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Re: [RFI] Smoke and CO alarms

To: Ed Richardson <ed_richardson@shaw.ca>
Subject: Re: [RFI] Smoke and CO alarms
From: dgsvetan@rockwellcollins.com
Date: Tue, 2 Oct 2007 14:54:59 -0500
List-post: <mailto:rfi@contesting.com>
This has become an interesting thread.  One common issue becomes apparent 
when reading about the various problems encountered:  too much RF close to 
the detectors.  It is good to know that at least some units have some 
capability to withstand external RF fields without false tripping.  It 
sure would be nice to know over what frequency range and at what field 
strengths those protected units can withstand.  If anyone on the reflector 
has data, please share it. 

My house (built 7 years ago) came equipped with wired smoke detectors, all 
connected in common on one of the AC power circuits.  Although they do not 
trigger a phone call to the fire department, they will all sound in unison 
(very loud!) if any one of them trips.  At present, my HF antennas are 50 
to 200 feet away from the house and I have no problems.  However, I 
initially had an inverted-V that was broadside to the house and only 
spaced about 30 feet away for the first two years.  No problems then, 
either, but max power was only 100 watts.

Keep in mind the "divide and conquer" theory of EMI sleuthing: 
1.  You have already "divided":  you know the victim (the smoke detectors) 
and the source (your antennas - not your rig, and not your coax UNLESS you 
are feeding a balanced antenna with coax and you have a lot of RF current 
flowing on the outside of the coax shield because you have no balun).

2.   For stand-alone detectors that run on batteries only, add shielding 
(if possible, but do not obstruct air flow to the sensor) or get another 

3.  For wired detectors, the example below and from others should work. 
Remember, though, if the problem is happening at HF, then you will need 
beads and cores that are effective at HF.

4.  Ed, it sounds as if you have a fair amount of RF coupled onto your 
alarm system wiring.  It's great that you were able to get rid of the 
false tripping, but it would be interesting to know the type and 
orientation of the antennas that are producing those fields, as well as 
bands and power levels used.  Is there any opportunity to relocate the 
antennas?  In difficult situations, if replacing the detectors with 
protected types is not an option, consider antenna relocation.  If I 
recall correctly, field strength in the near field (typical situation for 
160, 80, and 40 meters) varies inversely as the cube of distance.  That 
means that moving the antenna just a little farther away could make a lot 
of difference.  In the case of wire antennas, broadside versus end also 
makes a difference.

73, Dale


Ed Richardson <ed_richardson@shaw.ca> 
Sent by: rfi-bounces@contesting.com
10/01/2007 08:19 PM


Re: [RFI] Smoke and CO alarms

I was plagued by a similar problem, setting off the hardwired, monitored
smoke alarm when operating on 80m and 15m. Too solve the problem, I had
to follow a couple steps. 

First attempt was to install ferrite chokes on the smoke detector power
and sensor lines as they entered the alarm panel. This helped a lot but
did not totally cure the problem.

Second step, with the assistance of the alarm company technician, I
installed a couple .01 uF caps across the end of line resistors. It
appeared that the RF was strong enough that with the long leads, a
significant voltage drop was being seen across these resistors. The
alarm panel sees the voltage drop as an alarm and sets off the siren and
calls the alarm company etc. 

Together these two procedures have eliminated the problem. 

For the simple battery powered devices, I am not sure where to begin. A
schematic would be my first thought. There must be some component that
could benefit from some bypassing. 

As a word of caution, modifying a smoke or CO2 detector could cause
problems in terms if liability or insurance if something terrible was to
happen in the future. If the insurance company learned that you tampered
with a device and it failed to produce an alarm, I am sure they would
try to put the blame on you and claim the insurance void...

For that very reason I had the alarm technician test the system before
and after the modification and put it in writing that the system was
tested and found to be working.

Hopefully someone on here knows a product line that has better RFI


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