[RFI] Telephones and DSL

[Jim Brown]

Agreed on all counts.  W2RU went through DSL issues, including 160M, in 
a serious way a couple of years ago, reached good solutions, learned a 
lot, and published it, I think on a website.  Poke around to find it.

In the last couple of decades, Telcos have gotten quite careless (cheap) 
about maintaining twisted pair all the way to the user equipment. 
Twisted pair is a critical element in minimizing differential coupling 
onto signal wiring, and the best twisted pair around is CAT5/6/7 sorts 
of cable.

Also, differential mode coupling is nearly always the result of common 
mode current, so common mode chokes can help a lot. Considerable 
experience has shown that to be effective, the chokes must have quite 
high choking Z -- I've measured successful products in the 30K ohm 
range, and chokes in the 5K ohm range don't cut it.  There's also the 
issue of poor common mode rejection in the telco (DSL) equipment, again 
through both poor design and playing cheap.

Obviiously, it helps to make all these cables look as little as possible 
like effective receiving antennas.That includes keeping them as short as 
practical, breaking them up with common mode chokes so that they are 
nowhere near half-wave resonant at transmitting frequencies, running 
them low to the ground (but avoiding radial systems), route them away 
from TX antennas, etc. On the TX side, certainly choke all feedlines to 
kill common mode current.

Don't rule out coupling onto cables on the user side (Ethernet), and on 
power supply wiring, and choke them too.  For 160M, 20+ turns on a #31 
2.4-in o.d. core is a good starting point, and as few as16 turns will 
yield about 5K ohms on 160 and 80.

73, Jim Brown K9YC

On 12/11/2012 2:41 PM, Andy wrote:
> In THEORY, the DSL stuff should be all differential, so passing it
> through a common-mode ferrite choke should not affect or attenuate the
> DSL signals at all.  "Too much" ferrite should not be an issue, if you
> keep the filtering common-mode.
>
> On the other hand, a 160 meter tuned circuit or trap, inserted into
> the phone wires (as a differential filter), could indeed mess with the
> DSL.
>
> This all assumes that the line is well balanced.  If it isn't, then
> your 160m signal might be getting onto the line as a differential-mode
> component, that common-mode filters won't fix.  (That might be why
> N1BUG has problems in the rain.)
>
> Andy
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