[RFI] RFI With Smoke Detectors

[Jim Brown]

On Mon,8/17/2015 12:16 PM, Stan Zawrotny wrote:
> 433.93 MHz is one frequency used by smoke detectors. It is the 60th
> harmonic of 7.232 MHz, a 40 M SSB frequency. The chirping will occur when I
> am operating as low as the RTTY band (7.025 – 7.125).

It is VERY unlikely that the problem is caused by harmonics of the 
transmitter. FAR more likely that radiation from your antenna is 
coupling to wiring for the smoke detectors, which act as receiving 
antennas, and are exciting Pin One Problems in the detectors.

The most likely solution is the addition of a ferrite common mode choke 
to every wire connected to a detector that has the problem. The chokes 
MUST be multiple turns on #31 or #43 cores, with enough turns to move 
the resonance to the band(s) where you have problems. For more on this, 
study k9yc.com/RFI-Ham.pdf

If the alarm is AC powered, the power wiring is often the antenna. If 
your unit(s) can be run from batteries, install batteries and disconnect 
them from AC. If that cures the RFI, power wiring is the RX antenna. If 
you want to continue using AC power, you'll need to wind the choke on a 
small AC cable that you place in series between the unit and the AC 
line. If there's no room to install the choke, you're probably stuck 
with running on battery.

FWIW -- security system products are notorious for RFI susceptibility.

Another point -- by all means contact the installer and manufacturer and 
demand that they fix their problem, and at their expense. Any product or 
system that fails or does bad things in the presence of RF is defective.

73, Jim K9YC