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
rejection.
73,
Jim Brown K9YC
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