Abstract:
The object of the present invention is to achieve an antenna for both receiving and transmitting circularly polarized RF signals which is smaller and lighter than prior art antennas as well as to achieve one antenna for both receiving and transmitting circularly polarized RF signals which has better characteristics for a given physical length than prior art antennas. This is achieved by providing an N-helical-filar antenna with N radiating elements coaxially arranged and defining a cylindrical envelope where each individual radiating element is capacitively coupled to another radiating element.

Description:
TECHNICAL FIELD OF INVENTION 
     The present invention relates to an antenna device comprising capacitively coupled radiating elements and a hand-held mobile communication device comprising such an antenna in general, and more specifically to an antenna device and a hand-held mobile communication device comprising such an antenna for receiving and transmitting circularly polarized RF signals for communication with satellites. 
     DESCRIPTION OF RELATED ART 
     One of the driving forces of the mobile communication industry today is availability and another is size. A user of a hand-held mobile communication device requires to be reached wherever his location may be. This puts requirements on the operator to have good coverage of their mobile network, but for large unpopulated areas this is not possible with any reasonable economy. One solution for a user who frequently travels to unpopulated locations is to instead use a satellite telephone. 
     Such a user will still have requirements on the size of his satellite communication device as he undoubtedly will compare his ordinary cellular communication device with his satellite communication device. Since the distance to orbiting satellites is so great the antennas used will be larger compared to antennas for cellular communication devices, and will consequently take a considerable amount of the space of a satellite communication device. The need for reducing the size of the antennas for satellite communication devices is thus large and anyone being able to reduce the size for such an antenna will have a considerable competitive advantage. 
     In U.S. Pat. No. 5,191,352 is a quadrifilar radio frequency antenna disclosed for receiving signals from an earth orbiting satellite. The antenna has four helical wire elements shaped and arranged so as to define a cylindrical envelope. The elements are co-extensive in the axial direction of the envelope. 
     WO 96/06468 discloses an antenna device with a ceramic core with a relative dielectric constant of at least 5 where every second helical element is longer so that a self-phased antenna is achieved. Every second element is made longer through a meandering shape. 
     In the journal Microwave Engineering Europe June/July 1995 an antenna for personal hand-held terminals is disclosed. The antenna is of quadrifilar helix type. 
     RELATED PATENT APPLICATIONS 
     The following patent applications are related to the same technical field as the invention of this application, and are hereby incorporated herein by reference: 
     the Swedish patent application SE 9801754-4 having the title “An antenna system and a radio communication device including an antenna system”, filed in Sweden the same day as this application, May 18, 1998, applicant Allgon AB, 
     the Swedish patent application SE 9801753-6 having the title “Antenna device comprising feeding means and a hand held radio communication device for such antenna device”, filed in Sweden the same day as this application, May 18, 1998, applicant Allgon AB, and 
     the Swedish patent application SE 9704938-1, filed Dec. 30, 1997, applicant Allgon AB, having the title “Antenna system for circularly polarised radio waves including antenna means and interface network.” 
     SUMMARY OF INVENTION 
     The main object of the present invention is thus to achieve an antenna for both receiving and transmitting circularly polarized RF signals, which is smaller and lighter than prior art antennas. 
     Another object of the present invention is to achieve one antenna for both receiving and transmitting circularly polarized RF signals which has better characteristics for a given physical length than prior art antennas. 
     Another object according to one embodiment of the present invention is to achieve an antenna, which can receive and transmit RF signals in two different frequency bands. 
     Another object according to one embodiment of the invention is to achieve one antenna for both receiving and transmitting circularly polarized RF signals within a communication system where the RF band for receiving signals and the RF band for transmitting signals is spaced apart. 
     The problems described above, with how to achieve a smaller and more efficient antenna for receiving and transmitting circularly polarized RF signals is solved by providing an N-helical-filar antenna with N radiating elements, where N is an integer greater than one, coaxially arranged and defining a cylindrical envelope where each individual radiating element is capacitively coupled to another radiating element. 
     The problems described above, with how to achieve a smaller and more efficient antenna for receiving and transmitting circularly polarized RF signals, according to one embodiment of the invention, is solved by providing an N-helical-filar antenna with N radiating elements coaxially arranged and defining a cylindrical envelope where each individual radiating element has a meandering shape superimposed on the main helical form. 
     In more detail the objects of the present invention, with how to achieve a smaller and more efficient antenna for receiving and transmitting circularly polarized RF signals are obtained, according to one embodiment of the invention, by providing an N-helical-filar antenna with N radiating elements coaxially arranged and defining a cylindrical envelope where each individual radiating element has a meandering shape overlaid on the main helical form and where each individual radiating element is capacitivly coupled to its neighbor in at least one end distal from the feeding point. 
     An advantage with the present invention is that a smaller antenna can be achieved for receiving and transmitting circularly polarized RF signals. 
     Another advantage with the present invention is that one antenna can be used for receiving and transmitting circularly polarized RF signals in more than one band. 
     Another advantage with the present invention is that only one antenna is needed both for receiving and transmitting circularly polarized RF signals even when the band for receiving RF signals is widely separated from the band for transmitting RF signals. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings, which are given by way of illustration only and thus, are not limitative of the present invention and wherein 
     FIG. 1 shows a prior art antenna, 
     FIG. 2 shows a meandering radiating pattern antenna according to a first preferred embodiment of the invention, 
     FIG. 3 shows a meandering radiating pattern antenna with top capacitance according to a second preferred embodiment of the invention, 
     FIG. 4 shows a meandering radiating pattern antenna with top capacitance and a second line of capacitance according to a third embodiment of the invention, 
     FIG. 5 shows a meandering radiating helical antenna according to a fourth embodiment of the invention, 
     FIG. 6 shows a meandering radiating helical antenna with a disc according to a fifth embodiment of the invention, 
     FIG. 7 a ,  7   b  and  7   c  shows a support/capacitance disc disclosed in FIG.  5  and FIG. 6, 
     FIG. 8 shows a meandering radiating pattern antenna according to a sixth preferred embodiment of the invention, 
     FIG. 9 shows a hand-held communication device with an antenna according to the invention, 
     FIG. 10 a  and  10   b  shows different meandering patterns. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a prior art antenna. With  101  is a support denoted and with  102  is a feeding means denoted. The feeding means  102  comprises a first, second, third and fourth feeding points  102   a ,  102   b ,  102   c , and  102   d . Said feeding points are connected to a first, second, third and fourth radiating elements denoted  103   a ,  103   b ,  103   c  and  103   d , commonly denoted  103 . The radiating elements are coaxially wounded around a common axis defining a helical structure. RF signals are fed to the radiating elements  103  from a circuitry  104  through a phasing network  105 . The phasing network  105  converts the RF signal to four signals, each fed to one feed point respectively, with a phase difference of 360°/4=90° enabling the antenna to produce circularly polarized RF signals. The signals may be right-hand or left-hand polarized. The different polarization is achieved by winding the radiating elements in a right-hand or a left-hand direction and by feeding the RF signals accordingly. 
     Even though, throughout this description, mostly transmission of RF signals is described, the antenna device is of course also capable of receiving signals. 
     FIG. 2 shows an antenna according to a first preferred embodiment of the invention. With  201  is a support denoted and a first, second and third feeding points is denoted  202   a ,  202   b  and  202   c  respectively. Said feeding points are coupled to a first, second and third radiating elements  203   a ,  203   b  and  203   c  respectively, said radiating elements are commonly denoted  203 . Said radiating elements  203  are in this preferred embodiment molded directly onto said support using MID (Molded Intrusion Design) technology. Said radiating elements  203  are arranged so as to form a cylindrical envelope on said support. That is, each radiating element is wounded round a common axis, defined by said support, coextending in a cylindrical manner so as to define an helical form with a common radius and pitch. A meandering pattern is superimposed on said helical form construing a common helical form with meandering pattern. In other words, each of said radiating elements  203   a ,  203   b ,  203   c  comprises a number of small bends or turns without complete turns so as to define a stair-like pattern on said support. The meandering pattern increases the electrical length of the radiating element for the same physical length and capacitively couples each radiating element to its neighbors, thereby enabling the design of a shorter antenna with a given electrical length for a specific application such as for instance Iridium, Globalstar etc. A circuitry  204  feeds RF signals to said feeding points  202  through a phasing network  205 . Said phasing network  205  converts the RF signal to three different signals with a phase difference of 360°/3=120° and feeds said signals to each of said feeding points  202  respectively enabling the production of circularly polarized RF signals. The signals may be right-hand or left-hand polarized. The different polarization is achieved by winding the radiating elements in a right-hand or a left-hand direction and by feeding the RF signals accordingly. The meandering shape of the radiating elements may be arranged so that capacitive coupling occur between the different radiating elements. 
     FIG. 3 shows a second preferred embodiment according to the invention. A support is denoted  301  and a first and second feeding points, in a first end  305  of said support  301 , are denoted  302   a  and  302   b  respectively. A first and second radiating element is denoted  303   a  and  303   b  respectively commonly denoted  303 . Said radiating patterns  303  are arranged so as to form a helical cylindrical envelope on said support with an overlaid meandering pattern. That is each radiating element is wounded around a common axis, defined by said support, in a cylindrical manner so as to define a helical pattern. In other words, each of said radiating elements  303  comprises a number of small bends or turns back-and-forth without complete turns so as to define a stair-like pattern on said support. The radiating patterns are printed, etched or similar on a thin dielectric carrier. Said carrier is fixedly mounted on said support, for instance with an adhesive agent. Each radiating element  303  further comprises a coupling portion  304  for capacitively couple said first radiating element  303   a  to said second radiating element  303   b  in a second end  306  distal to said first end  305 . Said coupling portion  304  comprises a receiving member  307  and a extending member  308  where said extending member  308  fits into said receiving member  307  so as to construe a capacitance. The top capacitance enables the design of even shorter antennas for a given electrical length, it also improves the overall efficiency of the antenna. 
     FIG. 4 shows a third preferred embodiment according to the invention. With  401  is a support denoted, first, second, third and fourth feeding points are denoted  402   a ,  402   b ,  402   c  and  402   d  respectively and first, second, third and fourth radiating elements are denoted  403   a ,  403   b ,  403   c  and  403   d  respectively. A first end comprising the feeding points  402  is denoted  404  and a second end distal to said first end is denoted  405 . A first coupling portion is denoted  406  and a second coupling portion is denoted  407  said coupling portions  406  and  407  comprise receiving members and extending members similar to the receiving and extending members described in accordance with the second preferred embodiment and FIG.  3 . The antenna in FIG. 4 is arranged for receiving and/or transmitting RF signals in two different separate bands. The first coupling portion  406  construing a capacitive coupling between a first radiating  403   a  element and its neighbors, that is the first radiating elements neighbors is the second and fourth elements  403   b  and  403   d , is effectively lengthening the electrical length of said antenna, adjusted to a first band for receiving and/or transmitting RF signals, compared to the physical length. The second coupling portion  407  is arranged at a distance from said first or second end  404  or  405  so as to adjust said antenna to transmit and/or receive RF signals in a second band with increased efficiency. Said two bands may one be for receiving RF signals and the other for receiving RF signals or both may be for both receiving and transmitting signals. The invention thus make it possible to design a hand-held radio communication device with one single antenna for receiving and/or transmitting RF signals in two separate bands. 
     In FIG. 5, a fourth preferred embodiment according to the invention disclosed. With  501  is a support denoted and with  502   a  and  502   b  is a first and second feeding means denoted. With  503   a  is a first radiating element denoted and with  503   b  is a second radiating element denoted. Said first and second radiating elements are coaxially arranged and shaped so as to form a cylindrical helical envelope, further more each radiating elements comprises small bends or turns back-and-forth without any complete turns so as to define a meandering pattern superimposed on the helical structure. A first disc  504  is arranged in a first end and fixedly mounted to said support  501 , said feeding means  502  and said radiating elements  503  enabling coupling between the feeding means  502  and the radiating elements  503 . A second disc  505  is arranged on a second end distal to said first end and fixedly mounted to said support  501  and to said first and second radiating elements  503   a  and  503   b . Said second disc  505  may or may not comprise a capacitive coupling between said radiating elements  503   a  and  503   b.    
     In FIG. 6 is a fifth preferred embodiment according to the invention shown. This embodiment is similar to the embodiment just described with the difference of a third disc  601  enabling capacitive coupling between a first and second radiating element  602   a  and  602   b  at a distance L from a first end  604 . The distance L is chosen to improve the characteristics of the antenna for a second band for receiving and transmitting RF signals if the total length of the antenna is chosen for optimal performance for a first band for receiving and transmitting RF signals. Of course it might be beneficial to do some trade off in the performance for the first band to improve the characteristics for the second band. 
     In FIG. 7 a  and  7   b  is discs  701  disclosed with capacitive coupling  702  between the radiating elements  703 . FIG. 7 b  also discloses a disc for an antenna with four wires. FIG. 7 c  shows a disc where the capacitors are coupled to a common connection point  704  and also where the antenna elements are not symmetrically arranged but rather with 90° phase difference between a first radiator  703  and a second and third radiator  705  and  706 , the second radiator  705  having 90° phase difference to the first radiator  703  and 180° to the third radiator  706  and the third radiator  706  having 90° phase difference to the first radiator  703  and 180° to the second radiator  705 . 
     FIG. 8 shows a sixth preferred embodiment according to the invention. Five radiating elements  801  are arranged in a helical form construing a cylindrical envelope on a support  802 . In this embodiment a different radiating pattern is used with meandering edges. The pattern comprises alternating broader and narrower passages so that the edges of the pattern form a meandering shape. Thus, this type of pattern is also included in term meandering pattern or meander radiating element. Coupling portions  803  at a first end capacitively couples each radiating element to its neighbor, that is the first element is capacitively coupled to the second and fifth element, the second element is capacitively coupled to the third and first element and so on to the fifth element which is capacitively coupled to the fourth and first element. 
     FIG. 9 discloses a hand-held radio communication device according to the invention. 
     FIG. 10 a  and  10   b  shows different radiating patterns to be applied to a thin flexible carrier and fixedly secured onto a support using for instance a adhesive agent 
     The invention being thus described, it will be obvious that the same may be varied in many ways. For instance is it obvious that the radiating elements may be wounded in either clockwise or counter-clockwise direction even though only one direction is disclosed in the appended drawings. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.