Abstract:
A spiral antenna apparatus utilizes a noise suppression sheet that is interposed between the spiral antenna element and its ground plane. The noise suppression sheet permits an extremely compact spiral antenna apparatus while lessening antenna performance degradation.

Description:
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     This invention is assigned to the United States Government. Licensing inquiries may be directed to Office of Research and Technical Applications, Space and Naval Warfare Systems Center, Pacific, Code 72120, San Diego, Calif., 92152; telephone 619-553-2778; email: T2@spawar.navy.mil. Please reference Navy Case No. 100726. 
    
    
     BACKGROUND 
     Spiral antennas often lend themselves to conformal applications however minimizing the profile of these antennas can lead to negative antenna performance. It is desirable to be able to lessen the profile of a spiral antenna without compromising its performance. 
     SUMMARY 
     A spiral antenna apparatus utilizes a noise suppression sheet that is interposed between the spiral antenna element and its ground plane. The noise suppression sheet permits an extremely compact spiral antenna apparatus while lessening antenna performance degradation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an image of an example operational embodiment of a conformal antenna apparatus according to the description herein. 
         FIG. 2  portrays a side view of one embodiment of a conformal antenna apparatus according to the description herein. 
         FIG. 3  shows a side view of an additional embodiment of a conformal antenna apparatus according to the description herein. 
         FIG. 4  illustrates a double noise absorber material approach according to the description herein. 
     
    
    
     DETAILED DESCRIPTION 
     Spiral antenna engineers know that to achieve a unidirectional beam from a spiral antenna, a conducting cavity “behind” the spiral may be used. It is also known that as the distance between the spiral antenna element and the cavity is decreased below a quarter of the operating wavelength of the antenna, the performance characteristics of the antenna will substantially fall off. Under those conditions, circular polarization and gain can considerably suffer. It many cases, a “shallow” cavity-based spiral antenna apparatus is desirable as the apparatus can be made quite flexible and therefore be applied (adhered) directly to the surface of a vehicle such as an aircraft or marine vessel. Shallow cavity-based spiral antennas require however that measures be taken to enhance their operating characteristics. 
     The conformable antenna apparatus described herein utilizes a disc-shaped noise absorbing sheet that is interposed between a spiral antenna element and the ground plane of the antenna conducting cavity. The noise absorbing sheet may be designed to extend beyond the diameter of the spiral antenna element and can be placed either directly on the ground plane or in an intermediate position between the spiral antenna element and the ground plane. In either case, a spacer such as foam can be employed to position the noise suppressant sheet/spiral antenna element from the ground plane of the antenna apparatus. 
     Referring now to  FIG. 1 , there is shown a conformable antenna apparatus  10  having a conformable spiral antenna element  12 . Antenna element  12  has maximum diameter  14  when antenna element  12  is in a substantially planar configuration. As shown in this example, a disc-shaped noise suppression sheet  16  extends beyond diameter  14  of the antenna. The nature of the noise suppression sheet material and characteristics of usage of the material will be explained following a description of the various embodiments of the invention. 
     Referring now to  FIG. 2 , a side view of one embodiment of the conformable antenna apparatus according to the description herein is shown. In this embodiment, it can be seen that a disc-shaped conformable noise suppression sheet material  18  is placed in an intermediate position with respect to conformable spiral antenna element  20  and conformable ground plane  22  of this cavity-based antenna configuration. 
     A flexible spacer  24  is used to space the noise suppression sheet  18  from the antenna element  20 . While a variety of spacer materials may be utilized, foam and particularly white foam lends itself to such an application as the white foam has properties much like that of air. 
     In this embodiment, an additional flexible foam spacer  26  is placed between conformable noise suppressant sheet  18  and conformable ground plane  22  to position the antenna element and noise suppression sheet at predetermined distance from ground plane  22 . In this instance, the distance between antenna element  20  and ground plane  22  makes up the height of this cavity-based antenna apparatus. 
     The embodiment shown in  FIG. 3  is one wherein a disc-shaped conformable noise suppressant sheet  28  is adhered directly on (and placed immediately adjacent to) conformable ground plane  30 . As with the embodiment shown in  FIG. 2 , a flexible spacer  32  is used to space antenna element  34  from ground plane  30 . Though a variety of spacer techniques may be used, spacer  32  may be a foam and more particularly white foam. 
     In  FIG. 4 , a double noise suppression material approach is taken wherein a disc-shaped conformal noise suppression sheet  34  is surrounded by a co-planar ring-shaped conformal noise suppressions sheet  36 . Each of these sheets has a different magnetic permeability. 
     The noise suppression sheets used are commercially available and have magnetic permeability (mu) falling in the range of about 40 at 1 MHz to about 100 at 1 MHz. The absorbers used contain Fe and Si, one being of Fe—Si—Cr composition and another of Fe—Si—Al composition. 
     A test sheet of 0.5 millimeters thickness was used though thinner sheets are available and considered feasible as well. Additionally, one test version utilized a sheet with a hole but this was found to perform not as well as an un-perforated sheet. 
     The operational model depicted in  FIG. 1  operated at 1-18 Giga Hertz and had a total canister height (spiral antenna element to ground plate of 0.25 inches—working out to be about 1/47 th  of wavelength at 1 GHz. As a comparison, a conventional conformal spiral antenna apparatus requires the antenna element to ground plane to be spaced a quarter of wavelength at lowest operating frequency. 
     A version of the operation model used a double-noise material approach wherein the inner disc-shaped material had a magnetic permeability of 40 at 1 MHz and a surrounding outer co-planar ring-shaped noise suppression sheet having a magnetic permeability of 100 at 1 MHz. 
     Example materials are available through TDK such as their IRJ04 and IRJ09 flexible electromagnetic shield materials, respectively. Other manufacturers of similar materials are Mast Technologies of 6370 Nancy Ridge Drive, Suite 103, San Diego, Calif. 92121 U.S.A. identified as their MR51-0004-00 and MR51-0002-00 materials and Leader Tech of 14100 McCormack Drive, Tampa, Fla. 33626 U.S.A. identified as their EA 3200 material. 
     It was found that extending the absorber material beyond the maximum diameter of the spiral antenna element (when planar) was beneficial when using an 18 inch diameter spiral. Testing was additionally done of a six inch diameter spiral with absorbers of 7.5 inches diameter and 8 inches diameter (a twenty-five percent to 30 percent increase in diameter beyond that of the spiral antenna element). 
     It is to be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated to explain the nature of the invention by way of example, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.