μ 0 /∈ 0 ) 1/2  
 
where the subscript  0  indicates free space values associated with a vacuum. Air has very nearly the index of refraction and impedance of vacuum. Thus, the relative index of refraction and the relative electromagnetic impedance z of air are often taken to be equal to unity. Note that the permittivity and permeability can be found from the index and the impedance using the above relations, as ∈=η/z and μ=ηz.
 
     In addition to having low material losses, a material that is electromagnetically “transparent” will have both its index of refraction and impedance numerically close to that of the surrounding medium. Such a material is valuable for many applications. For example, airplanes may have a collision detection radar system mounted near their “nose.” This system operates inside a composite dome known as a radome that has a shape optimized for aerodynamic properties. The radar system must compensate for the lensing effects of the shaped radome composite material, which typically has a relative index of refraction that is significantly greater than unity. Such compensation requires effort and expense, and is subject to error. 
     By way of additional example, structural materials may be used to embed a sensor such as an array of antennas in a wireless communications device. Reflection and refraction effects in these structural materials are likewise undesirable. In both of these applications, material requirements, irrespective of their electromagnetic reflection and refraction properties, include physical properties such as strength, ductility, and resistance to heat, cold, and moisture. The prior art has had limited success in satisfying these needs. 
     Materials and methods for generally minimizing electromagnetic reflection and maximizing transparency have been proposed. For example, materials have been proposed that have a high absorption of incident radiation at microwave and other frequencies. In addition to preventing transmission of radiation, the strong absorbance of these materials often leads to a substantial reflected component. As a result, use of these materials is usually accompanied by irregular material shapes and surface angles required to direct the reflected component in a desired direction. The required irregular surface angles and shapes significantly limit the utility of such materials and methods. 
     Also, the prior art has employed particular naturally occurring media that may be found in nature or that can be formed by known chemical synthesis and that may have a low level of electromagnetic reflection over a particular frequency range. Use of such media is disadvantageously limited to these particular frequency ranges. Also, it is difficult to find media with significant permeability at RF and higher frequencies. These media may also be structurally unsuitable for many applications. 
     Previous study of the effects of so-called “artificial dielectric” materials on electromagnetic waves has been performed. For example, artificial dielectric materials based on arrays of substructures that collectively have a desired response to electromagnetic radiation have been studied. These arrays, which need not necessarily be periodic in nature, have in common that the dimensions and spacing of the scattering elements are less than the wavelengths over which the composite material will operate. It is found that by averaging the local electromagnetic fields over such a structure, an effective permittivity (and/or permeability) function can be applied that roughly describes the scattering properties of the composite. The procedure that arrives at this description is known in the literature as “effective medium theory.” 
     An example of a prior art artificial dielectric material is the “rodded” medium, used as an analogue medium to study propagation of electromagnetic waves through the ionosphere [See, e.g., R. N. Bracewell, “Analogues of an Ionized Medium”, Wireless Engineer, 31:320-6, December 1954, herein incorporated by reference]. An artificial medium based on conducting wires or posts has a dielectric function identical to that describing a dilute, collisionless neutral plasma. Accordingly, as used herein a medium based on conducting wires will be referred to as a “plasmonic” medium. More recently, artificial plasmonic media have been proposed using, for example, a periodic arrangement of very thin conducting wires. See, e.g., J. B. Pendry et al., “Extremely low frequency plasmons in metallic mesostructures”, Physical Review Letters, 76(25):4771-6, 1996; see also D. R. Smith et al., “Loop-wire for investigating plasmons at microwave frequencies,” Applied Physics Letters, 75(10):1425-7, 1999; both of which are incorporated herein by reference. 
     Other recent examples of artificial dielectrics include the use of random arrangements of metal “needles” suspended in a foam structure as a “lens” with an index of refraction greater than unity. Many foam-like materials have a refractive index approximately equal to unity. Adding needles serves to increase the index for low-frequency RF radiation as with radio astronomy. These materials, however, are not acceptable for applications requiring a degree of mechanical strength. 
     To date, these prior art efforts have not been successful in providing materials that have a low reflectance and good transparency at a desired wavelength in addition to having advantageous structural mechanical properties. 
     Unresolved needs in the art therefore exist. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a composite material comprising a host dielectric medium having an index of refraction greater than 1, and an artificial plasmon medium embedded in the host medium. The artificial plasmon medium has a dielectric function of less than one so that the permittivity of the composite material is substantially equal to that of the surrounding medium for incident electromagnetic radiation of a desired frequency. 
     Composite media of the invention thus can be of utility as materials that are highly transparent and exhibit minimal reflectance or refraction for electromagnetic waves in a desired frequency range. Also, composite media embodiments of the present invention can be “tuned” for achieving transparency and/or minimal reflection and refraction for electromagnetic waves in the desired frequency range through selection of particular conductor/host materials, conductor/host sizing and/or spacing, and conductor/host geometric configuration. Further, composite media of the present invention allow for achieving these desired electromagnetic properties (e.g., transparency and low reflection) while providing advantageous structural and mechanical properties, with the result that embodiments of the present invention will be well suited for applications such as radomes, antennas, and the like. 
     The above brief description sets forth broadly some of the features and advantages of the present disclosure so that the detailed description that follows may be better understood. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1(   a ) is a graphical representation of the relationship between a matrix dielectric constant and a normalized frequency; 
         FIG. 1(   b ) is a graphical representation of the relationship between the matrix dielectric constant and a bandwith; 
         FIG. 1(   c ) is a top plan cross section of a preferred embodiment of a composite material of the invention; 
         FIG. 2  is a side elevational cross section of the embodiment of  FIG. 1  taken along the line  2 - 2 ; 
         FIG. 3  is a top plan cross section of an additional preferred embodiment of a composite material of the invention; 
         FIG. 4  is a schematic perspective of the embodiment of  FIG. 3 ; 
         FIG. 5  is a top plan cross section of an additional preferred embodiment of a composite material of the invention; 
         FIG. 6  is a perspective schematic representation of the embodiment of  FIG. 5 ; 
         FIG. 7  is a perspective schematic representation of an additional preferred embodiment of a composite material of the invention; 
         FIG. 8  is a top plan schematic representation of the embodiment of  FIG. 7 ; 
         FIGS. 9(   a )-( c ) illustrate some alternative conductors of the invention; 
         FIG. 10  is a side elevational view of a portion of an additional preferred embodiment of the invention; 
         FIG. 11  is a top plan cross-section view of a portion of the embodiment of  FIG. 10 ; 
         FIGS. 12(   a ) and ( b ) are plots showing computer simulation based electrical properties of the embodiment of  FIG. 11 ; 
         FIG. 13  is a perspective view of a preferred radome embodiment of the invention; 
         FIG. 14  is a bottom plan view of the radome embodiment of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION: 
     In order to describe the best known modes of practice of the present invention, it will be useful to first discuss some relevant properties and relationships of physics. The wavelength λ, the frequency f of electromagnetic radiation, and the velocity v are related by:
 
v=λf
 
The angular frequency ω is related to the frequency by a constant:
 
ω=2πf
 
In dimensionless quantities, then, ratios of frequencies can be used interchangeably:
 
( f   1   /f   2 )=(ω 1 /ω 2 )
 
     In order to describe the presence of a material, Maxwell&#39;s equations must be solved in the presence of the material. The local electromagnetic response of a material—the exact electric and magnetic field distributions that occur near the atoms or elements that compose the material—will in general be very complicated. However, since the exact nature of the local fields in a material is usually unimportant to the behavior of the electromagnetic waves propagating through the material, the local fields are typically averaged to obtain a set of Maxwell&#39;s equations that includes the material properties in two parameters: ∈ and μ. 
     A simple example of an idealized medium is the Drude medium, which in certain limits describes such systems as conductors and dilute plasmas. The averaging process leads to a permittivity that, as a function frequency, has the form
 
∈( f )/∈ 0 =1 −f   p   2   /f ( f+iv )   EQTN. A
 
where f is the electromagnetic excitation frequency, f p  is the plasma frequency and v is a damping factor. In general, the plasma frequency may be thought of as a limit on wave propagation through a medium: waves propagate when the frequency is greater than the plasma frequency, and waves do not propagate (e.g., are reflected) when the frequency is less than the plasma frequency. Simple conducting systems (such as plasmas) have a dispersive dielectric response. The degree to which an artificial medium obeys EQTN. A must often be determined empirically and depends on the construction materials and on the geometric properties that determine f p  relative to the inter-element spacing of the metal scattering elements.
 
     The plasma frequency is the natural frequency of charge density oscillations (“plasmons”), and may be expressed as:
 
ω p   =[n   eff   e   2 /∈ 0   m   eff ] 1/2  
 
and
 
 f   p =ω p /2π
 
where n eff  is the charge carrier density and m eff  is an effective carrier mass. For the carrier densities associated with typical conductors, the plasma frequency f p  usually occurs in the optical or ultraviolet bands.
 
     The Pendry reference that has been incorporated herein by reference teaches a thin wire media—in which the wire diameters are significantly smaller than the skin depth of the metal—can be engineered with a plasma frequency in the microwave regime, below the point at which diffraction due to the finite wire spacing occurs. By restricting the currents to flow in thin wires, the effective charge density is reduced, thereby lowering the plasma frequency. Also, the inductance associated with the wires acts as an effective mass that is larger than that of the electrons, further reducing the plasma frequency. By incorporating these effects, the Pendry reference provides the following prediction for the plasma frequency of a thin wire medium: 
               f   p   2     =       1     2   ⁢   π       ⁢     (         c   0   2     /     d   2           ln   ⁡     (     d   r     )       -       1   2     ⁢     (     1   +     ln   ⁢           ⁢   π       )           )             
where c 0  is the speed of light in a vacuum, d is the thin wire lattice spacing, and r is the wire diameter. The length of the wires is assumed to be infinite and, in practice, preferably the wire length should be much larger than the wire spacing, which in turn should be much larger than the radius.
 
     By way of example, the Pendry reference suggests a wire radius of approximately one micron for a lattice spacing of 1 cm—resulting in a ratio, d/r, on the order of or greater than 10 5 . Note that the charge mass and density that generally occurs in the expression for the f p  are replaced by the parameters (e.g., d and r) of the wire medium. Note also that the interpretation of the origin of the “plasma” frequency for a composite structure is not essential to this invention, only that the frequency-dependent permittivity have the form as above, with the plasma (or cutoff) frequency occurring in the microwave range or other desired ranges. 
     Any conducting element that has an inductance can also be utilized as the repeated element that forms a plasmonic medium. In the thin wire medium, increased inductance is primarily achieved by making the wires very thin; However, the inductance can also be increased by other means, such as arranging inductive loops within the medium, or even the inclusion of actual inductive elements within the circuit. Thicker loop-wire media can be comprised, for example, of wire coils or wire lengths having periodic loops. 
     An embodiment of the present invention is directed to a composite, or hybrid, material comprised of a host dielectric with an artificial plasmon medium embedded therein, whereby the composite material has an index of refraction and impedance both substantially equal to that of the surrounding medium. As discussed below, it is assumed that the index of refraction and impedance of the medium are both measured relative to the surrounding medium, and accordingly the term “relative” as used herein in describing terms such as “index” and “impedance” is intended to refer to a comparison to the surrounding medium. An invention embodiment may be considered an artificial plasmon medium. Behavior of embodiments of the present invention is modeled on the assumption that the host dielectric has a uniform dielectric constant or function (it is noted that as used herein the terms dielectric constant and dielectric function are intended to be interchangeable). However, an effective dielectric function of the host medium can be substituted for the uniform constant and the properties in the frequency range of interest will be substantially unchanged. 
     The conductivity of the conducting elements of the composite embodiments of the present invention approaches infinity, but any good metal conductor such as copper or silver provides a close behavioral agreement to ideal simulations. For the composite material, the effective permittivity ∈ Ε is expressed as:
 
∈ Ε /∈ 0 =∈ H /∈ 0 −(ω p /ω) 2  
 
where ∈ H  is the permittivity of the host material and ω is the angular frequency of the electromagnetic radiation. Using the above relations, it may be derived that:
 
η=[∈ H /∈ 0 −( f   p   2   /f   2 )] 1/2  
 
     The composite materials of the present invention follow these relationships, and achieve good transparency and low reflectance for electromagnetic radiation in a desired frequency range. By way of example, a conductor of the present invention may be varied in spacing and/or geometry to control the plasma frequency ω p ,and thereby “tune” the composite of the invention. 
     In the absence of the dielectric, the only variable parameter for behavior of a plasmon medium is the plasma frequency f p ,with the index of refraction able to be expressed as
 
η=(κ) 1/2 =[1−( f   p   2   /f   2 )] 1/2  
 
where κ=∈/∈ 0 . The dielectric function of the composite of course changes upon addition of the dielectric. The presence of a dielectric matrix into which the plasmon medium is embedded will result in a polarization response that can be accounted for by introducing κ 0  such that:
 
κ=κ 0 −( f   p   2   /f   2 )
 
where κ is the effective dielectric constant of an ideal plasmon/dielectric composite material. The dipolar response term κ 0  is substantially equal to the effective dielectric constant of the polymer composite matrix in the absence of the integrated artificial plasmon medium when that medium closely obeys EQTN. A and also occupies a negligible volume fraction of the composite.
 
     With the addition of the dielectric host matrix, the dielectric constant or function κ takes a value of unity at a finite frequency f 1 =f p /(κ 0 −1) 1/2 . The frequency f 1  may be referred to as the “match frequency,” the frequency at which κ=1, the index η=1, and there is substantially no refraction at an interface between air and the ideal composite material. 
     The frequency at which κ=0 determines the onset of electromagnetic wave propagation. This “turn-on” frequency is given by: 
               f   0     =       f   p           κ   0     -   1                 FIGS. 1(   a ) and  1 ( b ) illustrate the dependence of f 0  and f on the matrix dielectric function.  FIG. 1  ( a ) shows the turn-on frequency f 0  (dashed line) and match frequency f 1  (solid line) as a function of the matrix dielectric constant κ 0  where the normalized frequency is in units of the plasma frequency f p , while  FIG. 1(   b ) shows the bandwidth as a function of the matrix dielectric constant κ 0  where the percent bandwidth is defined as (f n=1.1 −f n=0.9 )/f 1 . This illustrates the increased dispersion around n=1 as κ 0  increases.
 
     The present invention may be further described through reference to example structural embodiments. In considering the FIGS. Used to illustrate these structural embodiments, it will be appreciated that they have not been drawn to scale, and that some elements have been exaggerated in scale for purposes of illustration.  FIGS. 1(   c ) and  2  show a top plan cross section and a side elevational cross section, respectively, of a portion of an embodiment of a composite material  10  of the present invention. The composite material  10  comprises a dielectric host  12  and a conductor  14  embedded therein. It is noted that the term “dielectric” as used herein in reference to a material is intended to broadly refer to materials that have a relative dielectric constant greater than 1, where the relative dielectric constant is expressed as the ratio of the material permittivity ∈ to free space permittivity ∈ 0  (8.85×10 −12  F/m). In more general terms, dielectric materials may be thought of as materials that are poor electrical conductors but that are efficient supporters of electrostatic fields. In practice most dielectric materials, but not all, are solid. Examples of dielectric materials useful for practice of embodiments of the current invention include, but are not limited to, porcelain such as ceramics, mica, glass, and plastics such as thermoplastics, polymers, resins, and the like. 
     The term conductor as used herein is intended to broadly refer to materials that provide a useful means for conducting current. By way of example, many metals are known to provide relatively low electrical resistance with the result that they may be considered conductors. Preferred conductors for the practice of embodiments of the invention include aluminum, copper, gold, and silver. 
     As illustrated by  FIGS. 1 and 2 , the preferred conductor  14  comprises a plurality of portions that are generally elongated and parallel to one another, with a space between portions of distance d. Preferably, d is less than the size of a wavelength of the incident electromagnetic waves. Spacing by distances d of this order allow the composite material of the invention to be modeled as a continuous medium for determination of permittivity ∈. Also, the preferred conductors  14  have a generally cylindrical shape. 
     A most preferred conductor  14  comprises thin copper wires. These conductors offer the advantages of being readily commercially available at a low cost, and of being relatively easy to work with. Also, matrices of thin wiring have been shown to be useful for comprising an artificial plasmon medium, as discussed by Pendry et al., “Extremely Low Frequency Plasmons in Metallic Mesostructures,” Physical Review Letters, 76(25):4773-6, 1996; incorporated by reference herein. 
       FIG. 3  is a top plan cross section of another composite material embodiment  20  of the present invention. The composite material  20  comprises a dielectric host  22  and a conductor that has been configured as a plurality of portions  24 . As with the embodiment  10 , the conductor portions  24  of the embodiment  20  are preferably elongated cylindrical shapes, with lengths of copper wire most preferred. The conductor portions  24  are preferably separated from one another by distances d 1  and d 2  as illustrated with each of d 1  and d 2  being less than the size of a wavelength of an electromagnetic wave of interest. Distances d 1  and d 2  may be, but are not required to be, substantially equal. The conductor portions  24  are thereby regularly spaced from one another, with the intent that the term “regularly spaced” as used herein broadly refer to a condition of being consistently spaced from one another. It is also noted that the term “regularly spacing” as used herein does not necessarily require that spacing be equal along all axis of orientation (e.g., d 1  and d 2  are not necessarily equal). Finally, it is noted that  FIG. 3  (as well as all other FIGS.) have not been drawn to any particular scale, and that for instance the diameter of the conductors  24  may be greatly exaggerated in comparison to d 1  and/or d 2 . 
     As illustrated, the individual conductors may be thought of as organized in a plurality of planar layers separated from one another by the distance d 2 , as shown in the perspective schematic representation of  FIG. 4  where each planar layer  26  represents a plurality of parallel conductors  24 , and where the dielectric host  22  is illustrated as a transparent dashed line “box”. The embodiment  20  may also be thought of as having each plane of its conductors  24  in a single “dimension.” That is, the conductors  24  in each plane generally lie along a single axis of orientation (e.g., the x-axis). 
     Other embodiments of the invention will comprise conductors oriented along more than one axis of orientation. The composite material embodiment  50  represented by  FIGS. 5 and 6 , for example, illustrates the conductors  52  oriented along two axes and embedded in a dielectric host  54 . The conductors  52  in the composite material embodiment  50  may be thought to generally extend along both the x-axis and the y-axis. This is illustrated schematically in  FIG. 6 , with the conductors  52  represented as lines, and the dielectric host  54  represented as a dashed line box. Such a configuration thereby can also be considered to have a plurality of first conductors  52  organized into substantially planar rows, and a plurality of second conductors  52  organized into substantially planar columns. When laid out along an x and y axis as in the embodiment  50 , these planes are substantially normal to one another. The planar columns are preferably separated from one another by a distance less than a wavelength of electromagnetic wave of interest, with the planar rows likewise preferably spaced. 
     Other invention embodiments may additionally comprise conductors oriented along additional axes. By way of example, a composite material  100  is represented schematically in the perspective view of  FIG. 7  and the top plan view of  FIG. 8 . With reference to  FIG. 7 , a plurality of conductors  102  represented as lines may be oriented along the x, y and z axis to result in a “three dimensional” configuration. Those skilled in the art will appreciate that other conductor orientations are also possible within the present invention. 
     It will also be appreciated that conductors of embodiments of the present invention may comprise configurations other than substantially straight portions as shown in the embodiments  10 ,  20 , and  50 . Indeed, depending on a particular application it may be desirable to “tune” the composite material by altering the electrical properties of the conductor. By way of example, the diameter, geometry, and/or spacing of the conductor could be altered. With reference to  FIGS. 9(   a )-( c ) by way of example, alternate conductor shapes are illustrated.  FIG. 9(   a ) shows conductors  150  with a plurality of loops  152 . The loops  152  are preferably of substantially uniform diameter, and are preferably substantially regularly spaced along the length of the conductors  150 . That is, a substantially uniform distance preferably separates each loop  152  along a length of a conductor  150 . Those knowledgeable in the art will appreciate that the loops  152  comprise inductive elements, and thereby serve to increase the impedance of the conductors  150 . Varying the diameter and number of the loops  152  will of course alter the electrical properties of the conductors  150 , and may thereby be used to further “tune” a resulting composite material so that the composite refractive index and/or reflection coefficient is substantially equal to 1. 
       FIG. 9(   b ) shows conductors  153  in the form of spring-like coils. It will be appreciated that the conductors  150  or  153  may be used in combination with a dielectric host to comprise a composite material of the invention. By way of illustration, the conductors  150  or  154  could be used in any of the embodiments  10 ,  20 ,  50  or  100  of  FIGS. 1  ( c )- 8 .  FIG. 9(   c ), for instance, shows an additional alternate conductor  155  embedded in a host dielectric  157 . The conductor  155  is characterized in that each conductor  155  has a number of individual linked portions that are substantially straight, are at right angles to one another, with each of the portions lying along one of the x, y or z axes. 
     Those knowledgeable in the art will appreciate that many additional conductor geometries will be useful in practice of the invention. By way of example, non-cylindrical geometries comprising substantially square, rectangular, or eleptical cross sections may be of use. 
       FIG. 10  is a side elevational view of a portion of an additional embodiment  200  of the invention comprising a loop-wire artificial plasmon composite material. The embodiment  200  comprises a plurality of conductors  202  that may be considered to have the geometry of the conductors  150  or  154  of  FIG. 9(   a ) or ( b ). That is, the conductors  202  generally may comprise a plurality of connected loops, or may comprise coils. The conductors  202  are wrapped around a dielectric host, which is in the form of a plurality of elongated members  204  that may comprise by way of example nylon rods. The nylon rods are preferably substantially parallel to one another, and are preferably separated from one another by a substantially equal distance.  FIG. 11  is a top plan cross section of a portion of the embodiment  200 , illustrating the conductor  202  surrounding the dielectric nylon rod host  204 . 
     It will be appreciated that the composite material  200  of  FIGS. 10-11  is tunable by design by altering the wire conductor  202  diameter and spacing, for instance, to achieve an index of refraction and impedance as may be desired for electromagnetic waves in a desired wavelength range.  FIGS. 12(   a ) and ( b ) illustrate the result of computer simulations run on the composite material  200 , using thin copper wire as the conductor having vertical spacing between loops of about 8 mm, horizontal spacing between rods of about 8 mm, and using 6-32 nylon rods.  FIGS. 12(   a ) and ( b ) show a predicted matching condition close to 8 GHz. 
     One advantage of embodiments of the composite material of the present invention is that the composites can achieve mechanical strength and may be desirably conformed for particular applications. Indeed, those knowledgeable in the art will appreciated that using a preferred dielectric host such as a polymer and a preferred conductor such as thin copper wire, composite materials of the invention will lend themselves well to being readily configured to a multiplicity of applications. By way of example, a composite material of the invention may have utility as an electromagnetically transparent “window” for covering electronics. Examples include, but are not limited to, mechanically protective but electromagnetically transparent electronics housings and cabinets, antennae for communications devices such as cellular phones and transmission centers, building materials for structures used for communications such as satellite stations, “stealth” materials for military applications including airplanes, ships, submarines, land vehicles, individual armor; and the like. 
     A particular example is shown in  FIGS. 13-14 , where a composite material  250  of the invention has been configured in the general shape of a “dome” for use as a radome for covering radar equipment. The perspective view of  FIG. 13  shows the general “inverted bowl” shape of the radome  250 , with radar or other electronics equipment able to be covered by the radome  250 . The plan view of  FIG. 14  illustrates the general circular circumference of the radome  250 . The radome  250  is constructed of a composite material of the invention, which may comprise by way of example plastic or glass having an embedded thin wire conductor matrix therein. 
     The advantages of the disclosed invention are thus attained in an economical, practical, and facile manner. While preferred embodiments and example configurations have been shown and described, it is to be understood that various further modifications and additional configurations will be apparent to those skilled in the art. It is intended that the specific embodiments and configurations herein disclosed are illustrative of the preferred and best modes for practicing the invention, and should not be interpreted as limitations on the scope of the invention as defined by the appended claims. By way of example, electromagnetic transparency and reflection have been discussed herein for invention embodiments with the general assumption that measurements are relative to free space. Those skilled in the art, however, will appreciate that composite materials of the present invention will have utility in various environments other than free space. By way of example only, it is anticipated that composite materials of the present invention may have utility used in water, underground, and the like.