[RFI] Internet Cable interference
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