[UK-CONTEST] ADSL reverse QRM

[David]

I concur with Ian's comments. On my router I have ferrite rings on all the
cables attached to it. These reduced to the point of no longer heard
Ethernet birdies on 20m and increase the RF power level at which the router
failed to function correctly. I then had to add a balanced LPF on the
incoming BT line to finally make it "bomb" proof on 160m. 

I have been offered BT Infinity but I understand it uses spectrum up to
about 12MHz on the BT cable. My worry is that I will take out the "Infinity"
modem when operating on bands below 14MHz. Does anyone know better?

73 David G3YYD

-----Original Message-----
From: uk-contest-bounces at contesting.com
[mailto:uk-contest-bounces at contesting.com] On Behalf Of Ian White GM3SEK
Sent: 28 May 2012 08:28
To: uk-contest at contesting.com
Subject: Re: [UK-CONTEST] ADSL reverse QRM

Roger Dixon wrote:
>Hi All
>
>I've been having "random" issues with my ADSL connection.  When it 
>works well, I get a sync speed of around 6mbps and a BT line rate of 5mbps.
>
>
>
>However, from time to time I get bunches of multiple disconnects and 
>the line rate can drop to 250kbps.  Sometimes it recovers by its self, 
>on other occasions my ISP has to reset the lime.
>
>
>
>I now think that I've discovered the connection between these issues - 
>it would appear to be the 80m club championship events!!
>
>
>
>Before I start applying ferrites everywhere I thought I would see if 
>anyone else has suffered in this way.  The set up is a telephone 
>extension lead from the master socket to the router (Thomson TG585 v8), 
>a short Ethernet lead  to the main house computer, a long Ethernet 
>connection to shack computer upstairs and a wireless connection to a 
>Sky box and other itinerant laptops.
>

It depends on how the RF is getting into the router.

First of all, try disconnecting the ethernet cable from the router to the
PC. You always have the option of changing this to a wireless link (which
will also help reduce the many ethernet birdies on the higher bands).

If that doesn't solve the problem, it indicates that RF is reaching the
router by radiation from your antenna onto the overhead telephone line. 
QRM at 3.5MHz will then arrive at the line input of the router as a
combination of common-mode and differential-mode signals. Common-mode
currents need a ferrite choke while differential mode needs a lowpass filter
in the ADSL line. Sod's Law says you'll probably need both.

An effective ferrite choke for the low bands needs about 12 turns on an
FT240-31 toroid, or one of the chokes I described a while back using the
smaller oval type 43 toroids. DO NOT SUBSTITUTE OTHER CORES! See here for
further details:
<http://www.ifwtech.co.uk/g3sek/in-prac/index.htm#1005>

G3VMW has a very good design for an ADSL LPF (google for the link).



-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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