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Message-ID: <5.1.0.14.0.20021206113746.02895c58@gemini.herts.ac.uk>
Date: Fri, 06 Dec 2002 12:23:30 +0000
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
In-reply-to: <12a.1c772615.2b1fe0f8@aol.com>
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Subject: Re: LF: Re: ADSL EMC with LF receiving?
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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