Abstract:
A quadrifilar helix antenna of the present invention is used with a wireless device to transmit and receive signals. The antenna includes a plurality of helical antenna elements and an impedance transformer electrically connected to the antenna elements and connectable to the wireless device. The impedance transformer may be comprised of either a planar transmission line ¼ wave impedance transformer or a pi-network impedance transformer. The antenna may also be comprised of dual band quadrifilar helix antenna for transmitting or receiving two different frequency bands.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation application of Petitioners&#39; earlier application Ser. No. 09/160,481 filed Sep. 25, 1998, entitled ANTENNA FOR PERSONAL MOBILE COMMUNICATIONS OR LOCATING EQUIPMENT. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to wireless radio equipment. More particularly, though not exclusively, the present invention relates to a method and apparatus for providing improved quadrifilar helix antennas. 
     2. Problems in the Art 
     Some antennas, such as satellite antennas, require circular polarization because the orientation of the user with respect to the satellite is not usually predetermined. Circular polarization is typically independent of orientation. Few antennas are suitable for circular polarization, especially for use with handheld wireless equipment. Three types of prior art circularly polarized antennas include turnstile antennas, patch antennas, and axial mode helical antennas. The turnstile and patch antennas are riot suitable for use with handheld equipment because the antennas are larger than desired. Axial mode helical antennas are not suitable for use with handheld equipment because of the excessive diameter of the antenna. 
     The most desirable type of circularly polarized antenna is a quadrifilar helix antenna. To meet beam width and size requirements for personal satellite communications, quadrifilars are made with ½ turn, ¼ wave elements. Hybrid power dividers are required to establish the phase between elements of the quadrifilar helix antenna. However, hybrid power dividers require more space than is desirable. Therefore, there is need for a quadrifilar helix antenna which does not require hybrid power dividers. 
     FEATURES OF THE INVENTION 
     A general feature of the present invention is the provision of a method and apparatus for providing an improved quadrifilar helix antenna which overcomes problems found in the prior art. 
     A further feature of the present invention is the provision of a method and apparatus for providing an improved quadrifilar helix antenna requiring no hybrid power dividers. 
     Further features, objects, and advantages of the present invention include: 
     A method and apparatus for providing an improved quadrifilar helix antenna which includes a planar transmission line ¼ wave impedance transformer as a matching circuit. 
     A method and apparatus for providing an improved quadrifilar helix antenna which includes a pi-network impedance transformer as a matching circuit. 
     A method and apparatus for providing an improved antenna having dual quadrifilar helix antennas and a bandpass filter for isolation. 
     A method and apparatus for providing an improved antenna having dual quadrifilar helix antennas and pi-network impedance transformers for isolation. 
     A method and apparatus for providing an improved quadrifilar helix antenna including parallel resonant circuits in series with each helical element. 
     These as well as other features, objects and advantages of the present invention will become apparent from the following specification and claims. 
     SUMMARY OF THE INVENTION 
     A quadrifilar helix antenna for a wireless device is used with a wireless device to transmit or receive signals. The invention is comprised of a plurality of helical antenna elements and an impedance transformer electrically connected to the antenna elements and connectable to the wireless device. The impedance transformer may optionally be comprising of a planar transmission line ¼ wave impedance transformer or a pi-network impedance transformer. The antenna may also be comprised of dual band quadrifilar helix antenna for transmitting or receiving two different frequency bands. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a quadrifilar helix antenna of the present invention having ¼ wave helical elements and a ¼ wave transformer; 
     FIG. 2 illustrates a quadrifilar helix antenna of the present invention having ¼ wave helical elements and a pi-network impedance transformer; 
     FIG. 3 illustrates a quadrifilar helix antenna of the present invention having ¼ wave helical elements on two quadrifilars, two band pass filters, and two ¼ wave transformers; 
     FIG. 4 is a side view of the antenna shown in FIG. 3; 
     FIG. 5 illustrates a quadrifilar helix antenna of the present invention having ¼ wave helical elements on two quadrifilars with two pi-network impedance transformers; 
     FIG. 6 is a side view of the antenna shown in FIG. 5; and 
     FIG. 7 illustrates a dual band quadrifilar helix antenna of the present invention having a single quadrifilar for two frequency bands. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will be described as it applies to its preferred embodiment. It is not intended that the present invention be limited to the described embodiment. It is intended that the invention cover all alternatives, modifications, and equivalencies which may be included within the spirit and scope of the invention. 
     The present invention relates to a technique for feeding a quadrifilar helix antenna which reduces the size of the antenna components by eliminating the need for hybrid circuits. 
     FIG. 1 shows a left-hand circularly polarized (per IEEE standard) ½ turn, ¼ wave quadrifilar helix antenna  10 . The quadrifilar helix antenna  10  includes four helical elements  12 . A quadrifilar having ½ turn, ¼ wave elements is the smallest practical length achievable. As the diameter of a quadrifilar helix antenna decreases, the input resistance decreases, necessitating the inclusion of a quarter-wave transformer (described below). The helical elements  12  are approximately ¼ wavelength long and are wound around a coil form  14  made of low loss dielectric material. The leading helical elements are shorter than the lagging helical elements to provide a 90° phase shift between leading and lagging. The leading helical elements are shorter than the lagging helical elements by only approximately 0.015 inches. The 90° phase shift between each set of bifilars provides a cardioid pattern from the two sets of bifilar helix antenna elements  12  which comprise the quadrifilar. By switching which elements lead or lag, the direction (up or down) of the cardioid can be controlled. Examples of helical elements suitable as the elements  12 , are described in detail in U.S. Pat. No. 5,541,617 and in the publication “Reflections: Transmission Lines and Antennas”, published by American Radio Relay League, 1990, Chapter 22, which are incorporated by reference herein. 
     The helical elements  12  are connected to a flexible planar transmission line ¼ wave transformer  16  formed on a flexible circuit board  17 . The transformer  16  is comprised of a conventional ¼ wave transmission line. Preferably, the connection between the elements  12  and the transformer  16  is made by a solder joint. The connection between the transformer  16  and the helical elements  12  is relatively short compared to the wavelength and disturbs the antenna pattern minimally. The direct connection of the helical elements  12  of the antenna  10  to the transformer  16 , and the shortness of the connection allows the elimination of the hybrid power divider. As the diameter of the antenna decreases, the distortion of the fields caused by the horizontal feed elements from the transformer to the filars decreases, allowing the elimination of the hybrid power divider. 
     The opposite end of the transformer  16  is connected to an RF connector  18 . The RF connector  18 , in turn, is connected to a transmitter/receiver (not shown). The RF connector  18  has the same intrinsic impedance as the remainder of the transmitter/receiver. The entire antenna  10  shown in FIG. 1 is preferably enclosed in a protective housing (not shown). 
     Further size reduction of the antenna  10  can be achieved by replacing the quarter-wave transformer  16  with a discrete component pi-network. FIG. 2 shows an antenna  10 A which is similar to the antenna  10  shown in FIG. 1 except that the planar transmission line ¼ wave transformer  16  is replaced with a pi-network impedance transformer  16 A formed on a small circuit board  17 A. As shown, the ½ turn, ¼ wave quadrifilar helix antenna  10 A includes four helical elements  12 A wound around the coil form  14 A. 
     The helical elements  12 A are connected to the pi-network impedance transformer  16 A by a solder joint, similar to the joint on antenna  10 . The impedance transformer  16 A is comprised of a conventional pi-network impedance transformer. The opposite end of the transformer  16 A is connected to an RF connector  18 A. The IRF connector  18 A, in turn, is connected to a transmitter/receiver (not shown). 
     In some applications, dual band quadrifilar helix antennas are needed for separate transmit and receive bands on personal mobile satellite communications equipment. In one example, the transmit band is at a lower frequency and the receive band is at a higher frequency, or visa versa. The embodiments shown in FIGS. 3-7 Illustrate examples of dual band quadrifilar helix antennas of the present invention. 
     FIGS. 3 and 4 show an antenna  10 B which is similar to the antenna  10  shown in FIG. 1 except that antenna  10 B includes an upper quadrifilar  20  and a lower quadrifilar  22 . The quadrifilars  20  and  22  each are formed by helical elements  123  wound around coil forms  14 C. The helical elements  12 B of the upper quadrifilar  20  are connected to an upper flexible planar transmission line ¼ wave transformer  24  formed on a flexible circuit board  25 . The helical elements  12 B of the lower quadrifilar  22  are connected to a lower flexible planar transmission line ¼ wave transformer  26  formed on a flexible circuit board  27 . The transformers  24  and  26 , like the transformers  16  and  16 A, are comprised of conventional ¼ wave transmission lines. Preferably, the connections between the elements  12 B and the transformers  24  and  26  are made by solder joints. The transformers  24  and  26  are connected together at a junction  30 . Two band pass filters  32  and  34  connected between the junction  30  and the transformers  24  and  26  are used to isolate the transformers  24  and  26  from one another. Without the band pass filters  32  and  34 , transformers  24  and  26  would detune each another. The band separation between the quadrifilars  20  and  22  insures that mutual coupling is minimized. 
     The space on top of some devices, such as a phone, is limited, forcing the quadrifilars  20  and  22  to be coaxially located with respect to each other. The lower quadrifilar  22  has a coaxial transmission line  28  centered in the helical elements  12 B which connects to the junction  30  formed between the planar transmissions lines  24  and  26 . The symmetry of the antenna  103  is disturbed if the coaxial transmission line  28  is not centered along the axis of the quadrifilar  22 . FIG. 4 is a side view of the antenna  10 B showing the coaxial transmission line  28 . As shown, the coaxial transmission line  28  extends through the coil form  14 B, along side the transformer  26 , where it is soldered to the junction  30 . 
     FIGS. 5 and 6 show an antenna  10 C which is similar to the antenna  10 B shown in FIGS. 3 and 4 except that the ¼ wave transformers  24  and  26  are replaced by pi-network impedance transformers  36  and  38  formed on small circuit boards  37  and  39 . The quadrifilars  40  and  42  each are formed by helical elements  12 C wound around coil forms  14 C. The helical elements  12 C of the upper quadrifilar  40  are connected to the upper pi-network impedance transformer  36  formed on the circuit board  37 . The helical elements  12 C of the lower quadrifilar  42  are connected to the lower pi-network impedance transformer  38  formed on the circuit board  39 . The transformers  36  and  38  are comprised of conventional pi-network impedance transformers. Preferably, the connections between the elements  12 C and the transformers  36  and  38  are made by solder joints. The transformers  36  and  38  are connected together at a junction  30 C. Two band pass filters  32 C and  34 C connected between the junction  30 C and the transformers  36  and  38  are used to isolate the transformers  36  and  38  from one another, as described above with respect to FIGS. 3 and 4. 
     The lower quadrifilar  42  has a coaxial transmission line  28 C centered in the helical elements  12 C which connects to the junction  30 C formed between the transformers  36  and  38 . FIG. 6 is a side view of the antenna IOC showing the coaxial transmission line  28 C. As shown, the coaxial transmission line  28 C extends through the coilform  14 C, along side the transformer  38 , where it is soldered to the junction  30 C. 
     FIG. 7 illustrates a technique which allows the use of a single quadrifilar for dual frequency bands, further reducing the size of the antenna. Two series resonant circuits in parallel with one another but in series with each of the quadrifilar elements provides for electrical lengthening of the elements in the upper band and electrical shortening of the elements in the lower band. 
     FIG. 7 shows an antenna  10 D which is similar to the antennas described above. The antenna  10 D includes a quadrifilar  44  having four helical elements  12 D wound around a coilform  14 D and series-parallel resonant circuit  46  associated with each helical element  12 D. The series-parallel resonant circuits are comprised of conventional resonant circuits which would normally be used as bandpass filters, but also include a reactance to electrically lengthen or shorten the quadrifilar elements. 
     As shown, the antenna elements  12 D are connected to a dual band impedance transformer  48 . The impedance transformer  48  is comprised of conventional reactive elements  50  that serve to transform different impedances at each of the two frequency bands. 
     The preferred embodiment of the present invention has been set forth in the drawings and specification, and although specific terms are employed, these are used in a generic or descriptive sense only and are not used for purposes of limitation. Changes in the form and proportion of parts as well as in the substitution of equivalents are contemplated as circumstances may suggest or render expedient without departing from the spirit and scope of the invention as further defined in the following claims. 
     Thus it can be seen that the invention accomplishes at least all of its stated objectives.