Patent Publication Number: US-8982011-B1

Title: Conformal antennas for mitigation of structural blockage

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made under US Government contract No. W15P7T-06-9-P011 and therefore the US Government may have certain rights in and to this invention. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     U.S. patent application Ser. No. 13/242,102 filed on the same date as this application and titled “Conformal Surface Wave Feed”, which is hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     This invention relates to the placement of antennas on vehicles such as aircraft (airplanes, including unmanned aerial vehicles (UAVs), and airships), land craft (automobiles, trucks, etc.) and sea craft (boats, ships, etc.) that have limited space for mounting antennas and have (or will have) obstructions that will degrade the radiation patterns of conventional antennas. 
     BACKGROUND 
       FIG. 1   a  shows the fuselage of an aircraft fuselage. It is desirable to mount an antenna on the underside of the fuselage behind the landing gear. However, at least portions of the landing gear (particularly its support strut) block the antenna radiation in the forward direction. 
     There are many other instances where some element protrudes (or could protrude) from the body of a vehicle which protruding element interferes or obstructs (or could interfere or obstruct) RF reception to and/or transmission from an antenna also on the body of the vehicle. If the vehicle is currently being designed, perhaps it will be possible to move either the antenna or the interfering or obstructing element. Other times, that cannot be done and if the vehicle has already been built it can be very inconvenient to do so, if not impossible to do so. This invention relates to techniques which can be used to mitigate the effects of such elements which otherwise can interfere or obstruct RF reception to and/or transmission from an antenna also on the body of the vehicle. An interfering or obstructing element is generically referred to as a blockage herein. 
     The prior art includes:
     D. J. Gregoire and J. S. Colburn, “Artificial impedance surface antenna design and simulation”, 2010 Antenna Applications Symposium, pp. 288-303, the disclosure of which is hereby incorporated herein by reference.   Fong, B. H.; Colburn, J. S.; Ottusch, J. J.; Visher, J. L.; Sievenpiper, D. F., “Scalar and Tensor Holographic Artificial Impedance Surfaces”,  IEEE Trans. Antennas Prop ., vol. 58, pp. 3212-3221, 2010, the disclosure of which is hereby incorporated herein by reference.   Ottusch, J. J.; Kabakian, A.; Visher, J. L.; Fong, B. H.; Colburn, J. S.; and Sievenpiper, D. F.; “Tensor Impedance Surfaces”, AFOSR Electromagnetics Meeting, Jan. 6, 2009, the disclosure of which is hereby incorporated herein by reference.   

     Artificial impedance surface antennas (AISA) are formed from modulated artificial impedance surfaces (AIS). The AIS are typically fabricated using a grounded dielectric topped with a grid of metallic patches. The article by Fong presents a detailed description of the methods used for designing and fabricating linearly and circularly polarized AISAs using scalar and tensor impedance maps, respectively. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect the present invention provides a method of mitigating adverse transmission and/or reception effects that an obstruction would otherwise have upon a RF signal to be transmitted or received, the RF signal being available at a feed point. The obstruction is spaced from the feed point in a direction of desired transmission or reception. The method includes disposing an artificial impedance surface adjacent the feed point and the obstruction, and tuning or otherwise causing the artificial impedance surface (i) to have a spatially constant impedance function in a constant impedance region at least immediately adjacent the feed point and (ii) to have a spatially non-constant impedance function in one or more regions spaced from the feed point and closer to the obstruction. 
     In another aspect the present invention provides a method of radiating RF energy available from a feed point disposed on object having a obstruction which would normally interfere with radiation of the RF energy at said feed point, the method including emitting RF energy as surface waves on an artificial impedance surface from said feed point, the artificial impedance surface having a first regions with a first surface impedance function which supports said surface waves moving away from said feed point and having a second region with a second surface impedance function which causes said surface waves to leak or launch off the artificial impedance surface as the radiation of said RF energy away from said artificial impedance surface. 
     In yet another aspect the present invention provides an apparatus for mitigating an effect of a RF obstruction upon a RF signal emitted by a RF feed point, the apparatus including an artificial impedance surface relative having the RF feed point disposed or adjacent the artificial impedance surface and with the RF obstruction being disposed on or adjacent the artificial impedance surface, the artificial impedance having an essentially spatially constant impedance function in a region of the artificial impedance surface bounded by the RF feed point and the RF obstruction and with a spatially varying impedance function in regions not bounded by the RF feed point and the obstruction. 
     In still yet another aspect the present invention provides an artificial impedance surface antenna comprising an artificial impedance surface disposed adjacent a structural element which acts as a RF block, the artificial impedance surface having an impedance modulation that routs surface waves released upon the artificial impedance surface around said obstruction and into a radiating region unaffected by the obstruction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  depicts the fuselage of an aircraft. Antennas mounted on the fuselage underbelly will have their forward-directed radiation blocked by the landing gear strut. 
         FIG. 1   b  depicts a model intended to simulate the portion of the aircraft shown in  FIG. 1   a  between the antenna, the landing gear strut and the region immediately in front of the landing gear strut to test mitigation of the obstruction caused by the strut relative to the antenna&#39;s feed point by employing a surface-wave waveguiding region in front of the strut. 
         FIG. 2  depicts the measured radiation patterns of the antenna shown in  FIG. 1   b , the antenna being the curved surface due to the presence of a surface-wave waveguiding region in front of the strut, the feed to the antenna is the rectangular waveguide mounted behind the strut. The radiation intensity was measured with and without the strut obstruction in place (solid and dashed lines respectively) and for different frequencies (represented by different levels of black and gray). For each frequency tested, the radiation intensity with the strut in place (in solid lines) very closely follows the radiation intensity without the strut in place (in dashed lines). The angle θ is with reference to the flat portion of the AIS  10 , with θ=0° being normal to the flat portion of the AIS  10  and with θ=90° pointing forward parallel to the flat portion of the AIS  10 . 
         FIG. 3   a  is a bottom up view of the ASIA where a conventional antenna has been replaced with a surface-wave feed that feeds a surface wave onto an AIS  10 . The AIS  10  has a modulated impedance (indicated by the gray variation) that radiates into a desired radiation pattern. However, the impedance is not modulated until after the surface wave propagates into regions where the gear strut  3  obstruction will not affect the radiated energy. 
         FIG. 3   b  depicts how the AIS is preferably enhanced by adding a surface-wave waveguiding region that guides the surface wave around the obstruction and prevents any of the surface wave energy from being attenuated by the obstruction caused by the strut. The surface waves propagate past the obstruction caused by the strut to the radiation region. 
         FIG. 4  is a plan view of an AIS with an obstruction more or less disposed in or adjacent the middle of it. 
         FIG. 5  compares a couple of simulation methods to each other and to some measured data for an AISA designed to radiate at 60° off normal at 12 GHz. 
         FIG. 6   a  shows a cross section of a model of the nose of an aircraft with a curved line designating the profile of the test version of the curved AIS. 
         FIG. 6   b  presents some representative measured radiation patterns for the curved AIS of  FIG. 6   a  for various frequencies spanning the range 11.8 to 12.4 GHz and 82° to 95° off normal. 
         FIGS. 7   a  and  7   b  are graphs which compare simulated radiation patterns given the obstruction with using an AISA to mitigate the obstruction ( FIG. 7   a ) and without using an AISA to mitigate the obstruction ( FIG. 7   b ). 
         FIGS. 8   a  and  8   b  are representation of flat AISAs. In the case of  FIG. 8   a , from left to right are shown (i) the AIS alone, (ii) the flat AIS with the obstruction modeled thereon, and (iii) a perspective view of (ii). In the case of  FIG. 8   b , the representations from left to right are as in the case of  FIG. 8   a , but the flat AIS in this figure also has a SWG region. 
         FIGS. 9   a  and  9   b  are graphs of radiation measurements for the AISA with flat AIS as depicted by  FIGS. 8   a  and  8   b , respectively. 
         FIGS. 9   c  and  9   d  are graphs of radiation measurements similar to the graph of  FIGS. 9   a  and  9   b , but instead of measuring with the AISA in place, the graphs are based using a metal plate of the same size and shape as the AIS of  FIGS. 8   a  and  8   b.    
         FIG. 10   a  is a plot of radiation patterns at several frequencies in range from 10 GHz to 12.5 GHz for the AIS embodiment with the SWG region. 
         FIG. 10   b  shows the peak intensity for the blocked (gear) and unblocked (no gear) cases for the AIS embodiment with the SWG region. 
         FIG. 10   c  shows how the peak angle scans with frequency for both the blocked (gear) and unblocked (no gear) cases for the AIS embodiment with the SWG region. 
         FIG. 10   d  plots the difference in peak intensity showing that it drops uniformly as the frequency approaches the design frequency of 12 GHz for the AIS embodiment with the SWG region. 
         FIGS. 11   a  and  11   b  are representation of curved AISAs. In the case of  FIG. 11   a , from left to right are shown (i) the curved AIS alone, (ii) the curved AIS with the obstruction modeled thereon, and (iii) a perspective view of (ii). In the case of  FIG. 11   b , the representations from left to right are as in the case of  FIG. 11   a , but the curved AIS in this figure also has a SWG region. 
         FIGS. 12   a - 12   d  and  FIGS. 13   a - 13   d  are similar to  FIGS. 9   a - 9   d  and  10   a - 10   d , but are for the curved AIS of  FIGS. 11   a  and  11   b  as opposed to the flat AIS of  FIGS. 8   a  and  8   b.    
     
    
    
     DETAILED DESCRIPTION 
     As indicated above,  FIG. 1   a  shows the fuselage  1  of an aircraft. It is desirable to mount an antenna  2  on the underside of the fuselage behind the strut  3  which supports a landing gear wheel. However, the landing gear strut  3  will block radiation from antenna  2  in a forward direction (towards the landing gear strut  3 ). While  FIG. 1   a  shows a strut  3  causing blockage, there are any number of objects which can protrude from a vehicle, such as the aircraft shown in  FIG. 1   a , which can hinder or obstruct the transmission and/or reception of RF energy at antenna  2 , for example. While it is a landing gear strut  3  which is the particular object causing RF obstruction here, the obstructing object will often be referred to simply as an obstruction herein, it being understood that any manner of objects blocking or hindering the transmission and/or reception of FR energy can be mitigated using the technology disclosed herein. 
       FIG. 1   b  is representation of a mockup or prototype of the forward portion of the fuselage  1  of an aircraft to test if the AIS  10  of the present invention will mitigate the blockage caused by strut or obstruction  3  in that forward portion. Its design is meant to generically represent the front portion of an aircraft fuselage  1 . The depicted elliptical variations  4  pictorially represent a surface-wave impedance modulation that characterizes AIS  10 . The shapes of the depicted elliptical variations  4  will depend upon the shape and size of the obstruction  3  as well as its location relative to feed point  2 . The variations are dependent on: (1) the desired antenna properties, including radiation angle and frequency, (2) the material properties of the substrate and its thickness, and (3) the period, shape and mean size of the metallic patches that form the AIS. All of this information is included in the equations (1)-(3) below. The elliptical variation and the light and dark bands seen in the figures are formed with metallic patches of varying size. The larger the patch, the higher the surface-wave impedance. The darker bands in the depictions are caused by larger patches on the light underlying dielectric substrate. The results of testing, see  FIG. 2 , show that the obstruction  3  has little effect on the radiation pattern over a broad range of frequencies when a properly designed AIS  10  is utilized to move the radiation to be launched around the RF obstacle presented by the obstruction  3 . The prototype AISA  10  as measured on the fuselage mockup shows less than a 1 dB attenuation due to the obstruction caused by the strut. See  FIG. 2  which depicts the radiation intensity is measured with and without the strut obstruction in place (solid and dashed lines respectively) and for different frequencies (represented by different levels of black and gray). For each frequency tested, the radiation intensity with the strut in place (in solid lines) very closely follows the radiation intensity without the strut in place (in dashed lines). 
       FIG. 3   a  illustrates conceptually the method the invention uses to mitigate the antenna blockage problem discussed with reference to  FIG. 1   a . The antenna originally used on the aircraft  1  is replaced with a Artificial Impedance Surface Antenna (AISA) which preferably conforms to the shape of the aircraft  1 . The AISA function as an antenna. The feed is a device located at feed point  2  that launches the surface waves across the antenna surface formed by the AISA. The device at the feed point can be any number of things: a monopole antenna, a waveguide, or microstrip line feed, for example. The surface waves propagate across an impedance modulation (represented by the elliptical-looking patterns  4  in  FIGS. 1   b  and  3   a ) formed by varying the size of metallic patches on the dielectric substrate on the AIS  10  until they reach one or more radiation region(s)  12  that is(are) not affected by the gear strut obstruction  3 , since the antenna&#39;s radiation region is effectively moved in front of the strut or obstruction  3  (an area, for example, which is not affected by the gear strut obstruction  3 ). See the dashed-line ovals identified with numeral  12  in  FIG. 4  which more or less identify the radiation region  12  of the AIS  10  of that embodiment.  FIG. 3   b  depicts an embodiment that is further enhanced by adding a surface-wave waveguiding region  14  that guides the surface waves around the obstruction  3  and prevents any of the surface wave energy from being attenuated by the obstruction.  FIG. 3   b  shows depicts an embodiment of the invention that incorporates the waveguiding region  14 . 
     Artificial Impedance Surface Antennas (AISA) 
     Artificial impedance surface antennas (AISA) are realized by launching a surface wave across the AIS  10 , whose impedance is spatially modulated across the AIS  10  according a function that matches the phase fronts between the surface wave on the AIS  10  and the desired far-field radiation pattern. The resulting radiation pattern may be a pencil beam whose directivity, angle, beam width and side lobes are determined the details of the AISA geometry and its electrical properties. The AISA is an antenna since it launches electromagnetic radiation from all points on the its surface where there is the impedance modulation. See regions  12  in  FIG. 4 . The AISA discussed above was designed to work in the Ku frequency band and could certainly be designed to work in other frequency bands as desired. 
     It is desirable to direct the radiation pattern from the antenna feed point  2  as close as possible to the plane of the fuselage&#39;s bottom, thus overcoming the radiation pattern lift caused by finite and curved ground planes. The approach used is conceptually presented in  FIG. 4  which shows an AIS  10  with an obstruction  3  in the middle of it. The feed  2  launches surface waves across the AIS  10 . When the surface waves reach the modulated impedance region designated by the light and dark bands on the AIS  10 , they leak off the surface to form the antenna radiation. The effects of the obstruction  3  are mitigated by forming a non-radiative, constant-impedance region  15  adjacent the feed point  2  and, in some embodiments, in front of the obstruction  3 . The AIS  10  is modulated for radiation only in those areas where the obstruction  3  does not impede a line of sight between the AIS  10  and the desired radiation region  12  (on the surface of AIS  10 , the obstruction  3  is limited to the depicted dark circular region—the obstruction  3  widens as it moves away from the surface of the AIS  10  as can be seen in  FIG. 3   b ). In the embodiment of  FIG. 4 , a small portion of the surface waves is intercepted by the obstruction  3  (the depicted dark circular region at the based of obstruction  3 ).  FIG. 3   b  shows a technique to enhance blockage mitigation by creating a low-impedance, surface-wave guide (represented by the dark triangular region  14 ) in front of the obstruction  3  that guides the surface waves around the obstruction  3  to the radiating region  12  not affected by the obstruction  3  (for example, where radiation is showing as occurring in  FIG. 4  by the black sinusoidal waves which are launched in region  12 ). The waveguide region  14  is formed analogous to dielectric waveguides that consist of a relatively high-index material surrounded by a relatively low index material. 
     The basic principle of AISA operation is to use the grid momentum of the modulated AIS to match the wavevectors between a surface-wave and a plane wave. In the one-dimensional case, the condition on the impedance modulation is 
     
       
         
           
             
               
                 
                   
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     where k o  is the radiation&#39;s free-space wavenumber at the design frequency, θ o  is the angle of the desired radiation with respect to the AIS normal, k p =2π/λ p  is the AIS grid momentum where λ p  is the AIS modulation period, and k sw =n o k o  is the surface wave&#39;s wavenumber, where n o  is the surface wave&#39;s refractive index averaged over the AIS modulation. 
     The AIS modulation for the one-dimensional AISA radiating at the angle θ o  and the wavenumber k o  can be expressed as periodic variation in the surface-wave propagation index (n sw ). In the simplest case, it is sinusoidal.
 
 n   sw ( x )= n   o   +dn  cos( k   p   x )  (Eqn. 2)
 
     where dn is the modulation amplitude. For AISA surfaces of arbitrary shape, the modulation of Eqn. 2 can be generalized as
 
 n   sw ({right arrow over ( r )})= n   o   +dn  cos( k   o   n   o   r−{right arrow over (k)}   o   ·{right arrow over (r)} ).  (Eqn. 3)
 
     where {right arrow over (k)} o  is the desired radiation wave vector, {right arrow over (r)} is the three-dimensional position vector of the AIS, and r is the distance along the AIS from the surface-wave source to {right arrow over (r)} along a geodesic on the MS surface. For a flat surface, r=√{square root over (x 2 +y 2 )}. 
       FIG. 5  compares two simulation methods to each other and to some measured data for an AISA designed to radiate at 60° off normal at 12 GHz. AISAs excited by TM-mode surface waves are limited in their angular range to about ˜75° declination from the surface normal because the surface currents are parallel to the direction of propagation. The AISA that can radiate close to 90° off normal by curving the AIS  10 . In terms of placing the AIS  10  on the fuselage of an aircraft, if a forward landing gear strut is causing the obstruction, then it is very convenient to curve the AIS  10  to follow the curving aircraft fuselage normally found at the front of the aircraft. An AISA can readily be designed with curvature by applying the generalized impedance map of Eqn. 3. If the AIS  10  is simply curved in a single plane, then it can be easily fabricated by printing the impedance map on a flat substrate and then bending it around a form or mold. Fabricating AISAs with a complex curvature such as a spheroid, ellipsoid or paraboloid requires more extensive design and fabrication processes.  FIG. 6   a  shows a cross section of the fuselage  1  with a curved line  16  designating the profile of the test version of the curved AIS  10 . If the AIS  10  were planar, it would be impossible to direct radiation 90° off normal. By curving the AIS  10 , the feed point  2  preferably is still on the planar portion behind the obstruction  3 , but the upward curving portion is now a radiation aperture that can efficiently radiate in the forward direction. Some representative measured radiation patterns from such an AISA are shown in  FIG. 6   b  for various frequencies spanning the range 11.8 to 12.4 GHz and 82° to 95° off normal. 
     Surface-Wave Waveguides 
     As is discussed above with reference to  FIG. 3   a , the AIS  10  can be further enhanced by adding a Surface-wave WaveGuide (SWG) region  14  thereto. A SWG region  14  offers further advantages for mitigating antenna pattern blockage due to structural elements (such as obstruction  3 ). The SWG principle is analogous to making a dielectric waveguide where the wave is guided in a high index region surrounded by a low index region. Similarly, an SWG is formed by creating regions of varying surface-wave index. Utilizing the simple SWG region  14  seen in  FIG. 3   b  with an AIS  10  can be very effective at reducing the effect of obstruction  3  to a minimum. The SWG region  14  is a low-index region that excludes the surface waves. The impedance in the SWG region  14  is lower than the neighboring region  15 , and this tends to reflect surface waves to reflect off the boundary between the regions  14  and  15 . The SWG region  14  is triangularly-shaped region whose base has a width approximately equal to the width of the obstruction  3  at the surface of the region  14  and whose apex points towards feed point  2 . The surface waves are guided around the SWG region  14  and thereby avoid being intercepted by the obstruction  3 . They continue to propagate past the obstruction  3  where they can radiate unimpeded from the radiating regions  12  (see  FIG. 4 ) in front and to the sides of the obstruction  3 . The low-impedance region  14  is preferably realized with a bare dielectric. There are other methods of obtaining a low-impedance region  14 : (1) the thickness of the dielectric can be reduced in the SWG region  14  as this would decrease the impedance even farther and/or (2) a material with lower permittivity than the surrounding region can be used in region  14 . 
     The terms surface-wave impedance and surface-wave index are related by a simple formula n=(1+Z 2 ) 1/2  where n is the index and Z is the surface wave impedance. A high index corresponds to high impedance, and vice versa. The term impedance herein refers to surface-wave impedance. 
     A second principle used in blockage mitigation is to locate the radiation aperture  12  so that it is not affected by the obstruction  3 . This is illustrated in  FIG. 3   a  which shows a non-radiating, constant impedance region  15  in front of the obstruction. Surface waves move through this region without radiating until they pass the obstacle  3  and reach the radiation aperture region  12 . So there are two effects of the obstruction  3  that are being independently mitigated. One is that obstruction  3  blocks where radiation can be emitted from the AIS  10 . Second, it blocks surface waves traveling along the AIS  10 . The SWG is used to prevent the surface waves from hitting the portion of the obstruction that is sitting on the surface. Putting a non-radiative region  12  in front of the obstruction  3  prevents radiation form being created in a place where it will be blocked by the obstruction  3  extending above the AIS  10 . When the radiation is emitted from the region  12  located in front of the obstruction  3 , then there is no blockage to radiation to be emitted in the forward direction (in the direction of the sinusoidal waves which are launched in region  12  as depicted in  FIG. 4 ). 
     The shape and impedance-profile of the SWG region  14  was chosen as one way of demonstrating its effect on improving AISA blockage mitigation. The results show that its effects are beneficial and it is advantageous to explore and optimize such structures, especially to optimize it for specific AISA platform applications and geometries of the feed point  2 , the obstruction  3  and the shape of the surface between them. So while the triangular shape depicted for region  14  is clearly beneficial, other shapes for region  14  may yield further improvement or modifying the depicted triangular shape of region  14  may yield further improvement. 
     Simulation of Blockage Mitigation 
     Simulations were used to demonstrate the ability of the SWG techniques outlined above to mitigate antenna blockage.  FIGS. 3   a  and  3   b  show two AISA configurations (one without region  15  and one with region  15 ) which were simulated using software. In these simulations, the obstruction  3  is represented by a PEC rectangular obelisk 10 cm in width and 30 cm high.  FIG. 7   a  compares simulated radiation patterns for an AISA with and without the obstruction  3  caused by the idealized landing “gear”. For comparison,  FIG. 7   b  shows simulated radiation patterns for a dipole mounted on a Perfect Electrically Conducting (PEC) surface (that is, without the AIS  10 ) with and without the same obstruction  3 . The obstacle has a pronounced effect on the dipole on the PEC, but the AIS  10  with SWG region  15  blockage mitigation is only affected slightly. 
     Measurements of Blockage Mitigation 
     AISA technology for blockage mitigation was characterized with measurements of flat and curved AISAs with and without the low-impedance SWG region  15 . The radiation patterns were measured with and without a metal structure emulating the landing gear strut  3  seen in  FIGS. 1   a ,  1   b  and  6   a  in order to characterize the effectiveness of the mitigation region  15 . In general, the effect of the blockage was limited to a reduction of only 0.5 to 2 dB. In one case, the curved AIS  10  showed no reduction in radiation intensity when radiating at 90°. Compare that to the pronounced effects of the strut  3  on a waveguide or dipole feed on PEC plates with the same geometry as the AIS. 
     The flat AISAs, with (see  FIG. 8   b ) and without (see  FIG. 8   b ) the SWG region  15 , and with and without the strut  3 , are shown in  FIGS. 8   a  and  8   b .  FIGS. 11   a  and  11   b  show the same views with the curved AIS  10 . The AISA feed  2  is a waveguide which is centered along the short side of the left side of AIS  10  in  FIGS. 8   a ,  8   b ,  11   a  and  11   b . The feed  2  is directed directly at the strut  3 . This feed  2  is an expedient and suitable method for verifying and characterizing AIS  10  performance and radiation patterns; it is not meant to represent an optimum feed system. A preferred feed  2  would comprise a feed that is conformal to the surface. See U.S. patent application Ser. No. 13/242,102 filed on the same date as this application and titled “Conformal Surface Wave Feed”, which is hereby incorporated herein by reference. 
     The flat AIS  10  depicted in  FIGS. 8   a  and  8   b , has its radiation measurements shown in  FIGS. 9   a - 9   b  and  10   a - 10   d . Its far-field radiation patterns were measured with and without obstruction  3  (see  FIG. 9   a ). When radiating at 60°, the obstruction  3  attenuates the peak intensity by 2 dB. A 2×5 inch surface-wave guiding region  15  depicted in  FIG. 8   b , was then integrated into that AIS  10  and the measurements were repeated (see  FIG. 9   b ). The obstruction  3  attenuates the SWG-AISA&#39;s peak intensity by 1 dB. 
     For comparison,  FIGS. 9   c  and  9   d  show the same measurements performed on metal plate of the same size and shape as the AIS  10  of  FIGS. 8   a  and  8   b , using either a waveguide or dipole feed  2 . The strut  3  is placed in the same location relative to the feeds as in the AISA measurements depicted by  FIGS. 9   a  and  9   b . It can be seen that without the AIS  10  on the metal plate, the blockage of the strut  3  causes drastic changes to the far field radiation patterns. In fact, the scattering of the waveguide-fed plate with the strut attached dominates the radiation pattern. In the case of both feed arrangements, the peak intensity with the strut is in the backward direction, indicating a strong reflection of radiation by the strut  3 . 
     The effectiveness of the AIS  10  embodiment with the SWG region  15  is consistent across the frequency range where the intensity drops off by several dB. Radiation patterns at several frequencies in this range are plotted in  FIG. 10   a .  FIG. 10   c  shows how the peak angle scans with frequency for both the blocked (gear) and unblocked (no gear) cases. There is little difference between the two cases.  FIG. 10   b  shows the peak intensity for the blocked (gear) and unblocked (no gear) cases and  FIG. 10   d  plots the difference in peak intensity showing that it drops uniformly as the frequency approaches the design frequency of 12 GHz. 
     Measurements of Curved AISAs 
     Similar results (see  FIGS. 12   a - 12   d  and  FIGS. 13   a - 13   d ) were obtained with curved AISAs that are designed to conform to a fuselage profile and to radiate at 90° relative to the normal to the fuselage&#39;s bottom. While an antenna feed mounted on a curved metal plate is strongly blocked, distorted and reflected backwards by the obstruction  3  (see  FIG. 12   d ) resulting in a reduction of the forward peak by several dB, the curved AIS  10  shows less than 2 dB attenuation due to blockage ( FIG. 12   a ), and the radiation patterns of the curved AIS  10  with the wave guiding region  15  ( FIGS. 12   a  and  12   b ) show almost no degradation caused by the obstruction  3 . 
     One significant item to note in comparing the patterns from the waveguide feed on the flat metal plates and the curved metal plate ( FIGS. 9   d  and  12   d ) is that the curving of the metal plate causes even more of a lift in the radiation pattern because of the finite size of the ground plane. This lift of the radiation pattern when antennas are installed on finite and curved ground plane causes significant degradation in azimuth plane omni coverage. As seen dramatically in  FIGS. 12   a ,  12   b  and  12   c , the AIS  10  completely eliminates the pattern lift. 
     The radiation patterns of the curved AIS  10  with SWG region  15  at several frequencies are plotted in  FIG. 13   a .  FIG. 13   c  shows how the peak angle scans with frequency for both the blocked and unblocked cases. There is little difference between the two cases.  FIG. 13   b  shows the peak intensity for the blocked and unblocked cases and  FIG. 13   d  plots the difference in peak intensity. 
     Those skilled in the art will appreciate that this disclosure is based on analysis and modeling of techniques which can doubtlessly be applied in actual, full scale applications, such as real life embodiments of the aircraft  1  modeled herein. 
     This technology can be applied in many other applications. The obstruction  3  for the UAV is a fixed blockage, but this technology can also be applied to movable obstructions or objects which change shape or configuration. The spatial surface-wave impedance function  4  that characterizes the AIS  10  can be permanently designed into the AIS  10  so that it does not change or it can be variable using suitable control signals which control variable capacitors imbedded in or disposed on the AIS  10  for the purpose of controlling its spatial surface-wave impedance function. Those control signals can vary the surface-wave impedance function  4  as a function of how the obstruction  3  changes shape and/position relative to the feed point  2 . 
     This technology can be used to overcome objects, whatever they might be, which block, obstruct, interfere with or hinder the transmission and/or reception of RF signals available at or supplied to a feed point. Most objects of the types mentioned herein will just interfere with the transmission and/or reception of RF signal and not completely block those signals. It is to be understood that the terms ‘blockage’ and ‘obstruction’ used herein are intended to embrace the notion that the blockage or obstruction interferes with or hinters the transmission and/or reception of RF signals available at or supplied to a feed point without necessarily completely blocking such transmission and/or reception. 
     The shape of the antenna does not have to conform to the shape of the aircraft, vehicle or object with which it is associated or mounted upon. The fact that it can be made to conform is believed to be desirable in many applications and/or uses, but an optional feature which need not be utilized. 
     Having described the invention in connection with certain embodiments thereof, modification will now suggest itself to those skilled in the art. For example, the disclosed embodiment preferably conforms to a frontal portion of an aircraft and is used to circumvent RF blockage caused by a strut. But those skilled in the art will appreciate the fact that the disclosed antenna may conform to the shape of a portion of any aircraft, vehicle or object and moreover the fact that disclosed antenna does not need to conform to the shape of any any aircraft, vehicle or object to which it might be attached or otherwise associated, and still be used successfully to circumvent a RF blockage caused by some interfering or obstructing element. As such, the invention is not to be limited to the disclosed embodiments except as is specifically required by the appended claims.