[RFI] CO Detector

Jim Brown jim at audiosystemsgroup.com
Fri Feb 16 01:21:41 EST 2007

On Thu, 15 Feb 2007 19:50:30 -0500, Rick Low wrote:

>For the "ham" side of the equation, check station grounding, harmonics
>emission, and RF on the coax shield, including placing ferrite beads on the
>AC power lead to the transmitter (ok, transceiver in my case) and on the
>transmission line.

This is all a red herring, and meant to suggest that the ham might be at 
fault. When smoke detectors detect RF instead of smoke, the root cause is a 
problem with the detector, not the radio transmitter!  Period. 

>For the consumer end, unplug the CO detector and let it operate on battery
>to see if the house AC wiring is coupling RF into the detector, and place
>ferrite beads on the leads going to the detector if necessary 

This clearly indicates they don't know much about ferrite beads and 
suppression, or the help line didn't know what 20 meters is. Ferrite beads 
are essentially ineffective below 50 MHz unless you either 1) wind a lot of 
turns through them; or 2) use a LOT of them in series along the cable. 

More to the point, ferrite beads affect common mode, not differential mode. 
If the CO detector is a self-contained unit, there's no place for common 
mode current to go THROUGH the unit, so the problem is almost certainly 
differential mode. And that calls for a capacitor across the power line, not 
a choke. 

>(challenging since these detectors plug directly into the wall outlet, 


>but the previous posts on AC line filter might be possible).

In essence, you would need to attach power leads to a filter, plugging one 
into the outlet and plugging the filter into the other one. To be most 
effective, the power lead between the filter and the CO detector should be 
very short. 

Most good line filters do both common mode and differential mode filtering.

Jim Brown K9YC 

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