This application is a continuation in part of application Serial No. 08/192,343 filed February 7, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to surge arrestors and, more particularly, to gas discharge tube surge arrestors for coaxial transmission lines.

2. Discussion of the Relevant Art Numerous gas discharge tube surge arrestors have been developed over the years for protecting telephone lines from overvoltage conditions caused, for example, by lightning or fallen high voltage power lines. Such conventional surge arrestors, while suitable for telephone lines, are unsuitable for coaxial transmission lines which have unique characteristics and requirements. Several attempts have, however, been made to provide gas discharge tube surge arrestors for coaxial transmission lines.

Kawanami U. S. Patent No. 4,544,984 issued October 1, 1985 (Kawanami '984) discloses a gas discharge tube surge arrestor for a coaxial transmission line. According to the Kawanami '984 patent, conventional gas discharge tubes, while suitable as surge arrestors for telephone lines, cannot be used for high frequency coaxial transmission lines because (1) the gas discharge tube has a considerable amount of capacitance and (2) the nature of the required connection is such that it greatly changes the impedance of the coaxial transmission line and causes reflections in the transmission line. According to the Kawanami '984 patent, there has previously been no surge arrestor which could be used in a high frequency coaxial transmission line (column 1, line 57 to column 2, line 4) .

The Kawanami '984 patent discloses a surge arrestor which connects a gas discharge tube between the

inner and outer conductors of the coaxial transmission line in a direction orthogonal to the direction of signal transmission. The unwanted increased capacitance associated with the use of a gas discharge tube in a coaxial transmission line is compensated for by reducing the effective cross sectional area of the inner conductor at the place where the gas tube contacts the inner conductor by cutting out a portion of the center conductor to create a flat area on which the gas tube rests.

Kawanami U. S. Patent No. 4,509,090 issued on April 2, 1985 (Kawanami '090) also explains why conventional gas discharge tubes have not been successfully employed as surge arrestors in coaxial transmission lines and discloses the same type of structure disclosed in the Kawanami '984 patent, i.e., a device which connects the gas discharge tube between the inner and outer conductors of the coaxial transmission line in a direction orthogonal to the direction of signal transmission. In Figure 7 the Kawanami '090 patent provides information concerning the impact of reducing the effective cross sectional area of the center conductor at the place where it contacts the gas discharge tube, showing that small dimensional changes on the order of 1 or 2 millimeters have a significant effect on the voltage standing wave ratio (VS R) .

Mic elson U. S. Patent No. 4,633,359 issued on December 30, 1986 also discloses a surge arrestor for a coaxial transmission line in which a gas discharge tube is connected between the inner and outer conductors of the transmission line in a direction orthogonal to the direction of signal transmission. The asserted advantage of the Mickelson device is that it is "simpler and less expensive to fabricate." Like the Kawanami '090 and '984 patents, Mickelson uses a center conductor which is flattened at the place where the gas tube contacts the center conductor. In addition to serving as a seat for the gas tube, this flat area adjusts the inductance of the center conductor to

compensate for the distributed capacitance of the gas tube. Chamfers are provided adjacent the flat area to match the impedance of the surge arrestor to that of the transmission line. It is well known that maximum power transfer occurs when matched impedances are employed.

The present invention provides a new and improved surge arrestor for coaxial transmission lines in which the axis of the gas discharge tube is oriented parallel to the direction of signal transmission, rather than orthogonal to the direction of signal transmission as disclosed in the prior art, and the RF signal flows through the gas discharge tube. The coaxial surge arrestor of the present invention is sufficiently small that it can be incorporated within or made an integral part of existing coaxial connectors. Further, the present invention results in a much simpler, easier to manufacture and, therefore, less expensive device. At the same time, the present invention permits compensating for the unwanted capacitance introduced by the presence of a gas discharge tube in the coaxial transmission line and further permits matching the impedance of the surge arrestor to that of the coaxial transmission line so as to provide a device having a useful frequency range extending from 50 MHz to at least 1 GHz.

Therefore, it is an object of the present invention to provide a coaxial surge arrestor which has a characteristic impedance similar to that of the coaxial transmission line.

It is another object of the present invention to provide a coaxial surge arrestor which permits compensating for the unwanted capacitance introduced by the use of a gas discharge tube in a coaxial transmission line.

It is another object of the present invention to provide a coaxial surge arrestor which may be mounted within conventional coaxial cable components and which may be readily installed in existing coaxial transmission lines. it is another object of the present invention to

provide a gas discharge tube suitable for use in a coaxial surge arrestor.

It is another object of the present invention to provide a coaxial surge arrestor in which the RF signal flows through the gas discharge tube.

It is another object of the present invention to provide an economically constructed coaxial surge arrestor which includes fail safe protection so that overheating of the gas discharge tube will short the communication line to ground, thereby protecting the equipment to which it is connected.

It is still another object of the present invention to provide a coaxial surge arrestor which includes current limiting and/or low voltage protection.

SUMMARY OF THE INVENTION A coaxial transmission line surge arrestor according to the principles of the present invention comprises a hollow conductive body having coaxial connectors mounted thereon. A gas discharge tube is located in or forms an integral part of the conductive body. The RF signal passes through the gas discharge tube. The gas discharge tube comprises a hollow conductive housing having insulating ends which seal the housing and maintain an inert gas within the housing. A center conductor extends axially through the conductive housing in the direction of signal transmission. The insulating ends may be ceramic and the portions of the ceramic ends contacting the conductive housing and the central conductor may be metallized. At least a portion of the inner surface of the conductive housing and at least a portion of the outer surface the center conductor may be roughened to concentrate the electric fields and provide reliable operation of the gas discharge tube. Matching the impedance of the coaxial surge arrestor to that of the coaxial transmission line may be effected by varying the ratio of the inner diameter of the conductive housing to the outer diameter of the center

conductor along the length of the center conductor and by varying the length of the active gas discharge region of the device. The gas discharge tube may be fitted with a fail¬ safe mechanism employing a thermally sensitive electrical insulation which results in grounding of the transmission line if the gas discharge tube overheats. In addition, the coaxial surge arrestor of the present invention may incorporate current limiting and/or low voltage protection.

The subject matter which I regard as my invention is particularly pointed out in the claims at the end of the specification. My invention, including its method of operation and its numerous advantages, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like components. BRIEF DESCRIPTION OF THE DRAWING

In order that the invention may be more fully understood, it will now be described, by way of non-limiting examples, with reference to the accompanying drawings, in which:

Figure 1 is a cross-sectional view taken along the longitudinal axis of one embodiment of a gas discharge tube according to the principles of the present invention;

Figure 2 is an end view in elevation of the device shown in Figure 1; Figure 3 is a top plan view with the cover removed, partially broken away, of a gas discharge tube inserted within a housing having a pair of coaxial connectors affixed thereto;

Figure 4 is a side view in elevation, partially broken away, of the housing shown with the gas discharge tube disposed therein;

Figure 5 is a perspective view of a ground clip;

Figure 6 is a perspective view of a mounting clip used to hold the gas discharge tube within the housing; Figure 7 is a perspective pictorial representation

of the thermally sensitive insulation utilized between the gas discharge tube and the mounting clips;

Figure 8 is a cross-sectional view in elevation of an alternate embodiment of the gas discharge tube according to the principles of the invention;

Figure 9 is an end view in elevation of the device shown in Figure 8;

Figure 10 is a top plan view with the cover removed, partially broken away, of the gas discharge tube as shown in Figure 8, mounted in the housing;

Figure 11 is a pictorial representation, partially broken away, of the apparatus shown in Figure 10;

Figure 12 is a top plan view with the cover removed of an alternative housing apparatus with the connectors appearing on different surfaces of the housing;

Figure 13 is an end view in elevation of the ^• housing apparatus shown in Figure 12;

Figure 14 is a cross-sectional view of another alternate embodiment of the gas discharge tube of the present invention;

Figure 15A is an end view of a printed circuit board coaxial connector embodying the gas discharge tube of the present invention;

Figures 15B and 15C are cross-sectional views of two variations of the coaxial connector of Figure 15A;

Figure 16A is an end view of an in-line coaxial connector embodying the gas discharge tube of the present invention;

Figure 16B is a cross-sectional view of the coaxial connector of Figure 16A;

Figure 17A is an end view of a right angle coaxial connector embodying the gas discharge tube of the present inven ion;

Figure 17B is a cross-sectional view of the coaxial connector of Figure 17A;

Figure 18 is a schematic diagram of a coaxial

surge arrestor in accordance with the present invention including current limiting and low voltage protection;

Figure 19 is a cross-sectional view of a coaxial cable with a male coaxial connector incorporating the gas discharge tube of the present invention; and

Figure 20 is a cross-sectional view of a female- female coaxial connector having an integral surge arrestor. DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to Figures 1 and 2, there is shown a gas discharge tube 10, according to the principles of the present invention, which has an elongated hollow enclosure 12 that is cylindrically shaped and made of electrically conductive material. The inner circumferential wall 14 is preferably roughened for more reliable performance, as shown by the thread-like serrations in Figure 1, which concentrate the electric field in the discharge gap. An elongated electrically conductive electrode 16 extends from one end 18 to the other end 20 of enclosure 12.

Electrode 16 is provided with outwardly extending portions 22 and 24 which extend beyond the ends 18 and 20 of the enclosure 12 and are centrally disposed within apertures 26 provided in ceramic (nonconducting) sealing members 28 and 30 inserted in the ends 18 and 20 of the enclosure 12. Ledges 32 and 34 are provided proximate the ends 18 and 20 within the enclosure 12 so that the sealing members 28 and 30 may be accurately seated therein. The electrode 16 is also roughened along its outer circumference, as shown by the serrations in Figure 1, in order to provide reliable firing of the gas discharge tube. Once the pieces of the gas discharge tube described above are assembled, the unit is fired in a conventional manner to allow a complete sealing of the gas 36 within the enclosure 12. The gas 36 utilized is inert and typical of that used in conventional overvoltage breakover tubes.

Figure 3 shows a conductive housing 38 into which is placed the gas discharge tube 10 in a manner which will

be explained hereinafter. Housing 38 includes threaded input and output connectors 40 and 42 which are adapted to receive conventional threaded F-type coaxial connectors 44 and 46, although other conventional coaxial connectors such as BNC connectors may be employed. The coaxial connectors are aligned in the direction of transmission. Each male connector includes a threaded outer shell 48 and an insulating portion 50 having a centrally disposed conductor 51 that is inserted into receptacle portion 52 of clip 54 shown in more detail in Figure 6.

Clip 54 has a second receptacle portion 56 adapted to receive and removably hold therein the extending portions 22 and 24 of gas discharge tube 10. Clip 54 also has a plurality of fingers 58, 60, 62 and 64, which are curved and adapted to receive gas discharge tube 10 therein.

In order to insure the isolation of the conducting electrode 16 of gas discharge tube 10 so that it is not in electrically conductive contact with the clip 54, a thermally sensitive material 66 known as FEP is placed between the base portion 68 of clip 54 so that it extends over the fingers 58, 60, 62 and 64 to prevent electrically conductive contact with the metallic enclosure 12 of gas discharge tube 10.

Figure 7 discloses the configuration of the FEP insulator 66. Two apertures 70 and 72 are provided in insulator 66 so that the fingers 74 and 76 of ground clip 78 (shown in Figure 5) may come into electrically conductive contact with the metallic electrically conductive surface of the enclosure 12. Ground clip 78 is affixed to the conductive housing 38 in a conventional manner and thus, is in electrically conductive contact therewith and with the ground portion of connectors 40 and 42 and also, the connectors 44 and 46 affixed thereon completing the ground integrity of the system.

Figures 8 and 9 show an alternative embodiment of the gas discharge tube 80, which includes an elongated

hollow enclosure 82 that preferably is fabricated in three separate pieces. The enclosure 82 includes a first portion 84 preferably fabricated from an insulating material (ceramic) , a second central electrically conductive portion 86, generally referred to as the ground terminal, and a third portion 88, which is identical to the first portion 84. Each of the three pieces is generally tubular shaped and hollow. The inner surface 90 of the conductive portion 86 may also be roughened in order to achieve more reliable performance of the gas discharge tube in a manner similar to that set forth with regard to Figure 1.

Centrally located within the hollow opening 92 of the enclosure 82 is electrically conductive electrode 94 which is fabricated in three sections. The first and third sections 96 and 98 have the same structure and are connected together by an electrically conductive bridging pin 100 which forms the third section. Thus, electrically conductive contact is continuous from the first end 102 to the other end 104, via the bridging pin 100. End caps 106 and 108 provide the seal so that the gas 106 may be retained in the space provided between the electrically conductive electrode 94 and the enclosure 82. The end caps 106 and 108 are in electrically conductive contact with the conductive electrode 94, thus providing a continuous conducting medium from one end to the other, maintaining a continuous path therethrough.

Figure 10 is a top plan view of the housing 38 having the alternative embodiment of the gas discharge tube 80 inserted therein and with one of the coaxial connectors 46 removed from the connector 42 on the housing 38. The other connector 44 is connected to the female connector 40 on the housing 38. The clip 54 shown in Figure 6 is modified somewhat by replacing receptacle portion 56 with a pair of fingers 110 and 112 suitable for grasping the end caps 106 and 108 of the gas discharge tube 80. The remaining portion of clip 54 remains the same. Here again,

an insulator 66 formed from a thermally sensitive material such as FEP is utilized to electrically insulate the end caps 106 and 108 from the electrically conductive material from which the clip 54 is fabricated.

Figure 11 is a side view in elevation of the housing 38 partially in cross-section with the cover 114 in place to completely seal the housing 38. The ground clip 78 in Figure 11 is identical to the ground clip 78 in Figure 5. The surge arrestor shown in Figures 12 and 13 may utilize either gas discharge tube 10 or gas discharge tube 80, with the clip 54 being slightly modified from that shown in Figure 6, since the receptacle portion 52 of clip 54 is bent at right angles so that it may accommodate female connectors 40 and 42 appearing on the same surface of the housing 38. Alternatively, a connector 116 may be placed on the opposite wall of the housing 38 for convenience, if desired, with the clip 54 being modified as necessary and shown in the broken lines. Mounting ears 118 and 120 with apertures 122 and 124 may be provided on the housing 38 to allow for mounting the housing 38 in various locations. in operation, the parts of the gas discharge tube may be assembled and fired in a conventional manner sealing the gas within the enclosure. Thereafter, the assembly is placed in the housing utilizing the FEP insulator, mounting and ground clips so that the unit is ready for use in the field.

Figure 14 discloses another alternative embodiment of the gas discharge tube of the present invention which is suitable for use in a coaxial transmission line surge arrestor. The gas discharge tube 200 comprises a conductive housing 202, insulating ends 204 and a center conductor 206 extending through housing 202. The RF signal flows axially through the gas discharge tube 200. Although shown projecting beyond ends 204, center conductor 206 could terminate at ends 204 and external conductors could be attached thereto. As with the embodiment shown in Figure 1,

the insulating ends 204 are preferably formed from a ceramic material and seal the housing and an inert gas within the housing. In conventional gas discharge tubes the inert gas is a mixture of hydrogen and argon to provide a breakdown voltage of 250 to 350 volts DC. In a preferred embodiment of the present invention the inert gas is a mixture of neon and argon which provides a breakdown voltage of about 100 volts DC.

The insulating ends 204 are preferably metallized in the regions 208 where the ends contact the conductive housing 202. The insulating ends 204 are also preferably metallized in the regions 210 where the ends contact center conductor 206. It is also preferred that the insulating ends have annular recesses 212 in the exterior faces 205 thereof in the regions where conductor 206 projects through ends 204. These annular recesses are also preferably metallized.

The annular recesses facilitate the metallization step in the manufacturing operation. Thus, the entire outer surface of the insulating end 204 containing the annular recess can be metallized and the metallization can be removed in the area outside the annular recess by grinding down the outer surface of the insulating end.

As shown in Figure 14, a portion of the interior surface 214 of conductive housing 202 and a portion of the exterior surface 216 of center conductor 206 are roughened, for example by threads or other forms of serration, to concentrate the electric field and increase the reliability of the gas discharge tube operation. In addition, as with conventional gas discharge tubes, the surfaces 214 and 216 are preferably coated with a low work function material to reduce the breakdown voltage and enhance the firing characteristics of the gas discharge tube. The gas discharge occurs in the region "G" between surfaces 214 and 216. Region "G" is the active discharge region.

In addition to coating surfaces 214 and 216, it is

preferable to employ "striping" in the form of radial graphite lines on the interior surface of the insulating end 204 adjacent the active discharge region "G." This "striping" helps to initiate the voltage breakdown.

As also shown in Figure 14, the distance between the inner surface of the conductive housing 202 and the outer surface of the center conductor 206 varies along the length of the center conductor. Put another way, the ratio of the inside diameter D of housing 202 to the outside diameter d of center conductor 206 varies along the length of the center conductor. The ratio D/d may vary by a factor of 2 or 3 or more along the length of center conductor 206. This variation in the ratio D/d is used to adjust the impedance of the gas discharge tube and for matching the impedance of the surge arrestor in which the gas discharge tube is located to that of the coaxial transmission line to which the surge arrestor is attached.

The impedance of a coaxial transmission line is proportional to the logarithm of (D/K)/d, where "D" is the inside diameter of the outer conductor, "d" is the outside diameter of the inner conductor and "K" is the dielectric constant of the medium between the inner and outer conductors. In the case of the gas discharge tube shown in Figure 14, the medium is an inert gas which has a dielectric constant of approximately one. Therefore, the impedance of the gas discharge tube varies between the insulating ends as the logarithm of the ratio D/d. As noted earlier, the insulating ends 204 are preferably ceramic and ceramic has a dielectric constant of about eight. By varying the ratio D/d along the length of center conductor 206 one can compensate for changes in impedance caused by, inter alia, the dielectric constants of the insulating ends 204. The portion of gas discharge tube 200 that is used for impedance matching is designated by the letter "I", to distinguish it from the active discharge region "G" .

In addition to adjusting the ratio D/d within the

gas discharge tube, it is also possible to adjust the length of the active gas discharge region "G" relative to the length of the impedance matching region "I" to match the impedance of the gas discharge tube to that of the coaxial transmission line. Thus, for a 50 ohm coaxial transmission line the ratio of the region "G" to the region "I" may be on the order of one to one whereas, for a 75 ohm coaxial transmission line, the ratio of the region "G" to the region "I" may be on the order of one to two.

Some typical dimensions for the miniature coaxial gas discharge tube 200 shown in Figure 14 are: (1) overall length of center conductor 206 - one inch; (2) length of conductive housing 202 - 0.32 inches; (3) outside diameter of gas discharge tube 200 - 0.33 inches; (4) diameter of center conductor 206 - 0.035 inches.

Figures 15A through 15C show a coaxial surge arrestor 220 which incorporates the gas discharge tube 200 of Figure 14. Surge arrestor 220 is designed to connect between a coaxial transmission line using F-type coaxial connectors and a printed circuit board. Thus, one end 222 of surge arrestor 220 is threaded and is designed to receive a conventional male F-type coaxial connector, while the other ends has conductors projecting therefrom and is designed to be mounted on a printed circuit board or similar substrate.

In Figure 15B the impedance matching section "I" of gas discharge tube 200 is located to the left of the gas discharge gap "G", whereas in Figure 15C the impedance matching section "I" is located to the right of the gas discharge gap "G" . In Figure 15C the distance by which the center conductor 206 projects beyond the insulating end of gas discharge tube 200 may not be sufficient to permit connecting the surge arrestor to the printed circuit board, in which event an additional conductor 224 is employed which is electrically connected to center conductor 206. As also shown in Figures 15B and 15C, the surge

arrestor 220 has a cavity 226 located behind the gas discharge tube 200. This cavity can also be used for matching the impedance of the surge arrestor to that of the coaxial transmission line by appropriately dimensioning the cavity 226 and/or by filling the cavity with a material having a suitable dielectric constant.

Figures 16A and 16B show another coaxial transmission line surge arrestor 230 which incorporates the gas discharge tube 200 of Figure 14. The surge arrestor of Figures 16A and 16B is an in-line device designed to be connected between two coaxial transmission lines having male F-type coaxial connectors. The gas discharge tube 200 is secured within surge arrestor 230 by means of a set screw 232.

Figures 17A and 17B show another coaxial transmission line surge arrestor 240 which incorporates the gas discharge tube 200 shown in Figure 14. The surge arrestor of Figures 17A and 17B is a right angle device designed to be connected between two coaxial transmission lines having male F-type coaxial connectors. As shown in Figure 17B, the length of the center conductor 206 projecting from gas discharge tube 200 is insufficient and, therefore, it has been extended by electrically connecting a second center conductor 242 thereto. Surge arrestor 240 also has a cavity 206 which may be suitably dimensioned and/or filled with a dielectric material for matching the impedance of surge arrestor 240 to that of the coaxial transmission line.

Figure 18 is a schematic diagram of a coaxial transmission line surge arrestor system in accordance with the present invention. Figure 18 shows an RF transmission line having an input 250, an output 252 and a ground 254. Located in series in the RF transmission line is a gas discharge tube 256 in accordance with the present invention. As can be seen from Figure 18, the RF signal flows through the gas discharge tube 256 which may be any embodiment of

the present invention including, without limitation, the embodiments 10,. 80 and 200 shown, respectively, in Figures 1, 8 and 14.

The schematic diagram of Figure 18 shows the presence of a fail short protective device at 258 which may utilize a ground clip and FEP film as previously disclosed. Also shown is an inductor 260 and a resistor 262 for limiting the current which flows to the output 254 of the surge arrestor. In addition, a ferrite bead 264 and an avalanche diode 266 are connected between the center conductor and ground for low voltage protection. The ferrite bead 264 permits low frequency (e.g. 10 MHz and below) signals to go to ground but prevents high frequency (e.g. 50 MHz to 1 GHz) signals from going to ground. Avalanche diode 266 clamps low frequency signals to a voltage of, for example, five to ten volts.

Figure 19 shows another embodiment of the invention comprising a coaxial cable 27ύ having a male coaxial connector 272 attached thereto. Connector 272 contains gas discharge tube 200. The center conductor 206 of the gas discharge tube projects from the end of the male connector 272. The various parts of gas discharge tube 200 are as shown in Figure 14 and described earlier.

Figure 20 shows another embodiment of the invention which comprises a surge arrestor 280 having back- to-back female coaxial connectors 282 and 284. A gas discharge tube 200 is located between coaxial connectors 282 and 284. The embodiment shown in Figure 20 differs from the embodiments shown in Figures 15B, 15C, 16B, 17B and 19 in that the conductive housing 202 is an integral part of the conductive outer body of the coaxial surge arrestor. As also shown in Figure 20, the female coaxial connectors 282 and 284 have solid dielectric materials 286 and 288 located on either side of the gas discharge tube 200 which positions the gas discharge tube in the middle of the coaxial surge arrestor 280.

It will be understood that various changes in the details, materials, arrangement of parts and operating conditions which have been herein described and illustrated in order to explain the nature of the invention may be made by those skilled in the art without departing from the principles and scope of the instant invention.