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Author Topic: Smoke/fire alarm  (Read 643 times)
KC2IRX
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Posts: 3




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« on: December 29, 2001, 09:34:19 AM »

I just got my rig set up and started tuning up.  Imagine my suprise when my fire alarm went off.  Had to call the alarm folks and the fire department to register a false alarm.  Found that it will go off whenever I transmit.  Obviously, this is unacceptable.  Has anyone had a similar problem?  Possible solutions?  As of now, I climb the ladder, disconnect the smoke alarm and go on the air.  When I'm done, I reconnect and store the ladder.....  Not the best way to go on the air.
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WA7KQG
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Posts: 1




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« Reply #1 on: December 29, 2001, 10:46:50 AM »

First take care of the easy stuff.  Be sure your antenna feed is well away from the detector.   Ground your rig and be sure all the covers are in place on your equipment.  Get a good match between your rig and the antenna.  Do everyting you can to get all the RF from the transmitter to the antenna.

The next step depends on what type of fire alarm you are dealing with.

If this is a local (single station) detector used in homes it may be a simple matter to upgrade to a better detector.  If you have several detectors that go off at once (tandem) then you have to replace all of them.  These detectors are designed to warn the occupants and normally are not monitored to summon the Fire Department.  These typically run off of 120 volts, will have a 9 volt battery inside or both.  These detectors are often sold at the local hardware store for 9 to 15 dollars and are not what you would call loaded with RFI immunity.  I use commercial version of photoelectric detectors made by Gentex in my house and have not had any RFI problems.  But then I never transmit over 100 watts and most of my activity is at 5 watts.

You mentioned in your posting that you called the Fire Dpt to tell them it was a false alarm.  This would tend to point to the fact that this detector is monitored or integral to a detection system.  Be very careful here.  It is one thing to fiddle with a device and put your self at risk and quite another to put a complete building at risk.  Fire detection devices and systems are, by their very nature, life safety systems and strictly regulated.  You need to be very cautious about doing anything that might interfere with the proper operation of the device or system.  It is quite easy to move a wire here and add a resistor there and bypass all the down stream devices.  If this is a system, call a professional, preferably one who has been in the business long enough to have seen this problem in the late 70s when cheep detectors just stared being introduced.

Of course there is always QRP. ;-)
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N9AVY
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Posts: 66




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« Reply #2 on: December 29, 2001, 11:02:49 AM »

Been in the fire alarm business about 32 years.  My experience has been that you probably have one of those stand alone detectors which is an ionization type.  If you have a full blown alarm system we can cover that later.

Home type detectors are usually of the ionization type as they are cheaper to manufacture and sell. The best way to determine this is to look at label on back; if there is a radioactive symbol then it is an ionization type. The problem with these detectors is that they have a high impedence front end and are very susceptible to RFI.  The only solution is to find a photoelectric type which are available (think I saw some at Home Depot a couple years ago).  Photoelectric type detectors have a low impedence front end and are less prone to RFI.

Now, if you have a full blown alarm system about the only thing you can do is replace the detector with a newer model or a different manufacturer.  This has worked for me in many instances. Older smoke detectors tended to be sensitive to RFI and upgrading to a newer model usually solves the problem. Better models are made by System Senor and ESL/Sentrol. If you have a system, you might also try ferrite beads on leads entering control panel and/or .01 disc capacitors from each side of loop to ground.
(Don't try that on 110 VAC systems!)

You might also check your antenna system for high SWR. A friend of mine had an attic dipole and would always set off the smoke alarms. Perhaps you might also check ground on rig as well as reducing mic/processor gain.

If, having done all the above and you still set off the smoke detector; then, you may have to resort to a big hammer.  Good luck !

73,    Jerry  n9avy
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VK2GWK
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Posts: 196


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« Reply #3 on: December 29, 2001, 04:43:57 PM »

Had a burglar system installed and warned the installer about RFI. But still at the first test the thing went of.
What he did to correct it was:
ferrite beads at all terminals with capacitors to ground.
shielded cable from the sensors to the control panel.
That solved the problem.
Antenna and power that triggered the alarm was a 3 el tribander at 60 ft next to the house and 400 W output.
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RobertKoernerExAE7G
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Posts: 1435




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« Reply #4 on: December 31, 2001, 03:35:51 PM »

What bands?
What is, are your antennas?
How do you feed the antennas?
Where are your antennas?
Do you use a current balun?
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RobertKoernerExAE7G
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Posts: 1435




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« Reply #5 on: December 31, 2001, 03:36:15 PM »

What bands?
What is, are your antennas?
How do you feed the antennas?
Where are your antennas?
Do you use a current balun?
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RobertKoernerExAE7G
Member

Posts: 1435




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« Reply #6 on: December 31, 2001, 03:36:16 PM »

What bands?
What is, are your antennas?
How do you feed the antennas?
Where are your antennas?
Do you use a current balun?
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KC2IRX
Member

Posts: 3




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« Reply #7 on: December 31, 2001, 06:09:27 PM »

I have an ESL/Sentrol 449 alarm unit.  It is part of a whole-house system which I installed myself.  The alarm unit is photoelectric.  The alarm sounds on either of two antennas -- a dipole in the attic cut for 20 meters and another one outside also cut for 20.  The feed for the attic antenna is RG-8U (no balun) and the one outside is ladder line.  I use an AT-200 tuner and have very low SWR.  I tested the other day and found that the sensor actually latches --the RFI is not being picked up by the wiring.  If I disconnect the hot lead from the sensor, I can operate with no problem.  It just gets to be a pain to climb that ladder every time I want to go on the air.  I think I'll try another unit.

Thanks to everyone for their input.  
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