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2m and wired smoke detectors

blindfire

blindfire

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After my 2m antenna was mounted on the gable end of my 2 story colonial, I started noticing that when I transmit at 65W, my upstairs smoke alarms would chirp 3 times. All of the detectors are hard wired and interconnected.

I dropped down to 25W and the problem has gone away.

Well, the detectors are almost 20 years old, so I plan on replacing them anyway.

My wife did some googling and found that some say that if the antenna isn't grounded properly, this can introduce RF into the AC lines in the house. Not sure I believe that, but my antenna is not yet grounded. If that were true, all of my detectors would be chirping. I'm guessing that the proximity to the antenna is probably related and that they are somewhat resonant to the 146MHz freq.

I've got the ground rod in the ground, but I don't have my ground block mounted yet and I don't have the additional coax to do it anyway.

I'm headed up to HRO on Thursday to pick stuff up to remedy that part.

Has anyone else seen this before?
 
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Dont think they had them 20 years ago but there are smoke detectors that respond to RF or maybe infrared for testing. They did this so they could be tested with any TV remote. I am guessing that the receiver in these units is probably not to tight and may respond to just about any frequency if it was strong and close
 
I don't know enough about the problem described, but most smoke detectors only have a 10-year life span and as you stated, should be replaced.
 
I would be more concerned about the wiring. Although new detectors may cure the problem. Grounding is a good place to start, I would also think about using ferrite cores on the alarm wires but that's a lot of hunting and hoping.
 
Remember that 1 S-unit is a 4:1 change in signal strength. Going from S-7 to S-9 requires a 16x increase in power. The difference between 25W and 65W is less than 1/2 of one S-unit. Drop it down to 25W and don't sweat it. You'll never notice the difference.
 
My Baofeng HT causes my battery powered smoke detectors to chirp sometimes. I've asked about it on my local repeater and a few people have said that it happens to them, too. I think it's more interference with the unit itself than an electrical problem. Move your antenna away from the detector if you can and see if that helps.
 
New detectors and clamp on ferrite chokes to their power line (assuming they are all hard wired. Look up proper mix of ferrite that covers frequency in question.


Sent from my iPhone using Tapatalk
 
I don't have a problem with the choke idea...but we are talking about a wire harness on the detector with 3 wires; red, white and black.

Do I just wrap the choke around all 3 wires at once, or do I put one on the hot or ground wire?

Here is the harness on the detector.
MITHzYU.jpg
 
Clamp it around all 3. The usual ferrite material for VHF frequencies is #43. #31 is better at HF frequencies, but either will improve things.

If you have enough slack wire and the bead's hole is big enough, loop the wires so they pass through the hole twice or even 3 times. Think of these clamp on beads as a long toroid. The more turns the more inductance you'll have blocking the RF. Remember, the inductance goes up as the square of the number of turns. 2 turns is 4 times better than 1 turn.
 
cockpitbob said:
Clamp it around all 3. The usual ferrite material for VHF frequencies is #43. #31 is better at HF frequencies, but either will improve things.

If you have enough slack wire and the bead's hole is big enough, loop the wires so they pass through the hole twice or even 3 times. Think of these clamp on beads as a long toroid. The more turns the more inductance you'll have blocking the RF. Remember, the inductance goes up as the square of the number of turns. 2 turns is 4 times better than 1 turn.
Click to expand...

Awesome, thanks for the info.

Now, for argument's sake, say I put the #43 on the wires and it resolves the issue for the 2m stuff, but if I then move into the HF bands and see the same problem, can you then put on another choke for the lower frequencies? Or is there a choke the covers 160m up to 70cm?

Thanks again!
 
blindfire said:
Awesome, thanks for the info.

Now, for argument's sake, say I put the #43 on the wires and it resolves the issue for the 2m stuff, but if I then move into the HF bands and see the same problem, can you then put on another choke for the lower frequencies? Or is there a choke the covers 160 down to 70cm?

Thanks again!
Click to expand...
Yes, adding a low frequency bead in series will work. No one choke will do a great job from 160m to 70cm, but #31 is a pretty good universal material. It starts dropping off at VHF frequencies but it's still got enough permeability to block some RF. If you look at the data sheets for the material, you'll see curves of permeability vs. frequency.
Go here and look at the permeability curves for 31 and 43 to get an idea of how they perform.

I wouldn't worry about HF frequencies. I doubt the detectors are a good wide-band receiver and will be sensitive to an incredibly broad range of frequencies.
 
I can trigger my smoke detector in the hall when I use the Mt Graylock repeater. This seems to be the only frequency that this will happen on. Coincidentally it is also the repeater I use more then 5 watts.
 
New detectors are on order. We'll see how it goes.

Are there any hazards putting chokes onto these types of electrical wires? My wife is paranoid that THIS could induce heat and a fire risk.

I told her that I seriously doubted it given that chokes are on many electrical devices, including laptop power supplies. But, to put her at ease, I'll ask anyway. [grin]
 
blindfire said:
New detectors are on order. We'll see how it goes.

Are there any hazards putting chokes onto these types of electrical wires? My wife is paranoid that THIS could induce heat and a fire risk.

I told her that I seriously doubted it given that chokes are on many electrical devices, including laptop power supplies. But, to put her at ease, I'll ask anyway. [grin]
Click to expand...
No hazard at all. None.
 
JDL said:
I can trigger my smoke detector in the hall when I use the Mt Graylock repeater. This seems to be the only frequency that this will happen on. Coincidentally it is also the repeater I use more then 5 watts.
Click to expand...

Funny, that's the repeater that does it to me, too.
 
Update:

Well, I swapped out the smoke detectors, and while the chirping doesn't happen as much, it does still happen on one of the units.

On to the ferrite chokes.
 
Before you bother with the chokes, just unplug them from wired power and run them on the backup battery power to see if you still get interference. I bet you do--I don't think it's a grounding issue. If you still get interference on battery power, then the chokes on wired power aren't going to help.

The photoelectric or ionization detector (which do you have?) is probably picking up the transmission frequency through the air. I've seen this happen with other optical and energy based instruments. You might try enclosing the smoke detector in a Faraday cage (metal mesh)--provided it doesn't interfere with its ability to detect smoke.

Edit: maybe you can just shield the side of the smoke that faces the antenna with a metal plate--of tinfoil!

[tinfoil]
 
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Another_David said:
Before you bother with the chokes, just unplug them from wired power and run them on the backup battery power to see if you still get interference. I bet you do--I don't think it's a grounding issue. If you still get interference on battery power, then the chokes on wired power aren't going to help.

The photoelectric or ionization detector (which do you have?) is probably picking up the transmission frequency through the air. I've seen this happen with other optical and energy based instruments. You might try enclosing the smoke detector in a Faraday cage (metal mesh)--provided it doesn't interfere with its ability to detect smoke.

Edit: maybe you can just shield the side of the smoke that faces the antenna with a metal plate--of tinfoil!

[tinfoil]
Click to expand...
Very good point. Might as well determine if it's getting picked up by the wire or not first.
I believe all smoke detectors have both photo and ionization detectors, not that it matters here.
 
quincy said:
Have you tried a 1:1 choke Un-Un at the antenna feed point?


Sent from my iPhone using Tapatalk
Click to expand...

No...The problem, I think, is that it is a dual band antenna 2m/440. Wouldn't choking the feed point interfere with the 440 operation of the antenna? Sorry if that is a dumb question...still learning. [grin]
 
Another_David said:
Before you bother with the chokes, just unplug them from wired power and run them on the backup battery power to see if you still get interference. I bet you do--I don't think it's a grounding issue. If you still get interference on battery power, then the chokes on wired power aren't going to help.

The photoelectric or ionization detector (which do you have?) is probably picking up the transmission frequency through the air. I've seen this happen with other optical and energy based instruments. You might try enclosing the smoke detector in a Faraday cage (metal mesh)--provided it doesn't interfere with its ability to detect smoke.

Edit: maybe you can just shield the side of the smoke that faces the antenna with a metal plate--of tinfoil!

[tinfoil]
Click to expand...

That's easy enough to do.

One other question/observation.

The detector that is chirping is not the closest to the antenna. The unit is in my upstairs hall right next to my HVAC return vent (big metal box up in the atic). The two bedrooms that also have detectors do not seem to be chirping. Could the return vent be acting as an antenna if the wiring on the hall detector is in contact with the box?
 
blindfire said:
That's easy enough to do.

One other question/observation.

The detector that is chirping is not the closest to the antenna. The unit is in my upstairs hall right next to my HVAC return vent (big metal box up in the atic). The two bedrooms that also have detectors do not seem to be chirping. Could the return vent be acting as an antenna if the wiring on the hall detector is in contact with the box?
Click to expand...
Yes. Or it could act as a reflector directing the RF towards whatever is acting as the receive antenna (smoke detector wires or the detector itself).
 
quincy said:
Have you tried a 1:1 choke Un-Un at the antenna feed point?


Sent from my iPhone using Tapatalk
Click to expand...

blindfire said:
No...The problem, I think, is that it is a dual band antenna 2m/440. Wouldn't choking the feed point interfere with the 440 operation of the antenna? Sorry if that is a dumb question...still learning. [grin]
Click to expand...
As they say, the only dumb questions are the ones you don't ask. [grin]

If anything, putting a common mode choke (ferrite bead(s) on the coax, or coiling the coax) will improve things. It will keep the antenna from pushing RF down the coax shield. In my attic j-pole I coiled the extra coax in a foot diameter coil right at the antenna. Putting a few clamp on beads also works well at VHF frequencies.
 

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