Patent Publication Number: US-7714796-B1

Title: Hemispherical helical antenna and support frame therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) based on provisional application No. 60/708,169 filed Aug. 15, 2005. 
    
    
     BACKGROUND OF THE INVENTION 
     The helical antenna and its derivatives have been widely used in the field of communications for several decades. The helical antenna typically operates in the frequency ranges of 1 GHz and above. Many helical antennas resemble a coil of wire in the shape of a spring. As shown in  FIG. 1A , wrapping the antenna wire on a cylinder is a relatively easy way to form the coils of a helical antenna. As shown in  FIG. 1B , the cylinder used to construct the element is often incorporated into and becomes part of the antenna system. Helical antennas that operate at frequencies below 1 GHz tend to be more difficult to manage and construct because of their physical size and weight. For example, a 2 GHz helical antenna has a coil diameter of 2 inches while a 500 MHz helical antenna has a coil diameter of 8 inches. As a general rule, the larger coils of conventional helical antennas are supported by means of a main rod located in the center of the coil with smaller rods supporting the element. 
     The hemispherical helical antenna is a derivative of the helical antenna. Typically, hemispherical helical antennas exhibit a gain of approximately 8 dbi and are much smaller in axial length than a cylindrical helical antenna of similar gain. However, the coil shape of the hemispherical helical is significantly more challenging to form and support than the cylindrical helical antenna. 
     SUMMARY OF THE DISCLOSURE 
     A hemispherical helical antenna employs a support frame assembly which defines the geometry and provides the principal support structure for the antenna element. The support frame assembly includes a plurality of panels manufactured of dielectric material. The panels are configured to properly align and stabilize the spacing between the turns of the wire helical antenna element at specific locations above the ground plane of the antenna. Preferably, there are three panels disposed at a fixed angular orientation to one another about a central axis. Each panel is configured with a plurality of openings or supports that are equidistantly and radially spaced from a common center point on the particular panel. The openings may be slightly larger than the diameter of the element wire to allow the element to pass through the panels. A dome-like cover may enclose the element and the support frame. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a top plan view, partly in diagrammatic form, of a typical prior art cylindrical helical antenna; 
         FIG. 1B  is a side elevational view, partly in section and portions removed, of a typical prior art cylindrical helical antenna; 
         FIG. 2A  is a side elevational view, partly in representational form, of an embodiment of a hemispherical helical antenna; 
         FIG. 2B  is a top view, partly in representational and diagrammatic form, of the embodiment of  FIG. 2A ; 
         FIG. 2C  is a perspective view, partly in representational form, of the embodiment of  FIG. 2A ; 
         FIGS. 3A-3C  are elevational views of support components for the embodiment of  FIG. 2A , each Figure illustrating the configuration of a single support panel relative to a base; 
         FIG. 4  is a graph representing the gain for the antenna of  FIG. 2A ; and 
         FIG. 5  is a perspective view of a preferred embodiment of a hemispherical helix antenna including a support frame and a cover. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the drawings wherein like numerals represent like elements throughout the various views, a hemispherical helical antenna is generally designated by the number  10 . The hemispherical helical antenna  10  includes a support frame assembly  22  that is uniquely configured to provide for efficient and effective mounting of hemispherical helical antennas used in the broadcasting and receiving of UHF wireless band signals. Different views of an exemplary embodiment of the support frame assembly  22  are illustrated in  FIGS. 2A-2C . Generally, the hemispherical helical antenna  10  is principally comprised of a hemispherical antenna element  20 , a support frame assembly  22 , a ground plane  24 , a plurality of mounting brackets  26 , and a covering member  50 . A mounting assembly  52 , which may assume various forms and functions, optimally mounts the antenna  10  at a selected orientation as illustrated in  FIG. 5 . 
     The hemispherical antenna element  20  is preferably fabricated from a single uniform thickness wire, typically composed of copper. As shown in  FIG. 2A , the element  20  is configured to form a helix preferably having between 3 to 10 windings or turns, which are generally shaped in the form of half of a sphere (i.e., a hemisphere). The central axis  32  of the helix intersects the functional center  33  of the ground plane  24 . As illustrated in  FIG. 2B , the windings of the helix are diamentrally widest at points closer to the ground plane  24  and become considerably narrower as the element  20  reaches the top  31  of the axis at the center of the ground plane  24 . 
     In a preferred embodiment, the element  20  begins at a point that is farthest from the axis  32  and ends at the axis at the element point farthest from the ground plane  24 . It is also preferred that the windings of the element  20  extend in a counterclockwise rotation as viewed in  FIG. 2B . There is a fixed constant spacing S between adjacent turns of the element  20 . In a preferred embodiment, the spacing S between adjacent turns of the element  20  is 1.005 inches. The element  20  is spaced above the ground plane  24  at a distance D  24 . In a preferred embodiment the distance D is 1.050 inches. 
     Generally, the spherical coordinates of the helix wire along its axis is determined by the following equation: 
               r   =   a     ,           ⁢     θ   =       cos     -   1       ⁢           [     +     (       ϕ     2   ⁢   π   ⁢           ⁢   N       -   1     )       ]       ,           ⁢       2   ⁢   π   ⁢           ⁢   N     ≤   ϕ   ≤     4   ⁢   π   ⁢           ⁢   N             
The radius of the hemisphere is a.
 
The number of turns in the helix is N.
 
The circumference of the hemisphere=the wavelength of the antenna at its lowest working frequency.
 
The distance of the first point of the element from the ground plane=wavelength/0.0415.
 
     As shown in  FIGS. 2B and 2C , the ground plane  24  is a flat panel of metal that is substantially square. The functional center  33  of the ground plane  24  is where all points of the element  20  are derived. The distance W between the center  33  of the ground plane  24  and the edge  25  of the ground plane is approximately equal to one quarter of the antenna wavelength. In a preferred embodiment, W is 7 inches. 
     As shown in  FIG. 2B , the support frame assembly  22  is preferably comprised of at least three (3) substantially flat interlocking panels  34 ,  35 , and  36 , which are made of a dielectric substance. In a preferred embodiment, the panels are made of Lexan® material and the thickness of each panel  34  is between 1/32″ and 3/16″. It is also preferred that each panel  34 ,  35 , and  36  is fastened to the ground plane  24  in at least two points  30   a  and  30   b  by mounting brackets  26 . Preferably, the mounting brackets are “L” shaped brackets (such as Keystone™ #618). 
     As shown in  FIGS. 3A-3C , points  30   a  and  30   b  are located along the base  40  of each panel, at a distance that is approximately half the helix radius or greater from the center  33  of the ground plane  24 . Mounting points  30   a  and  30   b  are located opposite to each other, one on each side of the center  33  of the ground plane  24 . 
     It should be understood that each of the panels  34 ,  35 , and  36  of the support frame assembly is substantially perpendicular to the ground plane  24  and oriented with its respective center point  38  of the base  40  aligned with the center point  33  of the ground plane. Accordingly, each panel has a center line  44  that is substantially coaxial and parallel to the axis  32 . 
     Each panel  34 ,  35 , and  36  is further provided with a series of selectively positioned openings to accommodate the element  20 . In a preferred embodiment, each panel  34 ,  35 , and  36  has a series of discrete openings  34 A,  34 B,  34 C,  35 A,  35 B,  35 C,  36 A,  36 B,  36 C, etc. arranged in regular intervals along the substantially arcuate edge  37  of the respective panel. It is further preferred that these openings are slightly larger than the diameter of the element  20  and allow the element  20  to pass easily through the panel. The walls of the openings and particularly the lower portions thereof (relative to the ground plane  24 ) function as positional locators and supports for the element. Preferably, the element  20  is threaded through openings  34 A,  34 B,  34 C,  35 A,  35 B,  35 C,  36 A,  36 B,  36 C, etc. which are substantially circular holes in the panel surface. In another embodiment, the openings  42  may be thin slits or notches that extend radially inward from the edge of the panel. According to this embodiment, the element rests at the bottom of the notch. 
     In a preferred embodiment, the support frame assembly  22  is comprised of three (3) panels  34 ,  35 , and  36 , each panel being oriented at about 60 degrees from each of the other panels. Accordingly, one panel  34  ( FIG. 3A ) will intersect the element  20  at the 0 degree plane and at the 180 degree plane. The second panel  35  ( FIG. 3B ) will intersect the element  20  at the 60-degree plane and the 240-degree plane. The third panel  36  ( FIG. 3C ) will intersect the element  20  at the 120-degree plane and the 300-degree plane. It is further preferred that each of the panels  34 ,  35 , and  36  has a thin section of material removed along at least one portion of the center line  44  to form at least one slot  46  in each of the panels  34 ,  35 , and  36 . The relative configuration of the slots  46  will allow the panels to be interlocked and oriented in the manner previously described. It is preferred that the diameter of the slots  46  is approximately equal to twice the thickness of the material comprising the panels. Preferably, the panels can be interlocked without physical interference with the remaining panels and the preferred interlocking configuration provides structural stability to the support frame assembly  22 . 
     As shown in  FIGS. 3A-3C , each panel has a unique pattern in which the section of material is removed from along its center line  44  to form the slots  46 . In a preferred embodiment where the support frame assembly  22  has three panels  34 ,  35 , and  36 , one panel  36  will have a single slot  46 A that extends upwards from the base  40  of the panel. A second panel  35  will have two slots  46 A and  46 B, where one slot  46 B extends downwardly from the top and the second slot  46 A extending upwards from the base  40 . As shown in  FIG. 3B , it should be understood that a section of material remains in-between the slots  46 A and  46 B. In the third panel  34  the slot  46 B will extend downwardly from the top of the panel. 
     It should also be understood that the preferred configuration of medial slots  46  (described above) minimizes the number of components in the support frame assembly  22  by allowing each panel to pass through the center vertical plane while keeping the other panels intact. This will also provide for the efficient interlocking assembly and orientation of the three panels  34 ,  35 , and  36 . 
     In one preferred embodiment, the maximum diameter of the panels  34  is approximately 11½ inches and the vertical height of the arc above the ground plane  24  is approximately 5½ inches. In another preferred embodiment, the ground plane  34  may be configured as a regular octagon with a maximum diameter (i.e. the distance between two opposite vertices) of about 14 inches. Once the panels of the support frame assembly  22  are erected and secured to the ground plane  24 , the element  20  is sequentially passed through the openings to form the hemispherical helical antenna as illustrated in  FIGS. 2A ,  2 B and  2 C. 
     It should be understood that the support frame assembly  22  is not limited by the number or the shape of the panels. In another embodiment, the support frame assembly may be comprised of two panels. The two panels are perpendicular to each other on the ground plane. Similarly, the panels are not limited to substantially arcuate shapes. In other embodiments, the panels may be formed in a wide range of shapes having a number of precisely located openings to receive the element and form a hemispherical helical antenna. 
     The antenna support frame assembly may further include a covering member  50  which encloses the element  20  and the panels of the support frame assembly  22 . In one preferred embodiment, the covering member  50  is a black acrylic dome. The covering member  50  has a curvature which complements the arcuate curvature of the edges of the panels. 
       FIG. 5  depicts a hemispherical helical antenna  10  that incorporates one preferred embodiment of the foregoing support frame assembly  22  and employs an aesthetically pleasing covering member  50 . In one embodiment the combination of the antenna  10  and the support frame assembly  22  has RF performance characteristics that are extremely desirable for usage in the broadcast UHF wireless band, both as a transmitting and a receiving device. 
     Characteristics for one example of a hemispherical helical antenna  10  and support frame assembly  22  (shown in  FIG. 5 ) are described below: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Antenna 
                 Circular Polarized Helical 
               
               
                   
                 Gain 
                 8 dbi over most of the Wireless 
               
               
                   
                   
                 band 
               
               
                   
                 Bandwidth 
                 450-950 MHz 
               
               
                   
                 Frequency @Maximum Gain 
                 650 MHz (typically 9.5 dbi) 
               
               
                   
                 Polarization 
                 Circular 
               
               
                   
                 Connector Type 
                 N 
               
               
                   
                 Half Power Beam Width 
                 40 degrees 
               
               
                   
                 SWR 
                 1.5:1 (or better) 
               
               
                   
                 Power 
                 50 Watts 
               
               
                   
                 Radome Depth 
                 5.5″ 
               
               
                   
                 Ground Plane Diameter 
                 14″ (6061 Anodized Aluminum) 
               
               
                   
                 Mounting Bracket 
                 2″ × 1.5″ Black Nylon Block with 
               
               
                   
                   
                 ⅜-16 Helicoil and 2 Mic Stand 
               
               
                   
                   
                 adaptors 
               
               
                   
                 Radome Material 
                 Black Acrylic 
               
               
                   
                 Element 
                 Copper 
               
               
                   
                 Element Support 
                 Lexan ™ material 
               
               
                   
                 Hardware 
                 Stainless Steel 
               
               
                   
                   
               
            
           
         
       
     
     While preferred embodiments of the foregoing invention have been set forth for the purpose of description, the foregoing description should not be deemed a limitation of the invention herein. Accordingly, various modifications, adaptations and alternatives may be employed without departing from the spirit and scope of the invention.