Abstract:
A multi-mode, multi-band antenna system for a handheld wireless device includes a Quadrafilar Helix Antenna (QHA) that radiates circularly polarized waves is fed by a co-axial cable. The co-axial cable is also used in combination with the QHA as a monopole antenna. Because of the distinct electromagnetic field patterns of the QHA versus the combination of the QHA and the co-axial cable operating as a monopole antenna, the cross coupling between the two modes is low. In certain embodiments the co-axial cable can itself be formed into a helix in order to reduce the physical length of the antenna system while maintaining an electrical length desired to supported certain frequency bands in the monopole mode. According to certain embodiments a post which also serves to increase the effective electric length of the co-axial cable and thereby support a lower frequency band is provided along the centerline of the QHA.

Description:
RELATED APPLICATION DATA 
       [0001]    This application is based on provisional application Ser. No. 61/772,840 filed Mar. 5, 2013. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to wireless communication. 
       BACKGROUND 
       [0003]    While cellular telephone networks and wireless local area networks (LANs) provide ready access to global communication networks from cities, suburbs and even rural areas in the developed world, there are still vast areas of the world where access to communication via the aforementioned wireless communications or via regular telephone networks is not available. In such instances communications via satellites is a viable option. Satellite communications can be useful to a variety of civilian and military users. Certain communication satellites systems use directional antennas that cover a limited geographic region. For people who travel extensively it would be desirable to have portable wireless communication devices that are able to communicate using multiple communication systems e.g., terrestrial cellular systems and satellites. 
         [0004]    Additionally different types of communication services may be available in the same geographic from different sources (e.g., satellites, radio towers) and using different frequency bands. In order for the portable communication device to utilize each source it must include an antenna that exhibits the appropriate frequency response and has a gain pattern consistent with the frequency and the location of the source with which it is communing. For example while a gain pattern that is strong at relatively low zenith angles, is appropriate for communicating with overhead satellites, a gain pattern that is stronger at somewhat higher zenith angles may be more suitable for exchanging signals with a terrestrial antenna. Adding multiple antennas to a portable (e.g., handheld) device to handle multiple needs can lead to an excessively bulky and unwieldy device. Furthermore multiple antennas could interfere with each other. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0005]    The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention. 
           [0006]      FIG. 1  shows a wireless communication environment including multiple disparate wireless communication system infrastructure devices that communicate with a single wireless handset; 
           [0007]      FIG. 2  is a front view of a wireless communication handset according to an embodiment of the invention; 
           [0008]      FIG. 3  is a schematic of an antenna system and related circuits of the handset shown in  FIG. 2  according to an embodiment of the invention; 
           [0009]      FIG. 4  is a perspective view of the wireless antenna system shown in  FIG. 3  according to an embodiment of the invention; 
           [0010]      FIG. 5  is a fragmentary cross sectional view of the antenna system shown in  FIG. 4 ; 
           [0011]      FIG. 6  shows an enlarged portion of the antenna system shown in  FIGS. 4-5 ; 
           [0012]      FIG. 7  is a side view of the antenna system shown in  FIGS. 4-6 ; 
           [0013]      FIG. 8  is a schematic of the antenna system shown in  FIGS. 4-7  including an impedance matching network according to an embodiment of the invention; 
           [0014]      FIG. 9  is an equivalent circuit for the impedance matching network shown in  FIG. 8 ; 
           [0015]      FIG. 10  is a schematic of a feed network for a Quadrifilar Helical Antenna (QHA) included in the antenna system shown in  FIGS. 4-7  according to an embodiment of the invention; 
           [0016]      FIG. 11  is a polar gain plot for the antenna system shown in  FIGS. 4-10  when operating in dipole mode; 
           [0017]      FIG. 12  is a polar gain plot for the antenna system shown in  FIGS. 4-10  when operating in Quadrafilar Helix Antenna (QHA) mode; 
           [0018]      FIG. 13  is polar plot of axial ratio for the antenna system shown in  FIGS. 4-10  when operating in QHA mode; 
           [0019]      FIG. 14  is a graph of certain S-parameters for the antenna system shown in  FIGS. 4-10 ; and 
           [0020]      FIG. 15  is a partial cross sectional view of a variation on the antenna shown in  FIGS. 4-10 . 
       
    
    
       [0021]    Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. 
       DETAILED DESCRIPTION 
       [0022]    Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of apparatus components related to antennas. Accordingly, the apparatus components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
         [0023]      FIG. 1  shows a wireless communication environment  100  including multiple disparate wireless communication system infrastructure devices  102 ,  104 ,  106  that communicate with a single wireless handset  108 . The infrastructure devices  102 ,  104 ,  106  include a first communication satellite  102 , a second communication satellite  104  and a terrestrial radio tower  106 . The two communication satellites  102 ,  104  can support communications using different frequency bands and/or using different protocols. The terrestrial radio tower  106  may for example support cellular mobile telephone communications or municipal two-way radio communications. 
         [0024]      FIG. 2  is a front view of the wireless communication handset  108  according to an embodiment of the invention. The wireless handset  108  includes a housing  202 , a microphone  204 , a keypad  206 , a display  208 , a speaker  210  and an antenna housing  212  that encloses certain components of an antenna system  302  ( FIG. 3 ) that includes two tightly integrated antennas. The antenna system  302  ( FIG. 3 ) is effectively a “two-in-one” antenna. According to alternative embodiments of the invention, antenna systems according to the teachings of the present invention are incorporated in different types wireless communication equipment having form factors other than what is shown in  FIG. 2 . For example antenna systems according to teachings of the present invention could be included in laptop computers or in vehicle mounted radios. 
         [0025]      FIG. 3  is a schematic of the antenna system  302  and related circuits of the handset shown in  FIG. 2  according to an embodiment of the invention. The antenna system  302  includes a first communication circuit (e.g., transceiver)  304  coupled to a first antenna  306  through a transmission line  308  (e.g., co-axial cable). A second antenna  310  comprises the first antenna  306  and the transmission line  308 . A second communication circuit (e.g., transceiver)  312  is coupled to the second antenna  310  at an intermediate position  314  along the length of the transmission line  308 . The first antenna  306  and the second antenna  310  operate in completely separate modes and at different frequencies. 
         [0026]      FIGS. 4-7  show various views of an antenna system  402  that is one embodiment of the antenna system  302 . The antenna system  402  includes a quadrifilar helical antenna (QHA)  404  mounted atop a coiled (helically shaped) section  406  of a co-axial cable  408 . The co-axial cable  408  is used to couple signals to and/or from the QHA  404 . When used in the wireless handset  108  the QHA  404  and the coiled section  406  of co-axial cable  408  are suitably positioned in the antenna housing  212 . 
         [0027]    The QHA  404  includes a round circuit board  410  from which extend four helical antenna elements  412 . A phase shift network (not shown) which supplies the helical elements  412  of the QHA  404  with signals phase shifted at 0, π/2, π, and 3π/2 is implemented on the round circuit board  410 . 
         [0028]    An un-coiled section  414  of the co-axial cable  408  extends back in the direction away from the QHA  404  from the coiled section  406  to a feed end  416  that plugs into a main circuit board  418 . The first communication circuit  304  (not shown in  FIGS. 4-7 ) can be implemented on the main circuit board  418  and coupled to the QHA  404  through the feed end  416  of the co-axial cable  408 . The feed end  416  serves as the first of two feed points for the antenna system  402 . 
         [0029]    A second antenna  420  includes the QHA  404  and the coiled section  406  of the co-axial cable  408  as active elements. Thus no extra radiating antenna elements are required for the second antenna  420 . A feed point  422  for the second antenna  420  is located near the juncture of the coiled section  406  and the un-coiled section  414  of the co-axial cable  408 . At the feed point  422  signals are coupled to the second antenna  310  via a connection to the outer conductor  424  of the co-axial cable  408 . The co-axial cable  408  can be sheathed in an insulating jacket which can be partially removed to expose the outer conductor  424  at the feed point  422 . The second communication circuit  312  (not shown in  FIGS. 4-7 ) can be implemented on the main circuit board  418 . The second communication circuit  312  is coupled to the feed point  422  through an impedance matching network  800  shown in  FIG. 8 . 
         [0030]      FIG. 8  is a schematic of the antenna system  402  including an impedance matching network  802  according to an embodiment of the invention. A first signal source  804  which represents a part of the first communication circuit  304  is coupled to the feed end  416  of the co-axial cable  408 . A second signal source  806  which represents a part of the second communication circuit  312  is coupled through the impedance matching network  802  to the outer conductor  424  of the co-axial cable  408 . The impedance matching network  802  is a Pi network. The impedance matching network  802  includes an inductor  808  in series between the second signal source  806  and the outer conductor  424  of the co-axial cable  408 , a first capacitor  810  connecting the juncture of the inductor  808  and the second signal source  806  to ground and a second capacitor  812  connected the juncture between the inductor  808  and the outer conductor  424  to ground.  FIG. 9  is an equivalent circuit for the impedance matching network shown in  FIG. 8 . In  FIG. 9  the uncoiled section  414  of the co-axial cable  408  appears as a shunt inductive impedance which loads the impedance matching network  802  in parallel with the second antenna  310 . 
         [0031]    The QHA  404  radiates circularly polarized waves in a pattern that has strong gain in the upward direction aligned with the longitudinal axis of the QHA  404 . On the other hand the second antenna  420  emits a dipole radiation pattern having a null in the upward direction aligned with the longitudinal axis of the QHA  404 , and having larger gain in directions perpendicular to the longitudinal axis of the QHA  404 . A portion of the QHA  404 /co-axial cable  408  combination serves as a first monopole and the main circuit board  418  can serve as an opposite monopole or as a counterpoise for the first monopole, when the second antenna  420  is being utilized. 
         [0032]      FIG. 10  is a schematic of a feed network  1000  for the QHA  404  included in the antenna system  402  shown in  FIGS. 4-9  according to an embodiment of the invention. The feed network  1000  can be implemented on the round circuit board  410 . Referring to  FIG. 10  the feed network  1000  includes a balun  1002  that has an input port  1004  for receiving signals through the co-axial cable  408  from the first communication circuit  304 . The balun  1002  has a 0° output  1006  and a 180° output  1008 . The 0° output  1006  of the balun  1002  is connected to an input  1007  of a first 90° degree hybrid  1010  and the 180° output  1008  of the balun  1002  is connected to an input  1009  of a second 90° degree hybrid  1012 . The first 90° degree hybrid  1010  has a first output  1014  that provides an output at 0° and a second output  1016  that provides an output at 90°. The second 90° degree hybrid  1012  has a first output  1018  that provides an output at 180° and a second output  1020  that provides an output at 270°. The outputs  1014 ,  1016 ,  1018 ,  1020  of the 90° degree hybrids  1010 ,  1012  thus provide four signals spaced by 90° in phase to the four helical elements  412 . The outputs  1014 ,  1016 ,  1018 ,  1020  of the 90° degree hybrids  1010 ,  1012  are coupled to the four helical elements  412  through a set of four coupling capacitors  1019 . Each of the helical elements  412  is coupled to a ground plane of the round circuit board  410  (not shown in  FIG. 10 ) through one of four capacitors  1022 . When the second antenna  310  is being used and the four helical elements  412  are serving as an extension of the coiled section  406  of the co-axial cable  408 , radiating a dipole pattern, a displacement current passing through the four capacitors  1022 , as well as through inherent capacitance between the feed network  1000  and the ground plane (not shown) of the round circuit board  410  will serve to couple the four helical elements  412  to the coiled section  406  of the co-axial cable  408 . 
         [0033]      FIG. 11  is a polar gain plot for the antenna system  402  shown in  FIGS. 4-8  when operating in dipole mode associated with the second antenna  420 .  FIG. 12  is a polar gain plot for the antenna system  402  shown in  FIGS. 4-8  when operating in QHA mode.  FIG. 13  is polar plot of axial ratio for the antenna system  402  shown in  FIGS. 4-8  when operating in QHA mode. 
         [0034]      FIG. 14  is a graph of certain S-parameters for the antenna system  402  shown in  FIGS. 4-10 . Port  1  in  FIG. 14  corresponds to the feed end  416  through which signals are coupled to the QHA  404 . Port  2  in  FIG. 14  corresponds to the feed point  422  used to feed the second antenna  420 . Plot  1402  is the return loss (S 11 ) for the QHA  404  and plot  1404  is the return loss S 22  for the second antenna  420 . The QHA  404  supports an operating band centered at about 1.62 GHz and the second antenna  420  exhibits a fundamental resonance operating band at 400 MHz. The frequency of the operating band of the second antenna  420  can be adjusted by changing the length of the coiled section  406  of the co-axial cable  408 . The first communication circuit  304  is adapted to transmit and/or receive signals at a frequency corresponding to an operating band of the QHA, which in the case of  FIG. 14  is as shown, but can vary in other embodiments of the invention. The coiled section  406  of the co-axial cable  408  has a length chosen in view of the additional length provided by the QHA  404 , or post  1502  ( FIG. 15 ) to support an antenna resonance band at frequency corresponding to a frequency at which the second communication circuit  312  is adapted to send and/or receive signals. 
         [0035]    Plot  1406  is a plot of coupling between port  2  and port  1 . As shown the coupling is limited to a maximum of −40 dB. Thus the two ports are well isolated. Isolation is due in part to the fact that the near field radiation patterns of the QHA  404  and the second antenna  420  are largely uncorrelated (decoupled). Isolation is also due in part to the fact that operation of second antenna would tend to drive equal, in-phase (common mode) currents on all of the helical elements, whereas operation of the QHA drives the four antenna elements  412  with distinct quadrature phased signals, such that the signals on opposite pairs of antenna elements  412  are anti-symmetric. The coupling between the two antennas is preferably less than −25 dB, and more preferably less than −30 dB in the frequency bands of operation of the first communication circuit  304  and the second communication circuit  312  which correspond to the frequency bands of operation of the QHA  404  and the second antenna  420 . An added benefit of the antenna system  402  that arises from the isolation, is that the two antennas  306 ,  310  can be operated simultaneously. 
         [0036]      FIG. 15  is a partial cross sectional view of an antenna system  1500  according to an alternative embodiment of the invention which is a variation on the antenna shown in  FIGS. 4-7 . This embodiment includes a conductive post  1502  positioned on the centerline (longitudinal axis) of the QHA  404 . The conductive post  1502  is galvanically connected to a ground plane layer (not shown) of the round circuit board  410 , and the outer conductor  424  of the co-axial cable  408  is also galvanically connected to the aforementioned ground plane layer, so that there is a galvanic connection between coiled section  414  of the co-axial cable through to the conductive post. It should be noted that because the helical elements  412  are coupled through capacitors  1022  to the ground plane of the round circuit board  410  and in-turn to the coiled section  406  of the co-axial cable  408 , the electrical extension they provide for the purpose of the dipole radiation motion is somewhat less than indicated by their physical length. Because the conductive post  1502  is galvanically coupled there is no such shortening effect. 
         [0037]    In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.