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Re: [RFI] RFI in home security system

To: "rfi@contesting.com" <rfi@contesting.com>
Subject: Re: [RFI] RFI in home security system
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Sun, 06 Jul 2008 16:00:55 -0700
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On Sun, 6 Jul 2008 18:40:47 -0400, N3XX - Tim Heger wrote:

>I would first disconnect the telephone line and then xmit to see if the 
>alarm goes off.  If not, maybe the problem is the telephone line. 

Hang on a minute -- that telephone line is an ANTENNA!  OF COURSE it can 
be part of the problem, but not because there's something WRONG with it, 
only because it's acting as an antenna!   

Let's get our thought processes clear on these things. To paraphrase the 
politicians, "it's the antennas, stupid!" 

If common mode current from that antenna to the security system is the 
problem, a ferrite choke will solve it. However, if differential voltage 
is the culprit, a capacitor across the terminals of the security system 
input or output is the more likely cure. Both mechanisms are possible, 
sometimes only one is happening, sometimes both are happening. 

In general, if there's a path for common mode current from the antenna 
THROUGH the security device to "ground" or "power", a choke is a good 
idea. If, however, the security device is at the end of the line (for 
example, a sensor with active electronics that's powered from a battery 
or the line that connects it to the security system and not from a wall 
wart, the capacitor is the more likely fix.

Note that security systems and telephones are notoriously bad for RF 


Jim Brown K9YC

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