Abstract:
A coaxial feedthrough connector for connecting an RF signal through a wall in a hazardous environment and particularly to an antenna. The connector has a base with an axial passage. A coaxial transmission line extends through the passage and a nonconductive, preferably silicone sealing compound fills at least a longitudinal segment of the passage and sealingly engages both the transmission line and the interior surface of the passage. A sealant is used that is approved for use at the location in a hazardous environment where it will be used.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application Ser. No. 60/460,725 filed Apr. 4, 2003. 

   STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
   (Not Applicable) 
   REFERENCE TO AN APPENDIX 
   (Not Applicable) 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to electronic equipment adapted for use in environments which are hazardous because of the presence of flammable gases, more particularly relates to an RF feedthrough connector or sealing fitting for use with explosion proof enclosures, and most particularly is directed to a compact antenna and feedthrough assembly which is leakproof and therefore can be mounted near the hazard to facilitate wireless data communication with the hazardous site. 
   2. Description of the Related Art 
   Areas near or surrounding facilities for the manufacture, storage, transportation, or use of flammable materials are hazardous environments because of the possibility of accidental leakage and ignition by a spark or flame in the environment. Consequently, there are regulations and standards of construction for buildings and equipment in order to minimize the associated risks and make them explosion proof. Explosion proof means that they will not cause an explosion and, if one occurs within an explosion proof enclosure, it will be safely contained and the pressure safely relieved. The regulations include sealing requirements so that hazardous gases cannot reach an electric arc or spark and thereby ignite a fire. 
   The most common method to isolate a potential ignition source in a hazardous location is through the use of an explosion proof enclosure rated as NEMA  7 ,  8 ,  9 , or  10  which is strong enough to withstand the pressure of an explosion in the interior of the enclosure. Such an enclosure must be designed to safely vent the hot gases resulting from an explosion in a way that the gases are cooled below the ignition temperature of an explosive mixture outside the enclosure. In order to contain the pressures of an internal explosion, the enclosure is typically constructed of heavy cast steel or aluminum. Areas around various hazardous installations are ranked, based on distance, such as Class 1/Division 1 and the closer equipment is located to the hazard, the more rigorous are the regulations. There are regulations for electronic equipment, which must meet the rating standards for the place where it is located. 
   Often it is desirable to have wireless monitoring and control systems associated with equipment in hazardous environments such as oil rigs, fuel or other flammable-containing tanks or power plants. A wireless system avoids the need for installing long wires or transmission lines. However, a wireless system requires at least one antenna which is used for wireless data transmission to and from the monitoring and control equipment. Unfortunately, a wireless antenna can not be located within the explosion proof enclosure because the metal enclosure would shield the antenna and prevent transmission of the electromagnetic waves to a remote antenna in communication with the monitoring and control system. 
   It would be desirable to attach the antenna to the exterior of the explosion proof enclosure to permit radio communication. However, the use of RF transmitters in a hazardous or potentially explosive environment could cause a spark and/or a flame path out of instrument housing containing a transmitter or transceiver and associated instruments. Currently, no antenna is rated for Class 1, division 1, which is a location close to the hazard. The result is that, in the prior art, coaxial cable must extend to a location far enough away from the hazard that an available antenna that will meet the standards for the greater distance can be used. 
   The use of wireless monitoring and control systems in hazardous environments has always presented a problem. The exposed antenna creates a series of problems since, in the prior art, the typical antenna is attached to a coaxial connector outside of the explosion proof enclosure and is displaced a substantial and sufficient distance from the hazard and from the explosion proof enclosure that it will meet the less restrictive regulations and standards for the greater distance. Some manufacturers of wireless data systems have utilized a conduit with a sealed conduit fitting and having a coaxial connector at one end for the antenna connection and a ¾″ NPT thread at the other end. The sealed fitting utilizes a combination of both fibrous packing and sealing compound to seal the connector and coaxial cable allowing the fitting to be attached to the enclosure and allowing for a length of cable and a coaxial connector to be connected at one end to the transmitter inside the enclosure and at the other end to the antenna displaced from the enclosure. 
   It is an object and feature of the present invention to provide an explosion proof structure which permits an antenna for a wireless system to be attached directly to the explosion proof enclosure or pipe so that it can be located nearer to the hazard, avoid the need for long lengths of transmission line extending a greater distance from the hazard and yet could be certified by one or more of the licensing agencies for use close to the hazard. 
   It is a further object and feature of the invention to provide an antenna having an antenna/cable interface entering the explosion proof enclosure that has a sealing process and structure that does not allow the hot gases to be able to escape from the explosion proof enclosure through this path so that it maintains the explosion proof integrity of the enclosure. 
   It is a further object and feature of the invention to provide a coaxial cable connector for feeding through the wall of the explosion proof enclosure that has the above properties and can be used to connect to a coaxial cable on opposite sides of the wall of the explosion proof enclosure, instead of being connected on one side to an antenna, so that it may be used for a transmission line feedthrough or bulkhead connector were desired. 
   It is a further object and feature of the invention to provide a coaxial connector interface that may be connected to a pipe or housing containing the coaxial cable carrying the RF signal to an antenna. The interface is sealed in such a manner as to withstand the pressure of an explosion inside an instrument housing containing a RF transmitter or transceiver preventing hot gases from escaping through the antenna connection. The RF radiating element is DC grounded to prevent a potential spark that could take place at the radiating element. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention is a coaxial feedthrough connector for connecting an RF signal through a wall in a hazardous environment. The connector has a base with an axial passage defined by a passage interior surface. A coaxial transmission line extends through the passage and a nonconductive sealing compound, filling at least a longitudinal segment of the passage, is sealingly engaged to both the transmission line and to the passage surface. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a view in side elevation of an antenna including a coaxial feedthrough connector embodying the present invention. 
       FIG. 2  is a view in axial cross section taken substantially along the line  2 — 2  of  FIG. 1 . 
       FIG. 3  is a view in axial cross section taken substantially along the line  3 — 3  of  FIG. 5  and illustrating the outer antenna base member of the embodiment of  FIGS. 1 and 2 . 
       FIG. 4  is a side view of the outer base member of  FIG. 3 . 
       FIG. 5  is a left end view of the outer base member of  FIG. 3 . 
       FIG. 6  is a right end view of the outer base member of  FIG. 3 . 
       FIG. 7  is a view in axial section taken substantially along the line  7 — 7  of  FIG. 9  and illustrating the brass insert of the embodiment of  FIGS. 1 and 2 . 
       FIG. 8  is a side view of the brass insert of  FIG. 7 . 
       FIG. 9  is a left end view of the brass insert of  FIG. 7 . 
       FIG. 10  is right end view of the brass insert of  FIG. 7 . 
       FIG. 11  is a view in side elevation of an alternative embodiment of the invention comprising a coaxial transmission line feed through. 
       FIG. 12  is a view in axial cross section taken substantially along the line  11 — 11  of  FIG. 13 . 
       FIG. 13  is an end view of the embodiment of  FIG. 11 . 
   

   In describing the preferred embodiment of the invention which is illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific term so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2  illustrate the preferred embodiment of the invention. The structure is principally symmetrical about its longitudinal axis  8  with most of its structural features being cylindrical. It provides an external, 800 MHz to 6 GHz antenna for a flameproof housing. The antenna is somewhat similar to a MOBILE MARK, Body Mount 2.4 GHz ½ wave antenna with a gain of 2.5 dBi Such an antenna is small and ruggedly constructed. However, it is modified in accordance with the invention. 
   More specifically, the preferred embodiment has a base  9  which, in the preferred embodiment, comprises an outer base member  10  and an insert  24 . The outer base member is illustrated in more detail in  FIGS. 3–6 . The outer base member  10  has a threaded boss  12  at one end which threadedly engages a non-ferromagnetic radome  14  for the conventional purpose of protecting the radiating antenna element  16  and preventing contact with it. A gas tight seal is unnecessary between the radome  14  and the outer base member  10  but, preferably, an O-ring  15  forms a seal between the radome  14  and the base  9 . The opposite end of the outer base member  10  has a threaded boss  18 , forming a ¾″ NPT fitting, for threadedly engaging the explosion proof enclosure using a hub located on the top of the enclosure or a ¾″ pipe which is attached to enclosure. The outer base member  10  is preferably manufactured from nickel plated brass. The exterior surface of the outer base member  10  is formed with two opposite, parallel planar surfaces or flats to provide an external peripheral surface  17  for engaging a wrench to ease installation of the assembly onto the enclosure or pipe. 
   The outer base member  10  also has a first cylindrical bore  20  and a coaxial second, and smaller, cylindrical axial bore  22 . The brass insert  24  is matingly received within the first bore  20  with an interference, press fit. A conventional TNC coaxial transmission line connector  26  is threadedly engaged in the second cylindrical bore  22 . The brass insert  24  is illustrated in more detail in  FIGS. 7–10 . It has a cylindrical outer wall  28  and a coaxial, cylindrical interior passage  30 . It also has an annular, peripheral groove  32  for engagement by a set screw  34  to retain the brass insert  24  within the first bore  20 . 
   The interior passage  30  of the insert  24  together with the second bore  22  of the outer base member  10  form a passage through the base  9  defined by the interior surface of those component passages. This passage is substantially parallel to the axis of the base and preferably is concentric with that axis. 
   A central coaxial transmission line  40  extends through this interior passage and is electrically connected to the TNC connector  26 . This central coaxial transmission line  40  may have its central conductor extend into the radome to form an antenna radiating element or a separate radiating element may be connected to the central coaxial transmission line  40 . The illustrated radiating elements are an upstanding conductor  16  electrically connected to the central conductor of the transmission line  40  and a second radiating element  19  folded back and electrically connected to the outer conductor of the transmission line  40 . 
   A non-conductive sealing compound  42  is filled within the passage, and particularly within the passage  30  of the brass insert  24 . The sealing compound  42  not only fills the passage but it is sealingly engaged to both the central coaxial transmission line  40  within the passage and to the interior surfaces forming the passage. Preferably, the sealing compound also extends into the second, smaller bore  22  and into sealing engagement with the TNC connector  26 . This ensures the seal between the outer base member  10  and the TNC connector  26  to prevent leakage to the outside world. The preferred sealing compound is a silicone sealant and most preferably a two part, GE-RTV-627 potting and encapsulating compound sold under the trade name ADALET ADASEAL. This sealant has been found to possess the necessary strength and flame proof characteristics and cures better and at higher temperatures and has been approved by UL for explosion proof applications. 
   It is desirable, though not necessary in every case, that the antenna have a DC grounded antenna for the conventional purpose because it is the DC that poses a known spark hazard. The DC ground can be provided by an RF choke physically mounted to a PC board and making a dc short but RF open circuit across the antenna in order to prevent the DC spark hazard. Because the RF choke and the PC board are prior art technology, they are not illustrated with any detail. A round circuit board  56  contains the radiating element was designed with a surface mount RF choke on the top side of the circuit board. This assembly can be attached to the top of the brass insert  24  using three screws into 3–48 tapped holes  50 ,  52  and  54   
   The preferred embodiment illustrated in  FIGS. 1–10  is typically assembled by attaching the coaxial transmission line  40  to the TNC connector, pressing the insert  24  into the larger bore  20  of the outer base member  10  and then inserting the attached TNC connector and transmission line  40  into the smaller bore  22 . The sealant is then forced through a mixing nozzle into the radome end of the larger bore  20  (prior to attachment of the radome). The sealant is forced into the smaller bore  22  into sealing engagement with the TNC connector  26  and into an interior, annular shoulder  44  formed in the insert  24  adjacent the central passage  22  of the outer base member  10 . This shoulder  44  acts as a mold wall to form an annular flange within the annular shoulder  44  on the exterior of the solidified sealant  42 . This annular flange increases the sealant sealing area between the insert  24  and the outer base member  10  to improve the seal. 
   An important feature of the invention is the sealant around the transmission line formed within the base  9 . This feature can be used not only for a direct antenna feed through as illustrated and described above but also can be used for an RF feed through where a coaxial cable or transmission line is connected to both ends of an RF connector mounted in a wall. An RF feed through is essentially an RF transmission line path from one side of a barrier or wall, such as the cabinet for equipment, to the other side. It is also known as a bulkhead connector. 
     FIGS. 11–13  illustrate a feed through embodying the invention. It has a base  60  with a central passage  62 . Positioned within the central passage  62  is a semi-rigid, tin plated coaxial cable  64  having a central conductor  65 . A sealing compound  66  is filled within the passage  62  and is sealingly engaged to the walls of the passage  62  and to the central conductor  65 . The sealing compound is preferably the same as described above. TNC connectors  68  and  70  are threadedly engaged in the opposite ends of the central passage  62  and electrically connected to the coaxial cable  64 . Threaded bosses  72  and  74  are formed at the opposite ends for connection to a wall or bulkhead. 
   There are at least two types of coaxial transmission lines or coaxial cable which can be used embodiments the embodiments of the invention. Both have a central wire, Teflon or other plastic material around that, and an outer, cylindrical conductor around that. One type has a mesh outer tubular conductor having a tubular insulator over that, typically an outer flexible plastic insulator. The other type has a semi-rigid (solid) outer, tubular conductor. The semi-rigid coaxial cable is preferred because the sealant seats and sealingly bonds against it easily. The semi-rigid coaxial transmission line does not require that an outermost insulator be stripped off before the sealant is applied. However, the mesh type can be used, though its outer insulating layer should be stripped off and care must be taken to assure sealing contact with its mesh outer conductor. 
   While certain preferred embodiments of the present invention have been disclosed in detail, it is to be understood that various modifications may be adopted without departing from the spirit of the invention or scope of the following claims.