[RFI] Ethernet RFI

Jim Brown jim at audiosystemsgroup.com
Mon Jan 24 09:59:47 EST 2005

On Mon, 24 Jan 2005 07:34:36 -0800, sebdesn wrote:

>Jim, as an additional question near to this subject. The incoming telco wire
>for a dsl line;
>Is the overal length and make up critical?
> My line hits the house about three feet from the modem connection. However,
>it goes into the attic,then to the basement, and then back to the modem area
>probably a total of a 100' of 50 year old wire.Would I gain anything by
>getting into the box at the down lead and plugging a blocking filter on the
>wiring to the house, and then adding a separate cat 5 run for the ten feet
>directly to the modem? Any enlighten would be appreciated!

I don't consder myself an expert on this at all, but will offer these thoughts. 

1) The shorter any wire is, the less likely it is to couple RF from your rig to the 
equipment, especially at HF. So from that point of view, I would sure try to bang 
through the wall and make a more direct connection. 

2) From a data point of view, it is my understanding that the DSL signal on that 
line is relatively tolerant of line length up to the limits defined in industry 
guidelines for distance from the central office. On the other hand, we also know 
that shorter is better, and can allow higher data rates. I don't know the 
frequencies involved -- perhaps someone can jump in and enlighten us. 

3) I can't see how adding a ferrite choke to a DSL or other telco line could HURT, 
and it could solve (or prevent) overload of the DSL hardware. One of the guys I 
know from another list is currently trying to clean his 160M KW rig out of his 
cable or DSL modem. On the other hand, it is often wise to follow the course of "if 
it ain't broke, don't fix it." 

4) If you don't have problems, I would leave it alone. But if you poke around the 
wiring with a talkie that hears the ham bands and hear trash coming from that line,  
I would certainly work on cleaning that up. 

Someone could do us a great service by providing an overview tutorial on 
technologies like DSL from the ham and RFI perspective -- that is, what frequencies 
and modulation schemes do they use, is there any RF trash, and if so, where and 
what does it sound like? My guess is that someone on this list could either do this 
off the top of their head, possibly supplemented by some research and/or reading. 

As hams and working engineers, we need to keep up with the varous technologies that 
surround us. All of us can help by educating each other on those that we know 
about. I'm trying to do my part. 

Jim Brown  K9YC

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