Sunday, July 4, 2010

My burglar alarm system beeps when I transmit on 15-meter SSB...

Q Tom, AB5XZ, asks, “My burglar alarm system (installed in 1992, before I had a ham license) beeps (at the keypad) when I transmit on 15-meter SSB. If I talk long enough, it eventually will arm itself, which usually results in a false alarm. The manufacturer has no suggestions other than to ground (or unground) the cabinet containing the alarm panel. I’m hesitant to fiddle with it because I don’t want more false alarms. Do you have any suggestions as to how I can best approach this problem, other than by replacing the alarm system?”

A Sure sounds like a tough one. If you want to do some trouble shooting and fear false alarms, you may want to contact the alarm service in advance to let them know what you are doing.

Earth grounding the control unit may help, and should not cause a false alarm. Make sure your ground lead is less than about 1/10 wavelength on 21 MHz (about 5 feet or less), if at all possible. Failing that, if the interference happens only on 15 meters, this indicates that something is resonant. Look for a 10 to 20-foot wire connecting to the unit. Also, if the alarm company can tell you what the alarm indicates when it activates, that may tell you which wires to treat.

The easiest thing to try electrically is to make some ferrite chokes. Use FT-140-43 (1.4-inch outer diameter) for small wires, or FT-240-43 ferrite cores for larger wires and wrap about 10 turns of the wire onto them. Place them on the cables feeding the control unit and at any remote sensors that have active electronics in them. (Make sure to try them first on the wires corresponding to whatever alarm indication the alarm company reports to you.) You could also try applying some 0.005 μF ceramic capacitors across the wires that are suspect, and/or from each wire to chassis ground. I say this is easy electrically because there are often mechanical considerations—there is not usually enough slack in the wire to put 10 turns onto a ferrite. You can also try a filter on the ac connection to the controller.

If the alarm is arming itself only when you transmit on 15 meters, I think you will be reasonably successful with one or more of these techniques. You should also read the info in The ARRL RFI Book.

From QST September 1999