Abstract:
A surge suppressor for protecting electronic equipment by suppressing damaging surges of low frequency signals in a radio frequency (RF) transmission line, while allowing RF signals of a desired frequency range to pass through the transmission line. The surge suppressor can comprise a housing, a center pin connected to a stub, and at least one interface pin conductively coupled to the cable and capacitively coupled to the center pin. The surge suppressor can have a signal pass through bandwidth approximately 10 times exceeding the bandwidth of traditional quarter wavelength stub (QWS) devices, a higher return loss, and higher surge attenuation level. The surge suppressor can be symmetrically insertable into a cable providing an RF communication line.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to surge protectors, and more particularly to quarter wave stub (QWS) surge protectors employed in high-frequency signal transmission lines. 
     BACKGROUND OF THE INVENTION 
     In radio frequency (RF) signal transmission lines, typically transmitting electromagnetic signals with the frequencies over 1 MHz, undesirable effects can occur if a strong surge (e.g., caused by lightning) is transmitted to sensitive electronic devices coupled to the transmission line. Lightning can produce strong surge signals ranging in frequency from 0 (direct current) to 1 MHz. Therefore, a surge suppressor should prevent surges of low frequency signals from passing through the transmission line, while allowing the desired RF signals to pass freely. 
     Surge suppressors insertable into a transmission line in series with the equipment being protected can employ quarter wave stubs (QWS) which are seen as a short circuit to the ground by low frequency signals, while RF signals encounter input impedance corresponding to an open circuit. 
     Traditional QWS surge suppressors usually have very narrow bandwidth of the RF signals allowed to pass. Besides, the surge signals that can be allowed to pass by the traditional QWS surge suppressors can have energy levels which are dangerous for sensitive electronic equipment connected to the transmission line. Known enhancements intended to improve the bandwidth and the let-through energy usually introduce an element insertable into the communication line in series with the QWS, thus rendering the surge suppressor asymmetrical, i.e., requiring a unidirectional insertion of the modified QWS surge suppressor into the communication line. The asymmetrical insertion requirement can significantly increase the rate of installation errors. 
     Thus, a need exists for a surge suppressor which has a relatively wide pass through signal bandwidth with a return loss value more than 20 dB, low let-through energy and very high surge attenuation levels for low frequency signals. The need also exists for a surge suppressor which is symmetrically insertable into a communication line. 
     SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to provide a device for suppressing surges of low frequency electromagnetic signals in an RF transmission line, while allowing the desired RF signals to pass through. 
     It is a further object of the present invention to provide a device for suppressing surges of low frequency signals in an RF transmission line with a pass through signal bandwidth exceeding the bandwidth of the devices employing the conventional QWS design. 
     It is a further object of the present invention to provide a device for suppressing surges of low frequency signals in an RF transmission line with a high passband return loss and a high surge attenuation level. 
     It is a further object of the present invention to provide a symmetrical device for suppressing surges of low frequency signals in an RF transmission line, which is bi-directionally insertable into the transmission line which can be provided by a coaxial cable. 
     It is a further object of the present invention to provide a method of designing a surge suppressor possessing the above listed characteristics. 
     These and other objects of the present invention are attained by a surge suppressor insertable into a cable providing an RF transmission line. The surge suppressor can comprise a housing, a center pin connected to at least one stub, and at least one interface pin which is conductively coupled to the cable and capacitively coupled to the center pin. The surge suppressor can have a bandwidth approximately 10 times exceeding the bandwidth of traditional quarter wave stub (QWS) devices with a high passband return loss. In one embodiment, the surge suppressor can have a symmetrical design and thus be symmetrically insertable into a communication line. 
     The method of designing the surge suppressor can comprise the steps of specifying one or more design parameters, including a desired center frequency, a type of connector interface, a desired bandwidth, a desired return loss, a desired insertion loss, a desired surge attenuation level, and an allowable arc voltage level between the center pin and the interface pin; calculating the length of the stub; calculating a size of the gap between the center pin and the interface pin; and calculating a diameter of the interface pin. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a further understanding of the objects of the invention, reference will be made to the following detailed description of the invention which is to be read in connection with the accompanying drawings, where: 
         FIGS. 1   a - 1   b  illustrate cutaway and exploded views of one embodiment of the surge suppressor according to the invention; 
         FIG. 1   c  illustrates the surge suppressor according to the embodiment depicted in  FIGS. 1   a - 1   b , with the housing removed; 
         FIG. 2  illustrates a cutaway view of another embodiment of the surge suppressor according to the invention; 
         FIG. 3   a  illustrates a cutaway view of an embodiment of the surge suppressor with diameter steps for the impedance matching according to the invention; 
         FIG. 3   b  illustrates a zoomed-in cutaway view of coupled pins according to the invention; and 
         FIG. 4  illustrates a flow chart of a process of designing a QWS surge suppressor with coupled pins according to the invention. 
     
    
    
     The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views. 
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of a surge suppressor in accordance with the present invention is described referencing  FIGS. 1   a  and  1   b  which illustrate cutaway and exploded views of a symmetrical single-stub surge suppressor, and  FIG. 1   c  which illustrates a cutaway view of the surge suppressor with the housing being removed. A skilled artisan would appreciate the fact that the scope and spirit of the present invention include multi-stub designs of the surge suppressor. 
     In the embodiment shown in  FIGS. 1   a - 1   c , the surge suppressor  100  extending along a longitudinal axis  110 , is generally symmetrical relatively to the vertical axis  130 , the latter being the axis of symmetry of the stub  9 . The symmetrical design feature allows symmetrical bi-directional insertion of the surge suppressor  100  into a cable that provides the RF signal transmission. The symmetrical design feature further allows showing in the exploded view and describing only one component of each pair of the symmetrical components. A skilled artisan would appreciate the fact that the scope and spirit of the present invention include asymmetrical designs of the surge suppressor. 
     The surge suppressor  100  can generally comprise a metallic housing  8  which can incorporate most of the components of the surge suppressor. Unless explicitly stated otherwise, the components described herein infra can be made of suitable conductive metallic alloys. 
     The housing  8  can include a conductor portion  81  and a stub portion  82 . The conductor portion  81  of the housing  8  can generally extend along the longitudinal axis  110 . The conductor portion  81 , as best viewed in  FIG. 1   b , can have a central bore  84  designed to receive components which provide the RF signal transmission, including a center pin  7 , at least one support insulator  6 , at least one strike insulator  5 , at least one interface pin  4 , and at least one interface cap  3 . 
     A skilled artisan would appreciate the fact that while  FIGS. 1   a - 1   b  show the conductor portion  81  of the housing  8  having a form of a parallelepiped and the central bore  84  having a cylindrical form, the form factors shown do not limit the scope and spirit of the present invention. 
     The center pin  7  can have an elongated form and extend along the longitudinal axis  110 . The center pin  7  can further have an opening for receiving at least one stub  9  so that the stub  9  can be conductively coupled to the center pin  7 . In one embodiment, the stub  9  can extend in a direction orthogonal to the longitudinal axis  110 . 
     The center pin  7  can be supported within the central bore  84  by at least one support insulator  6  made of a dielectric material. The form factor of the support insulator  6  can be primarily defined by the form factor of the central bore  84 . The support insulator  6  can have a central opening designed to receive one end of the center pin  7 . 
     The center pin  7  can be capacitively coupled to at least one interface pin  4 . The interface pin  4  can be conductively coupled to the cable (not shown in  FIGS. 1   a - 1   c ) which provides the RF signal transmission. The interface pin  4  can have a form factor which allows the interface pin  4  to act as one plate of an isolation capacitor when being placed in a close physical proximity of one end of the center pin  7 , so that the end of the center pin  7  provides a second plate of the isolation capacitor. In one embodiment, the interface pin  4  can have a form of a cylindrical sleeve configured to receive one end of the center pin  7 . In another embodiment (not shown), the interface pin  4  can be received within one end of the center pin  7 . 
     In one embodiment, a strike insulator  5  made of a dielectric material can separate one end of the center pin  7  and an interface pins  4  and thus maintain a gap  13  of a pre-defined size (e.g., 0.01″) between the center pin  7  and the interface pin  4 , so that the interface pin  4  can be capacitively coupled to the center pin  7 . The strike insulator  5  can further have an opening around the center pin  7  which in operation will cause an electric arc to jump from a pointed end  71  of the center pin  7  to the interface pin  4 . In another embodiment, a support insulator  6  can support center pin  7  within the interface pin  4 . 
     In operation, the gap  13  can effectively prevent low frequency signals (e.g., lightning surges) with the voltage level less than a pre-defined threshold (e.g., 1 kV) from flowing between the center pin and the interface pin  4 . Increasing the size of the gap  13  will increase the voltage level of surges that can be blocked by the gap  13 . However, the insertion loss of the surge suppressor will increase as the width of the gap increases. 
     While the low frequency signals are prevented from flowing between the center pin and the interface pin  4 , the higher frequency RF signals can flow between the center pin and the interface pin  4 , since the center pin  7  is capacitively coupled to the interface pin  4  by an isolation capacitor composed by an end of the center pin  7  and the interface pin  4 , as described supra. 
     The housing  8  can have at least one stub portion  82 , which is now being described with references to  FIGS. 1   a  and  1   b . The stub portion  82  can generally extend in a direction orthogonal to the longitudinal axis  110 . Located within the stub portion  82  can be a stub  9 , a stub contact  10 , a stub cap  11 , and a stub insulator  12 . Stub cap  11  can be threadably attached to the stub portion  82 , as best viewed in  FIG. 1   a . A skilled artisan would appreciate the fact that any other suitable means of attaching the stub cap to the stub portion of the housing can be employed. A skilled artisan would further appreciate the fact that while  FIGS. 1   a - 1   b  show the stub portion  82  of the housing  8  having a cylindrical form, the form factor shown does not limits the scope and spirit of the present invention. Stub cap  11  can maintain the stub contact  10  firmly pressed against the stub  9 , while the stub insulator  12  can be inserted between the stub contact  10  and stub  9 , as best viewed in  FIG. 1   a . The stub insulator  12  can have a form factor configured to support and align the stub  9 . A skilled artisan would appreciate the fact that while  FIGS. 1   a - 1   b  show the stub insulator  12  having an annular form, the form factor shown does not limit the scope and spirit of the present invention. 
     The stub  9  can provide a short circuit to the ground for low frequency signals while deflecting the RF signals. The frequency range of the RF signals which would be deflected by the stub depends upon the impedance of the stub  9 , which in turn depends upon the length of the stub  9 . 
     In another embodiment, illustrated in  FIG. 2 , the stub portion  82  of the housing can be combined with the stub cap  11  of  FIG. 1   a  into a single part. A skilled artisan would appreciate the fact that other designs of the stub portion of the housing are within the scope and the spirit of the present invention. 
     Referring again to the conductor portion  81  of the housing best viewed in  FIGS. 1   a  and  1   b , at least one interface cap  3  can be received at one end of the conductor portion  81  of the housing. The interface cap  3  can be fastened to the conductor portion  81  of the housing. A skilled artisan would appreciate the fact that any other suitable means of attaching the interface cap to the conductor portion of the housing can be employed. The interface cap  3  can have a form factor matching the form factor of the central bore  84 . A skilled artisan would also appreciate the fact that while  FIGS. 1   a - 1   b  show the central bore  84  and the interface cap  3  having a cylindrical form, the form factor shown does not limit the scope and spirit of the present invention. 
     The interface cap  3  can be configured to receive a specific cable interface type. A skilled artisan would appreciate the fact that while  FIG. 1  shows the interface cap  3  suitable to receive a typical 50 Ohm coaxial cable connector (not shown in  FIG. 1 ), the interface cap  3  can be designed to be suitable to receive other types of cable interfaces. 
     At least one interface cap insulator  2  can support the interface pin  4  in the coaxial position. The interface cap insulator  2  can be made of a dielectric material and have a form factor conforming to the form of the interface cap  3 . A skilled artisan would also appreciate the fact that while  FIG. 1  shows the cap insulator  2  having an annular form, the form factor shown does not limits the scope and spirit of the present invention. 
     At least one interface ground contact  1  can provide the ground continuity with the cable received by the interface cap  3 . The interface ground contact  1  can have a form factor conforming to the form of the interface cap  3 . 
     To provide for a desired level of return loss (e.g., better than 25 dB), the surge suppressor can be matched to the line impedance at both interfaces. To achieve this, several diameter steps  302  can be provided on the stub  9 , the center pin  7 , and on the inside wall of the housing  8  as shown in  FIG. 3   a , thus providing return loss of 25 dB over a broad frequency band (e.g., between 600 MHz and 2500 MHz.) 
     In operation, the low frequency signal surges that are of higher voltage levels than the gap  13  can block will cause an electric arc to jump from an interface pin  4  to the pointed end  71  of the center pin  7 . This surge will then be diverted to the ground by the stub  9 , since the stub  9  is seen as a short circuit to the ground by low frequency signals, while the desired RF signals encounter input impedance corresponding to an open circuit. Thus, the energy surges having a voltage lower than the design voltage level will never hit the protected RF equipment. The frequency range of desired RF signals deflected by the stub  9  is determined by the length of the stub  9  and the length of the coupled section of the center pin  7 , as shown in  FIG. 3   b .  FIG. 3   b  illustrates the fragment  304  of  FIG. 3   a  being zoomed-in to show a cutaway view of one embodiment of coupling the interface pin  4  and the center pin  7 . The interface pin  4  having a form of a cylindrical sleeve can be configured to receive one end of the center pin  7 , with the gap  13  between the pins being maintained by the support insulator  6  and the strike insulator  5 . The desired bandwidth of the surge suppressor, exceeding the bandwidth of the traditional QWS design by 10 times or more, can be achieved by adjusting the design parameters, e.g., the length of the coupled section  310 , including the width  312  of the support insulator  6 , the size  314  of the gap  13 , and the width  316  of the strike insulator  5 . 
     The process of designing a QWS surge suppressor with coupled pins according to the invention is now described with references to the flowchart illustrated in  FIG. 4 . 
     At step  400 , the design parameters are specified. In one embodiment, the design parameters can include one or more of the following parameters: the desired center frequency, the type of connector interface, the desired bandwidth, the desired return loss, the desired insertion loss, the desired surge protection voltage level, and the allowable arc voltage level between the coupled pins. 
     At step  410 , the stub length is calculated. In one embodiment, the stub length can be calculated as being equal to one-fourth of the wave length of the signal transmission line at the specified center frequency. In another embodiment, the stub length can be calculated as being equal to one-fourth of the wave length of the signal transmission line at the specified center frequency, further divided by a square root from the value of the permittivity of the material of the stub insulator  12  of  FIG. 1   b.    
     For example, for a center frequency value of 2 GHz and the permittivity of the insulating material value of 4, the full wave length will be
 
λ= c /((2*10 9 )*4 1/2 )=3*10 8 /((2*10 9 )*4 1/2 )=0.075 m,
 
     wherein c is the speed of light in vacuum; 
     and the stub length will be equal to λ/4=0.01875 m. 
     At step  420 , the size of the gap  13  of  FIG. 3   b  between the coupled pins is calculated. In one embodiment, the size of the gap between the coupled pins can be calculated by dividing the allowable arc voltage level between the coupled pins by the breakdown voltage level of the material of the strike insulator  5  of  FIG. 1   b . For example, for an allowable arc voltage level of 1200V and the breakdown voltage level of 60 kV/inch, the size of the gap between the coupled pins will be 1200/60K=0.02″. 
     At step  430 , the multiplier k of the gap size is initialized with the value of 2. 
     At step  440 , the diameter of the interface pin is calculated. In one embodiment, the diameter can be calculated based on the following equation:
 
 D=D   s   +k*S,  wherein
     D is the interface pin diameter;   D s  is the standard pin diameter for the specified type of connector interface;   S is the size of the gap  13  of  FIG. 3   b  between the coupled pins; and   k is a real number which must be greater than or equal 2.   

     At step  450 , the design can be optimized, e.g., using simulation software. In one embodiment, the design can be optimized by adding additional impedance matching elements to meet the insertion loss and return loss specifications. 
     At step  460 , a sample surge suppressor is made and one or more of the values of return loss, insertion loss and bandwidth are tested. 
     At step  470 , one or more values measured on a sample surge suppressor during the testing are compared to the values specified at step  400 . If the specifications are not met, the method loops back to step  450 ; otherwise, the processing continues at step  480 . 
     At step  480 , the value of surge level is tested on the sample surge suppressor, by measuring, e.g., the throughput voltage or the let-through energy. 
     At step  490 , the value of the surge level measured on the sample surge suppressor is compared to the value specified at step  400 . If the specification is not met, the method branches to step  492 ; otherwise the method terminates at step  495 . 
     At step  492 , the value of the gap size multiplier k is incremented by a pre-defined value of Δ, and the method loops back to step  440 . In one embodiment, the value of Δ can be a real number from the range of [0.01; 1]. 
     At step  495 , the design of the surge suppressor is complete, and the method terminates. 
     While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.