Of course the phone line can be acting as an antenna. That's why I
suggested disconnecting it from the alarm system, then testing again. The
suggestion was not because I thought there was something wrong with the
telephone line.
73, Tim - N3XX
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <rfi@contesting.com>
Sent: Sunday, July 06, 2008 7:00 PM
Subject: Re: [RFI] RFI in home security system
> 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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