Abstract:
An artificial impedance structure and a method for manufacturing same. The structure contains a dielectric layer having generally opposed first and second surfaces, a conductive layer disposed on the first surface, and a plurality of conductive structures disposed on the second surface to provide a preselected impedance profile along the second surface.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. application Ser. No. 11/173,187, titled “Artificial Impedance Structures,” filed on Jul. 1, 2005, which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to conformal antennas. More particularly, the present invention relates to artificial impedance structures used with conformal antennas. 
     BACKGROUND 
     A common problem for antenna designers is the integration of low-profile antennas into complex objects such as vehicles or aircraft, while maintaining the desired radiation characteristics. The radiation pattern of an integrated antenna is the result of currents in both the antenna and the surrounding structure. In Prior Art, as shown in  FIG. 1   a , a flat metal sheet  15  excited by a quarter wavelength monopole antenna  16  produces a low gain (about 5 db) radiation pattern in the metal sheet  15  as shown in  FIG. 1   b . Therefore, controlling the radiation from currents generated in metal surfaces like metal sheet  15  can expand the available design space. 
     According to the present disclosure, artificial impedance structures may provide a more controllable radiation pattern than previous conformal antennas, by configuring the metallic surface to provide scattering or guiding properties desired by the antenna designer. According to the present disclosure, artificial impedance structures may be designed to guide surface waves over metallic surface and to ultimately radiate energy to produce any desired radiation pattern. 
     PRIOR ART 
     The prior art consists of three main categories: (1) holographic antennas, (2) frequency selective surfaces and other artificial reactance surfaces, and (3) surface guiding by modulated dielectric or impedance layers. 
     Example of prior art directed to artificial antennas includes:
     1. P. Checcacci, V. Russo, A. Scheggi, “Holographic Antennas”, IEEE Transactions on Antennas and Propagation, vol. 18, no. 6, pp. 811-813, November 1970;   2. D. M. Sazonov, “Computer Aided Design of Holographic Antennas”, IEEE International Symposium of the Antennas and Propagation Society 1999, vol. 2, pp. 738-741, July 1999;   3. K. Levis, A. Ittipiboon, A. Petosa, L. Roy, P. Berini, “Ka-Band Dipole Holographic Antennas”, IEE Proceedings of Microwaves, Antennas and Propagation, vol. 148, no. 2, pp. 129-132, April 2001.   

     Example of prior art directed to frequency selective surfaces and other artificial reactance surfaces includes:
     1. R. King, D. Thiel, K. Park, “The Synthesis of Surface Reactance Using an Artificial Dielectric”, IEEE Transactions on Antennas and Propagation, vol. 31, no. 3, pp. 471-476, May, 1983;   2. R. Mittra, C. H. Chan, T. Cwik, “Techniques for Analyzing Frequency Selective Surfaces—A Review”, Proceedings of the IEEE, vol. 76, no. 12, pp. 1593-1615, December 1988;   3. D. Sievenpiper, L. Zhang, R. Broas, N. Alexopolous, E. Yablonovitch, “High-Impedance Electromagnetic Surfaces with a Forbidden Frequency Band”, IEEE Transactions on Microwave Theory and Techniques, vol. 47, no. 11, pp. 2059-2074, November 1999.   

     Example of prior art directed to surface guiding by modulated dielectric or impedance layers includes:
     1. A. Thomas, F. Zucker, “Radiation from Modulated Surface Wave Structures I”, IRE International Convention Record, vol. 5, pp. 153-160, March 1957;   2. R. Pease, “Radiation from Modulated Surface Wave Structures II”, IRE International Convention Record, vol. 5, pp. 161-165, March 1957;   3. A. Oliner, A. Hessel, “Guided waves on sinusoidally-modulated reactance surfaces”, IEEE Transactions on Antennas and Propagation, vol. 7, no. 5, pp. 201-208, December 1959.   

     Example of prior art directed to this general area also includes:
     1. T. Q. Ho, J. C. Logan, J. W. Rocway “Frequency Selective Surface Integrated Antenna System”, U.S. Pat. No. 5,917,458, Sep. 8, 1995;   2. A. E. Fathy, A. Rosen, H. S. Owen, f. McGinty, D. J. McGee, G. C. Taylor, R. Amantea, P. K. Swain, S. M. Perlow, M. ElSherbiny, “Silicon-Based Reconfigurable Antennas—Concepts, Analysis, Implementation and Feasibility”, IEEE Transactions on Microwave Theory and Techniques, vol. 51, no. 6, pp. 1650-1661, June 2003.   

    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1   a  relates to Prior Art and depicts a metal sheet excited by a quarter wavelength monopole antenna; 
         FIG. 1   b  relates to Prior Art and depicts a low gain radiation pattern generated by the metal sheet of  FIG. 1 ; 
         FIG. 2  depicts an artificial impedance structure composed of a single layer of conductive structures in accordance with the present disclosure; 
         FIG. 3   a  depicts a hologram function defined by the interference pattern between a line source and a plane wave in accordance with the present disclosure; 
         FIG. 3   b  depicts a hologram function defined by the interference pattern between a point source and a plane wave in accordance with the present disclosure; 
         FIGS. 4   a - 4   f  depict exemplary conductive structures that may be used to design the artificial impedance structure of  FIG. 2  in accordance with the present disclosure; 
         FIG. 5  depicts a unit cell of one of the conductive structures of  FIG. 4   a  in accordance with the present disclosure; 
         FIGS. 6   a - 6   b  depict a dispersion diagram and an effective index of refraction, respectively, for a unit cell of  FIG. 5  in accordance with the present disclosure; 
         FIGS. 7   a - 7   b  depict plots of the surface reactance versus gap size for a periodic pattern of conductive squares, for two different values of the phase difference across the unit cell in accordance with the present disclosure; 
         FIGS. 8   a - 8   c  depict exemplary artificial impedance structures in accordance with the present disclosure; 
         FIGS. 9   a - 9   c  depict high gain radiation patters generated by artificial impedance structure of  FIGS. 8   a ,  8   b  and  8   c , respectively in accordance with the present disclosure; 
         FIG. 10   a  depicts a top view of an artificial impedance structure composed of a multiple layers of conductive shapes in accordance with the present disclosure; and 
         FIG. 10   b  depicts a side view of the artificial impedance structure in  FIG. 10   a  in accordance with the present disclosure. 
     
    
    
     In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of every implementation nor relative dimensions of the depicted elements, and are not drawn to scale. 
     DETAILED DESCRIPTION 
     Using techniques disclosed in this application, artificial impedance structures may be designed to guide and radiate energy from surface waves to produce any desired radiation pattern. According to the present disclosure, holographic antennas may be implemented using modulated artificial impedance structures that are formed as printed metal patterns. 
     Referring to  FIG. 2 , an artificial impedance structure  20  may provide nearly any scattering or guiding properties desired by the antenna designer. The artificial impedance structure  20  may be implemented using an artificial impedance surface  30  described in more detail below. 
     The artificial impedance structure  20  is designed so that the surface impedance of the artificial impedance structure  20  is formed as a pattern that represents the interference between a source wave and a desired wave. The source wave may be a plane wave represented by 
                 W   R     =     ⅇ           -   i     ⁢           ⁢   2   ⁢   π   ⁢           ⁢   n     λ     ⁢   x   *     Sin   ⁡     (   θ   )             ,         
a line source wave represented by
 
               W   o     =     ⅇ         i   ⁢           ⁢   2   ⁢   π   ⁢           ⁢   n     λ     ⁢   x             
as shown in  FIG. 3   a , a point source wave represented by
 
               W   o     =     ⅇ         i   ⁢           ⁢   2   ⁢   π   ⁢           ⁢   n     λ     ⁢           (     x   -     x   o       )     2     +       (     y   -     y   o       )     2                   
as shown in  FIG. 3   b , or any other source waves known in the art. The following symbol definitions apply to the above formulas: λ=wavelength; n=effective index of refraction; x,y=coordinates on the surface; θ=angle from the surface; W=wave function; i=imaginary number; π=3.1415 . . . .
 
     The desired wave is the radiation pattern that the surface of the artificial impedance structure  20  is intended to create. The two waves are multiplied together, and the real part is taken. The function H=Re(W O W R ) defines how the surface impedance varies as a function of position across the surface. Because this method only produces a normalized surface impedance, it may be scaled to the correct value of the impedance. Although impedance values in the range of 160 j ohms provide a good match to a waveguide source, the optimum average impedance depends on the source wave. Furthermore, a modulation depth of the impedance may determine the amount of energy that radiates from the surface, per length. Higher modulation depth may result in a greater radiation rate. For the source wave, it is assumed that a probe generates a surface wave that propagates with a phase velocity determined by the average effective refractive index as calculated in the unit cell simulations. For plane waves, it is assumed that the refractive index is that of the material surrounding the surface, which is often free space. 
     The surface impedance profile defined by the function H=Re(W O W R ) may be generated on the artificial impedance structure  20  with the artificial impedance surface  30  that comprises conductive structures  40  printed on a grounded dielectric layer  35  that is thinner than the wavelength of operation. 
       FIGS. 4   a , . . . ,  4   f  depict exemplary embodiments of conductive structures  40  that can be used for the artificial impedance surface  30 . The structures shown in  FIGS. 4   a , . . . ,  4   f  in general are called frequency selective surfaces, because they are often used in applications where they serve as a filter for microwave signals. Although the structures shown in  FIGS. 4   a , . . . ,  4   f  are typically used in a configuration where signals are passing through the surface from one side to the other, presently the structures shown in  FIGS. 4   a , . . . ,  4   f  may be used in a configuration where they are printed on a dielectric sheet (not shown) that has a conducting ground plane (not shown) on the opposite side, and where signals travel along the surface of the dielectric sheet rather than passing through the dielectric sheet. The present disclosure is not limited to the structures shown in  FIGS. 4   a , . . . ,  4   f . Other structures may be used to implement the disclosed embodiments. 
     The conductive structures  40  can be either connected or non-connected, and they may contain fine features within each unit cell such as capacitive or inductive regions in the form of gaps or narrow strips. The patterns of the conductive structures  40  are not limited to square or triangular lattices. The conductive structures  40  can also be connected to the ground plane using, for example, metal plated vias (not shown). 
     Referring to  FIG. 5 , the artificial impedance surface  30  may be designed by choosing a conductive structure, such as, for example, a small metallic square  60 , for a unit cell  50  and determining the surface impedance as a function of geometry by characterizing the unit cell  50  with electromagnetic analysis software. 
     The single unit cell  50  may be simulated on a block of dielectric  65  that represents the substrate under the small metallic square  60 . The bottom of the substrate may also be conductive to represent a ground plane (not shown). The electromagnetic simulation software used to characterize the unit cell  50  determines the Eigenmode frequencies of the unit cell  50 . The Eigenmode frequencies determine the effective index, 
               n   eff     =       ck   ω     =       c   ⁢           ⁢   ϕ       a   ⁢           ⁢   ω               
of a surface wave traveling across a surface comprising a plurality of the small metallic square  60 . The following symbol definitions apply to the above formula: n eff =effective index of refraction; c=speed of light in vacuum; k=wave number which equals 2*π/λ; ω=angular frequency which equals 2*π*frequency; a=unit cell length φ=phase difference across unit cell. The electromagnetic simulation software also determines the surface impedance,
 
                 Z   eff     =       ∫   Cell             ⁢         E   x       H   y       ⁢           ⁢     ⅆ   s           ,         
by the averaging ratio of the electric field (E x ) and magnetic field (H y ).
 
     Table 1 shows surface impedance values that were obtained for different square  60  lengths after the simulation of the unit cell  50  using electromagnetic simulation software. The squares  60  was simulated on a 62 mil sheet of Duroid 5880. The impedance of the square  60  is inductive, as seen by the positive imaginary part.  FIGS. 6   a  and  6   b  show a dispersion diagram and the effective index of refraction, respectively, based on the simulation of the unit cell  50 . 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Length 
                 Z TM   
               
               
                   
               
             
             
               
                   1 mm 
                 −0.1 + j 67.7  
               
               
                   2 mm 
                 −0.2 + j 71.9  
               
               
                 2.1 mm 
                 −0.1 + j 72.8  
               
               
                 2.2 mm 
                 0.2 + j 73.7 
               
               
                 2.3 mm 
                 −0.1 + j 75.0  
               
               
                 2.4 mm 
                 0.2 + j 76.6 
               
               
                 2.5 mm 
                 0.2 + j 78.8 
               
               
                 2.6 mm 
                 0.2 + j 81.6 
               
               
                 2.7 mm 
                 0.1 + j 85.2 
               
               
                 2.8 mm 
                 −0.1 + j 90.2  
               
               
                 2.9 mm 
                  0.3 + j 102.2 
               
               
                   
               
             
          
         
       
     
       FIGS. 7   a  and  7   b  plot the reactance of the surface in ohms versus the gap size between neighboring squares  60  that can be used to produce different surface impedances profiles based on the simulation of the unit cell  50 . The following equations may be obtained to fit the curves shown in the  FIGS. 7   a  and  7   b  respectively: 
             Z   =     63.7762   +     8.89729   γ     -     0.724152     γ   2               
and
 
             Z   =     107   +     62.5335   γ     -     12.7368     γ   2       +       0.943185     γ   3       .             
By inverting these equations, functions for the gap size versus desired impedance may be obtained.
 
     The unit cell  50  simulations provide a unit cell geometry as a function of the required surface impedance, and the function H=Re(W O W R ), disclosed above, defines how the surface impedance varies as a function of position across the surface. These two results can be combined to produce the unit cell geometry as a function of position to generate the artificial impedance structure  20 . 
       FIGS. 8   a ,  8   b  and  8   c  depict exemplary artificial impedance structures  70 ,  75  and  100 , respectively, designed to radiate at thirty (30) degrees and sixty (60) degrees using techniques described above. The artificial impedance structures  70  and  75  were excited with a waveguide probe (not show) placed against the microwave hologram surfaces  70  and  75 . As seen in the radiation patterns in  FIGS. 9   a  and  9   b , the artificial impedance structures  70  and  75  produce the expected result: a narrow beam at the desired angle and high gain represented by lobes  80  and  85 , respectfully. The artificial impedance structure  100  was excited by a quarter wavelength monopole antenna  101  disposed on the artificial impedance structure  100 . As seen in the radiation pattern in  FIG. 9   c , the artificial impedance structure  100  produces the expected result: a narrow beam at the desired angle and high gain represented by lobe  105 . 
     Although higher order diffraction lobes  90  and  95  also occur in the radiation patterns in  FIGS. 9   a  and  9   b , altering the impedance profile of the artificial impedance structure  70  and  75  so as not to be sinusoidal may eliminate the higher order diffraction lobes  90  and  95 . The alteration of the impedance profile may be done in a manner similar to that used to create optical diffraction gratings, and the angle for which the grating is optimized is known as the blaze angle. A similar procedure can be used for this microwave grating. It can also be considered as adding additional Fourier components to the surface impedance function that cancel the undesired lobes. 
     In addition to building artificial impedance structures using a single layer of conductive structures on a grounded dielectric substrate as disclosed above, an artificial impedance structures  150  may also be implemented using multiple layers  120  and  125  containing conductive structures  140  disposed on a grounded dielectric substrate  130 , wherein layers  120  and  125  are separated by an additional dielectric spacer layer  135 , as shown in  FIGS. 10   a  and  10   b . A conductive layer  155  may be utilized as a grounding layer for the grounded dielectric substrate  130 .  FIG. 10   a  depicts a top view of the artificial impedance structure  150  and  FIG. 10   b  depicts a side view of the artificial impedance structure  150 . The impedance of the artificial impedance structure  150  can be varied by varying the geometry of the conductive structures  140 , or by varying the thickness or dielectric constant of the spacer layer  135 , or by varying the thickness or dielectric constant or magnetic permeability of the grounded dielectric substrate  130 . 
     The artificial impedance structures presently described may be made using a variety of materials, including any dielectric for the substrates  35 ,  130 , and any periodic or nearly periodic conductive pattern for conductive structures  40 ,  140 , and any solid or effectively solid conductive layer  155  on the bottom surface of the substrate  130 . The top surface of the substrate  130  can also consist of multiple surfaces  120 ,  125  separated by multiple dielectric layers  135 . 
     The foregoing detailed description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise form(s) described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. The possibility of modifications and variations will be apparent to practitioners skilled in the art. No limitation is intended by the description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration of those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the Claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “step(s) for . . . ”