PATENT ABSTRACT
A spiral antenna system that is designed to have an increased upper frequency limit. The system includes a spiral antenna element having a feed end, and a helical antenna element having a helical portion electrically interconnected with the feed end of the spiral antenna element. In one embodiment, the helical antenna element comprises a coaxial cable having a portion of the outer conductor removed (e.g., tapered). For example, the helical antenna element could comprise a portion of the feedline that follows a substantially helical path. Preferably, the spiral antenna element defines a spiral axis, and the helical antenna element defines a helical axis substantially aligned with the spiral axis. The helical antenna element can be spaced from the helical axis a distance less than or equal to the radial distance of the feed end of the spiral antenna.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to antennas, and specifically to feedlines for spiral antennas.  
           [0002]    Spiral antennas are well known for being able to transmit and receive signals consistently over a wide range of frequencies. Typically, traditional spiral antennas operate over a 10:1 bandwidth, meaning the upper frequency limit of the antenna is approximately ten times that of the lower frequency limit. In traditional spiral antennas, the upper and lower frequency limits are highly dependent on the inner and outer radii of the spiral, respectively. The circumference or fineness of the spiral center determines the upper frequency limit.  
           [0003]    Manufacturing a spiral antenna to operate at millimeter wave frequencies (the frequencies in the range of about 18 GHz to about 60 GHz) becomes increasingly difficult because the upper frequency limit is so dependent upon the fineness of the spiral. The manufacturing tolerances on the spiral surface continue to diminish as cost for manufacturing grows.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention provides a spiral antenna system that is designed to have an increased upper frequency limit. More specifically, the system includes a spiral antenna element having a feed end, and a helical antenna element having a helical portion electrically interconnected with the feed end of the spiral antenna element. In one embodiment, the helical antenna element comprises a coaxial cable having a portion of the outer conductor removed (e.g., tapered). For example, the helical antenna element could comprise a portion of the feedline that follows a substantially helical path. Preferably, the spiral antenna element defines a spiral axis, and the helical antenna element defines a helical axis substantially aligned with the spiral axis. The helical antenna element can be spaced from the helical axis a distance less than or equal to the radial distance of the feed end of the spiral antenna.  
           [0005]    Other features and advantages of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a plan view of a spiral antenna embodying the present invention.  
         [0007]    [0007]FIG. 2 is a side view of the spiral antenna as shown in FIG. 1.  
         [0008]    [0008]FIG. 3 is a perspective view of an alternative feed structure.  
         [0009]    [0009]FIG. 4 is a perspective view of a plurality of feedlines included in a feed structure embodying the invention.  
         [0010]    [0010]FIG. 5 is a cross-sectional view of the plurality of feedlines shown in FIG. 4, taken along line  5 - 5 .  
         [0011]    [0011]FIG. 6 is a graph illustrating the predicted relationship between the upper frequency limit of a spiral antenna to the diameter of the feedline.  
         [0012]    [0012]FIG. 7 is a plan view of a broadband antenna system incorporating the spiral antenna as shown in FIG. 1  
         [0013]    [0013]FIG. 8 is a side view of the broadband antenna system as shown in FIG. 7, taken along line  8 - 8 .  
         [0014]    [0014]FIG. 9 is a side view of a second embodiment of the present invention.  
         [0015]    [0015]FIG. 10 is a side view of a third embodiment of the present invention. 
     
    
       [0016]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.  
       DETAILED DESCRIPTION  
       [0017]    A broadband antenna system  20  embodying the invention is illustrated in FIGS.  1 - 7 . The antenna system  20  includes a spiral antenna  24  having a plurality of spiral antenna elements or arms  28  defining a spiral axis  30 . In the embodiment shown, the antenna  24  is a planar equiangular spiral antenna and has four spiral arms  28 . In other embodiments, the antenna  24  can be an Archimedean spiral, a sinuous antenna, a log-periodic antenna or other antennas from the traveling wave or frequency independent antenna class. The antenna  24  can also include more or fewer arms  28  than the embodiment shown in FIGS.  1 - 2 . Each spiral arm  28  has a first or feed end  42  and a second or outside end  46 . The feed end  42  is spaced a radial distance from the spiral axis  30 . In other embodiments, the outside end  46  of each spiral arm is connected to additional electronics or circuitry, or connected to an electrical load.  
         [0018]    The antenna system  20  also includes a feed structure  58  having a plurality of helical feedlines  61 - 64  that define a helical axis, which in the illustrated embodiment is aligned with the spiral axis  30 . In other embodiments, the feed structure  58  includes a plurality of feedlines that take the form of a conical helix. The feedlines  61 - 64  electrically connect the plurality of spiral arms  28  to a receiving or transmitting network (not shown). In the present invention, the feed structure  58  includes the same number of feedlines as the number of arms  28  in the spiral antenna  24 . For the spiral antenna  24  illustrated in FIG. 1, the feed structure  58  includes four helical feedlines  61 - 64 . For illustrative purposes, only one feedline  61  is shown in FIG. 2 in solid line. The other three feedlines  62 - 64  are shown in dashed lines and are not labeled. All of the feedlines  61 - 64  are illustrated and labeled in FIG. 5. The feedlines shown in FIG. 2 form a helix having one turn. In other embodiments, the helical feedlines  61 - 64  can include more or fewer turns.  
         [0019]    [0019]FIG. 3 illustrates another feed structure having eight feedlines  65 . The feedlines each include a straight portion  66  and a helical portion  67 . In this embodiment, the helical portions each travel about one quarter of a turn. At least part of the helical portions  67  is unshielded so that the feedlines  65  can transmit and/or receive signals. The straight portions  66  can remain shielded. In this manner, the helical portions  67  essentially act as a miniature helical antenna element. The helical portions  67  are spaced from the axis  30  approximately the same distance as the feed ends  42 .  
         [0020]    The unshielding of the helical portions of the feedlines is illustrated in FIGS. 4 and 5. The feedlines  61 - 64  are preferably configured from coaxial transmission line. In other embodiments, the feedlines could be configured from microstrip transmission line or a similar transmission line. Referring to FIGS. 4 and 5, each feedline  61 - 64  includes an inner conductor  68 , a dielectric layer  72  and an outer conductor  76 . For ease of explanation, the feedlines  61 - 64  shown in FIGS. 4 and 5 are not arranged in a helix. The dielectric layer  72  surrounds the inner conductor  68 , and the outer conductor  76  surrounds the dielectric layer  72 . Each feedline  61 - 64  further includes a bottom end or input end  80 , a top end or output end  84 , and a transition section  88  found between the input end  80  and the output end  84 . The feedlines  61 - 64  are in a substantially uncoupled state at each of the input ends  80 . At the output ends  84 , the feedlines  61 - 64  are in a highly coupled state. The transition between the uncoupled state to the highly coupled state takes place during the transition section  88 . The outer conductor  76  of each feedline  61 - 64  is tapered in a manner such that the transition from one state to the other is smooth. The outer conductor  76  can be tapered linearly, exponentially or another manner that allows the states to transition smoothly. The illustrated tapering starts on the inside (i.e., the side facing the other feedlines) and moves toward the outside, but could instead be outside to inside or side to side. The dielectric layer  72  can also be tapered in the same fashion as the outer conductor  76 , tapered in a different manner than the outer conductor  76 , or not tapered at all.  
         [0021]    Tapering the feedlines  61 - 64  allows each feedline  61 - 64  to transition from a substantially uncoupled state at the input end  80  to a highly coupled state at the output end  84 . Having the feedlines  61 - 64  in a coupled state allows the feed structure  58  to better match the antenna input impedance to the feedline impedance, and can simultaneously match multiple antenna modes having different modal impedances. Also, at the output end or highly coupled end  84 , each feedline  61 - 64  is able to radiate when excited because the feedlines  61 - 64  are unshielded. It is believed that a helical feedline can increase the upper frequency limit of a spiral antenna  24  by a factor of two, allowing the antenna  24  to operate in the millimeter-wave frequency region.  
         [0022]    The diameter of the helical feedline and the number of antenna elements or arms both become a factor in determining the upper frequency limit of an antenna. The graph shown in FIG. 6 illustrates the predicted relationship between the upper frequency limit and the diameter of the feedline for various multi-element spiral antennas having a helical feed structure. The feedline diameter is represented on the x-axis  90  and the upper frequency limit is represented on the y-axis  92 . The first solid line  94  illustrates the relationship for a spiral antenna having four antenna elements, the second solid line  96  illustrates the relationship for a spiral antenna having six antenna elements, and the third solid line  98  illustrates the relationship for a spiral antenna having eight elements.  
         [0023]    Still referring to FIG. 6, the dashed line  102 ,  104 , and  106  illustrates the relationship between the upper frequency limit and the diameter of the feedline for various multi-element spiral antennas not including a helical feed structure. The first dashed line  102  illustrates the relationship for a spiral antenna having four antenna elements, the second dashed line  104  illustrates the relationship for a spiral antenna having six elements, and the third dashed line  106  illustrates the relationship for a spiral antenna having eight elements. The vertical lines  108  represent the diameters of commercially available or standard coaxial cable. As illustrated by the first solid line  94 , a spiral antenna having four antenna elements can include a standard coaxial cable with a large diameter (such as 0.047 inches) for the helical feedline and have an upper frequency limit of approximately 60 GHz. As illustrated by the first dashed line  102 , a spiral antenna having four antenna elements and not having the helical feed structure would have an upper frequency limit of approximately 20 GHz when using standard 0.047 in. coaxial cable for the feedlines.  
         [0024]    When the helical feedlines  61 - 64  are excited and start to radiate, the feedlines produce backfire radiation. In other words, the helical feedlines radiate in the opposite direction. As the number of turns in the helix increases, the directivity of the back lobe or rear beam increases and causes the front-to-back ratio (the ratio of the maximum directivity of an antenna to its directivity in a specified rearward direction) to decrease. Therefore, in one embodiment, the helical feedlines have approximately one quarter of a turn and a reflective element  110  (FIG. 8) is positioned beneath the helical feedlines to reflect the backfire radiation. The reflective element  110  is a metallic disc with an opening (not shown) or a series of openings (not shown) for the helical feedlines to pass through. In other embodiments, the reflective element  110  can vary in shape and size and can be configured from other materials with reflective properties.  
         [0025]    The antenna system  20  can also include a reflective cavity  112 . When a planar spiral antenna radiates, it typically produces equal radiation above and below the antenna. In order to produce one beam of radiation, the reflective cavity  112  is positioned substantially beneath the spiral antenna  24 . As shown in FIGS. 7 and 8, the reflective cavity  112  substantially surrounds the helical feedlines  61 - 64  and reflective element  110 . The cavity  112  includes a reflective base  114  and sidewall  118 . In other embodiments, the cavity  112  can vary in shape and size and include more or less sidewalls  118 . The cavity  112  can further include a single reflective base  114  of varying shape and size, such as a conical base  120 , shown in FIG. 9, or include a stepped base cavity  124 , shown in FIG. 10, with or without the additional inner side walls  128 . Also, the reflective base  114  can be substantially parallel to the spiral antenna  24  or not. In the embodiment of FIG. 8, a radio frequency absorber  132  is positioned within the reflective cavity  112  to avoid reflections that could degrade the antenna patterns over wide bandwidths. The absorber  132  can included one or more layers of a foam absorber, a honeycomb absorber, and/or a loaded material, as is known in the art. Typically, in the embodiments when the reflective base  114  is shaped, such as shown in FIGS. 9 and 10, the absorber  132  is not used. A layer or multiple layers of unloaded foam or honeycomb (not shown), in some embodiments, may be placed within the reflective cavity  112  to support the spiral antenna  24  and the reflective element  110 .  
         [0026]    Various features and advantages of the invention are set forth in the following claims.