Abstract:
Multi-band quadrifilar antennas that are suitable for satellite communication include composite elements each of which include multiple conductors operating at different frequencies connected to a bus bar. Each composite element is coupled to a signal feed and to a ground structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is based on provisional patent application No. 61/300,496 filed Feb. 2, 2010. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to the field of compact multiband antennas for satellite aided navigation and mobile satellite communications. 
         [0004]    2. Description of Related Art 
         [0005]    Currently in the mobile satellite communication and global navigation industries there is a need for compact multiband antennas that can be easily integrated into portable devices or more generally into mobile platforms and equipment. Ideally such antennas should provide a very controlled radiation pattern, with uniform coverage of the upper hemisphere and circular polarization purity, for multipath and noise rejection. The ideal antenna should also be electromagnetically isolated from the chassis or external conductive ground structures that it is mounted on, to enable integration on multiple platforms with minimal redesign. 
         [0006]    The fractional-turn Quadrifilar Helix Antenna (QHA) disclosed in US Patent Application Publication 2008/0174501 A1 assigned in common with the present invention, satisfies most of the above requirements.  FIG. 1  shows a conventional fractional-turn QHA. Its pattern is nearly hemispherical and can be shaped to favor a particular elevation angle, if needed. Circular polarization is almost ideal over a very wide range of elevation angle. The most compact variant of the QHA has four helical elements with electrical length of about ¼ wavelength fed by a 4-port phase shifting network enforcing the proper phase rotation. A detailed description of the possible implementation of the feeding network can be found in US 2008/0174501 A1 and is omitted here. 
         [0007]    When very compact dimensions are targeted an external matching network is necessary. The design of the matching network can be quite challenging because the strong coupling between the different arms requires that the four ports are matched simultaneously. Moreover, the design is intrinsically single band and the only way to cover multiple bands is to use as many antennas. Using multiple antennas, besides being impractical in many cases, is unacceptable in some particular applications, such as L1/L2 GPS navigation, since the difference in phase between the L1 and L2 signals needs to be accurately calibrated. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0008]    The present invention will be described by way of exemplary embodiments, but not limitations, illustrated in the accompanying drawings in which like references denote similar elements, and in which: 
           [0009]      FIG. 1  shows a conventional single band quadrifilar antenna and indicates the phasing of a 4 port feeding network for the antenna; 
           [0010]      FIG. 2  shows a quadrafilar antenna assembly according to a first embodiment of the invention in which each antenna element is coupled to a PCB structure by a feeding contact and a grounding contact; 
           [0011]      FIG. 3  shows a dual band antenna assembly that includes eight alternating shorter and longer elements that are uniformly spaced around a cylindrical surface according to an embodiment of the invention; 
           [0012]      FIG. 4  is a perspective view of a multifilar antenna element with tri-band response as it would appear if unwrapped from its cylindrical support surface and flattened out; 
           [0013]      FIG. 5  shows a return loss response of a dual band multifilar antenna according to an embodiment of the invention; 
           [0014]      FIG. 6  shows a 3-dimensional radiation pattern for the Right Hand Circular Polarization in the first band of operation for the antenna with the frequency response described in  FIG. 3 ; 
           [0015]      FIG. 7  shows a 3-dimensional radiation pattern for the Right Hand Circular Polarization in the second band of operation for the antenna with the frequency response described in  FIG. 3 ; 
           [0016]      FIG. 8  describes the radiation pattern in a vertical plane (containing the axis of the cylinder) in the first band of operation for the antenna with the frequency response described in  FIG. 3 ; 
           [0017]      FIG. 9  describes the radiation pattern in a vertical plane (containing the axis of the cylinder) in the second band of operation for the antenna with the frequency response described in  FIG. 3 ; 
           [0018]      FIG. 10  is a plan view of a co-planar printed circuit board showing how the ground contact can be embedded in the board, by branching the signal at the contact point, and connecting one arm to ground; 
           [0019]      FIG. 11  is an embodiment of the invention showing the geometry of the antenna element when the ground contact function is embedded in the PCB as shown in  FIG. 10 ; 
           [0020]      FIG. 12  is schematic illustration of a feed network that is used to feed quadrifilar antennas according to embodiments of the invention; 
           [0021]      FIG. 13  is an alternative embodiment of the structure described in  FIG. 2 , where the antenna elements wrap around a hemispherical surface; and 
           [0022]      FIG. 14  represents an alternative embodiment of the basic structure depicted in  FIG. 3 , in which the multifilar elements are wrapped around a frusto-conical surface. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention. 
         [0024]    According to the present invention compact quadrifilar antennas that do not require external matching are provided. Moreover according to embodiments of the invention multifilar antenna structures that provide multiband coverage while being fed like traditional QHA are provided. In each band multiband antennas according to embodiments of the invention produce very similar patterns and polarization characteristics and otherwise behave as a single band QHA. 
         [0025]      FIG. 2  shows an antenna assembly  200  according to an embodiment of the invention. Each of four elements  202  of approximately ¼ wavelength electrical length contact a circular PCB  203  at a signal feed location  204  and a ground location  206 . At the feed location  204  the signal is fed to the element  202  with a phase value chosen to enforce a clockwise or counterclockwise phase rotation around the elements and ultimately produce a Left Hand or Right Hand Circular Polarization. At the second location  206  the element is connected directly to a common ground  208  of the printed circuit board (PCB)  203 . A conductive bridge  210  in the form of a small horizontal conductive strip connects the feed and ground couplings providing an ohmic connection between them. The conductive bride is spaced from the printed circuit board  203 . The elements  202  are uniformly spaced in azimuth angle and shaped so as to wrap around a cylindrical surface (not shown in the figure) in a helical path. In practice the elements can be supported on an actual cylindrical dielectric body or the elements may be self-supporting. If an actual dielectric body is used, it is suitably made of a low loss tangent material such as ceramic or polycarbonate. According to alternative embodiments the shape of the surface is not necessarily cylindrical, but can be any surface of revolution generated by rotating a curve around the vertical axis of the antenna, including but not limited to conical and hemispherical shape for example as shown in  FIG. 13  and  FIG. 14 . 
         [0026]    In  FIG. 3  eight alternating shorter filar strip-like elements  302  and longer strip-like filar elements  304  are uniformly spaced in angle around a cylindrical surface (not shown in the figure). The longer filar elements  304  extend from coupling terminals (signal feed points)  310  formed on a PCB  306 . Each longer element  304  is connected to one shorter element  302  by a horizontal bus strip  308 , that extends parallel to and proximate the PCB  306 , forming a composite element. For example, the horizontal bus strip is suitably within λ/[100] of the PCB  306 . Each composite element is coupled by grounding conductor  312  to a ground plane (one form of ground reference structure) of the PCB  306 . The grounding conductor  312  is connected to the horizontal strip  308  at a location between the shorter element  302  and the longer element  304 . Each composite element, including one short basic strip-like element  302  and one long basic strip-like element  304 , provides a dual band response. The shorter element  302  supports a higher frequency band and the longer element  304  supports a lower frequency band. The center frequency of each band is controlled independently by the physical length of one of the two basic filar elements. If a third strip-like element (not shown) is added a third band of operation is introduced, associated with the length of the third strip-like element. The electrical length of each finger equals (2*n+1)*lambda/4 at the corresponding resonant frequency, where n=0, 1, 2, . . . and lambda is the effective wavelength at the resonant frequency. 
         [0027]      FIG. 4  represents the geometry of the basic building block  400  of a three band antenna according to alternative embodiment of the invention. In  FIG. 4  the basic building block  400  is shown unwrapped from a surface of revolution and flattened on a plane in order to more clearly illustrate its structure. The basic building block  400  includes three principle radiating elements  402 ,  404 ,  406 , including a first band radiating element  402 , a longer second band radiating element  404  and a yet longer third band radiating element  406 . A proximal end  408  of the first band radiating element  402  serves as a feed contact for the basic building block. In an assembled antenna the proximal end  408  of the first band radiating element will be coupled to a signal feed point of a feed network. Proximal ends of the three radiating elements  402 ,  404 ,  406  are connected by a bus strip  410 . A grounding strip  412  connects the bus strip  410  to ground. A quadrifilar antenna made from the basic building block  400  would have four such basic building blocks equally spaced in angle, and disposed in a helical configuration on a cylindrical (or other surface of revolution) surface (which may be virtual, or embodied by a physical dielectric support). 
         [0028]      FIG. 5  shows a graph  500  including a return loss response plot  502  for a dual band multifilar antenna according to an embodiment of the invention. The abscissa indicates frequency in Gigahertz and the ordinate indicates return loss in decibels. As shown the return loss includes a first band of operation centered at 1.225 GHz and a second band of operation centered at 1.575 GHz. 
         [0029]      FIG. 6  shows a 3-dimensional radiation pattern for the Right Hand Circular Polarization in the first band of operation for the antenna with the frequency response shown in  FIG. 3 . The radiation pattern is fairly even in the polar angle range 0.0 to 80 degrees varying from a minimum of −1 dB to a maximum of 3 dB. For GPS applications the polar angle range 0.0 to 80 degrees is considered important. 
         [0030]      FIG. 7  shows a 3-dimensional radiation pattern for the Right Hand Circular Polarization in the second band of operation for the antenna with the frequency response described in  FIG. 3 . This radiation pattern is also fairly even in the polar angle range 0.0 to 80.0 varying from a minimum of −1 dB to a maximum of 3.5 dB. 
         [0031]      FIG. 8  is a graph including polar plots  802 ,  804  of radiated intensity versus polar angle in a vertical plane (containing the axis of the cylinder) in the first band of operation for the antenna with the frequency response described in  FIG. 3 . A first polar plot  802  is for the Right Hand Circular Polarization (RHCP) component, and a second polar plot  804  is for the Left Hand Circular Polarization (LHCP) component.  FIG. 9  is a graph including polar plots  902 ,  904  of radiated intensity versus polar angle in a vertical plane (containing the axis of the cylinder) in the second band of operation for the antenna with the frequency response described in  FIG. 3 . A first polar plot  902  is for the RHCP component and a second plot  904  is for the LHCP component. As shown in the  FIG. 8  and  FIG. 9  graphs, in the polar angle range 0.0 to Pi/2 the RHCP component is strongly dominant over the LHCP component, with an axial ratio of less than 3 dB over the entire upper hemisphere. 
         [0032]      FIG. 10  is a fragmentary plan view that shows an alternative arrangement for providing the ground contact analogous to ground contact  206 ,  312 ,  314  described above. In the embodiment shown in  FIG. 10  the ground contract is provided as part of a co-planar printed circuit board  1000 . Referring to  FIG. 10  a signal line  1001  extends to a signal pad  1003 . The signal pad  1003  is connected to a helical antenna element ( 1104 ) of the type described above. A ground plane  1004  is disposed co-planar with and on both sides of the signal line  1001  and signal pad  1003 . A ground connection  1002  extends from the signal line  1001  to the ground plane  1004 . 
         [0033]      FIG. 11  shows an antenna  1100  that includes the printed circuit board  1000  such as shown in  FIG. 10  in which the ground connection  1002  is implemented in the printed circuit board  1000 . Note that the printed circuit board  1000  used in the antenna  1100  will have four arrangements of signal line  1001 , and ground connection  1002  such as shown in  FIG. 10 . The antenna  1100  includes four composite elements  1102 , each including a first element  1104  tuned to a first frequency and having a proximal end  1106  connected to one of four signal pads  1003 , and a second element  1108  that is connected to the first element  1104  by a bridge conductor  1110 . 
         [0034]      FIG. 12  represents a schematic of a possible implementation of a feeding network providing the incremental 90 degrees phasing between adjacent elements. The network employs a balun  1212  to convert a common signal into 2 signals having a differential phase relationship between them. Each one of the differential signals is fed to one of two 90 degrees hybrid couplers  1203 . The relative phase of each branch is indicated on the figure. The ground contacts  1210  are connected to the common PCB ground, such as for example the ground  306  shown in  FIG. 3 . A receiver and/or transmitter are coupled to the network through port  1201 . Four antenna coupling terminals (signal feed points)  1202 ,  1204 ,  1206  and  1208  are connected to the four feed points of the antennas described above, e.g.,  310  in  FIG. 3 . The four antenna coupling terminals  1202 ,  1204   1206 ,  1208  are spatially located on a printed circuit board implementation of the feed network (e.g.,  203 ) such that phase increases uniformly (e.g., in 90 degree steps) as a function of position (described by azimuth angle) around the printed circuit board (e.g.,  203 ). The feed network  1200  provides equal amplitude signals to the four antenna coupling terminals  1202 ,  1204 ,  1206 ,  1208 . 
         [0035]      FIG. 13  shows an antenna  1300  according to an alternative embodiment of the invention. The antenna  1300  comprises four helical antenna elements  1302  conforming to a hemispherical surface  1304 . Each antenna element  1302  includes a proximal end  1306  connected to a signal pad  1308  of a printed circuit board  1310  and is connected through a bridge conductor  1312  to a short ground conductor  1314  that extends up from a ground plane  1316  of the printed circuit board  1310 . 
         [0036]      FIG. 14  shows an antenna  1402  according to alternative embodiment. The antenna  1402  includes four composite antenna elements  1404 , each including a first frequency radiating element  1406  and a second frequency radiating element  1408 . The first frequency radiating elements  1406  are connected to signal pads of a printed circuit board  1410 . The second frequency radiating elements  1408  are coupled to the first frequency radiating elements  1406  through bridge conductors  1412 . The bridge conductors  1412  are coupled to a ground plane of the printed circuit board through four short ground conductors  1414 . The four composite elements  1404  are conformed to a frusto-conical surface  1416 . 
         [0037]    For proper functioning of the antenna it is important that the composite element is equipped with a direct contact to the reference PCB ground (e.g.,  412  in  FIG. 4 ), along with the feeding contact (e.g., proximal end  408  in  FIG. 4 ), coupling the signal. By means of the ground contact it is possible to attain an antenna matched to the same impedance (e.g., 50 Ohms) in all bands of operation. The value of the matching impedance is controlled by the spacing D, shown in  FIG. 4 , between the feed contact location (e.g.,  408 ) and the ground contact location (e.g.,  412 ). The value of the spacing D required to obtain a desired impedance Z can be determined by routine experimentation. 
         [0038]    Alternatively the ground contact can also be embedded in the PCB, by implementing a branching of the signal coupled to the composite element and connecting one of the paths to ground directly on the PCB, as shown in  FIG. 10 . In  FIG. 10  the signal line  1001  lies in the same plane as the ground plane  1004 . The antenna element is connected to the pad  1002 . The antenna pad is coupled to ground through the conductor  1003  travelling a distance D chosen to achieve the desired impedance matching. In this case the geometry of the antenna appears as depicted in  FIG. 11 . Whereas the embodiments described above include 4 antenna elements or 4 composite antenna elements alternatively more than 4 elements or composite elements can be provided.