Abstract:
A quadrifilar antenna for use in satellite communications comprises four conductive elements arranged to define two separate helical pairs, one slightly differing in electrical length than the other, defined by a cylinder of constant radius supported by itself or by a cylindrical non-conductive substrate. The two separate helical pairs are connected to each other in such a way as to constitute the impedance matching, electrical phasing, coupling and power distribution for the antenna. In place of a conventional balun, the antenna is fed at a tap point on one of the conductive elements determined by an impedance matching network which connects the antenna to a transmission line. The matching network can be built with distributed or lumped electrical elements and can be incorporated into the design of the antenna.

Description:
BACKGROUND OF THE INVENTION 
     This invention generally relates to quadrifilar antennas used for radiating or receiving circularly polarized waves. More particularly, this invention relates to an improved quadrifilar antenna and its feed system for coupling signals of equal magnitude and 90 degrees out of phase to one end of the antenna, and to a method of manufacturing such an antenna. 
     It is well known that helical antennas comprising a plurality of resonant elements arranged around a common axis are particularly useful in ground links with orbiting satellites or in mobile/relay ground links with geosynchronous satellites. Due to the arrangement of the helical elements, the antenna exhibits a dome-shaped spatial response pattern and polarization for receiving signals from satellites. This type of antenna is disclosed in “Multielement, Fractional Turn Helices” by C. C. Kilgus in IEEE Transactions on Antennas Propagation, July 1968, pages 499 and 500. This paper teaches, in particular, that a quadrifilar helix antenna can exhibit a cardioid characteristic in an axial plane and be sensitive to circularly polarized emissions. 
     One type of prior art helical antenna comprises two bifilar helices arranged in phase quadrature and coupled to an axially located coaxial feeder via a split tube balun for impedance matching. While antennas based on this prior design are widely used because of the particular response pattern, they have the disadvantage that they are extremely difficult to adjust in order to achieve phase quadrature and impedance matching, due to their sensitivity to small variations in element length and other variables, and that the split tube balun is difficult to construct. As a result, their manufacture is a very skilled and expensive process. 
     Therefore, there is a need for a quadrifilar antenna having a predetermined input impedance which could be manufactured on a production basis without the need for adjustment and costly individual tuning. Further, there is a need to provide a quadrifilar antenna having a simplified feed arrangement that avoids the complexities of conventional folded, stepped or split shield baluns. 
     The subject invention herein solves all of these problems in a new and unique manner which has not been part of the art previously. Some related patents are described below: 
     U.S. Pat. No. 5,635,945 issued to McConnell et al on Jun. 2, 1993 
     This patent is directed to a quadrifilar helix antenna comprising four conductive elements arranged to define two separate helically twisted loops, one slightly differing in electrical length than the other, to define a cylinder of constant radius supported by itself or by a cylindrical nonconductive substrate. The two separate helically twisted loops are connected to each other in such a way as to constitute the impedance matching, electrical phasing, coupling and power distribution for the antenna. 
     U.S. Pat. No. 5,191,352 issued to S. Branson on Mar. 2, 1993 
     This patent is directed to a quadrifilar antenna comprising four helical wire elements shaped and arranged so as to define a cylindrical envelope. The helical wires are mounted at their opposite ends by first and second printed circuit boards having coupling elements in the form of plated conductors which connect the helical wires to a feeder or semi-rigid coaxial cable on the first board, and with each other on the second board. The conductor tracks are such that the effective length of one pair of helical wires and associated impedance elements is greater than that of the other pair of helical wires, so that phase quadrature is obtained between the two pairs. 
     U.S. Pat. No. 4,008,479 issued to V. C. Smith on Feb. 15, 1977 
     This patent is directed to a dual-frequency circularly polarized antenna. The antenna comprises a longitudinal cylindrical non-conductive member supported at its top by four conductors each extending transversely from a center coaxial line. Two sets of the antenna conductors are attached to the non-conducting cylinder in a configuration of equally longitudinally spaced spirals. The two sets of conductors are conductively connected by pins such that one set corresponds to a half wavelength at one frequency and the other set corresponds to a half wavelength at another frequency. 
     U.S. Pat. No. 3,623,113 issued to I. M. Falgen on Nov. 23, 1971 
     This patent is directed to a tunable helical monopole antenna. The tunable helical monopole antenna comprises a winding having both an upper portion and a lower portion which are symmetrically substantially identical to each other. Connected to each end of the winding halves are cylindrical terminal dipole elements and connected to these terminal elements are shorting fingers. By synchronously moving the shorting fingers, the respective helical windings are effectively shorten or lengthen for tuning purposes. 
     U.S. Pat. No. 5,255,005 issued to Terret et al. on Oct. 19, 1993 
     This patent is directed to a dual layer resonant quadrifilar helix antenna. The antenna comprises a quadrifilar helix formed by first and second bifilar helices positioned orthogonally and excited in phase quadrature. Additionally, a second quadrifilar helix is coaxially and electromagnetically coupled to a first quadrifilar helix. 
     U.S. Pat. No. 4,148,030 issued to P. Foldes on Apr. 3, 1979 
     This patent is directed to a combination helical antenna comprising a plurality of tuned helical antennas which are coaxially wound upon a hollow cylinder, whereby the antennas are collocated. The antenna further comprises a printed circuit assembly having thin metal dipoles of the type used in a microwave strip line. The thin metal dipoles are resonating elements that are coupled to each other in a manner similar to end-fire elements of a microstrip filter. 
     While the basic concepts presented in the aforementioned patents are desirable, the apparatus employed by each to produce a quadrifilar antenna are mechanically far too complicated to render them as an inexpensive means of achieving an antenna having a predetermined input impedance which could be manufactured on a production basis without the need for adjustment and costly individual tuning and still present desired radiation characteristics during operation. 
     SUMMARY OF THE INVENTION 
     A quadrifilar antenna for use in satellite communications comprises four conductive elements arranged to define two separate helical pairs with both pairs being open circuited at one end, one pair slightly differing in electrical length than the other, to define a cylinder of constant radius supported by itself or by a cylindrical non-conductive substrate. The two separate helical pairs are connected to each other in such a way as to constitute the impedance matching, electrical phasing, coupling and power distribution for the antenna. In place of a conventional balun, the antenna is fed at a tap point on one of the conductive elements determined by an impedance matching network which connects the antenna to a transmission line. The matching network can be built with distributed or lumped electrical elements and can be incorporated into the design of the antenna. 
     Therefore, it is an object of the present invention to provide a simple matching network where the inductance of the conductor leading to the tap point is tuned out by a capacitor connected to the transmission line used to transfer radio frequency signals to and from the antenna. 
     An object of the present invention is to provide a quadrifilar antenna formed by a pair of helical elements where the coupling between the pair of helical elements is provided by a shared common current path. 
     A further object of the present invention is to have a quadrifilar antenna which has a simple feed method that does not require the use of conventional folded, stepped or split shield baluns. 
     Another object of the present invention is to provide a quadrifilar antenna formed by printed circuit boards which can be relatively accurately formed with predetermined shapes and dimensions, such that relatively little, if any, adjustment is required to obtain an antenna having the required electrical characteristics. 
     Yet, still another object of the present invention is to have a quadrifilar antenna which can be mass-produced to precise dimensions with high reproducibility of electromagnetic characteristics. 
     Still, yet another object of the present invention is to provide a quadrifilar antenna which is especially simple in construction, particularly light weight and compact in design. 
     A further object of the present invention is to provide a low cost antenna having a quasi-hemispherical radiation pattern of the type formed by two bifilar helices used in ground and orbital satellite telecommunication links or in mobile relay telecommunication links with geosynchronous satellites. 
     Another object of the present invention is to provide a method of making a radio frequency antenna having a plurality of helical elements formed through the use of alignment tabs for ease and accuracy in manufacturing. 
     Accordingly, it is an object of the present invention to provide an effective, yet inexpensive and relatively mechanically unsophisticated quadrifilar antenna, which is rugged yet lightweight, easily carried and used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above, as well as other, advantages of the present invention will become readily apparent to those skilled in the art from the following detailed descriptions of the preferred embodiment when considered in light of the accompanying drawings in which: 
     FIG. 1 is a perspective view of a quadrifilar helix antenna in accordance with the present invention; 
     FIG. 2 is a perspective view of one preferred embodiment of the quadrifilar helix antenna in accordance with the present invention; 
     FIG. 3 is a plan view of the conductive elements shown in FIG. 2; 
     FIG. 4 is a top plan view of one side of a first printed circuit board of the antenna of the present invention; 
     FIG. 5 is a top plan view of a second side of the printed circuit board shown in FIG. 4; 
     FIG. 6 is a perspective view of another preferred embodiment of the quadrifilar helix antenna in accordance with the present invention; 
     FIG. 7 is a top plan view of one side of a first printed circuit board of the antenna shown in FIG. 6; 
     FIG. 8 is a top plan view of a second side of a first printed circuit board of the antenna shown in FIG. 6; 
     FIG. 9 is a top plan view shown in FIG. 3 displaying a method of manufacturing the antenna; and 
     FIG. 10 is a top plan view shown in FIG. 4 displaying a method of manufacturing the antenna; and 
     FIGS. 11,  12 ,  13  respectively represent the radiation pattern and value of VSWR of an antenna built in accordance with the teachings of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings wherein like reference numerals refer to like and corresponding parts throughout, the quadrifilar antenna in accordance with the present invention is generally indicated by numeral  10 . Referring to FIG. 1, the quadrifilar antenna  10  comprises a generally elongated non-conducting cylindrical support tube  12  having four conductive elements  14 ,  16 ,  18  and  20  supported on an outer surface of tube  12  so as to make the antenna  10  right-hand or left-hand circularly polarized. Although not shown, it should be envisioned that the elements  14 ,  16 ,  18  and  20  could be self-supporting without tube  12  by the use of rigid wire or could be arranged against the inner surface of tube  12 . 
     Referring once again to FIG. 1, a first helical pair is formed by elements  14  and  18  and equal conductors  40  which are slightly longer than a second helical pair formed by elements  16  and  20  and equal conductors  42 . As shown in FIG. 1, the first and second helical pairs are not connected at one end, thereby forming an electrical open circuit. In this configuration, the first and second helical pair have two different electrical lengths translating into two different resonant frequencies which are chosen by design to result in an electrically 90 degree phase difference between the currents induced in each helical pair thus maintaining phase quadrature. A common section  38  is shared at one end by each helical pair and provides the coupling from the driven helical pair formed by elements  16  and  20  and equal conductors  42  to the other helical pair formed by elements  14  and  18  and equal conductors  40 . 
     Turning once again to FIG. 1, a coaxial transmission line  36  has its inner conductor  28  connected at one end  44  of a capacitor  46  whose other end  48  connects through a conductor  26  to a tap point  25  on element  20  to effectively impedance match antenna  10  without the use of a conventional balun. The placement and value of capacitor  46  and length and tap point of conductor  26  are predetermined from the desired input impedance presented by transmission line  36 . Although transmission line  36  is shown as coaxial, it may be any variety of transmission lines used to carry radio frequency signals. Therefore, the capacitor  46  and conductor  26  are used to tune out the reactance and inductance of the antenna  10  at the antenna frequency. An outer conductor  30  of transmission line  36  connects to the midpoint of common conductor section  38 . The shape of the antenna  10  may be cylindrically round or square or tapered without altering the intent of the invention. 
     It is understood by those familiar with the art that any method of feeding the antenna  10  with a variety of unbalanced transmission lines in addition to coaxial, such as microstrip or strip line can be accomplished by connecting the signal line to the capacitor  46  at capacitor end  44  and the ground or signal return side to the midpoint of shared common segment  38 . 
     It is also understood by those skilled in the art, that a transmission line is a common and practical way of transferring radio frequency electrical signals between circuits and antennae and is used herein as an example of how the invention can be utilized. However, the invention described here is placed very near to nearby circuits or adjacent to printed circuit boards directly where the coupling of signals to the antenna can be accomplished without the need for a conventional transmission line. 
     Referring now to the drawings, and more particularly to FIGS. 2 and 3, another preferred embodiment of the quadrifilar antenna  10  comprises a generally elongated longitudinal cylindrical substrate  12  having the four conductive elements  14 ,  16 ,  18  and  20  supported on its outer surface with the four conductive elements  14 ,  16 ,  18  and  20  not connected at one end and having mounted a printed circuit board  24  at the other end. As shown in FIG. 2, the conductive elements  14 ,  16 ,  18  and  20  respectively, are arranged as helical elements around the outer surface of the substrate  12  so as to make the antenna  10  right-hand circularly polarized. Although not shown, it should be envisioned that the antenna  10  could similarly be left-hand circularly polarized. 
     In the preferred embodiment, the cylindrical substrate  12  is made from a non-conductive material such as glass, fiberglass or the like, having a dielectric constant that corresponds to the width, length and material of the conductive elements  14 ,  16 ,  18  and  20  wherein each helical pair is preferably in a range of a quarter wavelength of the desired resonant frequencies. Using higher dielectric materials can result in significant shortening of the physical antenna structure. The cylindrical structure  12  can be formed as a tube or a flat structure rolled into a tubular shape and may have a cross section which is either circular or square as will be more fully described below. However, it should be well understood that the substrate or material can be varied without deviating from the teachings of the subject invention. The conductive elements  14 ,  16 ,  18  and  20 , respectively, may be made from copper, silver or like metals and are metal plated onto the substrate  12  by any type of coating technique known in the metallic plating arts. 
     Turning now to FIG. 3, the conductive elements  14 ,  16 ,  18  and  20 , respectively, are shown in a plane in order to further distinguish certain characteristics unique to the subject invention. As shown in FIGS. 2 and 3, the conductive elements  14 ,  16 ,  18  and  20 , respectively, are parallel and substantially equally transversely spaced from each other when plated onto the substrate  12 . As shown in FIG. 3, conductive element  18  is slightly longer then conductive elements  14 ,  16  and  20  wherein the length of conductive element  18  is predetermined from the desired input impedance and results in the antenna  10  being manufactured on a production basis without the need for adjustment and costly individual tuning as will be more fully described below. 
     Referring now to FIGS. 4 and 5, there is shown a first side  32  and second side  34  of the printed circuit board  24 , which is used to perform both the power distribution and impedance matching for the antenna  10 . The printed circuit board  24  comprises microstrip portion  29  over a ground conductor  30  shown in FIG. 5 on the second side of the board  24 , wherein the microstrip structure of  29  and  30 , respectively, are electrically coupled and connected to each other to form a ground return path  36 . 
     Turning now to FIG. 4, the transmission line  36  of the board  24  terminates into the midsection of generally rectangular portions  38 , the common section coupling the helical pairs, centered on the board  24 . The rectangular portions  38  have a first set  40  and a second set  42  of connecting lines, each set of connecting lines  40  and  42 , being electrically connected to a respective one of the conducting elements  14 ,  16 ,  18  and  20 , serving the same purpose as described in FIG.  1 . For electrical characteristic purposes, such as frequency bandwidth, the first set  40  of the connecting lines have a different electrical length, translating into two different resonant frequencies, than the second set  42  of connecting lines, and is a matter of design choice. Even though in the preferred embodiment, the connecting lines are shown as straight, it may be envisioned that the connecting lines may also meander to obtain longer electrical lengths. 
     Referring once again to FIG. 4, on the first side  32  of the board  24  is formed a first capacitive element  48  separated from the rectangular portions  38  and is connected to one of the connecting lines  42  through a feed line  26  to a tap point  25  which connects to conductive element  20 . Referring now to FIG. 5, on the second side  34  of the board  24  is a second capacitive element  44 . Elements  44  and  48  on each side of board  24  form a parallel plate capacitor whose function is the same as capacitor  46  in FIG.  1 . As shown in FIGS. 4 and 5, and as mentioned above, the feed line  26  supported by the board  24  is electrically connected to the conductive band  20  at the tap point  25  and is electrically connected to the first capacitive element  48  at the other end. The tap point  25  is connected to one of the second set  42  of connecting lines. The feed line  26  has a predetermined shape and position to impedance match the antenna  10  in association the length of conductive element  20  and with first capacitive element  48  which electrically couples to the second capacitive element  44  wherein the first and second capacitive elements,  48  and  44  respectively, have predetermined dimensions for matching out the inductance of the feed line  26  and the reactance of antenna  10 . 
     Although not shown, it may be envisioned that the quadrifilar antenna described above may be mounted to a printed circuit board electronic device by placing the second side  34  of the board  24  flush with the circuit board electronic device between the ground conductor  30  and second capacitive element  44  and electrically connecting the ground conductor  30  and second capacitive element  44  to the printed board electronic device by soldering or any electrical attachment means known in the arts. It should be appreciated that the antenna of the present invention eliminates the need for a conventional type transmission line between the antenna  10  and printed board electronic device. 
     A second preferred embodiment is shown in FIGS. 6 through 8 having the same conductive elements and feed structure described above with the addition of a transmission line  36 . The printed circuit board  24  now comprises a microstrip line  28  over an elongated ground conductor  30  formed on the other side of the board  24  wherein the microstrip structure of  28  and  30 , respectively, are electrically coupled to each other to form the microstrip transmission line  36  which serves the same purpose as transmission line  36  in FIG.  1 . As shown in FIGS. 7 and 8, the microstrip structure  30  of transmission line  36  inwardly tapers to connect to the rectangular portions  38  and microstrip structure  28  connects to second capacitive element  44  on the second side  34  of the board  24 , wherein the transmission line  36  is tapered solely for mechanical reasons for bending the flexible printed circuit board  24  away from the conductive elements  14 ,  16 ,  18  and  20 , respectively, and further does not interfere with the antenna radiation pattern. Typically, in the preferred embodiment the transmission line  36  will have an impedance of 50 ohms allowing the antenna  10  to be fed by a BNC connector or coaxial connector. 
     A method of manufacturing the antenna will now be described with references to FIGS. 9 and 10. Referring to FIG. 9, the substrate  12  having the four conductive elements  14 ,  16 ,  18  and  20  has a first extending tab portion  50  at one end and defines a first alignment slot  52  at the opposite end. In production the location of alignment slot  52  is such that the substrate  12  is rolled so that extending tab portion  50  is inserted into alignment slot  52  thereby retaining the substrate  12  into a cylindrical or tubular shape defining the proper radius for mounting the substrate  12  to printed circuit board  24  while simultaneously maximizing the electrical performance of the antenna. 
     Referring now to FIG. 10, circuit board  24  defines a second pair of alignment slots  54  and  56  at its sides to receive a second pair of alignment tabs  58  and  60  shown at the bottom of substrate  12  shown in FIG.  9 . Second alignment slot  54  is slightly longer then second alignment slot  56  and second alignment tab  58  is slightly longer then second alignment tab  60  so that when substrate  12  is placed upon board  24  and second alignment tabs  58  and  60  are inserted into second alignment slots  54  and  56 , the conductive element  20  is located at tap point  25 . In this configuration the antenna can now be soldered together. Lastly, referring to FIG. 10, the circuit board  24  additionally defines a pair of alignment indents  62  for use in locating and mounting the antenna against a printed circuit board electronic device. 
     FIG. 11 illustrates the radiation pattern of an antenna built in accordance with the present invention, obtained in the elevational plane at an approximate frequency of 1575 Mhz. A seen by the pattern, the axial ratio is 1.8 db at zenith, and the maximum circular polarized gain is 2.1 dBic. FIG. 12 illustrates the 80 degree off zenith conic pattern of the same antenna, wherein the maximum gain is shown at 130 degrees having an axial ratio of 2.8 dB and a circular polarized gain of 3.3 dBic. Lastly, FIG. 13 illustrates the impedance and return loss for this antenna with a VSWR of 1.15:1. The above data indicates that the antenna of the present invention performs comparably with conventionally designed quadrifilars. 
     Furthermore, since the antenna is practically matched at 50 ohms around the two resonance frequencies, the feed line in association with the printed circuit technology does not necessitate any specific assembly for additional matching. This frees the antenna from the drawbacks of conventional quadrifilar antenna designs. 
     There has been described and illustrated herein, an improved quadrifilar antenna formed by printed circuit boards which can be relatively accurately formed and mass produced with predetermined shapes and dimensions, such that relatively little, if any, adjustment is required to obtain an antenna having high reproducibility of electromagnetic characteristics. 
     While particular embodiments of the invention have been described, it is not intended that the invention be limited exactly thereto, as it is intended that the invention be as broad in scope as the art will permit. The foregoing description and drawings will suggest other embodiments and variations within the scope of the claims to those skilled in the art, all of which are intended to be included in the spirit of the invention as herein set forth.