[TowerTalk] [RFI] Lutron and RFI

[Tom Rauch]

> I think the idea is that if the supply and return from the 
> dimmer to the lamp are essentially a pair, then the 
> magnetic fields cancel, and it's sort of self shielding (a 
> balanced pair), since they are right next to each other, 
> and the "loop area" is small.

They are always the same distance apart, assuming the wire 
is the same wire.

Perhaps the assumption is the RFI is caused by differential 
currents, but in most cases for us they are either common 
mode or turn into common mode elesewhere in the system and 
radiate to the receiving antenna. If someone's antenna is 
not the primary source of ingress, they need to work on that 
first. We all know RFI is a lot different than ingress into 
audio systems, and audio systems are where sharing common 
outlets is a big problem.

> A shared neutral has more potential for having the supply 
> and return form a loop with some area, and making a better 
> radiator.

BPL advocates seem to think RF won't radiate from power 
lines with single returns also. Any line excited with RF 
currents will be a problem because eventually it will become 
unbalanced no matter how it is wired. In my experience 
worrying about wire routing is largely a waste of time when 
dealing with radio frequencies. Audio is another story, but 
again it is best handled by proper interconnects in audio 
lines.

If you excite anything other than a real transmission line 
that is properly installed, terminated (for balance), and 
fed.... it will radiate. And that is how things get into our 
receivers unless our stations happen to be messed up. For 
example people use longwire or other Marconi antennas with 
less than perfect grounds, and the power lines become part 
of the ground system. This allows ingress by "pumping" the 
ground with noise. In a case like this the antenna should be 
fixed. The same is true for Windoms or OCF dipoles or even 
regular dipoles without baluns.

The fact is we can run wires anyway we want through the 
house, and if a trashy device is connected to the line we 
will hear it. The system is so messed up it's pure dumb luck 
if a wiring change makes it better or worse.

The only sure cure is to make sure the antenna and feedline 
have minimal response to noise on house wiring, and make 
sure the things in our houses don't generate RFI. I sure 
wouldn't go rewiring anything to try to cure noise!

73 Tom