Micromagnetic circuit
BACKGROUND OF THE INVENTION 
     The present invention relates to micromagnetic circuits. More particularly, it relates to magnetic circuits in which a miniaturized magnetic conductor is disposed in the proximity of an electrical conductor for forming a magnetic path for a magnetic field related to the one produced by the electrical conductor. 
     Comprehension of the present invention will be enhanced by a description of the terms used herein. A magnetic field can be represented by lines called lines of induction. Lines of induction generally follow a path of least resistance (e.g., through higher permeability material) around the conductor producing the magnetic field. A line of induction closes on itself, not at a terminal as in an electric field. The region occupied by the lines of induction is called a magnetic circuit. 
     To create a path for the magnetic circuit, it has long been known to position a magnetic conductor in the magnetic field created by a coiled electrical wire. This technique is seen in traditional electromagnetic devices such as inductors and transformers. 
     For example, in the well-known toroidal ring seen in FIG. 1, an electrical conductor 2 is wrapped around a magnetic conductor 4 having a higher permeability than the air around it. When the electrical conductor is closely wound and an electrical current passed therethrough, practically all of the lines of induction are confined to the magnetic conductor ring 4, thereby creating the magnetic circuit indicated by the arrows inside the ring. 
     It is also known that the size of the magnetic conductor itself causes eddy current and hysteresis loses in the magnetic conductor. Techniques to reduce these losses have focused on improvements to the traditional magnetic devices such as partitioning the magnetic conductor and choosing magnetic conductor material having a higher permeability and higher electrical resistance. 
     The present invention completely revolutionizes these known techniques. Briefly, the magnetic conductor is now arrayed in strips about the electrical conductor to form paths for the magnetic field. The magnetic conductor forming the path may include amorphous material having very high permeability, low magnetic reluctance, and relatively high electrical resistance. Moreover, because the path for the magnetic circuit may be thousands of times more inviting for the lines of induction than the ambient air, only a microscopic strip of the magnetic conductor material may be needed. (A &#34;microscopic strip&#34; has a microscopic sized cross-sectional area and a length appropriate for the specific application.) For example, as seen in FIGS. 2 and 3, one or more strips 12 of high permeability magnetic conductor may be disposed in a proximate relation to an electrical conductor 14. The strips 12 may have any cross-sectional shape, such as a rectangle or a circle, and may range in cross-sectional size from several Angstroms thick (t) and several thousand Angstroms wide (w) to much larger dimensions in diameter. Each strip may be, for example, a deposited layer (i.e., a &#34;painted&#34; strip) or a small filament such as microwire. 
     The magnetic circuit of the present invention may take numerous forms and have countless applications. In its simplest embodiments, it may take two general forms; a closed circuit in which the strips generally form bands around the electrical conductor (see, for example, FIG. 2), and an open circuit in which the strips are arrayed linearly adjacent the electrical conductor (FIG. 3). The closed circuit form creates a closed path for a magnetic field and may be used to make electrical conductors more inductive in, for example, inductive wires, a variety of antennas, antenna ground planes, and inductive surfaces. In its open circuit form the present invention collects a portion of an existing magnetic field (i.e., by providing a high permeability path) and may be used in, for example, magnetic sensors and direction finding antennas. In either form, the circuit of the present invention is particularly useful for miniaturizing traditional inductive devices such as antennas and inductors. Further, the permeability of the microscopic strips is relatively insensitive to frequency variation and thus the present invention is particularly suited for antennas covering a wide range of frequencies. Smaller cross-sections may be appropriate when the present invention is to operate at relatively low power with frequencies up to several gigahertz. Larger cross-sections may be appropriate when the present invention is to operate with relatively high power at lower frequencies (e.g., a kilohertz). 
     Accordingly, it is an object of the present invention to provide a magnetic circuit that obviates the problems of the prior art and that provides a microinductive device for countless applications. 
     It is yet another object of the present invention to provide a magnetic circuit that creates microscopic paths for a magnetic field around an electrical conductor. 
     It is still another object of the present invention to provide a magnetic circuit that provides microscopic paths for electromagnetic waves. 
     It is a further object of the present invention to provide a miniaturized antenna that is responsive to electromagnetic waves having a wide band of frequencies. 
     It is still a further object of the present invention to provide an antenna with micromagnetic conductors for establishing the inductivity of the antenna along the length of the antenna. 
     It is a yet a further object of the present invention to provide a novel method for increasing the inductivity of a wire. 
     These and many other objects and advantages will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments. 
    
    
     THE DRAWINGS 
     FIG. 1 is a pictorial representation of a toroidal ring of the prior art. 
     FIG. 2 is a pictorial representation of an embodiment of the present invention in its closed circuit form with bands of magnetic conductor disposed about an electrical conductor. 
     FIG. 3 is a further embodiment of the present invention in its open circuit form with microscopic strips of magnetic conductor disposed adjacent an electrical conductor. 
     FIG. 4 is a partial pictorial representation of an embodiment of the present invention. 
     FIG. 5 is a chart showing the frequency responses of ferromagnetic materials having various cross-sectional sizes. 
     FIG. 6 is a partial pictorial representation of an embodiment of the present invention with a multistranded electrical conductor. 
     FIGS. 7A-C are partial pictorial representations of embodiments of the present invention in cross-section showing stacking techniques for the magnetic conductor strips. 
     FIG. 8 is a partial pictorial representation of an embodiment of a monopole antenna of the present invention. 
     FIG. 9 is a cross-sectional view of a multistranded cable antenna using plural antennas. 
     FIG. 10 is a partial pictorial representation of a dipole antenna of the present invention. 
     FIG. 11 depicts the steps for forming a single loop antenna having an open magnetic circuit of the present invention. 
     FIG. 12 is a multi-turn loop antenna of the present invention that may be formed using the steps depicted in FIG. 11. 
     FIG. 13 is a twin loop antenna of the present invention that may be formed using the steps depicted in FIG. 11. 
     FIG. 14 is a three loop antenna of the present invention that may be formed using the steps depicted in FIG. 11. 
     FIG. 15 is a direction finding antenna of the present invention that may be formed using the steps depicted in FIG. 11. 
     FIG. 16 is an exploded partial pictorial representation of a spiral antenna of the present invention shown in cross-section. 
     FIG. 17A is a top pictorial view of printed circuit board embodying the present invention. 
     FIG. 17B is a cross-sectional view of the embodiment of FIG. 17A. 
     FIG. 18A is a cross-sectional view of a loop antenna of the prior art. 
     FIG. 18B is a cross-sectional view of a loop antenna of the present invention partially enclosed in a twin coaxial cavity. 
     FIG. 19 is a pictorial representation of an inductor of the present invention with a spiral magnetic conductor strip. 
     FIG. 20 is a pictorial representation of a ground plane of the present invention. 
     FIG. 21 is a pictorial representation of a ground plane of the present invention illustrating a crosshatch mesh pattern. 
     FIG. 22 is a pictorial representation of a ground plane of the present invention illustrating a chicken wire mesh pattern. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to the figures where like elements have been given like numerical designations to facilitate an understanding of the present invention and particularly with reference to a first, relatively lower power, embodiment of the magnetic circuit of the present invention illustrated in FIG. 4, the magnetic circuit of the present invention may include one or more strips of a magnetic conductor 12 having submicron-sized cross-sections disposed adjacent an electrical conductor 14 with an insulative material 16 therebetween. 
     The submicron-size strips 12 have a thickness (t) of a few Angstroms (for example, about 4 to 10 Angstroms). The width (w) of each strip may be significantly less than the distance (d) between adjacent strips (for example, a width of about 2,000 to 4,000 Angstroms). The distance (d) and the number of strips (n) is determined for each specific application and may be optimized for a specific frequency range. The distance between strips (d) also may be established so that the magnetic field related to one of the strips has no substantial interaction with the magnetic field related to another one of the strips. 
     Each magnetic conductor strip 12 may be made of an amorphous magnetic material with high permeability, low magnetic reluctance, and relatively high electrical resistance. A material containing an alloy of iron, cobalt, and vanadium may be used, while materials containing alloys of iron, silicone and boron, or cobalt, silicon and boron, or cobalt, iron and silicon are also acceptable. It is desirable that the permeability of these materials be similar or better than that of crystalline permalloys and superalloys. Iron nickel alloys like Metglas™ 2826 MB, (Fe 40  40 Ni 38  Mo 4  B 18 ) exhibit effective permeabilities of 50,000 at 1 kilohertz. Addition of cobalt as the major constituent increases the effective permeability to greater than 100,000. Alloys such as Co 70 .5 Fe 4 .5 B 10  Si 15  and Co 61 .6 Fe 4 .2 Ni 4 .2 B 2  Si 10  exhibit permeabilities as high as 120,000. 
     In certain applications such as antennas, a ferromagnetic material may be used in the magnetic conductor strips. While it has heretofore been known that a ferromagnetic material is not desirable in a high frequency antenna, it has been shown in an antenna of the present invention that ferromagnetic strips are responsive up to several gigahertz. Eddy current losses are the primary reason for the failure of the ferromagnetic materials to be responsive at high frequencies. Eddy current losses decrease proportional to the square of the reduction of the magnetic path cross-sectional area. In other words, if the cross-sectional area is reduced by a factor of ten, eddy current losses will be reduced by a factor of one hundred. It is also known that eddy current losses increase with the square of the frequency. Thus, by using the submicron-size cross-section strips of the present invention, ferromagnetic materials in the magnetic conductor can be useful up to several gigahertz. As seen in FIG. 5, the frequency response of the present invention is a significant improvement over the frequency responses of traditional magnetic conductors such as solid and laminated metal and thin ribbons. 
     The submicron-size strips of magnetic conductor may be vacuum deposited and laser trimmed. Vacuum deposited strips provide a more amorphous magnetic conductor, that is less likely to crack and create gaps in the magnetic circuit than some other materials that may be used. Laser cutting may be used to trim the strips once they are deposited because it provides greater accuracy for submicron size strips. Photo-etching may also be used in applications where less accuracy is required. 
     In the first step of the laser trimming, traditional hot laser beams heat the material and provide rough cuts; that is, to dimensions not smaller than two- to three-thousandths of an inch. In the second step, a cool laser is used. Cool lasers may be pulse gas lasers with an output in the ultraviolet region of the spectrum. For example, an EXCIMER laser using krypton-fluoride lasing at 248 nanometers with an output power of 50 watts has been found to be acceptable. A cool laser removes and fine-trims the strips to meet the demanding submicron-size requirements. Cool lasers remove material through a nonthermal ablative process resulting in a high degree of precision with virtually none of the deleterious side effects of conventional hot laser processing. The cool laser controls line widths as well as controlling the ablation depth of the laser. Controlling the depth of the ablation is important because the magnetic material must be removed without damaging the insulator or electrical conductor. 
     An insulator 16 may be placed between the magnetic conductor strips 12 and the electrical conductor 14. The composition and configuration of the insulator is not critical to the present invention. The insulator, however, may have high dielectric strength, be able to prevent oxygen diffusion, and if the laser trimming process is to be used, be laser ablation resistant. If the strips 12 are to be vacuum deposited on the insulator, the insulator may be of a composition such that metal may be easily deposited thereon and once deposited have a high resistance to peeling. To this end, materials such as ACLAR™, polyimide film, amorphous fluoropolymer film, such as TEFLON A.F., epoxy, diamond film, silicon dioxide, and ceramic base powders are acceptable. 
     The strips 12 on the entire structure may be covered with a conformal coating (not shown) which protects the strips 12 from corrosion and physical damage. Non-conductive materials such as those which may form the insulator, above, may be used for the coating. The coating may also be heat protecting, such as a heat reflective coating, and may be reinforced with a braid having strands of KEVLAR or KEVLAR-like fibers. Heat reflective coatings often contain metal reflective elements but in an insufficient amount to form an electromagnetic shield. Thus, the coating can reflect heat but not substantially interfere with the use of the coated device as an antenna. 
     The electrical conductor 14 may include any known electrical conductor having low resistance such as copper or silver. The composition and configuration of the material depends on the specific application. The electrical conductor may take any shape, including without limitation, a solid wire, a tube, a deposited film or a thin multistranded layer supported by a carrying structure. With reference to FIG. 6, a multistranded electrical conductor 14 may provide flexibility to an antenna embodying the present invention. Each strand may be electrically insulated from the adjacent strands, except where it is operatively connected to the antenna&#39;s circuitry. 
     The electrical conductor 14 may also be a high or low electrical superconductor. For applications in antennas, the superconductor may be fabricated by depositing a thick film of ink containing Y Ba 2  Cu 3  O 7  onto a low loss ceramic substrate or may be deposited in a thin film of yttrium-barium-copper oxide by an appropriate laser beam vapor deposition technique. A matching network may be provided to match the impedance of the electrical conductor of the antenna to the impedance of the transmission line. The antenna and impedance matching network may then be enclosed within a single closed cycle liquid nitrogen cryostat. 
     As was mentioned earlier, the present invention is particularly suited for use in antennas. It is known that antenna elements one-quarter to one-half wavelength long generally provide the best gain in efficiency. The physical length of the antenna may be reduced by conventional ferrite or dielectric loading techniques. These methods, however, limit the shortening to a factor of five or six. By using the present invention to increase the inductivity of the electrical conductor in an antenna, the physical length of the antenna may be shortened by a factor of ten or more. In the present invention, the inductivity of the electrical conductor may be increased by increasing the density of the magnetic conductor strips 12 on the electrical conductor. For example, the density may be varied by varying the number (n) of magnetic conductor strips, the distance (d) between strips and, the width (w) and thickness (t) of the strips. 
     The inductivity of the electrical conductor may also be varied by stacking the magnetic conductor strips 12. With reference to FIGS. 7A-C, wherein an electrical conductor 14 and strips 12 are shown in cross-section, the inductivity may be varied linearly by uniformly spacing apart single strips 12 (FIG. 7A). The inductivity may be varied progressively by providing stacks of strips 12 of increasing height that are uniformly spaced (FIG. 7B). The inductivity variation may be both nonlinear and progressive when the electrical conductor is provided with stacked strips of increasing height that have nonuniform spacing (FIG. 7C). 
     When the present invention is to operate at relatively high power with frequencies lower than several gigahertz (e.g., at a kilohertz) the submicron-size magnetic conductor may no longer be optimal. Magnetic conductors having a larger cross-section, such as microwire having a diameter up to about 100 microns, may be used to form the magnetic circuit. 
     The present invention may be applicable in a variety of antenna applications. The embodiments of the present invention which follow are merely exemplary of those applications. As seen in FIG. 8, the closed form of the present invention may be employed in a monopole antenna or as an inductive wire that is part of an antenna system. Magnetic conductor strips 12 having a width (w) and a thickness (t) may be spaced apart at distance (d) on an insulator 16 overlying an electrical conductor 14. The electrical conductor may be deposited on a supporting structure 20 such as KEVLAR™ epoxy. In such an antenna, the distance (d) between strips may be very much less than the wavelength of the electromagnetic wave and the strip width (w) may be much less than the distance (d). 
     Plural monopole antenna embodiments or the present invention may be combined to form a multistranded cable antenna, as shown in cross-section in FIG. 9. The strands may be twisted (not shown) and insulated from each other with appropriate insulative material. This embodiment may have application in large bandwidth antennae. 
     Another embodiment of the closed form of the magnetic circuit of the present invention may be seen in the dipole antenna depicted in FIG. 10. Each pole of the antenna may include plural magnetic conductor strips 12 at the most spaced apart ends of the electrical conductors 14. Appropriate electrical leads 30 may be provided. The poles of the antenna may take various forms, such as those shown in FIGS. 6 and 8 and stacked such as shown in FIG. 7. 
     With reference now to FIGS. 11 through 15, various embodiments of the open circuit version of the present invention are disclosed. The method of manufacture of such antennas is shown in FIG. 11. A strip of insulation 16 such as polyimide may be provided (drawing A of FIG. 11) and a film of magnetic conductor may be deposited thereon (drawing B). Magnetic conductor strips 12 may be formed from the film using processes discussed herein (drawing C). The electrical conductor 14 may be provided (drawing D) and arrayed about the insulator 16 and strips 12 (drawing E). The strips 12 may be grouped in patterns, with each pattern having its own electrical conductor 14, to form a multiple antennae array. Each electric 1 conductor 14 may be provided with a separate electrical connection (not shown). Using this technique, various embodiments of the present invention may be provided to obtain electromagnetic sensors such as a single loop antenna (FIG. 11E), a multiple loop antenna (FIG. 12), a sensing twin loop antenna (FIG. 13), a 360° sensing triplet (FIG. 14), and a direction finding &#34;Y&#34; array (FIG. 15). Appropriate sensing means 41 (FIG. 12) may be provided to detect the electrical current formed in each electrical conductor when the strips are in the presence of electromagnetic waves. Multiple antennae arrays may be used for direction finding by using the phase and amplitude relationships of inputs from each antenna. 
     Another embodiment of the present invention is a spiral antenna shown in exploded form in FIG. 16. Magnetic conductor strips 12 may be deposited in a radial pattern above and below the electrical conductor spiral 14. One or more interlocking spirals of electrical conductor 14 may be provided so as to coordinate the density of the electrical conductor and the magnetic conductor and provide the correct frequency response for the antenna. Appropriate supporting structure 20 may also be provided, which may include a tuned cavity 22. 
     When used with a printed circuit board the present invention may take the form of a sandwich, as seen in cross section in FIGS. 17A-B, with magnetic conductor strips 12 deposited on both sides of the board 42 about an electrical conductor 14 to provide a nearly unbroken magnetic circuit. The strips 12 may be deposited using the method discussed in relation to FIG. 11 or may comprise suitably affixed microwire. 
     With reference to FIGS. 18A-B, the present invention may have application in a loop antenna 34 partially enclosed in twin coaxial cavities 36 shown in cross-section. The addition of the magnetic conductors 12 to the loop antenna 34 of the prior art (FIG. 18A) provides for the size reduction of, both the antenna 34 and the cavity 36 (FIG. 18B). The ground plane 38 forming the cavity 36 may be connected to an appropriate electrical connection such as a coaxial line 40. 
     In addition to applications in antennas, the present invention may have application in numerous other devices. For example, the present invention may be used to create a simple inductor out of a single strand of wire. See, for example, FIG. 2 in which one or more strips 12 may be disposed about a core 14. Also, as seen in FIG. 19, a single strip 12 may be wound around a core 14 which may be hollow. The winding may be in the form of a spiral, which may close on itself or be open, or may have an even spacing between turns as depicted in FIG. 19, or with variable spacing to meed specific applications. Plural windings may also be used. The magnetic conductor may also be wound about the electrical conductor so that portions of the magnetic conductor overlap, such as when winding magnetically conductive microwire about an electrical conductor. 
     As seen in FIG. 20, the present invention may have application in a ground plane for an antenna to miniaturize the ground plane in a manner similar to the antenna miniaturization discussed above. For example, electrical conductors and magnetic conductors may be arrayed in the ground about an antenna 32 in an appropriate mesh or separated pattern, such as the crosshatch mesh illustrated in FIG. 21 or the chicken wire mesh illustrated in FIG. 22. The strips and/or conductors may be overlaid with a protective layer (not shown), such as a conformal coating, or be placed between two protective liners (not shown) that may be porous for drainage. 
     The present invention may have further application in a magnetic sensor. See, for example, FIGS. 3 and 11-15. 
     While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and that the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those skilled in the art from a perusal hereof.