[UK-CONTEST] RFI-ADSL Modem

[justin at g4tsh.demon.co.uk]

I am no expert, but:

First make sure all your house telephone extensions are disconnected from the BT master socket.  If that cures the problem, fit a master socket with built in microfilters and reconnect the extensions to the filtered output.  If it returns, you will need to start adding ferrites to things.  Even then you may not be rid of it.

Also make sure your antenna feedlines are not radiating in your house.  You may need to review earthing and use choke "baluns" to keep braid currents off the outer of coax before it enters the building/shack.

I have noticed that after an upgrade to a faster ADSL+ modem my EMC problems returned.  Of course the faster modems use a wider chunk of the HF spectrum and the modem "front end" will be wider open and at some point external RF will get into the system and swamp the relatively weak DSL signals on the line.  Common mode chokes will go some way to curing this, but I suspect that some sort of low pass filter on the telephone line may be required to cure it(Anyone have a design for one of these?).  The side effect of this will be that you will get a slower connection.  My old 512 and 1 meg rate limited connections were OK, but now I have a so called "up-to-8meg" connection that barely gets me 2 meg of useable bandwidth and a new ADSL2+ modem.  The EMC problem is back.  I have even managed to kill it with 5 watts of RF on 40m!

Internal house wiring is bad news for ADSL even without strongish RF fields about.  I improved the S/N on my connection by over 7dB just by disconnecting it in my home.

I don't even want to think what might be happening to my neighbours ADSL connections!  Fortunately many are on cable, which seems to be more robust.

73
Justin, G4TSH



g4ujs at qsl.net wrote:
> Hi All,
> 
> I have a problem with an ADSL Modem.... its the LinkSys AM-300 ADSL2+ 
> Ethernet Modem.
> 
> It doesn't like QRO 80m......drops the sync immediately.
> 
> I am sure others have had similar probs with RF and ADSL.
> 
> I have tried ferrite on telephone line and DC connector.
> 
> Suggestions please
> 
> Regards
> 
> Bob Harrison
> 
> Telephone: +44 7876 791215
> 
> Website: http://www.qsl.net/g4ujs 
> 
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