Dear LF Group,
My colleague Dave Lauder, G0SNO, has spent some time investigating ADSL and
similar, and has occasionally discussed this in his EMC column in RadCom.
Apparently, the idea is that so long as the phone line is accurately
balanced, the differential-mode ADSL signal will generate minimal radiated
QRM. However, unbalance in the phone line or its termination will lead to
"mode conversion" producing common mode signals which can be radiated. This
is not too much of a problem for the phone line itself, basically a twisted
pair, but once it gets into the house, the UK phone extension wiring uses a
3 wire system which is inherently unbalanced. A splitter which separates
out the ADSL and phone signals where the phone line comes into the building
ought to minimise this, so the major concern is "splitterless" ADSL, which
may be the same as the "wires only" installation that was talked about.
I expect that, like most EMC issues, how much of a problem it actually is
will depend on the individual phone installation, how close it is to the LF
antenna, and the transmission characteristics of that particular phone line
to the exchange etc. I guess it would be mostly a near field source of QRM
rather than actually radiating - but with overhead phone lines, "near
field" could cover a wide area - probably your neighbor's ADSL would be
more of a concern then - you can't just unplug it!
Cheers, Jim Moritz
73 de M0BMU
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