Patent Publication Number: US-7583238-B2

Title: Radome for endfire antenna arrays

Description:
FIELD OF THE INVENTION 
     The present invention relates to radomes. More particularly, embodiments of the present invention relate to radomes for endfire antenna arrays. 
     BACKGROUND OF THE INVENTION 
     Many antenna applications require the installation of a radome over the antenna radiators. For a uniformly, well-constructed radome, the radome material does not significantly effect a broadside antenna&#39;s array gain. However, if the radome is located too closely to the radiators of an endfire antenna array, the radome may adversely effect the endfire antenna&#39;s array gain. This adverse effect is due, in large part, to the different phase shifts induced in the antenna array&#39;s signals by the dielectric effects of the radome material. 
       FIG. 1  is a schematic diagram of a broadside array  10  having an effective aperture  18 . Electromagnetic signals  14 ,  16  pass through radome  12  substantially perpendicular to the radome&#39;s surface, and, while the radome material&#39;s dielectric property shifts the phase of the electromagnetic signals  14 ,  16  to some degree, generally, the phase shift is relatively constant across the effective aperture  18  for all of the signals transmitted or received by broadside array  10 . Consequently, the array gain of broadside antenna  10  is not adversely effected by the radome material. 
       FIG. 2  is a schematic diagram of an endfire array  20  having an effective aperture  28 . Electromagnetic signals  24 ,  26  pass through radome  22  at different incident angles relative to the radome&#39;s surface. Consequently, the radome material&#39;s dielectric property shifts the phase of electromagnetic signals  24 ,  26  differently. The phase of electromagnetic signal  26 , which passes through more of the radome material, is shifted more that the phase of electromagnetic signal  24 , which passes through less of the radome material. Thus, antenna signals propagating to the lower portion of effective aperture  28  will experience larger phase shifts than the antenna signals propagating to the upper portion of the effective aperture  28 . For long antennas, the net cumulative shift can be as much as 180 degrees near the lower portion of the effective aperture  28 , which causes signals in the endfire aperture  28  to selectively cancel one another. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide a radome for an endfire antenna array that includes a honeycomb core with an inner skin and an outer skin attached thereto, a first set of conductive slats disposed on the inner skin of the honeycomb core and a second set of conductive slats that are disposed within the honeycomb core. The two sets of conductive slats are capacitively-coupled to one another to counteract the adverse effects of the dielectric property of the endfire radome. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other advantages of this invention will become more apparent by the following description of invention and the accompanying drawings. 
         FIG. 1  is a schematic diagram depicting a prior art broadside array and radome. 
         FIG. 2  is a schematic diagram depicting a prior art endfire array and radome. 
         FIG. 3  is a schematic diagram depicting an endfire array and radome in accordance with an embodiment of the present invention. 
         FIGS. 4   a  and  4   b  are depict endfire array beam patterns for two exemplary array element spacings. 
         FIG. 5  is a schematic diagram depicting an endfire array and radome in accordance with another embodiment of the present invention. 
         FIGS. 6A and 6B  present plots of the improvement in signal amplitude for an endfire and radome in accordance with the embodiment depicted in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide a radome for an endfire antenna array that includes two sets of conductive slats that counteract the adverse effects of the dielectric property of the radome. One set of conductive slats is located on the inner surface of the radome facing the antenna array, while a second set of conductive slats is located within the body of the radome, adjacent to, and capacitively-coupled to, the first set of conductive slats. The two sets of conductive slats may overlap one another to enhance the capacitive-coupling effect that reduces the phase shift experienced by antenna signals propagating through the radome toward the lower portion of the endfire array&#39;s effective aperture. The spaces between the slats in each set advantageously provide transmission windows for antenna signals propagating to the upper portion of the endfire array&#39;s effective aperture. 
       FIG. 3  is a schematic diagram depicting an endfire array  30  and a radome  40  in accordance with an embodiment of the present invention. 
     Generally, endfire array  30  includes an array of radiators  34  coupled to a ground plane  32 . In the depicted embodiment, endfire array  30  includes a single, linear array of identical monopole radiators  34  coupled to ground plane  32 . In order to achieve high gain and narrow beamwidth, the electromagnetic signals received or transmitted by the array of monopole radiators  34  should possess constant amplitude and phase. In alternative embodiments, endfire array  30  may include multiple, linear arrays of monopole radiators  34 . 
     In a preferred embodiment of the linear array, the spacing “d” between each monopole radiator is constant. For an exemplary spacing d=λ/2, the end fire radiation pattern  60  for a four-element array is depicted in  FIG. 4   a . Due to ambiguity, two main beams are present at 0° and 180°. When the spacing “d” is decreased, however, such that d&lt;λ/2, the ambiguity may be resolved, resulting in an end fire radiation pattern  62  depicted in  FIG. 4   b . As the beam steer angle for the end-fire array is changed from 0°, i.e., e.g., the lower portion of the endfire array effective aperture ( FIG. 2 ), to 15°, for example, i.e., e.g., the upper portion of the endfire array effective aperture ( FIG. 2 ), the electromagnetic signals propagating to the radiators at the rear of the linear array pass through more of the radome material than electromagnetic signals propagating to the front of the linear array. The additional propagation path through the radome, if uncompensated, induces undesirable phase shifts, as discussed above. 
     The radome  40  is typically a high-strength, low weight composite structure. In one embodiment, the radome  40  includes a honeycomb core  42  sandwiched between an inner skin or surface  43  and an outer skin or surface  44 . The inner and outer skins  43 ,  44  may be attached to the honeycomb core  42  using, for example, high-strength epoxy. Advantageously, the deleterious effects of radome-induced phase shifts are countered by attaching a first set of conductive slats  46  to the inner skin  43  of the radome  40 , and by positioning a second set of conductive slats  48  within the honeycomb core  42  itself, as depicted within  FIG. 3 . The conductive slats are preferably constructed using highly-conductive material, such as, for example, gold, silver, copper, etc., although other materials may be used. 
     In a preferred embodiment, the first and second sets of conductive slats  46 ,  48  are evenly-spaced, while in alternative embodiments, the slat spacing may be non-uniform and based upon other considerations, such as, for example, the distance of the particular spacing to the front of the endfire array. Optionally, the first and second sets of conductive slats  46 ,  48  may be constructed of dissimilar conductive materials. In one embodiment, the first and second sets of conductive slats  46 ,  48  overlap at the edges of each respective slat, as depicted in  FIG. 3 . 
     The first set of conductive slats  46  prevents a substantial portion of the electromagnetic field from entering the honeycomb core  42 , while the second set of conductive slats  48  are positioned, in close proximity to the first set of conductive slats  46 , in order to capacitively-couple the first and second sets of conductive slats together. In one sense, the dielectric property of the radome  40  effectively lengthens the electrical path along which the endfire electromagnetic field travels, which induces the undesirable phase shift described above. This effect is countered by the first and second sets of capacitively-coupled slats  46 ,  48 , which effectively shortens the electrical path along which the endfire electromagnetic field travels, which reduces the induced phase shift. 
       FIG. 5  is a schematic diagram depicting an endfire array and radome in accordance with an embodiment of the present invention. 
     In the depicted embodiment, endfire array  30  includes a single, linear array of monopole radiators  34 , spaced 3.75 inches apart, which generally supports a frequency range of 1.2 to 1.4 GHz. Radome  40  is positioned 6 inches above the ground plane  32 , and includes a fiberglass honeycomb core  42 , 0.9 inches in thickness, which is bonded to a fiberglass inner skin  43 , 0.063 inches in thickness, and to a fiberglass outer skin  44 , 0.063 inches in thickness. The first set of conductive slats  46  include individual slats that are 1 or 2 mils thick, 2.25 inches long, as wide as the antenna width of the antenna and evenly-spaced 1 inch apart. The second set of conductive slats  48  include individual slats that are 1 or 2 mils thick, 2.25 inches long, as wide as the antenna width of the antenna and evenly-spaced 1 inch apart. The second set of conductive slats  48  are positioned 0.6 inches above the first set of conductive slats  46 , and the edges of the first and second set of conductive slats overlap by 0.625 inches. The first and second sets of conductive slats are made from a conductive material, such as, for example, aluminum, copper, gold, silver, etc. 
       FIG. 6A  presents a plot of the improvement in signal amplitude for an endfire array having 108 radiators, at 1.21 GHz and nominal spacing, under three different conditions: the endfire array (curve  1 ), the endfire array with a prior art radome (curve  2 ), and the endfire array with radome  40  according to the embodiment depicted in  FIG. 5  and described above (curve  3 ). A comparison of these signal amplitude curves shows the signal cancellation at the far end of the endfire array (i.e., elements  0 ,  1 ,  2 , etc.) due to the adverse effects of the prior art radome, and the improvements derived from the advantageous effects of the present invention. The most efficient coupling would produce a flat signal response curve.  FIG. 6B  presents the improvement in signal amplitude at 1.3 GHz. 
     While this invention has been described in conjunction with specific embodiments thereof, many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth herein, are intended to be illustrative, not limiting. Various changes may be made without departing from the true spirit and full scope of the invention as set forth herein.