Abstract:
The invention as disclosed is a direct fed bifilar helix antenna. The bifilar helix is lengthened as needed to obtain the desired unidirectional pattern. The bifilar helix is employed as an infinite balun to bring a feed cable onto the antenna structure and eventually connect the feed cable to the antenna feed point. The bifilar elements are widened such that the combined width of each element is as wide as practically possible before the elements touch and/or overlap (approximately 98.5% of the available width) so that the practical lowest characteristic impedance value of approximately 50 ohms is obtained so that there is no need for a matching network.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     This patent application is co-pending with a related patent application entitled TAPERED DIRECT FED BIFILAR HELIX ANTENNA Ser. No. 13/194,382, by Michael J. Josypenko the same named inventor to this application. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention is directed to helical antennas. In particular, the present invention is directed to a direct fed bifilar helix antenna that is broadband with low characteristic impedance. 
     (2) Description of the Prior Art 
     There exists in the prior art a family of broadband quadrifilar helix antennas such as the antennas described in U.S. Pat. No. 6,246,379 (Josypenko), that have the characteristics of being broadband above a cut-in frequency, and having a low voltage standing wave ratio (VSWR) above a cut-infrequency about the characteristic impedance (Z 0 ) value of the antenna. Wide element quadrifilar helix antennas reduce the value of Z 0  to a practical lower limit of Z 0 =100 ohms, which can then feed into the Z 0 =100 ohms between the two center conductors of a one hundred eighty degree power splitter feeding a given bifilar. The wide element quadrifilar helix antenna taught in U.S. Pat. No. 6,246,379 (Josypenko), comprises two crossed bifilar helixes and a 50 ohm ninety degree power splitter feeding two 50 ohm one hundred eighty degree power splitters feeding their two 100 ohm outputs directly into the two crossed bifilar helixes making up the quadrifilar helix. The wide element quadrifilar helix antenna does not require a matching network. The antenna is directly fed via its power splitter feed network. 
     The broadband impedance properties exhibited by wide element quadrifilar helix antennas also apply to bifilar helixes, since the quadrifilar helix is an array of two crossed bifilar helixes. The bifilar helix is the basic building block of the quadrifilar helix. A difference in the characteristic impedance Z 0  between a wide element quadrifilar helix antenna (i.e., two crossed bifilars) and a wide element bifilar helix antenna is that when changing from two crossed bifilars to one, with the width of a bifilar element being the combined widths of the two quadrifilar elements it replaces, then the characteristic impedance is halved. 
     The halving of the characteristic impedance Z 0  is explained as follows with accompanying  FIGS. 1   a  and  1   b . Z 0  is calculated according to 
                 Z   0     =       L   C         ,         
where L is the series inductance per unit length of the helix and C is the shunt capacitance per unit length of the helix.  FIG. 1   a  shows a section  7  of quadrifilar helix unpitched the sources of capacitance per unit length of helix C along the helix length. The section is composed of sections  1 ,  2 ,  3  and  4  of the 4 elements of the helix of length  6  that is ⅛ wavelength or less, separated by small gaps G 12 , G 23 , G 34 , G 41 , all centered about helix axis  5 . The capacitance between radially opposite elements  1  and  3  is shown as C 13  between midpoints M 1  and M 3  of the element sections, capacitance C 24  exists between midpoints M 2  and M 4  of element sections  2  and  4 . Capacitance also exists between the elements at their gaps as C 12 , C 23 , C 34  and C 41  between element sections  1  and  2 ,  2  and  3 ,  3  and  4 , and  4  and  1 . Since the elements are much closer together at their gaps, the inter-gap capacitances are much larger than the radial capacitances. Thus, when finding the total capacitance between the midpoints of two radially opposite element sections, the radial capacitances C 13  and C 24  can be ignored. Thus, the capacitance between element sections  1  and  3  is the series capacitance of C 12  and C 23  in parallel with the series capacitance of C 41  and C 34 , or:
 
               C   Total     =       (     1       1     C   ⁢           ⁢   12       +     1     C   ⁢           ⁢   23           )     +     (     1       1     C   ⁢           ⁢   41       +     1     C   ⁢           ⁢   34           )             
with C 12 =C 23 =C 34 =C 41 =C from symmetry,
 
               C   Total     =       2       1   C     +     1   C         =       2     2   C       =     C   .               
This is the capacitance per unit length between either pair of radially opposite elements. When the quadrifilar helix is changed to a bifilar helix, gaps G 12  and G 34 , for example, are removed so that elements  1  and  2  combine to become the first element of the bifilar and elements  3  and  4  combine to become the second element of the bifilar. C 12  and C 34  are shorted out and disappear, so now the capacitance between only two element sections at new midpoints M 1 M 2 , and M 3 M 4  becomes: C Total =C 23 +C 41 =2C.
 
       FIG. 1   b  shows the quadrifilar elements E 1 , E 2 , E 3  and E 4  unwrapped and unpitched to more easily show the inductance per unit length L 1 , L 2 , L 3  and L 4  of the elements in section  7 . Due to symmetry, L 1 =L 2 =L 3 =L 4 =L. When the quadrifilar helix is changed to a bifilar case, element sections  1  and  2 , for example, combine to a first bifilar element and element sections  3  and  4  combine to a second bifilar element. Gaps G 12  and G 34  are filled and disappear, and now the ends of L 1  and L 2  are considered connected with virtual connections C 121  and C 122 ; ends of L 3  and L 4  are considered connected with virtual connections C 341  and C 342 . The inductance per unit length becomes the parallel combination of L 1  and L 2 , or L 3  and L 4 , or the inductance per unit length is 
     
       
         
           
             = 
             
               
                 1 
                 
                   
                     1 
                     
                       L 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                   
                   + 
                   
                     1 
                     
                       L 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                     
                   
                 
               
               = 
               
                 
                   1 
                   
                     2 
                     L 
                   
                 
                 = 
                 
                   
                     L 
                     2 
                   
                   . 
                 
               
             
           
         
       
     
     The characteristic impedance Z 0  for a loss less transmission line is found by 
               Z   0     =         L   C       .           
For the quadrifilar helix
 
               Z   0     =         L   C       .           
When the quadrifilar helix is converted to a bifilar helix, Z 0  becomes:
 
     
       
         
           
             
               Z 
               0 
             
             = 
             
               
                 
                   
                     L 
                     2 
                   
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     C 
                   
                 
               
               = 
               
                 
                   1 
                   2 
                 
                 ⁢ 
                 
                   
                     
                       L 
                       C 
                     
                   
                   . 
                 
               
             
           
         
       
     
     Thus Z 0  is halved when the quadrifilar helix becomes a bifilar helix. Note this is an approximation for the case when the gap width is small. An even more precise value of Z 0  can be obtained by adjusting the gap width, even if necessary to the point of negative gap width values, in which case the element edges overlap but do not touch. 
     A prior art bifilar helix antenna made of moderate width elements and a diameter of nine inches was investigated. To match the high characteristic impedance (Z 0 ) elements of the helix to 50 ohms, a quarter wavelength transmission line transformer is connected at the feed point of the bifilar antenna. The outer conductor connects to the feed point of the first element, the center conductor connects to the second element&#39;s feed point. The other end of the line is at 50 ohms over a certain bandwidth and connected to a 50 ohm cable. The whole length of the higher Z 0  cable connected to the 50 ohm cable follows the first element from its feed point to the unfed end fire of the antenna, exiting at a short placed across both elements at this end. Thus the bifilar antenna is used as an infinite balun to be able to bring a coaxial feed cable onto the antenna structure and eventually connect to its feed point. In the case of a bifilar helix antenna used as an infinite balun, the last quarter wavelength of cable before the feed point functions as a transformer that is a simple section of cable of Z 0  greater than 50 ohms. For optimal matching at a center frequency, the cable characteristic impedance is calculated according to the following equation: Z 0 =√{square root over (Z O feed cable *Z 0 antenna )}, wherein Z 0 feed cable =50 ohms. 
     Antenna patterns in the category of bifilar antennas are of cardioid shape, with only small differences in the shape between the bifilar antenna pattern and its corresponding quadrifilar antenna pattern. As stated above, the bifilar helix antenna can be made by simply removing one of the bifilars of a quadrifilar helix antenna. Among the differences between the two designs are that the bifilar will have poorer circular polarization and pattern symmetry in the azimuth plane, since there are only two versus four elements defining a circle. Also it has more undesirable backside radiation, since the arraying of two bifilar helixes in the quadrifilar helix helps reduce backside radiation. Also the bifilar must be fed in back fire mode and must be long enough to be a traveling wave antenna before unidirectional patterns of cardioid shape occur off of the fed end of the antenna. If the bifilar is too short, then lobes will come off of both ends creating a figure eight pattern along the antenna axis. A quadrifilar helix does not have this length requirement since it is an array of two interleaved bifilars. The phasing of the array can force unidirectionality by eliminating one of the two lobes of the figure eight pattern. 
     If lengthening of the filar elements is necessary to maintain the cardioid shaped pattern when changing from the quadrifilar case to the bifilar case, then there will be some change in the patterns. For low pitch angles (e.g. twenty to thirty degrees) the patterns become sharper. For high pitch angles (e.g. forty to fifty degrees) patterns will split more, which may require reducing the pitch angle if acceptable overhead patterns are desired. 
     SUMMARY OF THE INVENTION 
     It is a general purpose and object of the present invention to convert a direct fed wide element quadrifilar helix antenna to a direct fed bifilar helix antenna with approximately the same antenna pattern. 
     It is a further purpose and object to reduce the number of feed cables for use with an antenna from two to one. 
     It is a further purpose and object that a single cable feeds the antenna via an infinite balun that is formed by the antenna itself. 
     It is a further purpose and object to eliminate the need for power splitters. 
     It is a further purpose and object for the direct fed wide element bifilar helix antenna to have the practical lowest Z 0  value around 50 ohms, to be able to match the 50 ohm feed cable. 
     The above objects are accomplished with the present invention by removing one of the bifilars of a prior art quadrifilar helix antenna, lengthening the remaining bifilar as needed to obtain the desired unidirectional pattern, employing the bifilar as an infinite balun to bring a feed cable onto the antenna structure and eventually connect the feed cable to the antenna feed point, and widening the elements of the remaining bifilar such that the widths of the elements are as wide as practically possible before they touch and overlap (approximately 98.5% of the available width) thereby obtaining the practical lowest Z 0  value of approximately 50 ohms so that there is no need of a matching network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the invention and many of the attendant advantages thereto will be more readily appreciated by referring to the following detailed description when considered in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts and wherein: 
         FIG. 1   a  illustrates a cylindrical cross section of a quadrifilar helix antenna unpitched to show the sources of capacitance per unit length of element; 
         FIG. 1   b  illustrates the quadrifilar elements of the cross section of  FIG. 1   a  unwrapped and unpitched to more easily show the inductance per unit length of the elements; and 
         FIG. 2  illustrates the apparatus of the direct fed bifilar helix antenna of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 2 , there is illustrated the apparatus of the direct fed bifilar helix antenna  10 . In the figure, the antenna  10  at one end  122  consists of an optional support disc  12  made of a dielectric material at one end of an optional support cylinder  14  made of a dielectric material and whose other end  162  has another optional support disc  16  made of dielectric material at the other end of the optional support cylinder  14 . In an alternative embodiment where the metal parts of the antenna  10  are self supporting then the aforementioned optional support discs  14  and  16  and optional support cylinder  14  are not present. In an alternative embodiment, two insulating spacers  18  and  20  are bolted across the two bifilar elements of antenna  10  to hold the two bifilar elements in place relative to each other thereby providing minimum support. 
     At the feed end of the antenna where optional support disc  12  is located, the two radially opposite elements of the bifilar helix start as radial sections  22  and  24 . These sections  22  and  24  cover most of optional support disc  12 , except for a small gap  26  that separates them. The two radially opposite elements continue on optional support cylinder  14  as circumferential section  28  and circumferential section  30  approximately covering the entire cylinder circumference, and separated by gap  32  and gap  321 . In the alternative embodiment where there is no support cylinder  14 , insulating spacers, such as  182  and  202 , are placed along the length of helix  10  half way between the ends  122  and  162  of the antenna and bolted across gaps  32  and  321  between the circumferential sections  28  and  30 , to make the two sections a more solid structure, and to help prevent the circumferential sections  28  and  30  from unraveling or touching each other. For ease of viewing, the spacers have been drawn closer to end  162  in  FIG. 2 . Additional spacers may be added along the length of the gaps for further solidity. The circumferential sections  28  and  30  wrap about the cylinder length at pitch angle  34 . Cardioid antenna pattern shapes become broader as pitch angle  34  increases. If the element lengths become too long electrically and pitch angle  34  is large (roughly greater than or equal to forty degrees) the antenna patterns will start to split. 
     The radial sections  22  and  24  and the circumferential sections  28  and  30  of the bifilar elements are made of low loss conductive metal such as copper or silver. At the end  162  of optional support cylinder  14  the location of optional support disc  16 , the elements  28  and  30  are shorted by a metal disc  36  that is positioned on optional support disc  16 . Optionally, disks  36  and  16  may be combined as one disk if it is strong enough to support the antenna. In an alternative embodiment the short is a wide wire  38  and a section of the feed cable  44  connecting the ends and midpoints of circumferential sections  28  and  30 . In another alternative embodiment, the short is two wire shorts  40  and  42  that are placed across the gaps between the ends of the circumferential sections  28  and  30 . The feed cable  44  may be inserted onto the antenna  10  at one of these shorts, however, these types of shorts  40  and  42  are not optimal because it is preferable to insert the feed cable  44  onto the antenna  10  at a radio frequency point of zero that is at a symmetrical point on the antenna  10  somewhere on the axis of the antenna. Wire shorts  40  and  42  are not exactly at radio frequency points of zero, since they lie off axis. 
     The width (or circumference) of the elements is approximately 98.5% of the available width (or circumference), so that the antenna characteristic impedance is 50 ohms. The width of gaps  26 ,  32  and  321  comprise the remaining available width, which is 1.5%. Some adjustment of the gap width may be necessary for to obtain 50 ohms, since the impedance model discussed above is approximate. Also there is a small impedance dependence on pitch angle, and on the thickness of the bifilar elements  22  and  28 , and  24  and  30 . The edges of thicker elements will increase capacitance across the gaps and reduce the characteristic impedance. 
     The antenna is fed at the midpoints of the elements, on the radial sections  22  and  24 , on the axis of the antenna at feed point  46 . The feed point  46  is connected to a 50 ohm coaxial feed cable  44 , with the center conductor connecting to radial section  24  and the inside of the outer conductor connecting to radial section  22 . The feed cable  44  is snaked around the entire length of the antenna  10 , positioned at the centers of radial section  22  and circumferential section  28 , where its outer conductor is attached to the sections. It continues to the end of circumferential section  28  to a point  48  on metal disc  36  and continues to the center  50  of metal disc  36 , which is at a radio frequency zero (rf=0) point. The whole cable path from feed point  46  to the center  50  of metal disc  36  is an infinite balun, which allows the feed cable  44  to be introduced onto the antenna  10  and connect to the antenna&#39;s feed point  46 . At the center  50  of metal disc  36 , the feed cable  44  leaves the antenna for a section of length  52 . A radio frequency signal is applied to the antenna  10  at a point  54  on the end of cable  44 . The main beam of the pattern will come off of the feed point  46  end  122  of the antenna. 
     In an alternative embodiment, metal shorting disc  36  is removed and the antenna  10  is shorted just by the path established by the outside of the outer conductor of the section of feed cable  44  from point  48  to the center point  50  of the original metal disc  36  and by an added section of wide wire  38  of diameter similar to that of the cable from point  50  to the center of the edge of circumferential section  30  at  56 . In another embodiment, wherein wire shorts  40  and  42  are employed, instead of following a path from point  48  to point  50 , the feed cable  44  snakes from point  48  to wire short  40  and then leaves the antenna as a section of length of cable similar to section  52 . It is noted, however, that this is not the best method of feed the antenna  10 , since the feed cable  44  leaves antenna  10  at the radius of the antenna instead of at a symmetrical, on axis point. 
     In an alternative embodiment, the filar elements are made narrower so that a higher antenna Z 0  value results, so that the antenna can be matched to and fed with a higher Z 0  cable. For example, Z 0  can be raised to 75 ohms so that a 75 ohm cable can be used to feed the antenna. 
     The advantages of the antenna  10  of the present invention over prior art quadrifilar helix antennas is that the design is far less complex requiring no power splitters as opposed to three power splitters in the prior art antennas, only one versus two feed cables, and only two versus four antenna elements, while performing as a direct fed, 50 ohm broadband antenna with satellite coverage patterns. The advantage of the antenna  10  of the present invention over prior art bifilar helix antennas is there is no need for a matching transformer that may have limited bandwidth. 
     While it is apparent that the illustrative embodiments of the invention disclosed herein fulfill the objectives of the present invention, it is appreciated that numerous modifications and other embodiments may be devised by those skilled in the art. Additionally, feature(s) and/or element(s) from any embodiment may be used singly or in combination with other embodiment(s). Therefore, it will be understood that the appended claims are intended to cover all such modifications and embodiments, which would come within the spirit and scope of the present invention.