Patent Publication Number: US-6664868-B1

Title: Shim-tuned coaxial cable impedance transformer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to impedance matching transformers and, more particularly, to a shim-tuned coaxial cable impedance transformer. 
     2. Description of the Related Art 
     A generator, such as a transmitter, for example, is typically designed to operate into a specific impedance of a network. However, a load (e.g., an antenna) that is coupled to the generator usually does not provide the specific impedance in which the generator is designed to operate. 
     When the impedance of the load and the impedance as seen by the generator are equal, maximum power is transferred from the generator to the load over a transmission line coupling the generator to the load. If a mismatch between the impedances of the load and generator occurs, however, the power that is not transferred to the load may be returned towards the generator through the transmission line. These rearward-traveling waves may combine with their respective forward-traveling waves along the transmission line, and because of the phase differences along various positions within the line, may cause standing waves in the transmission line by the alternate cancellation and reinforcement of the voltage and current distributed along the transmission line. The larger the standing waves that occur along the transmission line, the greater the mismatch of the impedance of the load that is coupled to the generator. 
     In an attempt to compensate for this impedance mismatch between the generator and the load, series-tuned transformers, such as slug-tuned transformers, for example, have been used. These particular transformers, however, have been historically difficult to accurately construct and calibrate, thus resulting in a very limited improvement, if any, in impedance matching a generator to a load. Slug-tuned transformers are typically problematic because relatively large frequency shifts make it very difficult to match high standing wave ratio (SWR) values of the transmission line. Additionally, the slugs within the slug-tuned transformers cannot be changed or adjusted within the transformer without disassembly of the transformer. Accordingly, the slug-tuned transformer is difficult to calibrate as a result of the need to disassemble the transformer to replace and/or adjust the slugs. 
     The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is seen in a transformer for matching the impedance of a generator and a load coupled to the generator via a transmission line. The transformer includes an outer conductor having an inner surface and an inner conductor positioned within the outer conductor. The transformer further includes at least one shim disposed on the inner surface of the outer conductor and encircling the inner conductor. The at least one shim is slideable along the inner surface of the outer conductor for matching the impedance of the generator and the impedance of the load. 
     Another aspect of the present invention is seen in a system. The system comprises a generator for generating a signal and a load for receiving the signal generated by the generator. The system further includes a transformer coupled between the generator and the load. The transformer includes an outer conductor having an inner surface and an inner conductor positioned within the outer conductor. The transformer further includes at least one shim disposed on the inner surface of the outer conductor and encircling the inner conductor. The at least one shim is slideable along the inner surface of the outer conductor for matching the impedance of the generator and the impedance of the load. 
     Another aspect of the present invention is seen in a method for matching the impedance of a generator to a load coupled to the generator via a transmission line. The method comprises providing an outer conductor having an inner surface and providing an inner conductor positioned within the outer conductor. The method further comprises providing at least one shim disposed on the inner surface of the outer conductor and encircling the inner conductor, the at least one shim being slideable along the inner surface of the outer conductor for matching the impedance of the generator and the impedance of the load. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
     FIG. 1 shows a simplified block diagram of a wireless transmission network, including a shim-tuned transformer for impedance matching a transmitter to an antenna in accordance with one embodiment of the present invention; 
     FIG. 2 illustrates a more detailed representation the shim-tuned impedance matching transformer of FIG. 1; 
     FIGS. 3A-C provide a cross-sectional view of portions of the shim-tuned impedance matching transformer of FIG. 2 according to one embodiment of the present invention; 
     FIG. 3D provides a detailed representation of the shims of the impedance matching transformer of FIG. 2 incorporating mating teeth formed on the edges of the shims for combining the shims with one another; 
     FIG. 4 provides a cross-sectional view of the shim-tuned impedance matching transformer of FIG. 2 according to one embodiment of the present invention; and 
     FIG. 5 illustrates a process for designing the shim-tuned impedance transformer of FIG. 2 according to one embodiment of the present invention. 
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     Turning now to the drawings, and specifically referring to FIG. 1, a simplified block diagram of a transmission network  100 , employing a shim-tuned transformer, is shown in accordance with one embodiment of the present invention. In the illustrated embodiment, the transmission network  100  may be used for a variety of wireless applications including, but not necessarily limited to, AM, FM, SSB, TV, satellite, cellular, and PCS communications. In addition to the aforementioned examples, it will be appreciated that the transmission network  100  may operate in accordance with various other wireless transmission protocols without departing from the spirit and scope of the present invention. It will further be appreciated that the transmission network  100  may alternatively take the form of a receiving network for receiving signals either in addition to or in lieu of transmitting signals. 
     In one embodiment of the present invention, the transmission network  100 , in one of its simplest forms, comprises a transmitter  105  for generating signals, a transmission line  115  for carrying the signals generated by the transmitter  105 , and an antenna  120  for sending the signals generated by the transmitter  105  via a wireless communication medium to a receiver station (not shown). Although the network  100  of FIG. 1 is provided in the form of a transmission network, its application is not so limited. It will be appreciated that the transmitter  105  may take the form of any type of signal generator and the antenna  120  may take the form of any type of load. Accordingly, the transmission network  100  illustrated in FIG. 1 need not necessarily be limited to a wireless transmission network, but may take on a variety of other forms where the need for impedance matching a signal generator to a load is desirable. 
     In accordance with one embodiment of the present invention, the transmission line  115  that couples the transmitter  105  to the antenna  120  is provided in the form of a coaxial cable, such as RG8A coaxial cable, for example. It will be appreciated, however, that the transmission line  115  may include various other types of known transmission lines in lieu of a coaxial cable without departing from the spirit and scope of the present invention. 
     When the load impedance of the antenna  120  (i.e., the load) and the characteristic impedance Z 0  (as seen from the transmitter  105 ) are equal, maximum power is transferred via the transmission line  115  to the antenna  120 . If a mismatch of these impedances occurs, however, the power that is not transferred via the transmission line  115  to the antenna  120  may be returned towards the transmitter  105 . These rearward-traveling waves may combine with their respective forward-traveling waves on the transmission line  115 , and because of the phase differences along various positions within the transmission line  115 , may cause standing waves in the transmission line  115  by the alternate cancellation and reinforcement of the voltage and current distributed along the transmission line  115 . 
     To compensate for the impedance mismatch of the transmitter  105  and the antenna  120  that may occur, the transmission network  100  is provided with a shim-tuned transformer  110 . In accordance with the illustrated embodiment, the shim-tuned transformer  110  substantially matches the characteristic impedance as seen from the transmitter  105  to the load impedance of the antenna  120  to maximize the power that is transferred from the transmitter  105  to the antenna  120  via the transmission line  115 . 
     Turning now to FIG. 2, a more detailed representation of the shim-tuned transformer  110  is shown in accordance with one embodiment of the present invention. The transformer  110  comprises an outer conductor  205  and a center conductor  210  that is disposed lengthwise within the outer conductor  205 . In the illustrated embodiment, the outer conductor  205  takes the form of a copper tube. It will be appreciated, however, that the outer conductor  205  may be constructed out of other suitable conductive materials, as opposed to copper, without departing from the spirit and scope of the present invention. According to one embodiment of the present invention, the outer conductor  205  is provided with an elongated opening or slot (not viewable in FIG. 2) that runs lengthwise along the top surface of the outer conductor  205 . The functionality of this slot formed on the outer conductor  205  will be appreciated as the description proceeds. 
     Referring now to FIG. 3A, a cross-sectional view of a portion of the shim-tuned transformer  110  is provided. The outer conductor  205  encircles the center conductor  210 , and a slot  305  is formed therein lengthwise along the top surface of the outer conductor  205  so as to provide an elongated opening between the inside and outside of the outer conductor  205 . 
     Referring back to FIG. 2, the transformer  110  comprises a pair of shims  215 ,  220 , in accordance with one embodiment, that are moveably disposed on the inner surface of the outer conductor  205 . The shims  215 ,  220  (as illustrated in FIG. 2) are viewed as if one could see through the outer conductor  205 ; although in reality, the shims  215 ,  220  reside on the inner surface of the outer conductor  205  and are not viewable from the outside surface of the outer conductor  205 . Although two shims  215 ,  220  are illustrated in FIG. 2, it will be appreciated that the number of shims  215 ,  220  disposed on the inner surface of the outer conductor  205  may vary. For example, the transformer  110  may include three, four, or more shims  215 ,  220  disposed on the inner surface of the outer conductor  205  without departing from the spirit and scope of the present invention. In an alternative embodiment, the shims  215 ,  220  may be configured with mating teeth  320  (FIG. 3D) on each mating end of the shims  215 ,  220  such that the shims  215 ,  220  may be joined in a “locking” relationship so as to form one shim  215 ,  220  using various standard shim lengths. It will further be appreciated that the shims  215 ,  220  may be joined using other types of mating mechanisms, as opposed to the mating teeth herein described, without departing from the spirit and scope of the present invention. 
     The spacing between the shims  215 ,  220  is adjustable, along the outer conductor  205  to substantially match the characteristic impedance as seen by the transmitter  105  and the load impedance of the antenna  120  of the transmission network  100 . Once the proper spacing between the shims  215 ,  220  is set, the shims  215 ,  220  may then be moved as a unit along the outer conductor  205  to substantially match the impedances. Accordingly, both the spacing between the shims  215 ,  220  and their location along the outer conductor  205  are adjustable. 
     Referring to FIG. 3B, a cross-sectional view of a portion of the transformer  110  is shown where at least one of the shims  215 ,  220  is disposed therein. In the illustrated embodiment, the shim  215 ,  220  takes the form of a cylindrical shape and is disposed on the inner surface of the outer conductor  205  so as to encircle the center conductor  210 . 
     Turning now to FIG. 4, a side view perspective of the shim-tuned transformer  110  is shown in accordance with one embodiment of the present invention. The shims  215 ,  220  disposed on the inner surface of the outer conductor  205  respectively include header tabs  415 ,  420  that rise through the slot  305  that runs lengthwise along the top of the outer conductor  205 . The header tabs  415 ,  420  permit the shims  215 ,  220  to be moved along the inside surface of the outer conductor  205  by sliding their respective header tabs  415 ,  420  along the slot  305  that runs along the top of the outer conductor  205 . A cross-sectional perspective view of the header tabs  415 ,  420  are shown in FIG. 3B, protruding from the slot  305  of the outer conductor  205 . 
     The header tabs  415 ,  420  permit movement of the shims  215 ,  220  within the outer conductor  205  to calibrate the transformer  110  to match the characteristic impedance and the load impedance of the antenna  120  without the inconvenience of disassembling the transformer  110 . In accordance with one embodiment, the movement of the header tabs  415 ,  420  may be performed by human interaction. Alternatively, the transformer  110  may be configured with a motor-driven mechanism (not shown) to move the header tabs  415 ,  420  of the transformer  110 . 
     According to one embodiment of the present invention, a thin coat of polytetrafluroethylene (PTFE) may be applied to the inner surface of the outer conductor  205  to facilitate movement of the shims  215 ,  220  along the inner surface of the outer conductor  205 . PTFE is commercially made available by Dupont as Teflon®. It will be appreciated, however, that other types of coating materials that are suitable for facilitating the movement of the shims  215 ,  220  within the outer conductor  205  may be used in lieu of PTFE without departing from the spirit and scope of the present invention. 
     Referring again to FIG. 4, once the proper spacing of the shims  215 ,  220  is determined, a pre-sprung shielding material  430  may be placed within the slot  305  of the outer conductor  205  to prevent the header tabs  415 ,  420 , and their respective shims  215 ,  220 , from shifting within the outer conductor  205  of the transformer  110 . Referring to FIG. 3C, a cross-sectional view of a portion of the transformer  110  is shown. The shielding material  430  is pressed into the slot  305  of the outer conductor  205  to substantially prevent the shim header tabs  415 ,  420  of their respective shims  215 ,  220  from shifting within the slot  305  of the outer conductor  205  once the transformer  110  is calibrated for optimal impedance matching. 
     When it is desired to adjust the spacing of the shims  215 ,  220  within the transformer  110 , the shielding material  430  may be removed from the slot  305  of the outer conductor  205 . Subsequent to removing the shielding material  430  from the slot  305 , the spacing of the shims  215 ,  220  may then be adjusted by sliding the shim header tabs  415 ,  420  along the slot  305  of the outer conductor  205 . When the desired position of the shims  215 ,  220  is achieved by moving their respective shim header tabs  415 ,  420  along the slot  305  of the outer conductor  205 , the shielding material  430  may then be pressed into the remaining gaps of the slot  305  (i.e., the gaps in the slot  305  adjacent the shim header tabs  415 ,  420 ) to prevent the shims  215 ,  220  from shifting within the outer conductor  205  of the transformer  110  once calibrated. 
     According to one embodiment, the transformer  110  is further provided with connectors  440  on each end of the outer conductor  205  to permit connection of the transformer to the transmission line  115  of the transmission network  100 . In one embodiment, the connectors  440  are of the quick-change type, and the connectors  440  are fastened to the outer conductor  205  of the transformer  110  by set screws  445 . It will be appreciated, however, that the type of connectors  440  used for coupling the transformer  110  to the transmission line  115  and the manner in which the connectors  440  are fastened to the outer conductor  205  may vary without departing from the spirit and scope of the present invention. 
     In accordance with one embodiment of the present invention, the overall length of the shim-tuned transformer  110  is the sum of one-half the wavelength needed for phase adjustments, the optimum distance between the shims  215 ,  220 , and the combined length of the shims  215 ,  220 . Adjustments may be made with conventional impedance matching instruments such as watt meters, impedance bridges, and the like. By shortening the overall length of the outer conductor  205 , the length of the shims  215 ,  220 , and reducing the spacing between the shims  215 ,  220  may widen the bandwidth of the shim-tuned transformer  110 . This will, of course, limit the standing wave ratio (SWR) reducible to unity. The lengths of the shims  215 ,  220  may be cut shorter by a factor of 1/(ε) ½ , where ε is the velocity factor, to compensate for the slower speed of the electrons through the transformer dielectric in comparison to the speed of the electrons in air. For example, ε is typically measured at 0.66 for a transmission line  115  including RG8A coaxial cable. 
     Several different characteristic impedances may be produced for the transmission network  100  using the shim-tuned transformer  110  by varying the thickness of the shims  215 ,  220 . For example, a shim gauge of 15 is 0.0673 inches thick. When the shims  215 ,  220  (using gauge 15) are inserted within the outer conductor  205  having an outer diameter of 0.5 in., it reduces the inside diameter of the outer conductor  205  and produces a shim impedance (z t ) of 40.69 Ω and a characteristic impedance Z t  of 0.814. Alternatively, a shim gauge of 7 is 0.1793 inches thick, and when the shim  215 ,  220  (using gauge 7) is inserted within the outer conductor  205  it reduces the inside diameter of the outer conductor  205  by producing a shim impedance (z t ) of 21.17 Ω and a characteristic impedance Z t  of 0.423. From these examples, it will be appreciated that the impedance may be altered by using different thicknesses of the shims  215 ,  220 , inner diameters of the outer conductor  205 , and inner wire gauges for the center conductor  210 . 
     In one embodiment of the present invention, the shim-tuned transformer  110  having a length of 1.25 wavelengths with the shims  215 ,  220  having a total of one-fourth wavelengths (i.e., one-eighth wavelengths each) and having a ratio of shim impedance (z t ) to a characteristic impedance (Z 0 ) of 0.4 can match a transmission line  115  having a 40:1 voltage SWR. The minimum SWR occurs when the two shims  215 ,  220  are placed within the outer conductor  205  such that the spacing between them are conjugate and the shims  215 ,  220  are adjusted as a unit over the 1.25 wavelength distance of the shim-tuned transformer  110 . 
     Turning now to FIG. 5, a process  500  for designing the shim-tuned transformer  110  is provided in accordance with one embodiment of the present invention. The process  500  commences at block  505  where the resistance and the reactance of the antenna  120  is determined. According to one embodiment, the resistance and reactance of the antenna  120  may be calculated with a Numerical Electromagnetic Code method of moments antenna-modeling tool such as EZNEC 3.0, which is available by EZNEC Antenna Software, Beaverton, Oreg. At block  510 , the size of the center conductor  210  is determined to match the output impedance of the transmitter  105 . In the illustrated embodiment, the size of the center conductor  210  is selected based upon the current handling requirements at the RF frequency in which the transmitter  105  is tuned. 
     The process  500  continues at block  515  where the inside diameter of the outer conductor  205  is determined from the gauge size used for the outer conductor  205 . At block  520 , the thickness of the shims  215 ,  220  are determined based upon the outer diameter of the outer conductor  205 . 
     At block  525 , the amount of spacing between the shims  215 ,  220  is calculated using a Smith Chart® or Smith software, such as WinSmith®, available from Nobel Publishing Company, Atlanta, Ga. The transformer  110  is then constructed at block  530  and the spacing between the shims  215 ,  220  and the location of the shim assembly along the outer conductor  205  is adjusted for optimal impedance matching between the transmitter  105  and the antenna  120 . 
     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.