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LF: Re: Unwanted LF radiation

To: [email protected]
Subject: LF: Re: Unwanted LF radiation
From: "Howard Aspinall" <[email protected]>
Date: Sun, 12 Jun 2005 13:27:17 +0100
Delivery-date: Sun, 12 Jun 2005 14:09:39 +0100
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Thanks to all who've taken the trouble to reply.

I'm not aware of any water pumping stations in the area, and the LF signal
remains when everything is disconnected from the line other than the
analyser. DACS boxes are the subject of an interference debate off
reflector, but I'm not aware of any round here. I think they are
more prevalent in rural areas. The sound is quite unlike LORAN,
and I can't imagine LORAN giving me over 5 mV of signal (I hope not!).

It seem the LF signal on the BT line bears the hallmarks of broadband. The signal to 130 KHz is over 5 mV into 50 ohms on the analyser. There's the broadband "forbidden" gap showing between 130 and 140 KHz, and the signal above 140 KHz, going into medium wave is weaker, as I'd expect. There's no discernable radiation above 1.1 Mhz. Using Spectran, there are no 50 Hz or 100 Hz bars visible on the signal.
I accept what several people have said, that broadband shouldn't
cause any problems, but what about a fault condition? Supposing
the "A" on a pair feeding broadband has an earth or high capacity
to earth. Whilst the broadband and speech facilities would function, wouldn't the corresponding line unbalance increase the crosstalk to other pairs in the same cable? BT say cross talk between pairs is likely and normal but crosstalk giving more than 5 mV common mode signal on my BT line doesn't seem normal to me.
The relevant technical standards for the BT system appear
to be contained or referred to in a document ND1602:2002/11, downloadable on the Web. It makes good bedtime reading!
Incidentally, I gather that the prohibition on connecting non
"BT Approved" domestic customer equipment to the BT system has now been scrapped.
Apologies for the "broad" bandwidth. Inevitably noise and local
interference will continue to affect us all, more and more. I guess these problems would be more managable if amateur radio could acquire "protected" status from Ofcom, rather than "free" licences!
Again, many thanks to one and all who responded.

Howard    G3RXH











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