[RFI] Internet Cable interference

Jim Brown jim at audiosystemsgroup.com
Thu Sep 29 08:39:37 EDT 2005

On Tue, 27 Sep 2005 14:06:52 -0400, catwhiskr at aol.com wrote:

>I have an S3 signal every 61 khz away from 14031. 

I don't know about the +/- 61 kHz component, but the 14030 stuff 
is Ethernet-related, and is emitted by MANY devices that use 
Ethernet for their interconnection, including, but not limited to, 
cable modems. Some other frequencies I've identified are 10106, 
10120, 21052, 28016, and on 6 meters between 50 and 50.125. There 
are others -- I mostly work CW. ALSO -- the exact frequency varies 
from one system to another, depending on the clocks within the 
product that is radiating the trash. I've also identified some 
broadband hash as coming from Ethernet devices. 

At least two mechanisms are at play in putting this trash into 
your ham receivers. First, the trash is radiated as a COMMON MODE 
signal by any cables connected to the device producing the trash, 
including the power connection and the Ethernet cable. In other 
words, that wiring is a long wire antenna. This component of the 
radiation can be reduced in strength a lot by killing the current 
on the cable that is doing the radiating (all of the cables). 
Ferrite chokes can be used effectively to do that. See the 
applications notes and data on my website.  


The second mechanism is direct radiation from the noisy device 
itself, thanks to an enclosure that is rarely shielded. There are 
two effective solutions to this one. #1 -- shield it. #2 -- the 
bucket treatment (see **** below). 

If you live in a densely populated area like I do, you can hear a 
dozen or more of these Ethernet sources, each on a slightly 
different frequency, and some stronger than others. You may kill 
your own component of that trash with chokes, but you will still 
hear your neighbors unless you also suppress them. 

Another important point. The proximity between your ham RECEIVER 
and the source essentially doesn't matter. What does matter is the 
proximity between the source and your ANTENNA, as well as the 
directivity of both the antenna and the noise source. 

****  The bucket treatment.  Fill a bucket with water. Put the 
device in twice. Take it out once.  Some devices need bigger 
buckets than others.


Jim Brown K9YC

More information about the RFI mailing list