Patent Publication Number: US-7898357-B2

Title: Coaxial impedance matching adapter and method of manufacture

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/052,606, “Coaxial Impedance Matching Adapter”, by Pratibha Chaulagi Phuyal, Kendrick Van Swearingen, Albert Cox and Jeffrey D. Paynter filed May 12, 2008—currently pending and hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     RF systems components are generally configured for a standardized characteristic impedance, enabling interconnection of the various systems components with reduced power losses. 
     Standardized characteristic impedances common in the RF industry include 50, 75, 110 and 300 Ohms. At microwave frequencies, the preferred characteristic impedance is 50 ohms. Therefore, a large number of microwave frequency RF devices such as transceivers, antennas, interconnecting transmission lines and other devices configured for in-line connection are configured for 50 ohm characteristic impedance. 
     Prior impedance matching adapters have applied a range of different electrical circuitry and/or apparatus to transform characteristic impedance, for example, between 50 and 75 ohms. RLC lumped element impedance transformers, such as wound ferrite toroids, may generate undesirable parasitic effects as operating frequencies increase, for example above 500 Mhz. 
     Another impedance matching solution is application of a load inline with the transmission line. Impedance transformers of this type may introduce an insertion loss that is unacceptably high. 
     The dimensions of microstrip transmission lines may be manipulated to form low loss impedance transformers. Multi-section impedance matching transformers such as Chebyshev ¼ wavelength, coaxial, microstrip or stripline transformers apply a series of transmission line width steps, each spaced ¼ wavelength apart along the transmission line. Passage along the transmission line through each step raises or lowers the characteristic impedance depending upon the direction of travel. Depending upon the desired operating frequency(s) and acceptable insertion loss levels, a series of steps separated by ¼ wavelength each, suitable to arrive at the desired characteristic impedance transformation, may require a transmission line of considerable length. 
     Cost of manufacture, including materials costs and labor, may be a significant factor in commercial success in the impedance adapter market. 
     Therefore, it is an object of the invention to provide an apparatus that overcomes deficiencies in the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general and detailed descriptions of the invention appearing herein, serve to explain the principles of the invention. 
         FIG. 1  is a schematic external side view of a first exemplary embodiment of a Coaxial Impedance Matching Adapter. 
         FIG. 2  is a schematic partial cross section view of  FIG. 1 . 
         FIG. 3  is a schematic angled isometric view of internal components of the Coaxial Impedance Matching Adapter of  FIG. 1 , showing the second side, with the enclosure removed for clarity. 
         FIG. 4  is a schematic partial cross section exploded isometric view of the Coaxial Impedance Device of  FIG. 1 . 
         FIG. 5  is a schematic partial cross sectional isometric view of a second exemplary embodiment of a Coaxial Impedance Matching Adapter, shown attached to a coaxial cable. 
         FIG. 6  is a schematic partial cross sectional side view, the cross section normal to a plane of the printed circuit board, of the Coaxial Impedance Matching Adapter of  FIG. 5 . 
         FIG. 7  is a close-up view of area A of  FIG. 6 . 
         FIG. 8  is an exploded isometric view of the printed circuit board and ground block of  FIG. 6 . 
         FIG. 9  is an exploded isometric view of a first alternative printed circuit board and ground block configuration. 
         FIG. 10  is an exploded isometric view of a second alternative printed circuit board and ground block configuration. 
         FIG. 11  is a schematic cross sectional side view of a third exemplary embodiment of a Coaxial Impedance Matching Device. 
         FIG. 12  is a network analyzer plot of a printed circuit board microstrip assembly from the Coaxial Impedance Matching Device of  FIG. 5 , wherein measurements were taken with the printed circuit board separate from the enclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The inventor(s) have recognized that, due to the larger dimensions of 50 ohm characteristic impedance transmission lines and connectors, 50 ohm characteristic impedance systems have a significantly higher materials, manufacturing, logistics and installation cost than systems having higher characteristic impedances. Further, the inventors have recognized that a cost efficient inline coaxial impedance matching adapter with low insertion loss and signal degradation characteristics would enable use of, for example, 75 ohm characteristic impedance interconnecting transmission lines and connectors within an otherwise 50 ohm characteristic impedance system, thereby achieving a significant cost savings. 
     A first exemplary embodiment of a Coaxial Impedance Matching Adapter (CIMA)  2  is demonstrated in  FIGS. 1-4 . The CIMA  2  may be configured for in-line insertion between sections of coaxial transmission line and/or various RF devices such as radios and antennas. The CIMA  2  may be designed for insertion via standardized or proprietary coaxial connector interfaces or via direct interconnection with and/or between two coaxial cable transmission lines. The first exemplary embodiment demonstrates a CIMA  2  configuration for 50 ohm characteristic impedance transmission line at a first end  4  and for a standardized coaxial connector interface commonly used for 75 ohm characteristic impedance transmission lines/equipment at a second end  6 . 
     The first end  4  and the second end  6  are also applied herein as identifiers for respective ends of discrete elements of the CIMA  2 , to identify same according to their alignment along a longitudinal axis of the CIMA  2  extending between the first end  4  and the second end  6 . 
     A microstrip transmission line is contained within an environmentally sealed cavity  7  formed enclosed by a body portion  8  and a mating cap portion  10 . The microstrip transmission line is formed as a trace  12  on a first side  14  of a printed circuit board (PCB)  16  ( FIG. 2 ) having a ground plane  18  on a second side  20  ( FIG. 3 ). 
     To minimize the overall length requirements for the microstrip transmission line, the PCB  16  substrate material is preferably selected to be a microwave quality substrate having a high and uniform dielectric constant. Suitable PCB  16  substrates include glass filled PTFE substrates available from the Taconic Corporation of Petersburgh, N.Y., USA such as RF-60A, 30 mil (Dk=6.15) or the like. 
     The microstrip transmission line trace  12  may be derived from a series of Chebyshev ¼ wavelength spaced apart trace  12  width step(s)  22  that reduce the trace  12  width from a wide width at the low impedance end, here the first end  4 , to a narrower width at the high impedance end, here the second end  6 . The number of steps(s)  22 , each separated, for example, by ¼ wavelength of a desired operating frequency of the CIMA  2 , is determined by the desired operating bandwidth, with more step(s)  22  increasing the bandwidth characteristics of the CIMA  2 . Further tuning of the microstrip parameters may be applied to tune for specific portions of the desired frequency band with respect to specific step(s)  22 , bend(s)  24  and/or the placement of nearby ground field sources, such as the cavity sidewall(s)  13  and/or fastener(s)  29 . Alternative trace  12  solutions include ¼ wave stub resonator filters aligned in parallel or series configurations, and/or various microstrip bandpass filters. Any trace solution which net transforms the characteristic impedance may be applied, including bandwith tuned microstrip traces. 
     The inventors have recognized that, because the manufacture of an enclosing metal housing of extended length with a high tolerance bore and suitable strength characteristics may represent a significant portion of the resulting device cost, the overall length of the PCB  16  should be reduced. As best shown in  FIG. 2 , to reduce the length of the PCB  16 , the trace  12  is formed with a bended and/or doubled back sinuous path including a series of sharp and/or arcuate bend(s)  24  that route the trace  1   2  away from and then back to a centerline of the longitudinal axis of the CIMA  2 , resulting in the trace  12  having a length greater than a longitudinal axis length of the PCB  16 . 
     Each bend  24  may create an opportunity for signal degradation. To address the potential signal degradation introduced by each bend  24 , the bend(s)  24  of the present embodiment are demonstrated with shallow angles, such as 45 degrees, and a miter corner is demonstrated applied to the outer side  26  of each bend  24 . To further minimize signal degradation the step(s)  22  may be located at linear portions of the trace  12 , such as midpoints of linear trace  12  segments between each of the bend(s)  24 . 
     The body portion  8  and mating cap portion  10  that together form the surrounding enclosure are demonstrated as being formed with a coaxial bore that forms a cavity  7  for the PCB  16  with a first diameter  28  at either end and a larger second diameter  30  in a mid portion. The PCB  16  is dimensioned to seat within the cavity  7 , the PCB  16  dimensions matching a cross section of the cavity  7 , the trace  12  aligned with the longitudinal axis at the first end  4  and the second end  6  for interconnection with an inner conductor  31  of the respective coaxial transmission line  33  (see  FIG. 5 ) and/or coaxial connector interfaces applied to each of the first and second ends  4 , 6 . In the present embodiment, the connection to the inner conductor(s)  31  is via a first contact  42  surrounded adjacent the PCB  16  by a first end insulator  44  and a second contact  46  surrounded adjacent the PCB  16  by a second end insulator  48 . The first and second contact(s)  42 ,  46  may be coupled to the trace  12  by soldering a preferably less than half diameter section of each thereto, thus maintaining axial alignment of the first side  14  of the PCB  16 , and thus the interconnections with the trace  12 , to the enclosure. 
     As best shown in  FIG. 3 , the ground plane  18  applied to the second side  20  of the PCB  16  may be spaced slightly away from the PCB  16  periphery so that interconnection between the ground plane  18  and the enclosure sidewall(s)  13  is limited to ground contact(s)  32  placed proximate the first end  4  and second end  6  of the PCB  16 , respectively. The ground contact(s)  32  extend along the width of the PCB  16  on the second side  20  at each end and extend radially outward to the first diameter  28 . At the first diameter  28 , a plurality of spring finger(s)  34 , oriented to extend toward a center of the cavity, provide a secure electromechanical contact with the enclosure sidewall(s)  13 . 
     As demonstrated in  FIG. 4 , the CIMA  2  may be assembled by preparing a sub-assembly of the PCB  16 , ground contact(s)  32 , any supporting structure, the first and second contact(s)  42 ,  46  and respective first and second end insulator(s)  44 ,  48 . Insulator mating surfaces of the first and second contact(s)  42 ,  46  may be knurled to rotationally interlock them with their respective insulator. The sub-assembly may then be inserted into the bore of the body portion  8 , the PCB  16  seating within the first diameter  28  and second diameter  30 , the second end  6  ground contact  32  spring finger(s)  34  engaging the first diameter  28 , and the second end insulator  48  passing through and seating within an inner conductor bore  50 , mating surfaces of which may also be knurled to prevent rotation of the respective first and second end insulator(s)  44 ,  48  therein. 
     To further support the PCB  16  and ground contact(s)  32  during CIMA  2  assembly, installation and over extended periods of use, a supporting structure such as a support block  36  may be applied to the second side  20  and/or within the body portion  8 . A fastener screw  41  passing through the sidewall  30  may be applied to secure the support block  36  against the second diameter sidewall  30 . The support block  36  is configured to receive a tension bar  38  that extends between contact with each of the ground contact(s)  32  proximate the first diameter  28 . A tension screw  40 , also passing through the sidewall  30  and support block  36 , is positioned to retain and bias the tension bar  38  into secure supporting contact with the ground contacts  32  after the PCB  16  with attached ground contacts  32  is inserted into the cavity  7  of the body portion  8 . 
     The sub-assembly may be axially locked within the body portion  8  by an outward projecting shoulder  49  formed in the second insulator  48  (see  FIG. 6 ) and dielectric key(s)  52  that seat in a retaining groove  54  of the second end insulator  48  adjacent a second end  6  shoulder  58  of the inner conductor bore  50 . The dielectric key(s)  52  are then retained by a dielectric retainer  56  that fits between the dielectric key(s)  52  and the second end  6  bore sidewall, preventing radial movement of the dielectric key(s)  52  out of engagement with the retaining groove  54 . 
     The PCB  16  is sealed within the cavity  7  by placing the cap portion  10  over the first end  4  of the body portion  8 . As the cap portion  10  is seated, the first contact passes through a corresponding inner conductor bore  50  of the cap portion  10  and the first end insulator  44  is seated therein, the first end  4  of the PCB  16  seating within a first diameter  28  of the cap portion  10 , and the first end  4  ground contact  32  spring finger(s)  34  engaging the first diameter  28 . The cap portion  10  and the body portion  8  engage, for example, with overlapping annular shoulder portions that seat against each other. Once seated, the cap portion  10  and the body portion  8  may be permanently joined together, sealing the PCB  16  there within, for example by a swage operation bending the outer overlapping annular shoulder portion over the inner overlapping shoulder. Alternatively, the body portion  8  and the cap portion  10  may be provided with a threaded interconnection. 
     Depending upon the desired interconnection interface(s) provided at the first and second ends  4 , 6 , the first and second contacts  42 , 46  may each be provided with a spring basket  60  for receiving and securely gripping the inner conductor  31  of a mating connector or coaxial cable transmission line  33 . A compression member  62  may be provided on one or both of the connection interfaces to improve connection characteristics of the first and or second contact(s)  42 , 46 . The compression member  62  may be retained, for example, by a snap connection into an inner diameter annular compression member groove  64  proximate the respective end of the CIMA  2  along the extent of which the compression member  62  is axially movable. A wedge surface  66  formed on an inner diameter of an aperture of the compression member  62  is dimensioned to mate with a ramp surface  68  formed on distal ends of spring fingers extending from the dielectric retainer  56  over the spring fingers that together form the spring basket  60 . As the compression member  62  is shifted axially towards the CIMA  2  during cable or connector connection, the wedge surface  66  engages the ramp surface  68 , biasing the ramp surface  68  radially inward and thereby the spring basket  60  radially inward to more securely grip the mating inner conductor  31 . 
     The overall length of the PCB  16  is shortened as a function of the distance the bend(s)  24  allow the trace  12  to be spaced away from the longitudinal axis in a sinuous path, resulting in the trace  12  having a longer length than a longitudinal axis length of the PCB  16 . However, spacing the trace  12  away from the longitudinal axis also increases the required diameter of the surrounding enclosure. 
     A second exemplary embodiment, as shown in  FIGS. 5-11 , demonstrates an alternative trace  12  layout that includes a sinuous path in which bend(s)  24  are arranged to direct the trace  12  back upon itself and/or across the longitudinal axis to enable a reduction of the CIMA  2  overall length and/or second diameter  30  and thus the size of the surrounding enclosure, potentially with a trade-off with electrical performance. The bends are also demonstrated with a continuous width, i.e. inside and outside edges formed as arc segments with a common radius centerpoint. Further, to minimize the size of the surrounding enclosure, the trace  12  may be spaced away from the enclosure sidewall by, for example, a minimum of at least four times the trace  12  width from each edge. 
     The PCB  16  is supported in the second exemplary embodiment by a ground block  70  that also provides a secure electrical interconnection between the ground plane  18 , the cap portion  10  and the body portion  8 . As best shown in  FIGS. 6 and 7 , first and second ends  4 ,  6  of the ground block  70  are demonstrated formed with a projecting rim  72  that mates with, for example, a ground groove  74  formed in each of the cap portion  10  second end  6  and the body portion  8  first end, each proximate and coaxial with the inner conductor bore  50 . The projecting rim  72  may have a flexure characteristic, the projecting rim  72  formed with a slight, for example outward, misalignment with the ground groove  74 , resulting in a bias between the ground groove(s)  74  and the respective projecting rim(s)  72  when assembled, thereby, providing a secure electrical interconnection therebetween. As shown in  FIG. 8 , the ground block  70  may be coupled to the ground plane  18  on the second side  20  of the PCB  16  via a retainer  76  such as solder and/or conductive adhesive. 
     Alternatively, as shown in  FIGS. 9 and 10 , the ground block  70  may be dimensioned with a narrowed midsection and/or coupled with the assistance of fastener(s)  29  or orientation posts of the ground block  70  projecting through corresponding apertures of the PCB  16 . Also as demonstrated in  FIG. 10 , the ground block  70  may be provided with a cavity to further reduce materials cost and/or weight. One skilled in the art will appreciate that, due to the presence of the cylinder half section structure of the ground block  70 , the projecting rim  72  may be formed with a flexure characteristic without overhanging edges along a mold break line, enabling cost effective manufacture by die casting, metal injection molding or the like. 
     Further stabilization and/or support of the PCB  16  and ground block  70  may be provided by an annular protrusion  73  of the first end  4  of the second insulator  48  and the second end  6  of the first insulator  44  that keys with a corresponding annular groove  74  of the first and second ends  4 , 6  of the ground block  70  and/or the PCB  16 , as shown for example in  FIGS. 6 and 7 . In embodiments where the first and second contacts  42 , 46  are not required to resist high longitudinal and/or torsional forces, the insulator keylock arrangement may be simplified, for example as demonstrated in  FIG. 11 , where both ends of the CIMA  2  are configured as coaxial connector interfaces. 
     As shown in  FIG. 12 , a plot of the PCB  16  of  FIGS. 8-10  has been bench tested with a network analyzer separate from the enclosing metal housing, demonstrating viability of the trace  12  configuration for use over a target bandwidth of between at least 698 MHz and 3 Ghz. 
     In further alternative embodiments, one skilled in the art will appreciate that the CIMA  2  may similarly be configured with a multi-layer and/or multiple printed circuit board(s)  16 , for example in a strip line, rather than microstrip configuration. 
     One skilled in the art will also appreciate that the CIMA  2  may provide an improvement in the signal characteristics, materials and manufacturing costs of an in-line impedance matching device to a level that enables previously impractical substitution of lower cost higher characteristic impedance transmission line and components into multi-band microwave communications systems. Although an in-line connector terminated coaxial body embodiment has been described in detail, one skilled in the art will appreciate that any manner of enclosure may also be applied, including incorporating the PCB  16  into enclosures, for example, formed in non-coaxial configurations such as rectangular cast metal or polymeric enclosures provided with connection interfaces on desired or common sides. Further, the sub-assembly may be incorporated with further equipment and or circuits into existing enclosures with appropriate stand offs applied to isolate the PCB  16  from nearby electrical fields and or shorting surfaces. 
     
       
         
           
               
             
               
                   
               
               
                 Table of Parts 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 2 
                 Coaxial Impedance Matching Device 
               
               
                 4 
                 first end 
               
               
                 6 
                 second end 
               
               
                 7 
                 cavity 
               
               
                 8 
                 body portion 
               
               
                 10 
                 cap portion 
               
               
                 12 
                 trace 
               
               
                 13 
                 cavity sidewall 
               
               
                 14 
                 first side 
               
               
                 16 
                 printed circuit board 
               
               
                 18 
                 ground plane 
               
               
                 20 
                 second side 
               
               
                 22 
                 step 
               
               
                 24 
                 bend 
               
               
                 26 
                 outer side 
               
               
                 28 
                 first diameter 
               
               
                 29 
                 fastener 
               
               
                 30 
                 second diameter 
               
               
                 31 
                 inner conductor 
               
               
                 32 
                 ground contact 
               
               
                 33 
                 coaxial cable transmission line 
               
               
                 34 
                 spring finger 
               
               
                 36 
                 support block 
               
               
                 38 
                 tension bar 
               
               
                 40 
                 tension screw 
               
               
                 41 
                 fastener screw 
               
               
                 42 
                 first contact 
               
               
                 44 
                 first end insulator 
               
               
                 46 
                 second contact 
               
               
                 48 
                 second end insulator 
               
               
                 49 
                 outward projecting shoulder 
               
               
                 50 
                 inner conductor bore 
               
               
                 52 
                 dielectric key 
               
               
                 54 
                 retaining groove 
               
               
                 56 
                 dielectric retainer 
               
               
                 58 
                 shoulder 
               
               
                 60 
                 spring basket 
               
               
                 62 
                 compression member 
               
               
                 64 
                 compression member groove 
               
               
                 66 
                 wedge surface 
               
               
                 68 
                 ramp surface 
               
               
                 70 
                 ground block 
               
               
                 72 
                 projecting rim 
               
               
                 73 
                 annular protrusion 
               
               
                 74 
                 ground groove 
               
               
                 75 
                 annular groove 
               
               
                 76 
                 retainer 
               
               
                   
               
            
           
         
       
     
     Where in the foregoing description reference has been made to ratios, integers, components or modules having known equivalents then such equivalents are herein incorporated as if individually set forth. 
     While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus, methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of applicant&#39;s general inventive concept. Further, it is to be appreciated that improvements and/or modifications may be made thereto without departing from the scope or spirit of the present invention as defined by the following claims.