FIELD OF THE INVENTION 
     The present invention relates generally to an antenna system with broad-band operating characteristics and, more particularly, to a quadrifilar helix antenna for use in the Sirius Satellite Radio (2320-2332.5 MHz), XM Satellite Radio (2332.5-2345 MHz) and the like. 
     BACKGROUND OF THE INVENTION 
     An active quadrifilar helix (QFH) antenna is currently used in mobile satellite communication. QFH antennas are known in the art. As disclosed in “Fixed and Mobile Terminal Antennas”(by A. Kumar, Artech House, 1991, Chapter 5, pp.163-174), a QFH antenna comprises four helices, circumferentially and equally spaced on a dielectric cylinder or some dielectric disk support and fed with equal amplitude signals driven in phase quadrature. As shown in FIG. 1, the antenna requires a phasing network or balun, which connects to the four helices for providing signals having a 0°, 90°, 180° and 270° phase relationship to the helices and for matching the impedance of the helices to a coaxial feed line. The quadrifilar helix can be fed from the bottom, as shown in FIG.  1 . Currently, the phasing network for feeding the helices incorporates multiple 90° hybrids, as shown in “Modified Quadrifilar Helix Antennas for Mobile Satellite Communication” (1998 IEEE AP-S Conference on Antennas and Propagation for Wireless Communications, pp.141-144). A number of such hybrids are commercially available in both discrete form and single chip form. In the single chip form, there are four outputs extended from the chip for providing electrical connections to the helices. The insertion loss of the feed circuit incorporating the single chip is typically in the 0.75 to 1.25 dB range. Similar insertion loss is also found on the discrete hybrids. This level of insertion loss is unacceptable for use in either the Sirius or the XM systems. 
     Alternatively, the quadrifilar helix can be constructed as two orthogonally arranged bifilar helical antennae to be fed from the top, as shown in FIG.  2  and disclosed in “Fixed and Mobile Terminal Antennas”(by A. Kumar, Artech House, 1991, Chapter 5, p.168). As shown, the helix is fed from the top by running two coaxial cables to the lower end of the helices so that the bifilar antennae can be phased by a single hybrid. The high insertion loss, in this case, is mostly due to the length of the coaxial cables. Such an quadrifilar antenna is also unacceptable for use in the Sirius and XM systems. 
     It is, therefore, desirable to provide a phasing network, wherein the insertion loss can be reduced so that they can be used with the Sirius, XM and similar systems. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a radio antenna operable in the frequency range of Sirius and XM systems and the like, wherein in the insertion loss is greatly reduced. 
     It is another object of the present invention to provide a radio antenna system based on the known quadrifilar helix which is fed from baluns, wherein the baluns are simple and costeffective. 
     Thus, the first aspect of the present invention is a method of feeding a quadrifilar antenna having four helical arms circumferentially and equally spaced on a dielectric cylinder, wherein the arms have electrically connected first ends and separated second ends located on different quadrants of a circle. The method comprises the steps of: 
     providing a first balun having two feed points located on the opposite quadrants of the circle for feeding two of the helical arms, and 
     providing a second balun having two feed points located on the different opposite quadrants of the circle for feeding the other two of the helical arms. 
     Preferably, the first balun comprises: 
     a dielectric substrate having a first side and an opposing second side; 
     two electrically conductive planes located on the first side for separately providing the two feed points of the first balun; and 
     a first feed line located on the second side for electromagnetically coupling the electrically conductive planes of the first balun for providing a 180° phase differential between the two feed points of the first balun, and 
     the second balun comprises: 
     a dielectric substrate having a first side and an opposing second side; 
     two electrically conductive planes located on the first side for separately providing the two feed points of the second balun; and 
     a second feed line located on the second side for electromagnetically coupling the electrically conductive planes of the second balun for providing a 180° phase differential between the two feed points of the second balun. 
     Preferably, the method also comprising the step of combining the first and second feed lines at a common feeding point on a combiner, wherein the combiner has means for providing a 90° phase differential between the first and second feed lines. 
     The second aspect of the present invention is a radio antenna system based on a quadrifilar antenna having four helical arms circumferentially and equally spaced on a dielectric cylinder, wherein the arms have electrically connected first ends and separated second ends located on different quadrants of a circle. The antenna system comprises: 
     a first balun having two feed points located on opposite quadrants of the circle for feeding two of the helical arms, and 
     a second balun, orthogonally arranged relative to the first balun, wherein the second balun has two feed points located on different opposite quadrants of the circle for feeding the other two of the helical arms. 
     Preferably, the first balun comprises: 
     a dielectric substrate having a first side and an opposing second side; 
     two electrically conductive planes located on the first side for separately providing the two feed points of the first balun; and 
     a first feed line located on the second side for electromagnetically coupling the electrically conductive planes of the first balun for providing a 180° phase differential between the two feed points of the first balun, and 
     the second balun comprises: 
     a dielectric substrate having a first side and an opposing second side; 
     two electrically conductive planes located on the first side for separately providing the two feed points of the second balun; and 
     a second feed line located on the second side for electromagnetically coupling the electrically conductive planes of the second balun for providing a 180° phase differential between the two feed points of the second balun. 
     Preferably, the antenna system also comprises a single combiner for electrically connecting the first feed line and the second feed line at a common feed point, wherein the single combiner has means for providing a 90° phase differential between the first and second feed lines. 
     The present invention will become apparent upon reading the description taken in conjunction with FIGS. 3 to  6   b.   
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic representation illustrating a prior art quadrifilar antenna system. 
     FIG. 2 is a diagrammatic representation illustrating another prior art quadrifilar antenna system. 
     FIG. 3 is a perspective view illustrating the radio antenna system, according to the present invention. 
     FIG. 4 a  is a diagrammatic representation illustrating the first side of the first balun. 
     FIG. 4 b  is a diagrammatic representation illustrating the second side of the first balun. 
     FIG. 5 a  is a diagrammatic representation illustrating the first side of the second balun. 
     FIG. 5 b  is a diagrammatic representation illustrating the second side of the second balun. 
     FIG. 6 a  is a diagrammatic representation illustrating the first side of the combiner board. 
     FIG. 6 b  is a diagrammatic representation illustrating the second side of the combiner board. 
     FIG. 7 is a diagrammatic representation illustrating a prior art balun with two feed points. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment of the radio antenna system  1  of the present invention is shown in FIG.  3 . The antenna system  1  includes a quadrifilar helix antenna  10 , a first balun  20 , a second balun  40  and a combiner board  60 . Quadrifilar helix antennae are known in the art and, therefore, they are not part of the present invention. As shown in FIG. 3, the quadrifilar antenna  10  has four helical arms  11 ,  12 ,  13 ,  14  circumferentially and equally spaced on a dielectric cylinder  18 . The arms  11 ,  12 ,  13 ,  14  are electrically connected at a common point  100  at the first ends  101  of the arms. The second ends  103  of the arms are separately located at different quadrants of a circle  102 . The first balun  20  and the second balun  40  are orthogonally arranged under the circle  102  for feeding the quadrifilar antenna  10  at the second ends of the helices with signals with equal amplitudes but different phases. In particular, arms  11  and  13  are fed by the first balun  20  and arms  12  and  14  are fed by the second balun  40 . 
     As shown in FIGS. 4 a  and  4   b , the first balun  20  is printed on a dielectric substrate  120  which has a first side  22  and an opposing second side  23 . The substrate  120  has a slot  121  to allow the second balun  40  to be arranged orthogonally to the first balun  20 . As shown in FIG. 4 a , the first balun  20  has two electrically conductive planes  33 ,  34  separately located on different sides of the slot  121 . The conductive plane  33  has an upper tip  35  and a lower tip  37 . The conductive plane  34  has an upper tip  36  and a lower tip  38 . The upper tips  35 ,  36  of the conductive planes  33 ,  34  are electrically connected to opposing arms  11 ,  13  of the quadrifilar antenna  10  for feeding. The opposing arms  11 ,  13  are fed from the first balun  20  with signals having a 180° phase differential. As shown in FIG. 4 b , a feed line  24  is located on the first side  22  having an inner section  25  substantially aligned with the conductive plane  34 . The feed line  24  has an outer section  27 , which is substantially aligned with the conductive plane  33 . The feed line  24  also has an extended section  26  for connecting the outer section  27  to the inner section  25  so that the signals fed to the opposing arms  11  and  13  have a 180° phase differential when the conductive planes  33  and  34  are electromagnetically coupled by the feed line  24 . The feed line  24  has a terminal end  28 . 
     Similarly, the second balun  40  is printed on a dielectric substrate  140 , which has a first side  42  and an opposing second side  43 , as shown in FIGS. 5 a  and  5   b . The substrate  140  has a slot  141  complimentary to the slot  121  of the substrate  120  to allow the second balun  40  to be arranged orthogonally to the first balun  20 . As shown in FIG. 5 a , the second balun  40  has two electrically conductive planes  53  and  54  separately located on different sides of the slot  141 . The conductive plane  53  has an upper tip  55  and a lower tip  57 . The conductive plane  54  has an upper tip  56  and a lower tip  58 . The upper tips  55 ,  56  of the conductive planes  53 ,  54  are electrically connected to opposing arms  12  and  14  of the quadrifilar antenna  10  for feeding. The opposing arms  12  and  14  are fed from the second balun  40  with signals having a 180° phase differential. As shown in FIG  5   b , a feed line  44  is located on the first side  42  having an inner section  45  substantially aligned with the conductive plane  54 . The feed line  44  has an outer section  47  substantially aligned with the conductive plane  53 . The feed line  44  also has an extended section  46  for connecting the inner section  45  to the outer section  47  so that the signals fed to the opposing arms  12  and  14  have a 180° phase differential when the conductive planes  53  and  54  are electromagnetically coupled by the feed line  44 . The feed line  44  has a terminal end  48 . 
     The feed lines  24  and  44  are electromagnetically combined in such a way that the phase relation between the adjacent arms among arms  11 ,  12 ,  13  and  14  is 90° apart. For example, the phase relation in the arms  11 ,  12 ,  13  and  14  can be expressed as 0°, 90°, 180° and 270°, or 0°, −90°, −180° and −270°. As shown in FIGS. 6 a  and  6   b , the combiner board  60  has an upper side  62  and a lower side  63 , and four slots  81 ,  82 ,  83  and  84  for mounting the first balun  20  and second balun  40 . As shown in FIG. 6 a , a shorter conductive line  74  and a longer conductive line  72  are used to separately provide electrical connections to the inner section  25  of the feed line  24  on the first balun  10  and inner section  45  of the feed line  44  on the second balun  40 . The conductive lines  72  and  74  are jointed at a common feed point  76 . The conductive line  72 , in terms of phase shift, is 90° longer than the conductive line  74 . As shown in FIG. 6 b , the lower side  63  has a common ground plane  78  for electrically connecting the conductive planes  33 ,  34 ,  53  and  54  at the lower tips  37 ,  38 ,  57  and  58 . 
     Preferably, the first and second baluns  20 ,  40  are provided as printed circuits on dielectric substrates. As described in conjunction with FIGS. 4 a - 5   b , the feeding of the quadrifilar antenna  10  from the first and second balun  20 ,  40  is efficient in that the separation between the baluns  20 ,  40  and the second ends  103  of the helical arms  11 ,  12 ,  13 ,  14  is short. Thus, the insertion loss is significantly reduced. It has been found that the insertion loss in the antenna as system of the present invention can be reduced to the 0.2-0.4 dB range. 
     It should be noted that the shape of the conductive planes  33 ,  34 ,  53 ,  54  and the shape of the feed lines  24 ,  44  can be changed, while the phase relationship in the signals fed to the helical arms can be maintained. Similarly, the arrangement of the conductive lines  72 ,  74  on the combiner board  60  can also be changed without altering the phase relationship among the helical arms. 
     It should also be noted that, the quadrifilar antenna  10 , as described in conjunction with FIG. 3, is provided on a dielectric cylinder. However, it is not necessary to have such a dielectric cylinder for support. The quadrifilar antenna is well known in the art. Furthermore, a balun provided on a printed circuit is also known in the art. For example, a prior art balun with two feed points, as shown in FIG. 7, is disclosed in “A Printed Circuit Balun for Use with Spiral Antennas” (R. Bawer and J. J. Wolfe, IRE Transactions on Microwave Theory and Techniques, May 1960, pp.319-325). However, the balun, as shown in FIG. 7, cannot be used for the quadrifilar antenna system without modification. The subject matter of the present invention is the arrangement of the baluns in relation to the quadrifilar antenna, the use of a single combiner for providing the necessary phase differential. The subject matter of the present invention is a method of feeding the helical arms of a quadrifilar antenna in a low insertion loss fashion. 
     Thus, although the invention has been described with respect to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and various other changes, omissions and deviations in the form and detail thereof may be made without departing from the spirit and scope of this invention.
A radio antenna system having a quadrifilar antenna having four helical arms and a method of feeding the arms. The antenna system comprises a first balun having a feed line and two feed points with 180deg phase differential therebetween for feeding one opposite arm pair, and a second balun having a feed line and two feed points with 180deg phase differential therebetween for feeding the other opposite arm pair. The two feed lines are combined in a single combiner, which provides a 90deg phase differential between the feed lines.