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Thread: UHF/VHF Transmission Triggers my Smoke Alarms

  1. #1

    Join Date
    Mar 2017
    Silicon Valley

    Default UHF/VHF Transmission Triggers my Smoke Alarms

    I have about 10 smoke alarms in home. Two of them are combination smoke/carbon monoxide alarms (First Alert BRK SC-9120B Hardwire Combination Smoke/Carbon Monoxide Alarm with Battery Backup). Very often when I transmit on my 50W UHF/VHF rig at home, it triggers the 2 combination smoke/carbon monoxide alarms. The other plain smoke alarms are unaffected. I'm told that's because the First Alert alarms are tied to an interconnect wire that allows the alarms to communicate with each other so if one gets triggered, they all (both of them) get triggered. The interconnect wire in the house acts like an antenna, and the alarms respond to an RF signal. I don't feel like I need that interconnect, so I've disconnected the interconnect wire. That reduced the false alarms, but didn't completely eliminate them. I even tried grounding that interconnect input, which also reduced but didn't eliminate the false alarms.

    Can someone recommend a wired combination smoke/carbon monoxide alarm that isn't sensitive to RFI?

  2. #2

    Join Date
    Dec 2008
    Elgin, Illinois


    I think you should contact First Alert Customer Service as you would need to modify both units installing .1 uFd Capacitors to the circuit ground as bypass capacitors for VHF & .5 uFd Capacitors to the circuit ground as bypass capacitors for UHF. Use Ceramic Disc leaded Capacitors at 100 V. If these Smoke / Carbon Monoxide Alarms are connected to 121 VAC or 24 VDC power line the RF can be Detected this way too so these leads would need to be bypassed too.

    I wonder how close the coaxial cable and external antenna are to these Smoke / Carbon Monoxide Alarms? I've seen the Commercial Smoke Detectors in hospitals being set off with as little as 1 Watt RF on the UHF Band. I'm not aware of Smoke / Carbon Monoxide Alarms being sold that don't detect RF Interference.


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