Abstract:
Method for constructing a radome ( 110 ). The method can include the steps of providing a radome structure, wherein the radome structure can include at least one of a radome wall ( 115 ) and a radome frame ( 120 ). The radome structure can be impedance matched to an operational environment. The impedance match can be independent of the thickness and geometry of the radome structure.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to the field of radomes, and more particularly to efficient radomes of variable geometry.  
           [0003]    2. Description of the Related Art  
           [0004]    Conventional radomes are typically dome-like shells that can be used to protect enclosed electromagnetic devices, such as antennas, from environmental conditions, such as wind, solar loading, ice, and snow. Radomes, such as a solid laminate and sandwich radomes, can be rigid self-supporting structures. Mismatches between the impedance of free space and the radome can result in energy dissipation at the point of incidence. The energy dissipation can be the result of a reflective wave being generated at a medium boundary, such as the radome/free-space boundary.  
           [0005]    If an electromagnetic wave strikes a medium boundary at a point which is multiple of a half wavelength, energy dissipation at the boundary can be minimized. A material which minimizes reflections across medium boundaries by ensuring electromagnetic incidence occurs at half-wavelength multiples for a selected frequency utilizes an impedance transform. Advantageous transfer characteristics for conventional radomes are generally achieved through such a wavelength dependant impedance transform. More particularly, half-wavelength transforms can be advantageously used to achieve beneficial transfer characteristics.  
           [0006]    Relying upon such an impedance transform, however, results in radomes optimized for specific frequencies and places a limitation upon radome thickness. The further the deviation from the optimized frequency, the greater the perturbations caused by the exemplary conventional radome; since the half-wavelength transform cannot properly function for differing wavelengths. Consequently, conventional radomes are frequency dependant.  
           [0007]    Differing angles of incidence also substantially affect the transfer characteristics of conventional radomes. Different angles of incidence cause waves to travel different distances through a uniformly thick medium. For example, a wave at normal incidence passing through a 1.5 cm thick medium travels 1.5 cm.  
           [0008]    distance=thickness/sin(incident angle), so that  
           [0009]    distance=1.5 cm/sin 90=1.5 cm/1=1.5 cm  
           [0010]    Alternately, a wave at a 30 degree incident angle passing through the same medium (ignoring refraction) travels a distance of 3.0 cm.  
           [0011]    distance=thickness/sin(incident angle), so that  
           [0012]    distance=1.5 cm/sin 30=1.5 cm/0.5=3.0 cm  
           [0013]    Consequently, performance of conventional radomes is significantly affected by various incident angles.  
           [0014]    To minimize differences in incident angles, conventional radomes are often hemispherically shaped. Accordingly, if a radio frequency source is centrally placed within a hemispherical radome, waves generated by the source will strike the radome boundary at a substantially normal angle of incidence. Other shapes would result in differing angles of incidence, thereby degrading radome performance characteristics.  
           [0015]    A number of difficulties result from the necessity that conventional radomes be hemispherically shaped. For example, manufacturing and transportation considerations cause most large conventional radomes to be formed from multiple-curved panels that can be joined on-site to form the radome structure. The coupling planes at which adjacent panels are joined, however, can cause thickness variations. The thickness variations can result in decreased radome performance at the coupling planes—the coupling planes being the seams in a radome wall existing between joined radome panels. To minimize loss at panel boundaries, panels are made as large as practicable for a given situation. It can be very difficult to transport, install, and manufacture the large, rigid, and curved radome panels.  
           [0016]    Another negative aspect of conventional radomes relates to radome frames. A radome frame is a supporting framework that provides mechanical support to a radome. Such additional support can be necessary since radome walls, which utilize wavelength dependant impedance transforms, are thickness restricted, generally to multiples of half a wavelength of an optimized frequency. Conventional radomes can require support greater than that provided by material which is half a wavelength thick.  
           [0017]    For example, a large radome, such as the 140-foot diameter radome at Mt. Hebo, may need to be constructed of a dielectric material thicker than the lowest half wavelength, which would be 1.5 cm for a 10 GHz frequency. Increasing thickness of a radome wall to the next higher half wavelength multiple can significantly increase the cost to manufacture the radome wall. Additionally, increased losses due to the magnetic and electric loss tangents occur as the thickness of a radome increases. Accordingly, load bearing radome frames are often used in conjunction with radome walls.  
           [0018]    Losses attributable to radio frequency waves striking radome frames can be called scatter loss. Scatter loss of conventional radomes with radome frames can be as great as 10 times the wall pass loss. While many different approaches have been taken to minimize scatter loss, scatter loss remains a significant problem for conventional radomes with radome frames.  
         SUMMARY OF THE INVENTION  
         [0019]    The invention concerns a method for constructing a radome. The method can include the steps of providing a radome structure and impedance matching the radome structure to an operational environment, wherein the impedance match is independent of the thickness and geometry of the radome structure. The impedance match can be achieved independent of a frequency and an angle of incidence of radio frequency signals that pass through the radome structure. Additionally, a plurality of panels can be joined to form the radome structure. The coupling plain between adjacent ones of the plurality of panels can be impedance matched to the operational environment. The radome structure can be subdivided into a plurality of segments for shipping. The thickness of at least a portion of the radome structure can vary across the surface of that portion.  
           [0020]    According to one aspect of the invention, the radome structure can include at least one of a radome wall and a radome frame. An electrical characteristic can be selected for the radome structure from a permittivity, a permeability, a loss tangent, and/or a reflectivity. The selected electrical characteristic can be adjusted to achieve the impedance matching for the radome structure. For example, a relative magnetic permeability of the radome structure to can be adjusted to approximately equal a relative electrical permittivity of the radome structure. The radome structure can also be formed from a dielectric material within which a plurality of voids can be created. Further, a plurality of magnetic particles can be inserted into selective ones of the voids.  
           [0021]    The invention also concerns a radome. The radome includes a radome structure, wherein electrical characteristics of the radome structure result in an impedance match with an operational environment, where the impedance match is independent of the thickness and geometry of the radome structure. The radome structure can include a radome wall and a radome frame. The impedance match can be independent of the frequency and the angles of incidence of radio frequency waves which pass through the radome structure. A relative magnetic permeability of the radome structure can approximately equal a relative electrical permittivity of the radome structure.  
           [0022]    At least a portion of the radome structure can be formed from a dielectric material that includes magnetic particles. The magnetic particles can include a ferroelectric material, a ferromagnetic material, and/or a ferrite. At least a portion of the dielectric material can also include a plurality of voids. The radome structure can further include a plurality of panels, wherein a coupling plane joining adjacent ones of the plurality of panels is impedance matched to the operational environment. The radome structure can be of variable thickness. The radome can be subdivided into a plurality of segments for shipping.  
           [0023]    The invention also concerns a method for minimizing reflection of a radio frequency signal as it traverses a radome boundary. The method includes the steps of interposing at least one radome panel in the path of a radio frequency signal and selecting a permeability and a permittivity of a material forming the radome panel. The permeability and permittivity should be selected so that a ratio of the relative permeability to the relative permittivity is substantially equal to a ratio of a relative permeability to a relative permittivity of an environment surrounding the radome panel. The relative permittivity and the relative permeability of the radome panel can be selected to be substantially equal. The radome panel can be formed from a dielectric material having a plurality of voids, each void being between about one millimeter and one nanometer in size.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    There are shown in the drawings embodiments, which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.  
         [0025]    [0025]FIG. 1A is a schematic diagram illustrating an exemplary variably shaped radome in accordance with the inventive arrangements disclosed herein.  
         [0026]    [0026]FIG. 1B is an enlarged view of a cross section view of the radome of FIG. 1A.  
         [0027]    [0027]FIG. 2 is a schematic diagram illustrating waves passing through the radome of FIG. 1A.  
         [0028]    [0028]FIG. 3A is a schematic diagram illustrating one shape for the radome of FIG. 1A.  
         [0029]    [0029]FIG. 3B is a schematic diagram illustrating another shape for the radome of FIG. 1A.  
         [0030]    [0030]FIG. 3C is a schematic diagram illustrating yet another shape for the radome of FIG. 1A.  
         [0031]    [0031]FIG. 3D is a schematic diagram illustrating transport characteristics for the radome of FIG. 1A.  
         [0032]    [0032]FIG. 4 is a schematic diagram illustrating a system including a wave at normal incidence passing across two boundaries separating three mediums.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]    [0033]FIG. 1A is a schematic diagram illustrating an exemplary radome system  100  in accordance with the inventive arrangements disclosed herein. The system  100  can include an electromagnetic device  105  and a radome  110 , which includes a radome wall  115  and a radome frame  120 . The electromagnetic device  105  can be a transceiver coupled to an antenna.  
         [0034]    The radome  110  can be an environmental shell configured to be substantially transparent to radio frequency radiation in the frequency range of interest. The radome  110  protects the enclosed electromagnetic device  105  from environmental conditions. Radome  110  can be a variety of types including, but not limited to, a space frame radome, a sandwich radome, and a solid laminate radome. The radome  110  can be designed for particular performance characteristics relating to radio frequency radiation. For example, radome  110  can be impedance matched to the surrounding environment (i.e. free space). Accordingly, radome  110  need not utilize impedance transforms that are wavelength dependent. Therefore, radome  110  can efficiently operate even when electromagnetic waves strike the radome structure at different angles of incidence. Consequently, radome  110  can be of variable thickness and shape.  
         [0035]    The radome wall  115  can be designed for specific electrical characteristics that result in desired performance characteristics for the radome  110 . For example, the radome wall  115  can have a relative electrical permittivity equal to a relative magnetic permeability resulting in an impedance match with free space. Electrical characteristics can include a permittivity, a permeability, a loss tangent, and/or a reflectivity. The radome wall  115  can comprise a single surface or multiple surface segments, each of which can be formed from the same or different materials. Various materials can be used to construct the radome wall  115 . The selected material can depend upon necessary electrical characteristics required for the radome wall  115  to achieve desired performance characteristics for the radome  110 .  
         [0036]    The radome frame  120  can be a load bearing structure that provides mechanical support to the radome  110 . The radome frame  120 , unlike traditional radome frames, can be impedance matched to the environment in a manner similar to the radome wall  115 . As used herein, the radome frame  120  can be any structure which provides greater mechanical support than the structure defined as the radome wall  115 . Appreciably, since both the radome wall  115  and the radome frame  120  can have a variable thickness, traditional distinctions between the radome wall  115  and the radome frame  120  can be blurred as applied herein.  
         [0037]    For example, in one embodiment, the radome frame  120  can be indistinguishable from the radome wall  115 , except that the radome frame  120  is thicker than the radome wall  115 , resulting in enhanced structural support. In another embodiment, the radome frame  120  can be an equivalent thickness to the radome wall  115 , yet formed from a different material selected to provide enhanced structural support.  
         [0038]    [0038]FIG. 1B is an enlarged view of a cross section of the radome wall  115 . A dome material forming the radome wall  115  can comprise numerous voids  140  some of which are filled with magnetic particles  135 . Voids  140  can provide low dielectric constant portions within the dome material since voids  140  generally fill with air, air being a very low dielectric constant material. Other voids  140  can be filled with a filling material resulting in portions of the dome material having tailored dielectric properties that differ from the bulk properties of the dome material. The fill material can include a variety of materials which can be chosen for desired physical properties, such as electrical, magnetic, or dielectric properties. Voids  140  can occupy regions as large as several millimeters in area or can occupy regions as small as a few nanometers in area.  
         [0039]    The voids  140  can be selectively filled by the magnetic particles  135  in a variety of manners. For example, particle filling may be provided by microjet application mixing techniques known in the art, where a polymer intermixed with magnetic particles  135  is applied to voids  140 . Photonic radiation can be used to remove macroscopic or microscopic regions in the dome material to create voids  140  using various mechanisms, such as polymeric end group degradation, unzipping, and/or ablation. A CO 2  laser is preferred when creating voids by utilizing a laser. An optional planarization step may be added if filling initially results in a substantially non-planar surface and a substantially planar surface is desired.  
         [0040]    Magnetic particles  135  include materials that have a significant magnetic permeability, which refers to a relative magnetic permeability of at least 1.1. Magnet particles  135  can be metallic and/or ceramic particles and can have sub-micron physical dimensions. Preferably, magnetic particles  135  comprise a ferroelectric material, a ferromagnetic material, and/or a ferrite.  
         [0041]    Ferroelectric materials, which contain microscopic electric domains or electric dipoles, can exhibit a hysteresis property so that the relationship between an applied electric field and the relative dielectric constant of the cross section  125  is non-linear. Ferroelectric compounds include, for example, potassium dihydrogen phosphate, barium titanate, ammonium salts, strontium titate, calcium titanate, sodium niobate, lithium niobate, tunsten trioxide, lead zirconate, lead hafnate, guanidine aluminium sulphate hexahydrate, and silver periodate.  
         [0042]    Ferromagnetic materials, which contain microscopic magnetic domains or magnetic dipoles, can form a hysteresis loop when selected energetic stimuli are applied to create an applied magnetic field across the dome material. The hysteresis loop being a well-known effect of variation of an applied magnetic field. The hysteresis loop results from a retardation effect based upon a change in the magnetism of the dome material lagging behind changes in an applied magnetic field. Ferromagnetic materials include, but are not limited to, cobalt, iron, nickel, and mumetal.  
         [0043]    Ferrites are a class of solid ceramic materials with crystal structures formed by sintering at high temperatures stoichiometric mixtures of selected oxides, such as oxygen and iron, cadmium, lithium, magnesium, nickel, zinc, and/or with other materials singularly or in combination with one another. Ferrites typically exhibit low conductivities and can possess a magnetic flux density from 0 to 1.4 tesla when subjected to a magnetic field intensity from negative 100 A/m to positive 100 A/m.  
         [0044]    The selection and placement with which the magnetic particles  135  are incorporated into the dome material can determine the electrical characteristics of the dome material, thereby determining the performance characteristics of the radome  110 . The magnet particles  135  can be uniformly distributed or can be otherwise dispersed (e.g. randomly distributed) within the dome material.  
         [0045]    In one embodiment, the dome material can be a metamaterial. A metamaterial refers to composite materials formed from the mixing or arrangement of two or more different materials at a very fine level, such as the angstrom or nanometer level. Metamaterials allow tailoring of electrical characteristics of the dome material, which can be defined by effective electromagnetic parameters comprising effective electrical permittivity ε eff  (or dielectric constant) and the effective magnetic permeability μ eff .  
         [0046]    [0046]FIG. 2 is a schematic diagram illustrating waves  205  and  210  passing through wall  115  to demonstrate that radome  110  efficiently operates at any angle of incidence. FIG. 2 includes wave  205  with an incident angle A with respect to the wall  115  and wave  210  with a normal angle of incidence.  
         [0047]    As previously noted, conventional radomes use an impedance transform based upon a multiple of a determined wavelength. Such an impedance transform requires that the conventional radome be of a predetermined thickness, such as a half multiple of a wavelength for a selected frequency. Notably, the distance a wave travels through the radome can vary according to the angle at which the wave strikes the conventional radome, i.e. distance=thickness/sin(incident angle). Therefore, a conventional radome, which must be of a particular thickness, can efficiently operate only for a predefined frequency and a specified angle of incidence, such as a normal angle.  
         [0048]    In contrast, the radome wall  115  can be impedance matched to the surrounding environment (i.e. free space) and can be of variable thickness. Moreover, the distances B and C that waves  205  and  210  travel through the radome wall  115  is not significant to the efficient operation of radome  110 . Accordingly, the radome wall  115  can efficiently operate for any angle of incidence, such as angle A, thereby allowing for variably shaped radomes.  
         [0049]    It should be noted as an aside, that the magnetic and electrical loss tangents for the radome  110  can be affected by the angle A and the thickness of radome wall  115 . Hence, performance characteristics for radome  110  are not entirely independent of the thickness of the radome wall  115  and/or the angle of incidence. When the radome  110  is sufficiently thin, however, the magnetic and electrical loss tangents can result in minimal losses.  
         [0050]    [0050]FIG. 3A is a schematic diagram illustrating radome  300  depicting one of the possible shapes of the radome of FIG. 1A. Radome  300  can include a radome wall  305  and a radome frame  310 . Radome  300  illustrates that each side of a magnetic radome need not be of uniform thickness. For example, one side of radome  300  contains radome wall  305 , which is thinner than the surrounding radome frame  310 . Other sides of radome  300  can lack a radome wall and can be of the same thickness and composition as the radome frame  310 . The radome wall  305  can be formed from the same material as the radome frame  310  or can be formed from a different material.  
         [0051]    [0051]FIG. 3B is a schematic diagram illustrating radome  320  depicting another of the possible shapes of the radome of FIG. 1A. As shown, the radome  320  can include a radome wall formed of many panels  322 , each panel  322  supported by a radome frame  325 . Radome  320  illustrates that each side of an impedance matched radome can comprise multiple panels  322  interspersed with radome frame  325  elements. Different ones of the panels  322  can be constructed with different electrical characteristics. Likewise, the frame  325  of the radome  320  can comprise different sections differentially constructed. Of course, radome  320  can be of any shape and is not restricted to the square shape indicated and panels  322  can be curved and/or flat.  
         [0052]    Constructing radome  320  as a series of panels  322  can allow the size of the radome  320  to be adjusted by adding or subtracting panels  322  to various sides of the radome  320 . Since the radome  320  is frequency independent, operational radomes of any size and frequency range can be constructed from a plurality of standardized panels  322 . The ability to standardize panels  322  of the radome  320  can promote manufacturing efficiencies, resulting in less costly radomes that nevertheless possess desired performance characteristics.  
         [0053]    [0053]FIG. 3C is a schematic diagram illustrating radome  326  depicting another of the possible shapes of the radome of FIG. 1A. Radome  326  demonstrates that variably shaped radomes can be molded and/or constructed to conform to any shape and/or housing. Such a housing can be integrated into a protected device or structure. For example, the radome  326  can protect a microstrip antenna contained within a cellular telephone. The radome  326  can include a radome frame  328  and a radome wall  330 .  
         [0054]    In particular embodiments, the radome  326 , need not be a separate enclosure for the electromagnetic device, but can instead be integrated with the protected electromagnetic device. For example, the radome  326  can be integrated with a cellular telephone so that various electronic components necessary for operating the cellular telephone can be embedded within the surface material of the radome  326 .  
         [0055]    [0055]FIG. 3D is a schematic diagram illustrating transport characteristics for the radome of FIG. 1A. More particularly, FIG. 3D shows a radome  335 , frame elements  340 , panel sections  342 , and a transport symbol  345 . The radome  335  can be easily segmented to facilitate transportation. Radome  335  is depicted as a pyramidal radome with three sides, each of which can be segmented into sections comprising frame elements  340  and panel sections  342 . Each of the shown sections  340  and  342  can additionally be decomposed into smaller sections (not shown). Although shown as a pyramid shape, the radome  335  can be any shape and/or size. Once decomposed, the radome  335  can be easily transported  345  since any segmentation size is possible.  
         [0056]    In contrast, conventional radome panels can be very large in order to minimize the number of seams created. Sometimes individual radome panels are so large as to not be transportable via standard transport channels. Even when standard transport channels can be used, because each panel is curved, bulky, and thin, special shipping packaging is often required to safely ship a conventional radome. Custom packaging is not required for radome  335 .  
         [0057]    Further, the assembly of conventional radomes is problematic with large fragile panels needing to be positioned in precise orientations using minimal inter-panel couplings. Radome  335 , however, can be designed to include hinges, interlocking edges, and other coupling mechanisms that facilitate assembly. For example, the radome  335  can be hinged to ‘collapse’ into a flat structure to be later re-assembled. The radome  335  is not limited to any particular manner of decomposition or segmentation shape, size, or intersegment coupling mechanism resulting in enhanced flexibility in design, manufacture, transport, and installation.  
         [0058]    [0058]FIG. 4 is a schematic diagram illustrating a system  400  including a wave  408  at normal incidence passing across two boundaries separating three mediums. FIG. 4 details how a radome (depicted as medium  404 ) can be impedance matched to free space (mediums  402  and  406 ). The system  400  can include boundary  420  separating medium  402  and medium  404  and boundary  430  separating medium  404  and medium  406 . Mediums  402 ,  404 , and  406  have relative permittivity values of ε 1 , ε 2 , and ε 3  and relative permeability values of μ 1 , μ 2 , and μ 3 , respectively.  
         [0059]    Whenever the equation μ 2 ε 1 =μ 1 ε 2  is satisfied, transmission of radio frequency waves at normal incidence can occur across boundary  420  without significant reflection. Similarly, when μ 2 ε 3 =μ 3 ε 2  is satisfied, transmission of radio frequency waves at normal incidence can occur across boundary  430  without significant reflection. While, the above equations may not be dependant on length  410 , observable loss will occur as a function of length  410  resulting from non-zero electric and magnetic loss tangents. Accordingly, length  410  should generally be kept as short as possible.  
         [0060]    For example, assume medium  402  and  406  are both air and that medium  404  is a radome wall. The relative permeability and permittivity of air is approximately one (1). Accordingly, μ 1  and μ 3  are approximately equal one (1) and ε 1  and ε 3  are approximately equal one (1). Assume that the exemplary radome wall, which is represented by medium  404 , has an electrical permittivity of two (2). Thus, when the radome wall has a magnetic permeability of two (2), a wave  408  with a normal angle of incidence can be transmitted across boundary  420  without significant reflection. Furthermore in this example, because medium  402  and medium  406  are equivalent dielectric mediums (both air), boundary  430  will also be impedance matched, since the intrinsic impedance is identical in mediums  404  and  406 .  
         [0061]    The relationship for complete transmission across an ideal boundary  420  for an ideal wave  408  at normal incidence can be determined as follows. The intrinsic impedance (η) for a given medium can be defined as η=(μ/ε) 1/2  so that the intrinsic impedance for medium  402  is η 1 =(μ 1 /ε 1 ) 1/2  and intrinsic impedance for medium  404  is η 2 =(μ 2 /ε 2 ) 1/2 . Next, the reflection coefficient (Γ) for a plane wave  408  normal to boundary  420  can be defined as Γ=(η 2 −η 1 )/(η 2 +η 1 ). All energy can be transmitted at across boundary  420  if the reflection coefficient is zero; that is Γ=(η 2 −η 1 )/(η 2 +η 1 )=0.  
         [0062]    Using the above formulas, the following calculations can be made:  
         (η 2 −η 1 )/(η 2 +η 1 )=0  (1)  
         (η 2 −η 1 )=0  (2)  
         η 2 =η 1   (3)  
         (μ 2 /ε 2 ) 1/2 =(μ 1 /ε 1 ) 1/2   (4)  
         (μ 2 /ε 2 )=(μ 1 /ε 1 )  (5)  
         μ 2 ε 1 =μ 1 ε 2   (6)  
         [0063]    Equation (1) sets the reflection coefficient equation to zero. Equation (2) results from multiplying both sides of equation (1) by (η 2 +η 1 ). Equation (3) results from adding η 1  to both sides of equation (2). Equation (4) results from substituting in the defined values for η 2  and η 1  into equation (3). Squaring both sides of equation (4) results in equation (5). Equation (6) results from multiplying both sides of equation (5) by (ε 1 )(ε 2 ). Accordingly, when equation (6) is satisfied, an intrinsic impedance match between medium  402  and medium  404  will result. Accordingly, normally incident wave  408  is fully transmitted as no reflection loss results for normally incident wave  408  at the ideal boundary  420  when equation (6) is satisfied.  
         [0064]    As seen in the above example, when μ 3 ε 1 =μ 1 ε 3 , matching the impedance of medium  404  to medium  402  at boundary  420  can result in an impedance match of medium  404  to medium  406  at boundary  430 . However, when mediums  402  and  406  have dissimilar electrical permittivity and magnetic permeability values, it is not generally possible to perform an impedance match at boundaries  420  and  430  using the above formulas alone. In such a situation, an impedance transform can be utilized.  
         [0065]    The present invention can be embodied in other forms without departing from the spirit or essential attributes thereof. Accordingly, reference should be made to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.