Abstract:
A frequency selective low loss transmission system for communicating a signal using a coaxial cable of one impedance to a device of different impedance. A connector with a matching transformer is integral to the connector which terminates with a standard interface. The invention also includes a coupling mechanism to couple the coaxial cable with the connector. The invention can also include series open stub conductors for capacitive coupling to the conductors of the coaxial cable. In addition to low losses over a broad frequency range, the connector facilitates connector installation due to the series open stub conductor while reducing cost and complexity of both coaxial cable and connector.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a transmission line system that is optimized for low loss. More particularly, the invention relates to a transmission line system and a connector for communicating a coaxial cable of one impedance with a device of another impedance with low losses. 
   2. Description of the Related Art 
   A communication industry transmission standard is a 50 ohm impedance for communication systems. A 75 ohm coaxial transmission cable, however, has lower attenuation characteristics and a higher operating frequency than a 50 ohm coaxial transmission cable, thus making the 75 ohm transmission cable a better choice for some broadcast applications and CATV industries. To employ a transmission cable with higher impedance, broadcast systems may require separate matching transformers to convert the impedance back to a typical 50 ohm device and CATV systems require 75 ohm mating connectors and amplifiers to integrate the 75 ohm cables into the respective systems. One specific application is the use of telecommunication cables in the PCS band for mobile telephones. The frequency band for this service is 1850 to 1990 MHz in the United States. This band involves very high frequencies, but not high enough to justify the cost of waveguides or tower loading to lower the attenuation. Therefore, a system is desired that reduces signal loss while having low product and implementation cost. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a communication system comprising a signal on a coaxial transmission line which provides lower attenuation given the frequency of the signal, and a mating connector. The connector includes an integral connector transformer with optimized impedance for matching a low loss cable such as the 70 ohm coaxial transmission line to 50 ohm devices through an interface. The 70 ohm transmission cable typically includes low-density foam and a smooth hollow tube center conductor. A corrugated tube or solid wire could be used depending on the overall diameter of the cable. The outer conductor of the cable is typically made of an annular corrugated copper tube configured to simplify connector installation and provide flexibility. Other designs for the outer conductor are possible, designs such as smooth or helical corrugations. The connector includes means for attaching the connector to the cable as will be discussed further. 
   In one embodiment, the connector comprises an integral quarter wave transformer designed for the desired frequency of operation and standard means of attaching the connector to cable conductors by providing electrical contacts. In another embodiment, there is a series quarter wave open circuit inner stub that capacitively couples to the hollow center conductor of a coaxial transmission line, along with an integral transformer. Alternatively, the stub is reversed for a solid center conductor with a hollow center conductor of the connector. In yet another embodiment, there is an integral transformer and a series quarter wave open circuit outer stub that capacitively couples to an outer conductor of a coaxial transmission cable. Additionally, there is an embodiment which includes both a series quarter wave open stub inner conductor, a series quarter wave outer conductor, and an integral quarter wave transformer. 
   The use of the series quarter wave open stub conductors and the integral transformer provide additional tuning to allow a wider frequency band of operation and still have a Voltage Standing Wave Ratio, or VSWR, of less than 1.02:1. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the present invention will be apparent from the following description taken in connection with the accompanying drawings, wherein: 
       FIG. 1  is a cross sectional view of an embodiment of the invention using a connector coupling design incorporating an integral quarter wave transformer; 
       FIG. 2  is a cross sectional view of an embodiment of the invention showing a series open circuit outer stub; 
       FIG. 3  is a cross sectional view of an embodiment of the invention showing a series open circuit outer stub disposed inside the outer conductor of the coaxial transmission line; 
       FIG. 4  is a cross sectional view of an embodiment of the invention showing a series open circuit inner stub; 
       FIG. 5  is another configuration of the series open circuit inner stub; 
       FIG. 6  is a cross sectional view of an embodiment of the invention comprising a series open circuit outer and inner stubs; and 
       FIG. 7  is a cross sectional view of an embodiment of the invention showing series open circuit outer and inner stubs, and an outer conductor choke. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An exemplary first embodiment will now be described with reference to the drawings. A cross sectional view of a frequency selective low loss coaxial electrical connector  100  is shown in FIG.  1 . The connector  100  is used to connect a first coaxial transmission line  180  with a first impedance to an electrical device (not shown) with a second impedance. By way of example, the first coaxial transmission line  180  has an impedance of 70 ohms and the electrical device is a second coaxial transmission line with the communication industry standard impedance of 50 ohms. The impedance of coaxial transmission line  180  is selected to provide the minimum attenuation depending on the construction and material used. It is noted that the first coaxial transmission line  180  and the electrical device can take on different impedance values than the ones above. 
   First coaxial transmission line  180  includes a typically smooth hollow tube center conductor  182 A surrounded by an insulation  184  with a dielectric constant ∈ 1 . The insulation  184  is made of any suitable dielectric, including, for example, solid polyethylene, foamed polyethylene, Teflon (polytetrafluoroethylene), fluorinated ethylene propylene, and foamed fluorinated ethylene propylene, or any material in combination with air. The choice of material and final foamed density will determine the dielectic constant and, therefore, the impedance that provides the lowest attenuation for a given size cable. The dielectric provides support to maintain the inner conductor on the axis of the cable. Surrounding the insulation  184  is an outer conductor  186 . The outer conductor  186  is typically made of an annular corrugated copper sheet to provide flexibility and ease in attaching standard connectors. Surrounding the outer conductor  186  is a protective cover  188 . 
   First coaxial transmission line  180  is coupled to the connector  100 . The connector  100  comprises a substantially cylindrical body  200  having a spaced first end portion  210 , second end portion  220 , and an elongate center portion  230  including a transformer section  700 . It is noted that the substantially cylindrical body  200  is electrically conductive. The elongate center portion  230  is disposed between the first end portion  210  and the second end portion  220 , and has an axial bore  240  therethrough. Additionally, there is a dielectric bead  250  with a dielectric constant ∈ 2  fixed inside the axial bore  240  at an end of the center portion  230 . As with the insulation  184  of the first coaxial cable  180 , the dielectric bead  250  is made of any suitable dielectric, including, for example, solid polyethylene, foamed polyethylene, Teflon, fluorinated ethylene propylene, and foamed fluorinated ethylene propylene. By way of example, the dielectric bead  250  is made of solid Teflon. The bead  250  may or may not be part of transformer section  700 . 
   There is a metal member  300  within the dielectric bead  250  and extending coaxially within the axial bore  240 . The metal member  300 , which is an inner conductor of the connector  100 , has first and second end portions  310  and  320  corresponding to the first and second end portions  210  and  220  of the cylindrical body  200 , and a center portion  330  corresponding to the center portion  230  of the cylindrical body  200 . In the axial bore  240 , the metal member  300  is fixed in place and electrically insulated from the cylindrical body  200  by the dielectric bead  250 . The first end portions  210  and  310  interfit with the first coaxial transmission line  180 . 
   Specifically, the first end portion  210  of the cylindrical body  200  mates with the outer conductor  186  in metal-to-metal electrical contact through a clamping ferrule  590 , and spring-type contacts of the first end portion  310  of the metal member  300  mates with the center conductor  182 A in metal-to-metal electrical contact. There are numerous standard means in the art to connect cable and connectors in metal-to-metal electrical contact that will not be described in detail. 
   Further, there is a coupling mechanism  500  to mate the coaxial transmission line  180  to the cylindrical body  200 . It is noted that there are numerous standard means in the art to couple cables and connectors, and they will not be described. 
   The second end portions  220  and  320  are shaped to interfit or mate with an electrical device. By way of example, the second end portions  220  and  320  comprise a standard 7-16 DIN-type cable interface to interfit with the electrical device. In another configuration, the second end portions  220  and  320  comprise a standard N-type cable interface (not pictured). 
   The center portions  230  and  330 , and the dielectric bead  250  cooperatively provide for a transformer impedance for matching the first impedance of the first coaxial transmission line  180  and the second impedance of the electrical device. To provide a matching impedance, the connector  100  has a characteristic impedance calculated by EQN. 1 below.
 
 Z   char   =√{square root over (Z     i     ·Z     o     )}   EQN. 1
 
wherein Z char  is a characteristic impedance of the transformer section in the connector,
         Z i  is an impedance of a coaxial transmission line; and   Z o  is an impedance of an electrical device.
 
In other words, the maximum power is transferred when the load impedance, i.e., impedance of the electrical device, is the complex conjugate of the source impedance, i.e., impedance of the coaxial transmission line.
       

   For the first embodiment, Z char  is the transforming impedance of the connector  100 , Z i  is the impedance of the first coaxial transmission line  180 , and Z o  is the impedance of the electrical device  900 . 
   The characteristic impedance of a electrically conducting coaxial body is given by EQN. 2. 
               Z   char     =       138     ɛ       ·     log   ⁡     (     D   d     )                 EQN   .           ⁢   2             
 
wherein D is an inside diameter of an outer conductor,
         d is an outside diameter of an inner conductor, and   ∈ is a dielectric constant of a dielectric between the inner and the outer conductors.       

   By way of example, the inside diameter of the center portion  330  is D and the outside diameter of the center portion  230  is d. The dielectric constant of air surrounding the center portion  230  is ∈. Applying EQN. 2 to the center portions  230  and  330 , and taking into account an impedance imparted by the dielectric bead  250 , provide the relationships between some of the physical dimensions of the center portions  230  and  330 . For example, a D substantially equivalent to the diameter of the outer conductor  186  of the first coaxial transmission line  180 , results in a center portion  330  of the metal member  300  having a d different than the outside diameter of the center conductor  182 A to provide for a Z char  satisfying EQN. 1, when using a 70 ohm coaxial transmission line and a 50 ohm electrical device. Alternatively, the center portions  230  and  330  may have different configurations as long as their respective dimensions satisfy EQNS. 1 and 2. 
   In other words, center portions  230  and  330 , and the dielectric bead  250  comprise a matching transformer section  700 . As shown in  FIG. 1 , the components of the matching transformer section  700 , i.e., center portions  230  and  330 , and the dielectric bead  250  are integral to the connector  100 . 
   To minimize signal losses in the connector  100 , a transforming length L including the center portions  230  and  330 , and the dielectric bead  250  has a value depending on the frequency of the signal carried in the connector  100 . Electrically, the distance of the transforming length L is from a first impedance transition A between the first impedance and the matching impedance, to a second impedance transition B between the matching impedance and the second impedance. For the embodiment shown in  FIG. 1 , the first impedance transition A is at the abutting terminal end of the first coaxial transmission line  180  and the second impedance transition B is at a side of the dielectric bead  250  abutting the second end portions  220  and  320 . 
   By way of example, a 1920 GHz signal requires a transforming length L of 1.014 inches with solid polyethylene filling the complete cavity of transformer length. In comparison, a connector without the dielectric bead  250  included in the transformer length L of one quarter wavelength in air, requires a length of 1.475 inches for a 1920 GHz signal. In effect, the presence of the dielectric bead  250  allows for a shorter transforming length L and therefore a shorter connector. The final length of bead or percentage of dielectric will be determined by mechanical integrity and cost. 
   By way of example, a quarter wave transformer can provide a VSWR of approximately 1.02:1 for a signal in the frequency band of 1850 to 1990 MHz. VSWR is the result of reflected waves, and a lower VSWR ratio translates into lower levels of undesirable signal reflections resulting from the connection of transmission lines or devices with mismatched impedance. It is noted that in another configuration (not pictured), the transforming length L can comprise an integral multiple of quarter wavelengths depending on the desired bandwidth. 
     FIG. 2  illustrates another embodiment of the invention. With respect to the embodiment shown in  FIG. 1 , this embodiment differs in the following. Instead of a first end portion  210  of the cylindrical body  200  in electrical contact with the outer conductor  186  (FIG.  1 ), there is a series open circuit outer stub  212 A capacitively coupled to the outer conductor  186 . The capacitive coupling is created by the larger inside diameter of the first end portion  210  of the cylindrical body  200  of the connector  100  surrounding the cable  180 . This cavity is preferably lined with a dielectric lining  214 A to maintain the proper alignment of components between the series open circuit outer stub  212 A and the outer conductor  186  and to prevent electrical contact. The dielectric lining  214 A is made of a suitable dielectric material such as polyethylene. 
   Additionally, the embodiment includes a resilient gland  510 A disposed at a distal end of the dielectric lining  214 A. Specifically, the coupling mechanism  500  has a hollow inner cavity and a step along the inner surface of the hollow inner cavity in which the resilient gland  510 A is disposed. When the connector  102  is coupled to the cable  180 , i.e., when the coupling mechanism  500  is tightened with respect to the cylindrical body  200  and the cable  180 , the resilient gland  510 A is compressed. As the resilient gland  510 A is compressed, the gland  510 A deforms, and protrudes into a corrugation of the outer conductor  186 . In such an arrangement, the resilient gland  510 A grips the corrugated outer conductor  186  of the coaxial transmission line  180  to hold the same in place and provides a moisture barrier. 
   Another embodiment of the invention is shown in FIG.  3 . This embodiment differs with respect to the embodiment shown in  FIG. 2  in the following. Capacitive coupling is created by an inner diameter of the outer conductor  186  of the coaxial cable  180  that is larger than the outside diameter of an open circuit outer stub  212 B of a connector  103 . Similar to the embodiment described in  FIG. 2 , the open circuit outer stub  212 B is preferably covered with a dielectric  214 B to maintain the proper alignment of the components. In this embodiment, the outer body of the cylindrical body  200  is substantially spaced apart from the cable outer conductor and the series open circuit outer stub  212 B to create a quarter wave choke. In this embodiment, the center conductor  182 B of the coaxial transmission line  180  is solid and in electrical contact with a center portion  332 A of a metal member  300 . 
   This stub design requires a special tool to cut the cavity in the foam  184 . This type of tool is common in CATV cable connector installation. Alternatively, in another embodiment, the series open circuit outer stub  212 B is designed to cut the cavity into the foam  184  to eliminate the need for a special tool. 
   Additionally, there is a conductive member  520  disposed between the resilient gland  510 B and a distal end of the outer body the connector  103 . The conductive member  520  provides a more effective open circuit outer stub  212 B by creating an electrical contact between the outer conductor  186  of the cable  180 , the outer surface of the cylindrical body  200 , i.e., the outer body of the connector. The resilient gland  510 B in this case is conductive to provide electrical contact to the cable  180 . 
     FIG. 4  illustrates another embodiment of the invention. This embodiment of the connector  104  differs from the embodiment shown in  FIG. 1  in the following regard. Instead of a first end portion  310  of the metal member  300  in electrical contact with the center conductor  182 A (FIG.  1 ), there is a series open circuit inner stub  312 A capacitively coupled to the center conductor  182 A. In this embodiment, the outer diameter of the series open circuit inner stub  312 A is less than the inside diameter of the hollow cavity in the center conductor  182 A. Preferably, there is a dielectric sleeve  314 A of suitable material such as polyethylene to maintain the series open circuit inner stub  312 A in proper alignment with respect to the center conductor  182 A and to prevent electrical contact. 
   Alternatively, an another embodiment is shown in FIG.  5 . This embodiment is different from the embodiment shown in  FIG. 1  with respect to the following. In a connector  105 , there is a series open circuit inner stub  332 B at the center portion  330  of the metal member  300 . The series open circuit inner stub  332 B has a hollow cavity in which a projecting solid end portion of an inner conductor  182 B of the coaxial transmission line  180  is disposed. The inside diameter of the hollow cavity is greater than the outer diameter of the solid inner conductor  182 B. A dielectric lining  324  is preferably disposed on the inside surface of the hollow cavity to maintain proper alignment of the components and to prevent electrical contact. This design is applicable to smaller cables that are made with solid center conductors. 
     FIG. 6  illustrates yet another embodiment of the invention. With respect to the embodiment shown in  FIG. 2 , this embodiment differs in the following respect. This embodiment combines the inner capacitive coupling configuration shown in  FIG. 4  with the outer capacitive coupling configuration shown in FIG.  2 . In the connector  106 , the impedance property of each of the two stubs  212 C,  312 C will normally need to be modified when the two stubs are combined to maintain the correct impedance to conjugate the reactance of the transformer section  700  over the desired bandwidth. 
   To impede the flow of radiation and current toward the outside of the outer stub, a yet another embodiment of the invention is shown in FIG.  7 . This embodiment differs from the embodiment described in  FIG. 6  with respect to the following. Radially around the series open circuit outer stub  212 D, there is an outer choke  600 , i.e., a short circuit stub. Preferably, the choke  600  is a dielectric layer such as an air gap, preferably, or a dielectric sleeve, that is disposed within first end portion  210  of the cylindrical body  100  of the connector  107 . With an air gap, the choke  600  is physically longer than quarter wavelength dielectric loaded stub. Further, the embodiment includes the conductive member  520  and conductive gland  510 B. The conductivity of the gland  510 B need not be high since the gland  510 B is disposed at a high-impedance position where low current exists. In an alternative embodiment, the resilient gland  510 B may replace the conductive member  520  depending on the conductivity of the resilient gland  510 B. 
   In all the embodiments shown in  FIGS. 2-7 , the length of the series open stub inner conductors and the series open stub outer conductors is electrically one quarter wave long. By way of example, if the dielectric lining  214 C and the dielectric sleeve  314 C shown in  FIG. 4  are made of polyethylene, the quarter wave in polyethylene is 1.014 inches long for a 1920 MHz signal. In such a configuration, the inner stub can provide less than 10 ohm impedance and the outer stub will be approximately 25 ohms impedance with a corrugated outer conductor. The exact physical length of the stub is usually determined by test since the volume of cavity created by conductors and connector is a combination of dielectric and air to maintain the slip fit requirement for field installation of connector. 
   The cable of the present invention has low losses given the state of the art of the materials for cables such as foam polyethylene with densities below 0.18 g/cm utilized to effect the invention. The use of at least one series open circuit stub conductor as in  FIGS. 2-7  provides improved bandwidth characteristic over a connector using only a simple quarter wavelength transformer (FIG.  1 ). For example, the series open stubs and the integral transformer as shown in  FIG. 6  of the present invention allows for a greater bandwidth covering the worldwide PCS band of 1700 to 2300 MHz with a VSWR of less than 1.02:1. On the other hand, a connector without the series open stubs, i.e., embodiment shown in  FIG. 1 , covers a frequency band of 1850 to 1990 MHz with a VSWR of about 1.02:1. 
   Physically, the incorporation of the series open stub conductor allows for simplified connector installation by allowing for less precise cutting of the coaxial transmission cable and less critical torque requirements to install the connector. The utilization of a non-metallic connector contact through the use of a dielectric sleeve allows the connector to be hand tightened. Furthermore, capacitively coupling both inner and outer conductors eliminates all passive intermodulation (PIM) from the most likely source while eliminating the most expensive and complicated parts of the connector. 
   In use, the connector only needs to be hand tightened to properly connect the coaxial transmission line to the connector because the use of open circuit stubs reduce the need for precise electrical metal to metal contact between the coaxial transmission line and the connector. 
   The invention is described in terms of the above embodiments which are to be construed as illustrative rather than limiting, and this invention is accordingly to be broadly construed. The principle upon which this invention is based can also be applied to other frequency bands of interest. 
   It is contemplated that numerous modifications may be made to the present invention without departing from the spirit and scope of the invention as defined in the following claims.