[RFI] Non-RF sensitive hardwired smoke/fire alarms?

[Jim Brown]

On 7/9/2014 8:25 PM, Kelly Johnson wrote:
> I would also be interested to hear about others' experience with hardwired
> smoke/fire/CO2 detectors.  Building codes now require new construction to
> have hardwired/interconnected smoke detectors.  I read lots of horror
> stories about hardwired smoke detectors on the web.  All of the ionization
> type smoke detectors and CO2 detectors in my house have no RFI/EMI
> problems, but both of the photoelectric fire detectors chirp when I run
> QRO.  I have to pull out the battery when I run QRO.  That wouldn't be so
> easy with a hardwired detector, so I'd like to know how to avoid problems.
> Will ferrite beads on the leads be sufficient?  Are there brands that are
> known to be less problematic?

Anything i've seen from the alarm world has been a mess for RFI. Some 
random thoughts.

1) Avoid anything that uses the power line for carrying data from a 
sensor to a main unit.

2) Us very good twisted pair for ALL interconnect wiring. CAT5/6/7 is 
VERY good twisted pair.

3) Yes, do use common mode chokes at both ends of every cable, winding 
them to place the resonance(s) in the range where you will operate.  No, 
"a ferrite bead on the leads" will not be effective on the HF bands. 
Study my RFI tutorial. :)

4) In general, it is the wires attached to active equipment (anything 
with electronics in it) that act as antennas to receive our RF, and to 
radiate their RF trash. Thus, a wireless interconnect system could be 
superior to one that is wired. (But pay attention to #1 above).

5) If you're dealing with a contractor for this stuff, get it in writing 
that the system will neither cause RF interference nor receive RF 
interference, and make clear that you are talking about legal limit ham 
radio.  That language should part of your contract, and it should him 
responsible for all materials and labor.

73, Jim K9YC