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Re: [RFI] RFI With Smoke Detectors

To: rfi@contesting.com
Subject: Re: [RFI] RFI With Smoke Detectors
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Mon, 17 Aug 2015 13:45:35 -0700
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
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
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