Patent Publication Number: US-6215449-B1

Title: Systems and methods for coaxially coupling an antenna through an insulator

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to application Ser. No. 09/848,434, now U.S. Pat. No. 6,069,588 to the present inventor entitled “Systems and Methods for Coaxially Coupling an Antenna Through a Window and Amplifying Signals Adjacent and Inside The Window”, filed concurrently and assigned to the assignee of the present application, the disclosure of which is hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to coupling systems and methods and more particularly to systems and methods for coupling antennas through an insulator. 
     BACKGROUND OF THE INVENTION 
     Antenna coupling systems and methods are widely used to couple an antenna to a receiver through an insulator. For example in mobile radiotelephone communications, a radiotelephone such as a cellular radiotelephone may be mounted in the interior of a vehicle. An antenna may be mounted on the exterior of the vehicle. Coupling systems and methods are used to couple the antenna to the radiotelephone through an insulator such as a window. 
     It will be understood by those having skill in the art that although the present application will describe the coupling of mobile radiotelephones to antennas through a vehicle window, the present invention may be applied to any radio transceiver and the insulator need not be limited to windows. Moreover, the present invention need not be used in a mobile environment but also may be used to couple an antenna outside a building to a transceiver inside a building through an insulator. 
     It is known to use a capacitive plate on either side of a window to capacitively couple high frequency signals through the window. See U.S. Pat. No. 4,621,243 to Harada entitled “Transmission Channel Coupler for Antenna”. This patent also describes the use of loop coils and helical resonators to transmit high frequencies through a window. 
     Multiple plates also may be used on either side of the window to capacitively couple signals therethrough. See for example, U.S. Pat. No. 2,829,367 to Rychlik entitled “Television Lead-In Coupler” and U.S. Pat. No. 4,764,773 to Larsen et al. entitled “Mobile Antenna and Through-the-Glass Impedance Matched Feed System”. 
     Finally, it is also known to provide a collinear radiator mounted on one surface of a dielectric such as the window of a vehicle and a tunable coupling circuit disposed internally of a conductive housing mounted on the opposite surface of the dielectric. See U.S. Pat. No. 839,662 to Hadzoglou entitled “Cellular Mobile Communication Antenna”. 
     In providing antenna coupling systems and methods, it generally is desirable to couple the antenna to the transceiver through the insulator with low insertion loss. For example, in satellite radiotelephones which may provide low link margins, it may be desirable to maintain insertion loss through the insulator of about 0.5 dB or less. Unfortunately, many conventional coupling systems produce an insertion loss of 2 dB or more. 
     Moreover, many high performance antennas employ two or more conductors rather than a single conductor. For example, a quadrifiler helix antenna may employ two or more conductors. Accordingly, it may be desirable to couple radio frequency signals via a coaxial cable from inside the insulator to a coaxial cable outside the insulator. In conclusion, notwithstanding the above described solutions, there continues to be a need for coaxial-to-coaxial coupling systems and methods that can provide high performance and low insertion loss. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide antenna coupling systems and methods that can provide low insertion loss across an insulator. 
     It is another object of the present invention to provide antenna coupling systems and methods that can couple coaxial cables through an insulator. 
     These and other objects are provided, according to the present invention by coaxial coupling systems and methods that couple a first coaxial cable that includes a first inner conductor and first shield conductor to a second coaxial cable that includes a second inner conductor and a second shield conductor, through an insulator that includes first and second insulator surfaces. A first center plate and a first surrounding plate are adapted for attachment to the first insulator surface such that the first surrounding plate surrounds the first center plate on the first insulator surface. The first center plate is electrically connected to the first inner conductor and the first surrounding plate is electrically connected to the first shield conductor. A second center plate and a second surrounding plate are adapted for attachment to the second insulator surface such that the second surrounding plate surrounds the second center plate on the second insulator surface, the first and second center plates are adjacent one another with the insulator therebetween and the first and second surrounding plates are adjacent one another with the insulator therebetween. The second center plate is electrically connected to the second inner conductor and the second surrounding plate is electrically connected to the second shield conductor. 
     It has been found according to the present invention, that coaxial coupling systems and methods as described above can allow communication signals to pass with low insertion loss, over a desired frequency range, between an antenna mounted outside a window and a radio transceiver mounted inside the window. Transmission takes place via a coaxial transmission line on each side of the window. Moreover, by allowing two or more conductors to be capacitively coupled. a “single port” or “multiport” circuit may be provided for high performance antennas. 
     In a preferred embodiment, the first and second center plates are first and second disks and the first and second surrounding plates are first and second rings. More preferably, the first and second rings are first and second continuous rings. Moreover, a first inductor is preferably electrically connected between the first center plate and the first surrounding plate and a second inductor is preferably electrically connected between the second center plate and the second surrounding plate. A pair of first pads may be included, a respective one of which is on the first center plate and on the first surrounding plate so that the first inductor is electrically connected between the pair of first pads. A pair of second pads also may be included, a respective one of which is on the second center plate and on the second surrounding plate so that the second inductor is electrically connected between the pair of second pads. 
     A preferred configuration of the present invention can provide reduced insertion loss over a wide band, while reducing unwanted stray couplings. In particular, the first inner conductor is electrically connected to the first center plate at a first position thereon and the first inductor is electrically connected to the first center plate at a second position that is remote from the first position. The second inner conductor is preferably electrically connected to the second center plate at a first position thereon and is preferably electrically connected to the second center plate at a second position that is remote from the first position. 
     More preferably, the first position on the first center plate is adjacent the second position on the second center plate and the second position on the first center plate is adjacent the first position on the first center plate. Even more preferably, the first shield conductor is electrically connected to the first surrounding plate at a first position thereon and the second shield conductor is electrically connected to the second surrounding plate at a second position thereon that is remote from the first position on the first surrounding plate. 
     Most preferably, the first shield conductor is also electrically connected to the first surrounding plate at a first position thereon and the second shield conductor is electrically connected to the second surrounding plate at a second position thereon that is opposite the first position on the first surrounding plate. Thus, the first and second coaxial cables preferably emerge from the coupling system in opposite directions to reduce unwanted parasitic coupling. The first and second inductors preferably also are located at opposite locations from one another to reduce mutual inductance. 
     In another preferred aspect of the invention, the first center plate and the first surrounding plate define a first gap therebetween and the second center plate and the second surrounding plate define a second gap therebetween. The first shield conductor preferably extends into the first gap and the second shield conductor preferably extends into the second gap. More preferably, the first shield conductor extends midway into the first gap and the second shield conductor extends midway into the second gap. 
     The first center plate and the surrounding plate may be contained in a first housing. The second center plate and the second surrounding plate may be contained in a second housing. An alignment key may be provided on at least one of the first and second housings to facilitate alignment of the first housing and the second housing relative to one another on the respective first and second surfaces of the insulator. 
     The present invention may be used to couple an antenna to a radiotelephone through a window including an outside surface and an inside surface. Thus, as described above, the first coaxial cable includes a first inner conductor and a first shield conductor that are coupled to the antenna. The second coaxial cable includes a second inner conductor and a second shield conductor that are coupled to the radiotelephone. However, as described above, the present invention may be used in other coupling applications. 
     Methods according to the present invention may be used for coupling a first coaxial cable that includes a first inner conductor and a first shield conductor to a second coaxial cable that includes a second inner conductor and a second shield conductor, through an insulator that includes first and second insulator surfaces. A first center plate and a first surrounding plate are attached to the first insulator surface such that the first surrounding plate surrounds the first center plate on the first surface. The first center plate is electrically connected to the first inner conductor and the first surrounding plate is electrically connected to the first shield conductor. A second center plate and a second surrounding plate are attached to the second insulator surface such that the second surrounding plate surrounds the second center plate on the second surface, the first and second center plates are adjacent one another with the insulator therebetween, and the first and second surrounding plates are adjacent one another with the insulator therebetween. The second center plate is electrically connected to the second inner conductor and the second surrounding plate is electrically connected to the second shield conductor. The first center plate also may be inductively coupled to the first surrounding plate and the second center plate also may be inductively coupled to the second surrounding plate. Coaxial coupling systems and methods may thereby be provided to couple coaxial lines through an insulator with low insertion loss over a desired frequency range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A,  1 B and  1 C are a top view, a cross-sectional view and a bottom view respectively, illustrating systems and methods for coupling a first coaxial cable to a second coaxial cable through an insulator according to the present invention. 
     FIGS. 2A-2C illustrate alternate arrangements of coupling systems and methods according to the present invention. 
     FIGS. 3A-3C illustrate alternate uses of coupling systems and methods according to the present invention to couple an antenna on the exterior of a vehicle to a radiotelephone within a vehicle. 
     FIG. 4 graphically illustrates simulated attenuation loss for coupling systems and methods according to the present invention. 
     FIG. 5 graphically illustrates simulated return loss for coupling systems and methods according to the present invention. 
     FIG. 6 is a Smith Chart that graphically illustrates simulated input impedance for coupling systems and methods according to the present invention. 
     FIG. 7 is an equivalent circuit of coupling systems and methods according to the present invention. 
     FIG. 8 graphically illustrates measured data for coupling systems and methods according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. It will be understood that when an clement is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. 
     Referring now to FIGS. 1A,  1 B and  1 C, a top view, cross-sectional view and bottom view, respectively, illustrating systems and methods for coupling a first coaxial cable to a second coaxial cable through an insulator. As shown in FIGS. 1A-1C, coupling systems and methods  10  couple a first coaxial cable  12  that includes a first inner conductor  14  and a first shield conductor  16  to a second coaxial cable  22  including a second inner conductor  24  and a second shield conductor  26 , through an insulator  32  such as a windshield or other glass that includes first and second insulator surfaces  32   a  and  32   b  respectively. As is well known to those having skill in the art, each coaxial cable also may include an inner insulator and an outer jacket. 
     A first center plate  42  and a first surrounding plate  44  are adapted for attachment to the first insulator surface  32   a  using adhesive, fasteners and/or other conventional attaching means, such that the first surrounding plate  44  surrounds the first center plate  42  on the first surface  32   a . The first center plate  42  is electrically connected to the first inner conductor  14  using solder  34   a  and/or other conventional electrical connecting means. The first surrounding plate  44  is electrically connected to the first shield conductor  16  using solder  34   b  and/or other conventional electrical connecting means. 
     A second center plate  52  and a second surrounding plate  54  also are adapted for attachment to the second insulator surface  32   b  using adhesive, fasteners and/or other conventional attaching means such that the second surrounding plate  54  surrounds the second center plate  52  on the second surface  32   b . As shown, the first and second center plates  42  and  52  respectively are adjacent one another with the insulator  32  therebetween. Also, the first and second surrounding plates  44  and  54  are adjacent one another with the insulator  32  therebetween. The second center plate  52  is electrically connected to the second inner conductor  24  using solder  34   c  and/or other conventional electrical connecting means. The second surrounding plate  54  is electrically connected to the second shield conductor  26  using solder  34   d  and/or other electrical connecting means. 
     As shown in FIGS. 1A-1C, the first and second center plates  42  and  52  respectively, preferably are first and second disks. The first and second surrounding plates  44  and  54  respectively, preferably are first and second rings. As also shown in FIGS. 1A-1A, the first and second rings  44  and  54  respectively, preferably are first and second continuous rings. However, polygonal shaped center plates and surrounding plates, including but not limited to square shaped center plates and surrounding plates may be used, and gaps may be present in the center plates and/or surrounding plates so that they are not continuous. 
     A first inductor  46  is electrically connected between the first center plate  42  and the first surrounding plate  44  using solder  44   e ,  44   f  and/or other conventional electrical connecting means. A second inductor  56  is electrically connected between the second center plate  52  and the second surrounding plate  54  using solder  54   g ,  54   h  and/or other conventional electrical connecting means. More than one inductor also may be electrically connected between a center plate and a surrounding plate as will be described below. 
     The coaxial cables  12  and  22  and the inductors  46  and  56  may be electrically connected to the center plates and surrounding plates at any arbitrary position thereon. However, preferably, they are connected as illustrated in FIGS. 1A-1C to reduce and preferably minimize unwanted couplings and parasitics. More specifically, the first inner conductor  14  preferably is electrically connected to the first center plate  42  at a first position  42   a  thereon and the first inductor  46  preferably is electrically connected to the first center plate  42  at a second position  42   b  that is remote from the first position. The second inner conductor  24  preferably is electrically connected to the second center plate  52  at a first position  52   a  thereon and the second inductor  56  preferably is electrically connected to the second center plate  54  at a second position  52   b  that is remote from the first position  52   a . Moreover, the first position  42   a  on the first center plate  42  preferably is adjacent the second position  52   b  on the second center plate  52 . The second position  42   b  on the first center plate  42  preferably is adjacent the first position  52   a  on the second center plate  52 . 
     The first shield conductor  16  preferably is electrically connected to the first surrounding plate  44  at a first position  44   a  thereon. The second shield conductor  26  preferably is electrically connected to the second surrounding plate  54  at a second position  54   a  thereon that is remote from, and preferably opposite, the first position  44   a  on the first surrounding plate  44 . Moreover, the first inductor  46  preferably is electrically connected to the first surrounding plate  44  at a second position  44   b  that is remote from, and more preferably opposite, the first position  44   a . The second inductor  56  is preferably connected to the second surrounding plate  54  at a second position  54   b  that is remote from, and more preferably opposite, the first position  54   a . Thus, as shown, the first and second coaxial cables preferably emerge from opposite directions and the first inductors preferably are located remote from one another. 
     As also shown in FIGS. 1A-1A, the first center plate  42  and the first surrounding plate  44  preferably define a first gap  48  therebetween and the second center plate  52  and the second surrounding plate  54  preferably define a second gap  58  therebetween. The first shield conductor  16  preferably extends into the first gap and the second shield conductor  26  preferably extends into the second gap  58 . More preferably, as shown, the first shield  16  preferably extends midway into the first gap  48  and the second shield  26  preferably extends midway into the second gap  58 . 
     Still referring to FIGS. 1A-1C, the first and second positions on each of the first center plate  42 , first surrounding plate  44 , second center plate  52  and second surrounding plate  54  may be defined using a tab such as a projecting tab. The tab can facilitate solder connection at the appropriate place on the center plates and surrounding plates. The tabs may be raised and may have a shape that enhances soldering. Multiple layers may be used for the tabs. However, it will be understood that the first and second positions on each of the center plates and surrounding plates need not be defined by specific features such as tabs. 
     Finally, a first housing  50  contains the first center plate  42  and the first surrounding plate  44 . A second housing  60  contains the second center plate  52  and the second surrounding plate  54 . The first housing  50  also may contain the coaxial cable  12  and a first coaxial cable connector  18 . Similarly, the second housing  60  may also contain the second coaxial cable  22  and a second coaxial cable connector  28 . It will be understood however, that the coaxial cable connectors  18  and  28  need not be contained within or be adjacent the housings, and may be eliminated entirely. Similarly, the coaxial cables  12  and  22  themselves may be outside the housings  50  and  60 . 
     In order to facilitate alignment of the first housing  50  and the second housing  60  to one another on opposite surfaces of the insulator  32 , an alignment key such as a pair of dimples  36   a ,  36   b  may be provided on a respective housing  50  and  60 . Alternatively, alignment keys  36   a ,  36   b  may be painted or otherwise inscribed on the housings  50  and  60  and also may be provided by virtue of the overall shape of the housings  50  or  60 . Alternatively, alignment keys need not be provided at all. 
     The materials and dimensions of the center plates, surrounding plates, inductors and housings may be varied depending on a particular application. However, the center plates and surrounding plates preferably comprise stamped copper and the housing preferably comprises plastic. The surrounding plates may have an outer diameter of about 45 mm and an inner diameter of about 20 mm. The center plate may have a diameter of about 15 mm so that a 2.5 mm gap is present. The plates may be less than one mm thick. The housings should preferably maintain a clear area above and below of about one cm. The inductors may be meandering line inductors rather than coils. 
     It will be understood that more than one inductor may be used to couple a respective center plate to a respective surrounding plate. Alternative arrangements of center plates, surrounding plates, and positioning of coaxial cables and inductors are shown in FIGS. 2A-2C. The inductance may be distributed to reduce the difficulty of fabricating small inductor values. Thus, for example, four-20 nH coils may be used to achieve a 5 nH coil. 
     FIGS. 3A-3C illustrate the use of coupling systems and methods according to the present invention to couple an antenna on the exterior of a vehicle to a radiotelephone within a vehicle. As shown in FIG. 3A, coupling  10  is used to couple a first coaxial cable  12  that is connected to an antenna such as a quadrifilar helical antenna  110  on the roof of a vehicle  100 , through the rear window  32  of the vehicle  100 , to a second coaxial cable  22  that itself is coupled to a radiotelephone  120  within the vehicle  100 . FIG. 3B illustrates a similar coupling except that the quadrifilar helical antenna  110  or other antenna is directly mechanically attached to the coupler housing on the rear window  32 . FIG. 3C illustrates a similar embodiment to FIG. 3A except that a patch antenna  110 ′ is used on the roof of the vehicle  100 . It will be understood that other antennas may be used and other mounting positions for couplers, antennas and transceivers such as radiotelephones may be used. Coupling through windows other than the rear windshield also may be used. 
     As described above, the present invention may be used to coaxially couple two or more conductors through a window. A two-conductor circuit can provide for signal excitation and signal return to complete a circuit. This is known as a “single-port”. Components having input ports and output ports, known as “two-ports” or “multiports” may be cascaded from single ports to modify the signal delivered to the output ports. Examples of such two-ports are transmission lines, duplexers, filters, as well as quadrature matching networks. A low loss, two conductor coupling according to the invention can enable these above-referred components to become part of the external network. 
     Additional design considerations for coupling systems and methods according to the present invention will now be provided. As was described, a center conductor capacitor plate is formed on each side of the glass together with an annular shield conductor capacitor plate around the center conductor plate. On either side of the capacitor plates, shunt inductors are placed from the center capacitor plate to the shield capacitor plate. Thus, a high-pass Pi-circuit is formed which can be equivalent to a short portion of transmission line. 
     There are several factors that may be considered in the design for a given frequency band. For example, for L-Band (1500-1700 MHz) the capacitor plates are somewhat large at the desired wavelength and may not be considered strictly as a lumped element. That is, there is a distributed nature to the capacitor due to its size. Moreover, coaxial cable is unbalanced so the capacitance of the surrounding plates may be desired to be larger than the center plates. It also may be advantageous to provide extra isolation space between the center plates and the shield plates. Further, the coaxial cables on each side of the glass should be physically isolated from each other in order to reduce extraneous conduction modes. The position of the inductors also may be selected to reduce propagation of extraneous conduction modes. Beyond these positioning guidelines, there may be relatively good tolerance to component value variations because there need not be narrowly tuned resonators in the coupler. 
     Couplers according to the invention can support the feed line requirements for circularly polarized antennas that use coaxial or at least two wire connections to the radiating structure. For example, an L-Band transceiver operates between 1500 and 1700 MHz. The present invention may be scaled for this frequency range. Below 1500 MHz there may be gradually increasing transmission loss due to the high-pass Pi-circuit. Above 1700 MHz the transmission loss may eventually increase due to other microwave modes that can be propagated via the structure. 
     The invention also can be scaled to higher or lower frequency ranges. Lower frequency ranges may include cellular radiotelephone frequency bands. Higher frequency ranges may include for example the PCS ranges around 1800 and 1900 MHz. Satellite radiotelephone transceivers in the lower S band also may be used with the present invention. 
     In order to design the circuit, an assumption may be made that the energy is to be sourced from a 50 Ω coaxial transmission line and the energy is to be delivered to a 50 Ω transmission line. An automotive windshield generally has a nominal dielectric of 7.5 at room temperature. Thus, a square centimeter area may have a capacitance of 1.24 pF/cm 2 . Capacitance may be treated as part of a lumped element transmission line equivalent. 
     A simulation of a coupler according to the present invention was performed for L-Band. This simulation was based on the coaxial center window capacitance of 2.1 pF and shield window capacitance of 15 pF. The shunt inductors on each side of the window across the center to shield conductors had a value of 9.8 nH. In all component cases, component Q values were set at 50. The Q values contribute to a loss in the network. 
     FIG. 4 graphically illustrates simulated attenuation loss from 1500 MHz through 1700 MHz. The network is basically a high pass structure with a shunt inductor, series capacitor and shunt inductor. The cutoff frequency of the network may be determined by the value of the series capacitor. The series capacitor was chosen with a sense of a tolerable size and in consideration of the shield capacitor that surrounds the center capacitor. In the case of the series capacitor, 2 nH of series inductance was provided for. 
     FIG. 4 illustrates that a simulated insertion loss of about −0.3 dB may be obtained for values between 1500 MHz and 1700 MHz. FIG. 5 graphically illustrates return loss that shows a very good match over the frequency range of interest. FIG. 6 is a Smith Chart that shows the input impedance with the network terminated at 50 Ω. Accordingly, FIGS. 4-6 indicate that a transmission loss of less than 0.5 dB may be realized for L-Band coaxial coupling. 
     FIG. 7 is an equivalent circuit of couplers according to the present invention. M indicates mutual coupling between elements. The first and second surrounding plates  44  and  54  respectively are broken into semicircles for purposes of the equivalent circuit. 
     Actual measurements were taken on a coupler according to the present invention using the above described parameters. In these measurements, the calibration used a small connector for the through connection. In this way, the loss of an equivalent length of RG-223 coaxial cable may be subtracted from the coupling with cable measurement. The coupling loss values in the right hand column of the following table result from these measurements. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 
               
               
                   
                   
               
               
                   
                 Frequency 
                 Coupling with 
                   
                   
               
               
                   
                 (MHz) 
                 Cable Loss 
                 Cable Loss 
                 Coupling Loss 
               
               
                   
                   
               
             
            
               
                   
                 1525 
                 0.8024 dB 
                 0.3750 dB 
                 0.4274 dB 
               
               
                   
                 1559 
                 0.7994 dB 
                 0.3903 dB 
                 0.4091 dB 
               
               
                   
                 1626 
                 0.9095 dB 
                 0.3974 dB 
                 0.5121 dB 
               
               
                   
                 1660 
                 0.9762 dB 
                 0.3946 dB 
                 0.5816 dB 
               
               
                   
                   
               
            
           
         
       
     
     Measured data from 1.0 to 3.0 GHz is graphically illustrated in FIG.  8 . This data includes a length of coaxial cable similar to the coupling with cable loss data above. The data shows the nature of the high pass filter up to about 2 GHz. The distributed nature of the coupling tends to cause a low pass relation to the curve toward 3 GHz. A wide bandwidth was therefore obtained, with low insertion loss. 
     Accordingly, coupling systems and methods of the present invention can allow communication signals to pass with low insertion loss, over a desired frequency range, between an antenna mounted outside a window and a radio transceiver mounted inside the window. Transmission takes place via a coaxial transmission line on each side of the window. Accordingly, single-port or multiport coupling may take place. 
     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.