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

To: rfi@contesting.com
Subject: Re: [RFI] Non-RF sensitive hardwired smoke/fire alarms?
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Wed, 09 Jul 2014 21:06:37 -0700
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
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
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