The present invention relates to a fitting for connecting an antenna. More particularly, it relates to a fitting for connecting an antenna in a hazardous area environment to an electrical enclosure such as an explosion-proof enclosure, a safe area enclosure, or to an electrical conduit.
The length of an antenna depends on the frequency of the transmitted signal. Therefore, for different frequencies of transmission, and for different radio receivers and transmitters, it may be necessary to have different length antennas. Standards for installation of electrical equipment in hazardous area environments require that the installation comply with two types of protection:
an intrinsically safe barrier which will suppress an electrical surge which may potentially travel through the device, and
an explosion-proof device enclosure to withstand the maximum anticipated force of an explosion.
Prior art methods for complying with these standards as they relate to antennas include the use of a capacitive block circuit inside the explosion-proof or safe area enclosure to provide the intrinsically safe barrier, and/or an antenna which has been completely encased in a hazardous area sealing compound. This encased antenna is intended to be mounted directly to the electrical enclosure. If the electrical enclosure is not immediately adjacent to the antenna, and is instead separated from the enclosure by a conduit, then sealing fittings are required both where the antenna mounts to the conduit and where the conduit connects to the enclosure. The installation of sealing fittings is expensive, labor intensive, and time consuming (the sealing compound must be allowed to cure after it is injected into the sealing fitting).
This prior art method is highly inflexible. If the encased antenna was incorrectly specified or supplied, or if the receiver or transmitter must be replaced with one of different frequency, or if the encased antenna is accidentally damaged, there is no simple manner to replace the antenna. The entire encased antenna (and any sealing fittings installed adjacent to the antenna) must be replaced. This is expensive and time consuming.
The present invention provides an intrinsically safe, explosion-proof antenna fitting which may be threaded directly into an explosion-proof enclosure, a safe area enclosure, or an electrical conduit, without the need for further sealing fittings required at the connection point with the antenna fitting. A standard antenna may then be connected to the receptacle projecting from the antenna fitting. Should the antenna become damaged or in need of replacement, it is a simple, inexpensive, and quick matter to unscrew the old standard antenna and replace it with a new one or one of the correct length.
In a preferred embodiment, the antenna fitting is made of a corrosion resistant grade of stainless steel, threaded at one end for screwing into the enclosure or conduit. The antenna fitting defines a hollow cavity. Inside this cavity, a circuit board with a capacitive block circuit provides the intrinsically safe barrier. The cavity is then filled with a two-part compound. The first compound is a hazardous area sealing compound to withstand the expected force of an explosion. The second compound is an electrical potting compound which surrounds the capacitive block circuit and serves as an electrical insulator, protects the components from moisture, and provides strain relief for the coaxial cable projecting from a first end of the fitting. This coaxial cable connects to a radio transmitter or a receiver. Projecting from a second end of the fitting is a standard, reverse polarity SMA receptacle antenna connector to which a standard antenna may be easily and readily connected.
In a preferred embodiment, the circuit board components have a dielectric rating of 1500 volts and the capacitive block circuit prevents fault condition pass-through of alternating or direct current up to 250 volts. The circuit board components also provide a second function as they provide a means for optional radio attenuation if the radio transmits at too high a power level. This option can be jumpered out if the radio is internally attenuated.
In a preferred embodiment, the antenna fitting is designed to withstand a 6,000 PSIG hydraulic pressure test. Thus, the antenna fitting and the hazardous area sealing compound within the antenna fitting are designed to withstand the force of an explosion, eliminating the need for a separate potted sealing fitting at the point where the antenna fitting connects to the electrical conduit or to the electrical enclosure.
Referring now to
Potted fittings 18 are placed at both ends of the conduit 24 connecting the explosion proof enclosure 14 to the safe area enclosure 22 to make sure that no hazardous-area environment may travel through the conduit 24 either into the explosion proof enclosure 14 or into the safe area enclosure 22.
On the right hand side of
It may also be appreciated that the antenna 12 is connected directly to a radio unit 26 inside the enclosure 22 via the antenna fitting 10, without first going through a separate capacitive block circuit inside the explosion-proof enclosure 14 or safe area enclosure 22 to provide the required intrinsically safe barrier. This is because, as described in more detail later, the antenna fitting 10 includes a circuit board 42 with a capacitive block circuit 44 which provides the intrinsically safe barrier to suppress an electrical surge which may potentially travel through the device.
Referring to
An electrical circuit 42, which includes a capacitive block circuit 44, is housed inside the cavity 36 of the housing 30. This electrical circuit 42 is in electrical continuity with an electrical conductor such as a coaxial cable 46, which projects beyond the second end 34 of the housing 30, and which may be used to connect directly to the radio unit 26. This electrical circuit 42 is also in electrical continuity with a threaded RPSMA receptacle connector 48, which projects beyond the first end 32 of the housing 30. A standard antenna 12 may be readily screwed onto the RPSMA receptacle connector 48.
Once the electrical circuit 42 is placed inside the cavity 36 of the housing 30, a first sealing compound 50, which is a hazardous area sealing compound, is added to the cavity 36 to encapsulate at least a portion of the electrical circuit 42 and hold it within the housing 30. A second sealing compound 52, which is an electrical potting compound, is then added to finish encapsulating the electrical circuit 42. This electrical potting compound 52 serves as an electrical insulator, protects the components from moisture, and provides strain relief for the coaxial cable 46 projecting from the second end 34 of the antenna fitting 10.
While the embodiment described above shows an antenna fitting 10 for use in a typical “standard” application, various modifications could be made. For instance, the integrally formed nut 40 could be substituted by some other means for tightening the antenna fitting 10 onto the enclosure or conduit. The two-part sealing compound could be substituted by a three (or more) part sealing compound, or even by a single part sealing compound which provides both the structural integrity to resist the force of an explosion and provides the required electrical insulation characteristics. The standard, threaded RPSMA receptacle connector may be substituted by any other connector which meets the functional requirements of the antenna 12 which is to be coupled to the antenna fitting 10. It will be obvious to those skilled in the art that other modifications may be made to the embodiments described above without departing from the scope of the present invention.
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