[RFI] 40m dsl modem update

Christopher E. Brown cbrown at woods.net
Sat Feb 14 03:44:16 EST 2009

On Fri, 13 Feb 2009, W0MU Mike Fatchett wrote:

> It looks like I will need to build a filter.  Just on a lark I found a Kcom
> RF1 telephone filter and it did stop the 40m signal from knocking the dsl
> off the internet.  It also reduced the downstream speed from 7186 to about
> 168.  I also put some ferrites on the cables but that had no noticeable
> effect.
> Thanks for all the tips so far!
> Mike W0MU

ADSL2+ operates at up to 2.25Mhz, telephone RFI filters often cover down 
into this range.

In my case, underground service but space limited (all antennas < 50 feet 
from telephone cabling, primary < 30 feet).

I started out with mild interferance to the phones, and line errors or 
even loss of sync depending on freq running 100w.


1) Added a proper ADSL2+ line splitter to the NID.  This is a balanced 
line splitter, audio is passed on to one set of ports, 10Khz and up to the 
other.  The provider stocks them, as they can "fix" problematic services, 
by isolating the analog devices from the RF side, eliminating the issue 
with pumping RF into the in-home wireing.

2) Fresh CAT5 plenum (100% teflon insulation) run from the DSL side port 
to the DSL modem location.

3) Replaced all of the rj11 outlet plates with new units, with special 
prep.  I ran 5 turns of single twisted pair through a fair-rite 2631540002 
type31 cylinder.  This is small enough to be glued to the back of the 
plate.  I also added the same to the audio side feed in the NID.

4)  On the rj11 plate for the DSL feed and on the DSL side of the splitter 
in the NID I did similar but with 6 turns (should put the peak impedance < 

Since I do happen to work for the provider of the DSL line, and have 
alternate connectivity, I was able to monitor not just the minimal stats 
available on the modem, but the much more useful stats on the DSLAM while 
on the air.

Installation of the ADSL2+ rated splitter / moving the DSL modem onto a 
dedicated feed upped the max sync rate for a 9db u/d SNR setting from ~ 
5300 to over ~ 9000kbps while maintaining the same very low error rate. 
It also eliminated the disconnect issue, and lowered the error rate when I 
was on the air.  (I assume this is due to the RF isolation between the 
extensive house wireing playing antenna and the DSL modems new feed).

Adding the toroids got rid of any trace of RFI on the phones, and the line 
errors.  Looking at the DSLAM stats, cannot tell if I am transmitting or 

The DSL modem already had power and ethernet lines with multiple turns 
through type31 snap ons to take care of RF coming from the device.

Total out of pocket, about $25 for the cable, ferrite and rj11 plates. 
The splitter was provided, they are kept on hand for use one problematic 

All that being said, I am not an expert, YMMV, every situation is 
different and the hammer I chode just happened to fit the problem.

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