Abstract:
An apparatus and method for coupling energy from a transmission line is provided. The apparatus includes a contact designed to “tap” into an inner conductor of the transmission line 100 through an aperture in an outer conductor of the transmission line. A portion of the contact may be coiled (e.g., a spring) and the coil&#39;s characteristics may be varied to control the insertion loss and coupling loss of the apparatus. For example, the wire size, coil diameter, number of turns, and pitch design of the coil may be controlled. The apparatus may also include a secondary transmission line connected to the coil and the secondary transmission line may allow additional control over the coupled energy.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 09/563,328, filed May 3, 2000, which claims the benefit of U.S. Provisional Patent Application No. 60/169,722, filed Dec. 8, 1999. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates in general to radio frequency devices and in particular to methods and devices for coupling radio frequency energy from transmission lines.  
         DESCRIPTION OF THE RELATED ART  
         [0003]    Until this invention, coaxial taps and couplers were installed by cutting and connectorizing RF cable using coaxial jumpers. The primary disadvantage of this methodology is the resulting excessive loss to the host cable. Stein et al, U.S. Pat. No. 5,729,184, subsequently taught that a tap can be used without connectorization; however, the Stein et al. invention still caused losses of over 1 dB to the host cable. Stein et al did mention the theoretical ability to devise taps with coupling losses up to 20 dB but did not describe a method for the manufacture of such devices.  
           [0004]    What is needed are methods and devices embodying the ability to select the coupling loss and accompanying insertion loss in RF systems. In particular, such methods and devices should allow a wireless system not only to be tuned but should also allow minimization of the number of amplifiers and active devices required to RF illuminate a structure.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention relates generally to a coupling device for obtaining energy from a transmission line. In one embodiment, the coupling device comprises a contact for contacting an inner conductor of the transmission line through an aperture in an outer conductor of the transmission line. At least a portion of the contact includes a coil of a preselected configuration, where the configuration defines at least one property of the transferred energy. The coupling device also includes a connector having an inner conductor coupled to the contact.  
           [0006]    In another embodiment, the coupling device includes a wire of a preselected configuration positioned between the contact and the connector. The wire is spaced from a ground plane to create a selected parasitic capacitance and the configuration of the wire at least partially defines a center frequency of the coupling device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0008]    [0008]FIG. 1A is a schematic of a coupling device according to the principles of the invention;  
         [0009]    [0009]FIG. 1B is a schematic diagram of a second coupling device according to the principles of the invention;  
         [0010]    [0010]FIG. 1C is a schematic diagram of a third coupling device according to the principles of the invention;  
         [0011]    [0011]FIG. 1D is a schematic diagram of a fourth coupling device according to the principles of the invention;  
         [0012]    [0012]FIG. 2 shows an assembly and section view of the coupling device according to the principles of the invention;  
         [0013]    [0013]FIG. 3A shows an electronic assembly of an ultra low insertion loss, high coupling loss coupling device such as that shown schematically in FIG. 1B;  
         [0014]    [0014]FIG. 3B shows an electronic assembly of a low insertion loss, medium coupling loss coupling device such as that shown schematically in FIG. 1B;  
         [0015]    [0015]FIG. 3C shows an electronic assembly of a low insertion loss, low coupling loss coupling device such as that shown schematically in FIG. 1C;  
         [0016]    [0016]FIG. 3D shows an electronic assembly of a low insertion loss, high frequency coupling device such as that shown schematically in FIG. 1A;  
         [0017]    [0017]FIGS. 4A and 4B illustrate a cutaway side view and a top view, respectively, of a fifth coupling device;  
         [0018]    [0018]FIGS. 5A and 5B illustrate a cutaway side view and a top view, respectively, of a sixth coupling device;  
         [0019]    [0019]FIGS. 6A and 6B illustrate a cutaway side view and a top view, respectively, of a seventh coupling device;  
         [0020]    FIGS.  7 A- 7 C illustrate a cutaway side view, a top view, and a close up view, respectively, of an eighth coupling device; and  
         [0021]    [0021]FIG. 8 illustrates an alternative embodiment of the coupling device of FIGS.  7 A- 7 C.  
         [0022]    [0022]FIG. 9 is a graph illustrating two representative samples of insertion loss using variations of the coupling device of FIG. 8.  
         [0023]    [0023]FIG. 10 is a graph illustrating two representative samples of coupling responses using variations of the coupling device of FIG. 8.  
         [0024]    [0024]FIGS. 11 a - c  illustrate a cutaway unassembled side view, an assembled side view, and a top view, respectively, of a ninth coupling device.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in FIGS.  1 - 3  of the drawings, in which like numbers designate like parts.  
         [0026]    [0026]FIGS. 1A and 3D respectively show a schematic and layout of a coupling device for coupling RF energy from a coaxial cable to a second coaxial cable, RF radiator or RF amplifier. Although a coaxial cable is represented, it is understood that any transmission line can be substituted and tapped. A hole is drilled into the host transmission line outer conductor  100  and a contact  104  (shown in FIG. 3D at  300 ) is inserted to make contact with the host transmission line center conductor  102 . The contact might be spring loaded, but it is understood that any means of contacting the center conductor will suffice. It is preferable that the center conductor contact  104  ( 300 ) be insulated, but it is not necessary to meet the principles of the invention. Insulation on the shaft of the contact  104  ( 300 ) is provided to prevent inadvertent contact with the outer conductor  100 .  
         [0027]    The coupler internal transmission line  106  (shown in FIG. 3D at  326 ) is a low loss wire. The length and diameter of the wire determine the frequency response and to some degree, the coupling loss and insertion loss of the device. The transmission line wire may be insulated to allow longer length for lower frequencies and still meet the intent of the invention.  
         [0028]    One principle of the invention is the use of highly conductive wire. This prevents dielectric loss through insulation.  
         [0029]    The wire is connected to the center conductor pin  111  ( 310 ) of an output connector represented by outer conductor  110  and center conductor  111  ( 310 ). It is understood that the output may be a hard-wired cable, a directly connected antenna, amplifier or a dummy load. Any of these will meet the principles of the invention.  
         [0030]    Loss element  112  ( 314 ) is connected between the center pin  111  ( 310 ) of the output connector and the outer shield  110  to provide a closer impedance match to the device connected to the output connector. The loss element adds to the performance of the invention, but is not required to meet the principles of the invention.  
         [0031]    The configuration of FIGS. 1A and 3D are used for coupling devices with coupling values from near −15 dB to −6 dB. The loss element of the internal transmission line  106  ( 306 ) is a low loss, wire. The length and diameter of the wire determine the frequency response and to some degree, the coupling loss and insertion loss of the device. The transmission line wire may be insulated to allow longer length for lower frequencies and still meet the intent of the invention. FIGS. 1B, 3A and  3 B are respectively schematic and layout diagrams of an alternate coupling device for coupling a minimum amount of RF energy from a host cable to an output connector while minimizing the insertion loss in the host cable in accordance with the principles of the invention.  
         [0032]    A hole is drilled into the host transmission line outer conductor  100  and a contact  104  ( 300 ) is inserted to make contact with the host transmission line center conductor  102 . The contact might be spring loaded, but it is understood that any means of contacting the center conductor will suffice. It is preferable that the center conductor contact  102  be insulated, but it is not necessary to meet the principles of the invention.  
         [0033]    The internal transmission line  114  ( 306  and  320  in FIGS. 3A and 3B) is a low loss, non-insulated wire but may be insulated for longer lengths to accommodate lower frequencies and still meet the principles of the invention. The transmission line wire is not to be in contact with any dielectric except where it is connected to the terminal points.  
         [0034]    The configuration of FIGS. 1A and 3D are used for coupling devices with coupling values from near −15 dB to −6 dB. The loss element of the internal transmission line  106  ( 326 ) is a low loss wire. The length and diameter of the wire determine the frequency response and to some degree, the coupling loss and insertion loss of the device. The parasitic capacitors  105  are formed by the diameter of the wire and the distance from a ground plane  108  ( 308 ) ( 202 , FIG. 2) shown in FIG. 3D. The parasitic capacitance and the configuration of the wire determine the center frequency response of the device. The transmission line wire may be insulated to allow longer length for lower frequencies and still meet the intent of the invention. As shown in FIG. 3D, the PC board  312  includes holes  316  for purposes that will be described  
         [0035]    One principle of the invention is the use of highly conductive wire. This prevents dielectric loss through insulation. Still another principle of the invention is to prevent the transmission line wire from contacting any dielectric surface except at the point of connection.  
         [0036]    The wire is connected to the center conductor pin  111  ( 310 ) of an output connector represented by outer conductor  110  and center conductor  111  ( 310 ). It is understood that the output may be a hard-wired cable, a directly connected antenna, amplifier or a dummy load. Any of these will meet the principles of the invention.  
         [0037]    A further principle of the invention is to not connect the transmission line to the center contact  102  ( 300 ), but using capacitive coupling, sample the field around pin  102  as shown in detail in FIGS. 3A and 3B at  302  and  318 . The greater the sampling, the greater the coupling energy.  
         [0038]    In FIG. 1B, an element  132  represents a complex impedance, dc blocked connection between the transmission line  114  and the pin  104  connecting the center conductor  102  of the host cable. This connection is further shown in FIGS. 3A and 3B. As seen in FIG. 3A, the connection can be small allowing a small amount of power to be coupled (from 20 to 30 dB) or larger per FIG. 3B allowing coupling values of from 15 to 20 dB. The high coupling loss causes insertion losses from 0.3 to 0.05 dB.  
         [0039]    The configuration of FIGS. 1C and 3C allows a coupling device to pass several selected frequencies with accompanying low insertion loss at those frequencies. In FIG. 1C the internal transmission line is shown at  116  and in FIG. 3C at  322 . The lumped impedance  117  on FIG. 1C and the coil  325  shown in FIG. 3C allows the coupling device to be configured to emphasize selected frequencies while minimizing the insertion loss at selected frequencies.  
         [0040]    A further principal of this invention is that using the lumped impedance input, such as shown in FIGS. 1C and 3C and the selected coupling of FIGS. 1B and 3A and  3 B, allows the designer to not only select the coupling, insertion loss, but also allows him or her to select the required frequencies so that several frequencies can be sent and received on the same cable.  
         [0041]    [0041]FIG. 1D generally relates to this invention with a dc blocked, complex impedance  119  at the input of the coupled port. This allows the designer to configure the coupling device to customize the return loss and to some extent the frequency response. Here, the transmission line (internal) is shown at  118 .  
         [0042]    [0042]FIG. 3D generally relates to the invention for coupling devices used for single frequencies at frequencies around 2 GHz. The principals requiring different wire sizes to select the coupling loss and insertion loss apply to this device as for the other devices described herein. It is understood that any combination of the principals of this invention are included as part of this invention.  
         [0043]    [0043]FIG. 2 generally relates to the mechanical aspects of the invention. The package consists of 3 plastic parts, the bottom  210 , the top  206  and the top seal  214 . The coupled port connector  200  is shown as a type “N”, but any applicable RF connector can be used. The connection to the coupled port may also be a “clamp-on” or “hard-wired”. The connection to the host cable is  208 , but it is understood that any probe or other means of contacting the host center conductor will meet the principals of the invention.  
         [0044]    Captive screws  212  are used to connect the top and bottom of the device to the host cable. Captive screws are used to facilitate installation.  
         [0045]    Screws  216  are disposed on opposite corners of the connector flange extending through holes  316  in PC board  312  ( 204 , FIG. 2), and act as anti-rotation as well as providing a ground path from the host cable to the outer conductor of the coupled port. Although the anti-rotation is not required to allow the device to function, it adds to the overall strength. The ground is not required for operations above 400 mHz, but does add to the overall electrical stability. The screws  216  will generally be partially installed at the time of manufacture and will be finally installed at the time of installation.  
         [0046]    Referring now generally to FIGS.  4 - 9 , further embodiments are illustrated and will be discussed in greater detail.  
         [0047]    Referring now to FIGS. 4A and 4B, in one embodiment, a coupling device  400  utilizes a wire-wound coil  402  (e.g., a spring) to contact a center conductor of a coaxial cable (not shown). The coupling device  400  may include a housing comprising a plastic or non-ferromagnetic material, but the housing is not shown for purposes of clarity. The spring  402  may comprise a non-ferromagnetic material of constant or variable pitch. In the present example, the spring  402  includes a coiled portion  412 , a relatively straight extension  414  at the top of the coiled portion  412 , and a relatively straight extension  416  at the bottom of the coiled portion  412 . The wire diameter, coil diameter, and number of turns of the spring  402  may be selected based on desired results such as coupling and insertion loss.  
         [0048]    The bottom extension  416  of the spring  402  is connected through a secondary transmission line  404  to a center conductor pin  406 . A printed circuit board (PCB)  408  may be used to provide a mounting surface for the spring  402 , secondary transmission line  404 , and center conductor pin  408 . In the present example, an RF interface connector  410  is mounted on the side opposite the spring  402  and is connected to the spring  402  through the center conductor pin  408  and secondary transmission line  404 . One or more apertures (not shown) in the PCB  408  may provide signal connection pathways between the two sides of the PCB  408 , as well as mounting holes.  
         [0049]    In operation, the spring  402  may transform an impedance level from a characteristic transmission line impedance (e.g., approximately fifty or seventy-five ohms) of the coaxial cable to a higher desired value. The transformation is accomplished primarily in the imaginary plane and the complex impedance of the spring  402  establishes the overall frequency response and the amount of energy extracted from the coaxial cable. More specifically, the transformation is in the imaginary plane because the complex impedance is mostly series inductance with parasitic, turn-to-turn, capacitance. Accordingly, there is generally little or no resistive, real plane, component to the impedance.  
         [0050]    The ratio of the magnitude of the complex impedance to the transmission line impedance governs the amount of energy extracted from the transmission line. This complex impedance is, in part, a function of the diameter, pitch, number of turns, and wire size of the spring  402 . In addition, the top and bottom extensions  414 ,  416  of the spring  402  enable a second order control of the total complex impedance. Furthermore, the secondary transmission line  404  may be used to complete the complex impedance transformation to achieve the desired value. For example, the secondary transmission line  404  may control the frequency response and the power extracted from/inserted to the coax cable.  
         [0051]    Referring now to FIGS. 5A and 5B, in another embodiment, a coupling device  500  includes a coil  502 , a secondary transmission line  504 , a center conductor pin  506 , a PCB  508 , and an RF interface connector  510  that are connected in a similar manner to that described in reference to FIGS. 4A and 4B. In the present example, the secondary transmission line  504  may be provided in any configuration that allows the desired complex impedance over the required frequency band or bands. For example, while the coil  502  serves as the primary impedance transformer, the secondary transmission line  504  can be a transmission line or any passive component (such as a lumped element resistor, capacitor, or inductor) that may be used to achieve a desired insertion and coupling loss.  
         [0052]    Referring now to FIGS. 6A and 6B, in yet another embodiment, a coupling device  600  includes a coil  602 , which may be similar to the coils  402  and  502  described in reference to FIGS. 4 and 5, respectively. The coil  602  may comprise a single non-ferromagnetic coil of fixed or variable pitch and may have a fixed or variable diameter. The coil  602  is attached directly to a center pin  604  of an RF interface connector  606 . As previously described, the insertion loss and coupling loss of the coupling device  600  may be determined by the wire size, coil diameter, number of turns, and pitch design of the coil  602 .  
         [0053]    The present example may be constructed without the use of a PCB. This may simplify the manufacture of the coupling device  600 , reduce costs, and provide similar benefits. In addition, the direct connection of the coil  602  to the RF interface connector  606  may prevent energy losses that may occur if the connection is routed through a PCB. Furthermore, the frequency response enabled by the coil  602  may be broadband. The broadband frequency response may occur partly because the direct connection approach described above removes the circuit board and precludes the use of a secondary coil/transmission line, which reduces the total secondary/parasitic impedance. This reduction allows the self resonance of the coil  602  to be moved up in frequency (out of the band of interest), resulting in a broadband frequency response.  
         [0054]    Referring now to FIGS.  7 A- 7 C, in still another embodiment, a coupling device  700  includes a coil  702  that is attached directly to a center pin  704  of an RF interface connector  706 . A portion of the coil  702  may be encapsulated in a material  708 , such as a low-loss plastic (e.g., polystyrene). In the present example, the majority of the upper portion of the coil  702  is encapsulated, while a smaller portion near the bottom is not.  
         [0055]    The upper portion of the coil  702  acts as the principal impedance transformer and its complex impedance may be held invariant by mechanically constraining the dimensions of the coil with the material  708 . The lower portion of the spring  702  acts as a secondary impedance transformer but is allowed to compress, as it is the portion of the coil  702  that maintains contact with the center conductor of the host cable. Referring specifically to FIG. 7C, for purposes of illustration, the coil  702  comprises fourteen turns of American Wire Gauge (AWG) 25 wire with an outer diameter of 0.120 inches. The portion of the coil  702  denoted by the reference numeral “A” represents the upper 12.5 turns and is encapsulated by the material  708 . The portion of the coil  702  denoted by the reference numeral “B” represents the lower 1.5 turns and is not encapsulated.  
         [0056]    This encapsulating feature enables control over the coil  702  while allowing the coupling device  700  to be mounted on coaxial cables with varying dielectric jacket thickness (e.g., the unencapsulated portion can compress or expand to engage a cable). Furthermore, the frequency response enabled by the coil  702  may be broadband. The broadband frequency response may occur partly because the direct connection approach described above removes the circuit board and precludes the use of a secondary coil/transmission line, which reduces the total secondary/parasitic impedance. This reduction allows the self resonance of the coil  702  to be moved up in frequency (out of the band of interest), resulting in a broadband frequency response.  
         [0057]    Referring now to FIG. 8, in still another embodiment, the coupling device  700  of FIGS.  7 A- 7 C includes a tubular extension  710  that may extend from the device  700  into the coaxial cable. The extension  710  may be formed as a part of the coupling device  700  or may be added as a separate component. The extension  710  may serve a variety of functions such as acting as a stabilizer for the coil  702  and as an anti-rotation device.  
         [0058]    In addition, a cavity  712  may be provided in the housing  714  of the coupling device  700 . The cavity  712  may be sized to adjust the parasitic capacitance, which serves to fine-tune the frequency response. More specifically, the cavity  712  may form an electromagnetic resonant circuit. When the coil  702  (or a transmission line) is introduced inside the cavity  712 , the fields surrounding the coil  702  are constrained (e.g., there are electromagnetic boundary conditions that may not exist in an unconstrained space). Accordingly, the cavity  702  will exhibit a largely imaginary complex impedance, which may be capacitive.  
         [0059]    Referring now to FIG. 9, a representative insertion loss from a tap is illustrated by a graph  900 . The graph  900  includes an x-axis  902  representing frequency in MHz and a y-axis  904  representing insertion loss in dB. Two samples  906  and  908  each represent an exemplary behavior pattern of two different variations of the coupling device  700  of FIG. 8. The exemplary behavior of the sample  906  illustrates a result when a nominal amount of power is being extracted, while the sample  908  illustrates a result when the amount of power being extracted is increased by approximately 3 dB.  
         [0060]    Referring now to FIG. 10, a representative coupling response from a tap is illustrated by a graph  1000 . The graph  1000  includes an x-axis  1002  representing frequency in MHz and a y-axis  1004  representing coupling loss in dB. Two samples  1006  and  1008  each represent an exemplary behavior pattern of two different variations of the coupling device  700  of FIG. 8. The exemplary behavior of the sample  1006  illustrates a result when a nominal amount of power is being extracted, while the sample  1008  illustrates a result when the amount of power being extracted is increased by approximately 3 dB.  
         [0061]    The samples  906 ,  908  and  1006 ,  1008  in the graphs of FIGS. 9 and 10, respectively, are based on two variations of FIG. 8. The samples  906  and  1006  are the corresponding results from a single variation, and the samples  908  and  1008  result from an additional variation. For example, the variation represented by the samples  906  and  1006  may be created with a baseline coil length, coil inner diameter, coil wire size, and coil number of turns. Having established this baseline, the samples  908  and  1008  may result when a variation is created with the same coil length but 20 percent reduction in coil turns, 10 percent increase in coil diameter, and a 5 percent increase in coil wire size. Both variations are based on constant diameter and constant pitch coils. Similar results can be achieved by utilization of one or both of these parameters instead of, or in combination with, the parameters that were varied. Furthermore, it is understood that a variety of parameters may be utilized to produce a desired variation.  
         [0062]    Referring now to FIGS. 11 a - c , in still another embodiment, an exemplary coupling device  1100  includes a coil  1102 , a secondary transmission line  1104 , a center conductor pin  1106 , a PCB  1108 , and an RF interface connector  1110  that are connected in a similar manner to that described in reference to FIGS. 4 and 5. As described previously, the secondary transmission line  1104  may be provided in any configuration that allows the desired complex impedance over the required frequency band or bands. For example, while the coil  1102  serves as the primary impedance transformer, the secondary transmission line  1104  can be a transmission line or any passive component (such as a lumped element resistor, capacitor, or inductor) that may be used to achieve a desired insertion and coupling loss.  
         [0063]    The device  1100  includes a housing  1112 . In the present example, the housing  1112  comprises a lower housing  1112   a , an upper housing  1112   b , and a top plate  1112   c . The top plate  1112   c  may be fastened to the upper housing  1112   b  by a plurality of screws  1114  and the upper housing  1112   b  may be fastened to the lower housing  1112   a  by a plurality of screws  1116 . Other fastening means may be used to replace or complement the screws  1114  and  1116 .  
         [0064]    The device  1100  may also include a tubular extension  1118  and a cavity  1120  as described in reference to FIG. 8. The tubular extension  1118  may extend from the device  1100  into the coaxial cable. The extension  1118  may be formed as a part of the coupling device  1118  or may be added as a separate component. The extension  1118  may serve a variety of functions such as acting as a stabilizer for the coil  1102  and as an anti-rotation device. The cavity  1120  may be provided in the housing  1112  of the coupling device  1100 . For example, the cavity may be formed in the upper housing  1112   b  as illustrated. The cavity  1120  may be sized to adjust the parasitic capacitance, which serves to fine-tune the frequency response as previously described.  
         [0065]    Although the invention has been described with reference to a specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. It is therefore, contemplated that the claims will cover any such modifications or embodiments that fall within the true scope of the invention.