[UK-CONTEST] ADSL reverse QRM

[Ian White GM3SEK]

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