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
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|