EPIRB tester

Discussion in 'The Projects Forum' started by artificer, Sep 17, 2009.

  1. artificer

    Thread Starter New Member

    Sep 17, 2009
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    Hi everyone,

    I need to create a device for an EPIRB (Emergency Position Indicating Radio Beacon), which are used onboard various sea going vessels to signal maritime distress (for more info: http://en.wikipedia.org/wiki/Distress_radiobeacon). Once activated, the EPIRB transmits a signal via 406 MHZ to overhead satellites and aircraft. To transmit this signal when not in distress is illegal as well as a potential PITA. I work as a machinery technician for the United States Coast Guard and have an upcoming event before crab season where I will be testing EPIRB's for commercial fisherman (a fully functional test, not the self test feature they can do themselves). I have a shielded box where you place the EPIRB for testing. How it works is like this: the EPIRB's I will be testing can be manually activated with a switch or activated by going into the water. On the underside of the top of the unit there are several recessed screws. Water creates continuity between these screws and activates the unit. When we have tested them in the past, you would place the EPIRB in a bracket in a shielded box and use a stiff length of wire to create continuity between the screw heads and activate the device. then you would shut the shielded box, blocking the signal from reaching the satellite. The only access to the box is what looks like a coaxial cable connection mounted in the side where you attach the computer so that it can analyze the EPIRB's signal. EPIRB's will transmit usually every 50 seconds to one minute, theoretically allowing one to activate it and seal the box, then monitor the signal output via computer so you could deactivate it in the interval. Unfortunately, in practice, this doesn't always go so seamlessly, since some EPIRB's will transmit immediately upon activation.

    To remedy this I am trying to fabricate a circuit and switch to allow a person to close or open the circuit from outside the sealed box. The EPIRB would rest in a fabricated bracket, with two stiff strands of mechanics wire making contact with the heads of the screws. A lead of some sort (maybe an alligator clip) would be attached to each length of mechanics wire; connected to each lead would be a single wire, which then would be attached to a knife switch so you could open and close the circuit. My problem is routing the wires to the exterior of the shielded box. If I just run them under the lid of the box and seal it on top of the wires, I am fairly certain the wires will be severed because of the box's design necessitates containing the EPIRB's transmission (there is a pronounced knife edge running around the interior of the lid. I am extreley hesititant try and drill the box and make some manner of shielded cable run, since the box itself is absurdly expensive and I don't want to render it incapable of containing the radio transmission. I have thought about putting a sort of timed breaker on the circuit, but that would necessitate having one timer to close the circuit and another timer several minutes later to open the circuit, giving the tester time to seal the box, test the EPIRB, then wait for the timer to break the circuit before opening the box.

    I know that was all long winded and if you made it this far I appreciate your attention. If my ideas thus far are pretty crude I apologize; I am a mechanic by trade and my electrical experience usually involves boat and engine rigging (splicing and running cable, finding shorts, etc.) I would be thankful for any suggestions you guys could provide.

    Best Regards
     
  2. bepobalote

    New Member

    Sep 2, 2009
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    Maybe you can make a little timer (2x cascaded NE555): it will activate your device only after 'x' minutes, so you have all of the time you need to close your box without requiring any modification to it. After 'y' minutes the second NE555 will deactivate the output (relay).
    I think this solution is cheap and reasonable.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    If you can make the contacts to the bolt heads inside the shielded case, you can use a BNC bulkhead connector for the on/off switching. Just place a shorting cap on the BNC connector when you want the EPIRB to run. A bulkhead BNC bolts into a 3/8" hole and makes a solid contact with the case material. Using a shorting cap to make the contact between bolt heads means there is no leakage path for the RF. They are also cheap and very sturdy.

    Run one lead to the center contact and the other to the included terminal washer.

    Digi-Key has a #ARFX1064 BNC bulkhead @ $2.56, and a 50 ohm cap #ARF1075 @ $3.28.
     
    Last edited: Sep 17, 2009
  4. artificer

    Thread Starter New Member

    Sep 17, 2009
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    thank you both for your helpful replies...I am researching both options.

    just out of curiosity, my set up using the two lengths of stiff mechanics wire to contact the screw heads and then clipping the leads in is a little hokey. Since the screw location is variable depending on the EPIRB (except for them always being recessed up on the underside of the units head), it was my quick fix option. However, I wonder if there would be a more polished way to do this. It would be easy if the location was constant, but unfortunately that isnt the case. The screw heads are fairly small and are recessed in about half an inch on the average. I had thought about some manner of modified clamp to bring the probe into contact with the screw and then wrap around the top. Another idea I had been playing around with was to create a sort of bread board on either side of the holding brackets frame, using various lengths of thin, stiff metal stock, the leads running to the switch each from the own breadboard. This would perhaps make the device a little easier for end use (though it would be more complicated, as well as having to keep spare stock to trim up for the various odd ball EPIRB).

    Any ideas on this would be appreciated.

    Thanks again for the ideas and your time.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Welcome to the Forums, and Thank You for your Service.

    You are correct; you cannot simply run wires outside the enclosure without causing an RF leak, and subsequent dire consequences.

    You also cannot simply drill a hole in the enclosure and run wires through, or mount a switch.

    Any proposed modifications to the enclosure need to be reviewed by a qualified RF Engineer, and any modifications to the enclosure would result in it having to be re-certified for RF emission levels prior to it's return to service.

    An alternative might be to use a magnetic reed switch on the inside to open/close the circuit, and a large permanent magnet on the outside to activate it. You might also use a battery-powered coil on the inside, away from the activation reed switch (so that it doesn't hold it latched closed) to activate a reed switch on the outside to get some feedback on whether it actually engaged or not, as you said their may be a delay from when the EPIRB is initally activated until it starts transmitting. Shielding the reed switch with MU metal from all directions except where the activation magnet is to be placed would help prevent accidental activation.

    Another suggestion is that while seawater is conductive, it is not nearly as low resistance as a piece of copper wire. In order to more accurately simulate the unit being submerged, you would need to test a mockup of the cover in a sample of seawater. You could perhaps just get a plastic electrical enclosure or the like, drill/countersink some holes from the inside, install some brass or copper screws of similar dimensions to those in the EPIRB's cover, partially fill the enclosure with seawater and measure the resistance. Use a similar amount of resistance in line with the switch.
     
    Last edited: Sep 17, 2009
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Beenthere,
    I considered something very similar to your proposal, but I feel that drilling any holes through the enclosure should really be a last resort. At a former employer, one of their functions was certifications of emergency aircraft beacons. Someone made a modification to the cage, and it caused an RF leak, and search/rescue teams showed up during a test. :eek:

    I'm afraid that if our OP made such a mod to the enclosure and something went awry, he would be up the proverbial creek.
     
  7. artificer

    Thread Starter New Member

    Sep 17, 2009
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    thank you for the welcome and the advice. i assume from your name and picture that you are (or were) in the Marine Corps. thank you for your service as well. is/was your MOS related to electronics?

    I never considered the resistance of copper wire as opposed to seawater. I am glad you brought that up, i will work on knocking together an experiment like you proposed so as to accurately simulate sea water activation during the test. just to make sure, insofar as modifying the resistance in line with the circuit, would this be best accomplished by simply splicing in the approprate resistor in each lead?

    This is the rf screened box I am using: http://www.sartech.com/testequip_fc2qrt.asp

    I am looking into the reed switch idea. there is a good place to mount it behind a foam pad near the top, near where the installed bnc connecter for the coaxial cable from the computer is mounted. i suppose i would attach it with its mu-metal shielded enclosure using an epoxy of some sort so as not to affect the integrity of the box. after you mentioned the reed switch i had to smile because i hadn't even considered it even though it was right in front of me the whole time. a similar setup is used when the epirb is mounted on a ship to prevent accidental activation, with a magnet in the EPIRB's bracket. if the boat sinks there is an installed hydrostatic release which pops it free to float the surface and begin transmitting.

    I had initially assumed i could tell if the circuit was closed or not be waiting to see if the EPIRB was transmitting via the computer. however, i suppose that would only tell me if it was transmitting or not, and would not prove very useful in determining if the problem the unit itself or the circuit (at least not in any economical manner). so your idea for integrating a coil so as to indicate if the circuit is functioning or not is now being put into the plan. to clarify though, would i mount an electromagnet on the inside in line with the testing circuit with a reed switch mounted on the outside as my indicator? Or are you referring to another method?

    Thanks again for your post.
     
  8. artificer

    Thread Starter New Member

    Sep 17, 2009
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    Indeed....without a paddle. the accidental transmission wouldn't be the end of the world, inasmuch as disabling a relatively expensive piece of equipment. I am not ruling it out completely, but it would be a very last resort, and i wouldn't even start drilling the pilot hole until the department head signed of on it.
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    If you do need to lead wiring outside the case, there are such things as feedthrough capacitors that should act to leave any RF inside. But you need holes in the shielding again.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    I spent six in the Corps; EAS was a number of years ago. MOS 6657; OMA tech, missile/fire control radars on F-4J/S Phantom II's. Spent a number of years after that continuing in the same line of work in civvieland.

    Well, just so that the resistance in the complete circuit was roughly the same as you measured. You might want to go 20%-30% higher, just to be reasonably certain that it will trigger under most conditions.

    Ahh, I see. A miniature anechoic chamber. At one point in my career, I was working in a very large anechoic chamber where communications satellites were tested. Try to avoid handling/disturbing the foam any more than absolutely necessary.

    I was thinking that while testing it, you could simply use Ordinance Tape to secure it, and more OT to hold some mu-metal against it. The mu-metal may or may not be overkill.

    Mu-metal can be hard to find and expensive, but Electronics Goldmine has some in stock: http://www.goldmine-elec-products.com/prodinfo.asp?number=G16600A

    And here I was thinking that I came up with the idea it first! :rolleyes: :D

    Reed switches should only carry very small amounts of current; a few mA's. Otherwise, the contacts will either rapidly burn up, or get welded together. Either would be A Bad Thing, but the latter would be worse. If the contacts welded together, you might have to wait a couple of weeks for an EPIRB battery to discharge enough to stop transmitting, so you could open the enclosure without causing search & rescue to be alerted.

    A resistor in line with the reed switch would limit the current; you'll need one anyway to simulate the resistance of seawater. But to activate a battery-powered coil to generate a magnetic field that could be detected externally ... that'll require some kind of a driver circuit and perhaps a relay that will handle the current required. The more complexity that's put in, the more likely it is that something will break.

    On using the wire to connect to the screws; I was thinking it might be convenient to just use a couple of short lengths (maybe 3/4" to 1" long) of AWG 4 solid copper wire, ends rounded off, and solder wires to them.

    Stick the rounded ends into the countersunk holes to contact the screw heads, and use ACE wrap bandages, or perhaps sections cut from inner tubes or other flat, non-conductive stretchy material like exercise bands to wrap around the transmitter box and hold the copper contacts in tension against the screw heads. It doesn't have to be fancy; just hold them in there securely without damaging the box.
     
  11. artificer

    Thread Starter New Member

    Sep 17, 2009
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    fair enough. as i said, i am really a novice when it comes to electronics, and any and all advice is appreciated. to clarify something, when i was looking at the installed bnc connecter already mounted on the box for the computer, the plug had a square plate on both sides where it was mounted with four small screws. on the inside of the box the output was encased in a clear spongy substance, almost like a semi-firm silicone coating. is this merely to protect the soldered wire end, or is the coating used to contain radio frequencies inside the enclosure? Is the flat plate with the four holes a better configuration for the bnc in these types of applications, or is it irrelevant? Also, if one were to install a bnc connector, would you wrap or coat the plug in anything where it mounts in the hole to help create a better seal (sorry if seal isn't the right term when concerning radio frequencies)? Also, do you think using a handheld rf detector (i don't know its technical name, we use it to pick up and anylze the EPIRB's self test frequency, which is when it transmits a test message to the satellite) after installing a set up like this would be a good way to check the integrity of the system?

    i am not ruling this option out; if the situation calls for it, i will kick it upstairs to the boss man and see if he bites off (it isn't the first time i have had to go shadetree to get something to work). just wanted to make sure i was exploring all potential options before i waded into it. figured it was worth running it by some folks with experience in these sorts of matters, and thus far I am very happy i did (it has been pretty educational). thanks again.
     
  12. artificer

    Thread Starter New Member

    Sep 17, 2009
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    Good idea with the mounting so as to maintain tension on the wires.

    Is the foam an integral part of the box's screening capabilities?

    fair enough on the secondary coil. time depending, i will fart around with the idea, but this whole system will be getting dragged around marina's and ship yards so keeping it as simple as possible might be prudent.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Is it a BNC connector, or TNC? They are a similar size. BNC is a bayonet-style connection with two lugs; it takes a push and a 1/4 turn to connect or disconnect. TNC are threaded. They provide better, more positive shielding than BNC, but take longer to screw/unscrew.

    Bulkhead connectors: the square flange with four holes/screws is a typical mounting. But they can leak RF if installed incorrectly, are overtightened or are loose. Fasteners must be installed at the proper torque to help prevent warping of the flange, which would cause RF leakage. When building test fixtures, we would frequently silver solder the flanges to the enclosure to make a more positive RF seal. Silver solder makes for a much better electrical and physical connection than SN63/PB37 or other typical electronic solders. [eta] Since your enclosure is made from aluminum, silver soldering is not an option. Silver epoxy MAY be an option.

    The clear, spongy substance is likely either RTV silicone or a potting compound, to help relieve stress on the coaxial cable and to help prevent the bulkhead flange nuts from becoming loose. One of the flanges may be soldered to the enclosure.

    Any time you make holes in what is supposed to be an RF-tight enclosure, you risk RF leaks. Also, if you install another BNC bulkhead connector, you know that some goofball will sooner or later mix up the RF and switch connections, or remove the wrong one first.

    If not silver solder, perhaps conductive epoxy; something like this:
    http://www.mgchemicals.com/products/8331.html
    That is not an endorsement, simply an example.

    Not really, no.

    The last thing I want to happen here is for you to get your keister in a crack over something that you did as a "field expedient" attempting to get your mission accomplished.

    In the "good old days", we could get away with almost anything. Nowadays, you can get hung out to dry just trying to get your job done - if you screw anything up.

    Whatever you do, submit a written problem statement and proposed solutions (including drawings and parts lists) to your NCOIC and/or OIC for signature, and don't do anything before you get written approval. This is to CYA.
     
    Last edited: Sep 17, 2009
  14. SgtWookie

    Expert

    Jul 17, 2007
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    The "exercise bands" I mentioned are available at your local China-Mart for around $10. After more thought, ACE wraps wouldn't be good, as they could absorb moisture/saltwater. Bicycle inner tubes could be cut into strips for use, and are not very expensive.

    Do you get a resistance measurement when you place meter probes a few mm's apart on it? If yes, then yes. If no, then no.

    KISS does apply. ;)
     
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